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POWER SYSTEM STABILITY AND CONTROL

Editorial Board

Advisor:

Dr. R. Nagaraja

Editor:

M.M. Babu Narayanan

Members:

Faraz Zafar Khan

Poornima T.R.

Venkatesh H.R.

Maheedhar Patnala

Rajesh Kanchan

Thimmappa N.

Inside This Issue

From MD's Desk……………………………………………........…………………..…………......….…. 2

Detailed Case Study to Understand the Concepts of Transient Stability Analysis...……………………………………………………………..…………………………………… R. Nagaraja 4

Low Frequency Oscilla�ons in Power Systems and their Mi�ga�on …….. .......................................................................................................... K.R. Padiyar 13

Enhancing Power System Stability and Control Using Special Protec�on Systems………………………………………………….……..... Nitesh Kumar. D and Faraz Zafar Khan 16

Stability Studies for Industrial Power Systems ............................................................................................................ Maheedhar Patnala and T. Guru Charan Das 20

PRDC Provides LED Ligh�ng Solu�on for the Holy City of Puri ............................. 26

PRDC Annual Day - April 2015 ............................................................................... 27

Our Exper�se in Training ……………………………………………………………….………………... 28

Indian Power Sector Highlights …………………………………………………..…………….…….. 29

About the Authors ................................................................................................ 30

R.N.I No. KARENG/2013/51589

April - September, 2015April - September, 2015April - September, 2015Quarterly NewsletterQuarterly NewsletterQuarterly Newsletter Issues - 2&3Issues - 2&3Volume - 5Volume - 5Issues - 2&3Volume - 5

Power Research and Development Consultants

Newsletter

From MD's Desk

Dear Friends,

More o�en than not, the electric power

systems are subjected to faults of

various kinds, and hence it is

extremely important for power

engineers to be well-versed with the

stability condi�ons of the power

system. Through this column, I thought

of dwelling upon the various facets of

power system stability and control while

hopefully, readers would get a trea�se

on the specific aspects of transient

stability studies through the technical

ar�cles in this special issue.

An electrical power system comprising

synchronous generators, transmission

& distribu�on network and loads can be

analyzed for its steady state and transient

behaviour. The steady state behaviour of

the power system is fairly well

analyzed and understood as part of

short term and long term planning

studies and also through opera�onal

studies using the load flow program.

Most of the power systems would

perform well during steady state

condi�ons. However, its real behaviour

would be tested while performing

t rans ient stud ies where in the

disturbances are applied to the power

system, similar to subjec�ng oneself to

treadmill test to ascertain the human

heart behaviour.

The interconnected power system is

analogous to a spring - mass problem.

The generators are masses and the

transmission lines behave as springs.

When we excite the spring-mass system

with a disturbance, oscilla�ons are seen

everywhere. The generator which is

close to the disturbance would swing

more and the generator away

from the disturbance would swing less.

Here again, the machine with higher

mechanical iner�a will swing less, while

the machine with lesser iner�a will

swing more. However, all the generators

in the system will swing with respect to

one another.

These types of oscilla�ons in the

electrical system, wherein the electrical

system transients are coupled with the

machine mechanical system dynamics

are cal led e lectro-mechanical

transients. Power system transient

stability program is the tool to analyze

the behaviour of the power system

during large disturbances, wherein the

exc u rs i o n s o f ro t o r a n g l e s o f

synchronous machines are studied with

respect to each other. This type of

studies also reveals varia�ons in

frequency, voltage, line loading etc.

and the response of the associated

control systems like AVR, turbine-

governor, FACTS, HVDC etc. during and

post disturbance periods.

The first and foremost purpose of

transient stability study is to determine

the cri�cal clearing �me following a

three phase fault in the power system.

Cri�cal clearing �me is defined as the

�me within which the fault should be

detected and cleared so that the

associated synchronous machines do not

lose their synchronism. A power system

is said to be secure if the voltage,

frequency and the line loadings are

within the acceptable limits for the

credible system con�ngencies that are

most likely to occur. In line with this

concept, the sta�c system security

assessment is done using the

con�ngency analysis whereas the system

dynamic security assessment is

performed using the transient stability

studies. At �mes, con�ngency analysis

performed using the steady state load

flow analysis may give favourable results,

but a dynamic security assessment will

only reveal the cri�cality of the outage

/ con�ngency. While designing and

se�ng up of a new power plant,

transient stability studies are done to

finalize the combined iner�a constant of

the generator and the turbine, various

control parameters of the AVR and

turbine-governor control systems.

Sensi�vity analysis performed using the

transient stability study iden�fies the

Power Research and Development Consultants Newsletter

secure, reliable and stable power system.

I thank a l l those who have

contributed to this issue of PRDC

Newsle�er though their technical

ar�cles. I wish all the readers, their family

and friends a happy fes�ve season

ahead.

connected on the grid-side would get

affected. For grid connected CPPs,

transient stability studies are performed

to arrive at the proper se�ngs for the

islanding relays and also devise scheme

for under frequency load shedding or

generator tripping as the case may be,

in case of excess load or surplus

genera�on.

Finally, the importance of stability studies

is emphasized in the context of

protec�ve relay se�ng based on cri�cal

clearing �me for ensuring system

stability. Special protec�ons schemes are

best designed with the help of stability

studies. Also, as a measure of preven�ng

undesirable trippings during power

swings in transmission lines, it is the

general prac�ce to block the distance

relay tripping when the power swing

enters zone 3 or zone 2 and allow the

tripping only when the power swing

enters zone 1. However, the best

prac�ce is to block all the zones for

power swings, as a distance relay should

operate only during the fault condi�on.

A separate out-of-step protec�on

scheme has to be designed to safeguard

the system during unstable power

swings. Stability studies are performed

to determine the out-of-step relay

se�ngs in such cases as well. To

conclude, transient stability study is one

of the important aspects of the power

system studies and u�li�es, industries

and prac�cing engineers should give

adequate emphasis to conduct the

requisite studies to design and operate a

bandwidth of these control parameters

to give best system performance.

Another typical applica�on of the

transient stability study is the analysis

of the behaviour of a cap�ve power

plant (CPP) with its process load

connected through one or two lines to

the U�lity grid. Most of the CPPs are

synchronized with the grid mainly to

support their manufacturing processes

consis�ng of �me-varying or cyclic loads

wherein grid support is essen�al viz.,

arc furnace loads, rolling mills;

processes where no interrup�on of

power is envisaged and cases wherein

grid support provides enough system

strength for star�ng of large motors. In

case of certain grid disturbances or

even failure of grid, the process of

disconnec�ng the CPP from the U�lity

grid is called islanding of the CPP. Grid

islanding scheme consists of a single or

a set of protec�ve relays connected at

the point of islanding (also called point of

common coupling) which will sense the

disturbance in the grid and give a trip

command to the islanding breaker

whenever the set parameters exceeds

the limit. By opening the islanding

breaker, the CPP and CPP side loads are

isolated from the grid for secure

opera�on of CPP with its cri�cal loads.

The load on the grid side will survive if

the grid survives during the islanding.

However, if the U�lity grid collapses

following the disturbance, only part of

the plant load (o�en referred to as non-

cri�cal loads) that was originally

Dr. R. Nagaraja

Managing Director

PRDC, Bangalore


Newsletter

2. Sample System

To understand the various aspects of

transient stability study, typical steel plant

system shown in figure 1 is considered. The

sample system consists of an industrial plant

having its own cap�ve genera�on. The

industrial plant is connected to the 220 kV

grid though 220 kV double circuit line of

zebra conductor of 100 km length. Cap�ve

generators are of 2x120 MW capacity;

genera�on voltage being at 11 kV and the

genera�on is stepped up to 220 kV using

141.5 MVA genera�ng transformer (GT). Unit

auxiliary transformer (UAT) load at 6.6 kV is

fed by a 16 MVA transformer. UAT load

consists of 6 MW lumped load at 0.9 power

factor and a largest boiler feed pump (BFP)

motor of 3.6 MW ra�ng.

In the transient stability studies, one is

generally interested in the rotor angle swing

whereas in the dynamic stability study, the

performance of the various control func�ons

to bring down the oscilla�ons of different

state variables in the system is studied.

Reference [1] gives the elaborate concept of

the power system stability and control, being

wri�en by a prac�cing power system

engineer. Reference [2] gives the various

aspects of power system stability studies.

This paper is wri�en to help the prac�cing

system study engineers to understand the

concept of stability studies through a typical

case study. Emphasis is given to understand

the physical concept and interpreta�on of

results rather than detailed mathema�cal

analysis.

1. Introduc�on

Electrical power system is one of the most

dynamic and complex human made systems

on earth. Complexity is due to different

voltage levels, amount of power being

handled and the varie�es of equipment

being used. Dynamic is because of the �me

frame and response to system disturbances,

which is several days for energy resource

dynamics and of the order of micro seconds

to nano seconds during fast and very fast

transients in the power system. Power

system stability studies fall under electro-

mechanical oscilla�on studies. These studies

are further classified into transient stability

studies for large disturbances and small

signal stability study or dynamic stability

studies for small disturbances in the system.

Technical Article

Detailed Case Study to Understand the Concepts of Transient Stability AnalysisR. Nagaraja

Figure 1: Sample system to understand the aspects of stability study


and line loadings are within the permissible

limits and about 100 MW of power is being

exported to grid.

4. Transient analysis for typical

case of single machine

connected to infinite bus

Transient stability concepts are be�er

understood through classical representa�on

of all the machines, i.e. constant voltage

behind the transient reactance xd'. The

analysis is similar to single machine

connected to infinite bus, grid being treated

as infinite bus having an equivalent machine

with fault level of about 4000 MVA and large

iner�a constant of 1000 MJ/MVA on 100

MVA base. A three phase to ground fault is

considered at the 220 kV grid bus occurring

at 1 second from the start of the simula�on.

Fault is cleared at 1.1 second (corresponding

to zone 1 fault clearing �me of 5 cycles).

Figure 2 shows the plot of machine terminal

voltage. When the fault occurs at 220 kV grid

bus, voltage at 220 kV grid bus is zero during

the fault and the plant generator terminal

voltage comes down to almost 55%, as the

machine feeds to the fault. Once the fault is

removed, the voltages restore to pre-fault

values.

governor system control blocks are

considered and all relevant data is furnished

in annexure. SVC control block considered is

taken from reference [1] and values are fine

tuned to minimize the oscilla�ons in the

system. SVC control block schema�c and the

transfer func�on parameters are also

furnished in annexure. All the simula�on

studies have been performed using the

MiPower™ so�ware package.

3. Steady state load flow results

For any transient study simula�on, it is

essen�al to define the steady state condi�on

and analyse the load flow problem to

establish the ini�al condi�on to solve the

differen�al equa�ons being used in the

transient problem. Figure 1 also depicts the

base case load flow results. Both the

generators are scheduled to generate 110

MW each, with machine terminal voltage set

at 1 pu. The GT taps are set at 105% to push

the required reac�ve power to the system.

For the load flow condi�on, the average

power of the varying load has been

considered. BFP motors are set to operate at

2% slip. SVC opera�on is not considered for

steady state simula�on. It is seen from the

load flow results, that all the bus voltages

The industrial process load consists of non-

varying power plant auxiliary load and other

clean and firm loads of steel plant and

varying rolling mill load. The fixed load is

distributed at 33 kV through a 220/33 kV,

100 MVA power transformer. The sta�on

auxiliary load is not explicitly shown in the

diagram, as lower voltage buses are not

explicitly depicted in the sample system and

all lower voltage loads are lumped at 33 kV.

The rolling mill load is cyclic in nature having

a cyclic period of 200 seconds. The varying

load is connected at 33 kV through a

dedicated 220/33 kV transformer. While

designing the industrial system, it is always

be�er to segregate the varying load and the

fixed process loads to different transformers,

so that the voltage varia�ons and harmonics

of the varying load do not affect the plant

auxiliary loads. In this case study, the average

power of varying load is 21.75 MW at 0.707

power factor. Figure 1 also shows the MVA

ra�ng of the transformer in the system along

with the percentage impedance value on

transformer MVA ra�ng.

One more important aspect of the industrial

system design is to iden�fy the islanding

breaker with a view to isolate the essen�al

plant load and genera�on from the rest of

the system for severe grid faults. In the

sample system considered, one of the

generators along with the varying load and

grid lines are connected onto the 220 kV grid

side bus. The second generator and the fixed

part of the plant load are connected onto the

220 kV plant side bus. Bus coupler breaker

connects both the 220 kV buses and gets

isolated for severe gird faults/disturbances.

Annexure gives the various data considered

in the sample system. Informa�on given in

the SLD and the data given in annexure is

adequate to re-produce the results using any

power system analysis tool. Sta�c Var

Compensator (SVC) of 100 MVAR is

considered at 33 kV bus of rolling mill load.

IEEE Type 1 excita�on system and turbine Figure 2: Classical representa�on - machine terminal voltage plot for three phase to

ground fault


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ensuring the system stability. In figure 3, area

of the hatched por�on in red colour indicates

the energy available for the rotor angle to

accelerate and the area of the hatched

por�on in green colour indicates the energy

available for the rotor angle to decelerate. As

long as the decelera�ng area is more

compared to accelera�ng area, the system

angular stability is ensured. Cri�cal clearing

�me is defined as the maximum fault

clearing �me, at which the fault should be

isolated in order to ensure the system

angular stability. If the fault is isolated at the

cri�cal clearing �me, area under the

power Pe will suddenly increase and at this

instant it will be more than the mechanical

power Pm. Under this condi�on, machine

speed starts de-accelera�ng and rotor angle

decreases. As power system engineer, one

s h o u l d a p p re c i a t e t h i s b e a u �f u l

phenomenon in-built in our electro-

mechanical system. Fault has occurred

somewhere in the system and the protec�on

system si�ng in the vicinity of the fault has

operated and isolated the fault. At the

machine terminal, without any control

ac�on, the rotor angle which started

increasing automa�cally starts coming down,

It is quite interes�ng to observe as what

happens to machine electrical power output

during the fault. Electrical power output of

the machine is given by the expression,

Wherein, E and V are machine internal and

terminal voltages respec�vely, X is the

reactance of the machine and δ is the angle

between internal and terminal voltages. As

soon as the fault occurs, the terminal voltage

comes down. As the classical machine model

is considered in the study, the internal

voltage E remains the same. However, as

terminal voltage V comes down due to fault,

electrical power output of the machine also

decreases. Figure 3 shows the plot of

generator 1 mechanical power and electrical

power varia�on. Since no turbine governor

effect is considered in this case, the

mechanical power remains constant and

only electrical power varies.

Now, let us look at the machine swing

equa�on, given by the expression,

Wherein, H is the iner�al constant of the

rota�ng masses of the generator, Pm and Pe

are the mechanical and electrical power of

the generator, respec�vely.

During the fault, as electrical power output is

less compared to mechanical power output,

the rotor starts accelera�ng and rotor angle

increases. Figure 4 shows the plot of the

machine swing curve. At 1.1 second, the fault

is removed and hence terminal voltage

restores to its original value (Ref figure 2). By

this �me, the rotor angle δ would have

increased and since sinδ is more now

compared to pre-fault condi�on, electrical

Figure 3: Generator mechanical and electrical power for three phase to ground fault - Classical machine representa�on

Figure 4: Classical representa�on - swing curve plot for three phase to ground fault


Even though the classical representa�on of

the machine with constant voltage behind

the transient reactance xd' is adequate to

understand the basics of power system

transient stability, detailed representa�on of

the machine including the damper winding

effect and field winding effect (sub-transient

model) is adopted to apply the transient

stability problem to prac�cal systems.

Further, if the transient stability simula�on

�me period is less than 1 second and one is

interested only in the first swing of the

machine to conclude on the system stability,

it is generally not required to model the AVR.

However, if the simula�on �me period is

more than 1 second, AVR should be

modelled. Besides, if the simula�on �me

period is more than 3 seconds, it is generally

advised to model the turbine-governor

control system as well. In the transient

stability study, it is assumed that the

abundant steam pressure or water head is

available and therefore, the boiler dynamics

are generally not modelled. In the

subsequent case studies, the sub transient

model of the plant machines is considered

including the AVR and turbine-governor

control system. Grid machine is con�nued to

be represented as classical model.

generator should not trip and to ensure the system stability and stable opera�on, the generator protec�on system should be properly co-ordinated for external faults and generator should be the last one to trip for the external system faults.

5. Sub transient modeling of machine with AVR and Turbine-Governor System

region is equal to the area under the decelera�ng region. This concept is called “Equal Area Criteria”, generally taught and answered in the power system stability class.

It is quite interes�ng to observe what happens to machine frequency, which is indica�ve of the rotor speed. Figure 5 shows the frequency of the generator 1 plo�ed along with the grid generator frequency. As the grid iner�a constant is very high, the generator 1 frequency oscillates with respect to the grid frequency. It is further observed from figure 1 that the generator 1 con�nues to oscillate with respect to the grid machine. The analogy is similar to a table top puppet as shown in figure 6, having ideal spring. The large base is equivalent to the infinite grid and the puppet head is analogous to generator 1. When the puppet head is pushed down and released, it con�nues to oscillate forever, as the spring is ideal. Similar to this, for the single machine connected to infinite bus case, as there is no field winding and damper winding effects as also without the AVR and governor control func�ons, there is no damping to this electro-mechanical system and oscilla�ons con�nue for ever. From figure 5, the �me period of oscilla�on is found to be 0.66 second resul�ng in the natural frequency of oscilla�on of this electro-mechanical system as 1.52 Hz.

It is general prac�ce to observe the machine electrical power output transients ge�ng recorded using the power plant digital control system (DCS), whenever major grid disturbance occurs and power plant trips or when damage occurs to the power plant equipment during the system disturbance. Power plant operators observe at �mes that a 700 MW generator delivers 1200 MW, 62 MW generator delivers 100 MW and perhaps even conclude as 'either DCS is faulty' or 'this high power delivery has resulted in the damage to the system'. However, from figure 3, it is clear that this phenomenon is quiet natural and as soon as the fault is cleared, the electrical power jumps and in the sample system considered, it has touched almost 200 MW, for a 120 MW generator. Further, for any of these momentary transients in the system, the

Figure 5: Generator frequency plot for three phase to ground fault - Classical representa�on

Figure 6: Analogy of single machine connected to

infinite bus


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to slip. If the load torque at the star�ng is

more than the star�ng electrical torque,

motor will not start. Opera�ng point of the

motor is at that s l ip, wherein the

transformer with lower impedance, etc.

should be studied as alterna�ves to arrive at

a suitable op�on. Figure 9 shows the plots of

motor electrical and load torque with respect

Figure 7 shows the plot of the swing curves

when the fault at the grid 220 kV bus is

isolated at 0.l second and in another case, at

0.3 second. In both the cases the system is

stable, even though the angular excursion is

high at the la�er case. It can be found that

the cri�cal clearing �me for the sample

system for the fault at 220 kV grid bus is

around 0.36 second. Most of the grid codes

specify the maximum fault clearing �me, for

which the system should be stable. For

example, in the Indian gird code, it is

specified that for 220 kV faults, the system

should be stable for fault clearing �me of

0.16 second. While performing the transient

stability study for the interconnected system,

the system design should ensure stability for

the specified fault clearing �me. From figure

7, it is observed that even a�er 10 seconds,

the rotor angle con�nues to oscillate, with

low frequency oscilla�on of about 1.5 Hz.

This oscilla�on can be curtailed by judicious

deployment of power system stabilizer (PSS)

in the system. Discussion on the PSS, its

tuning and applica�on are beyond the scope

of this paper. Reference [1] gives the detailed

discussion on PSS.

6. Motor Star�ng

While designing the industrial system, one

should always conduct the motor star�ng

studies to ascertain the extent of voltage dip

while star�ng the largest motor within the

plant, while other motors and loads are s�ll

connected to the same bus at which the

largest sized motor is connected. Figure 8

shows the voltage dip at the 6.6 kV power

plant auxiliary bus, when the 3.6 MW BFP

motor is started. It is assumed that the motor

load torque varies as the square of the motor

speed. It is concluded that the voltage dips to

the extent of 0.91 pu on 6.6 kV base and the

motor star�ng �me is around 5 seconds. If

the voltage dips to less than 0.85 pu or as

specified in the industrial system design

standards, remedial measures like star-delta

star�ng, so� star�ng, resistance star�ng,

auto-transformer star�ng, series capacitor

star�ng, higher ra�ng of the incoming

Figure 7: Detailed model of plant machine - swing curve under stable opera�on

Figure 8: Voltage dip during motor star�ng

Figure 9: Motor electrical and load torques


lines from the grid. The reac�ve power

varia�on is minimized by installing the

dynamic var compensa�ng devices like SVC.

U�lity should ascertain from the system

studies that the transient and steady state

voltage dip & voltage flicker level at the PCC

and current harmonic distor�on limits in the

grid lines are within the acceptable limits as

per the grid code. Even though the ac�ve

power is varying, within the demand block

period of say 15 minutes, industries are

required to maintain the scheduled demand

i.e., either import or export of ac�ve power.

It is quite un-fortunate that at �mes without

knowing the power system behaviour,

u�li�es insist on limi�ng the sudden ac�ve

power varia�on and also curtail the voltage

harmonic limits at PCC. Current harmonic

distor�on can be curtailed by the industry

and voltage harmonic distor�on needs to be

ascertained by the u�lity. As seen in figure 3,

even during system fault, there is sudden

varia�on in the ac�ve power and it cannot

be curtailed. Transient and steady state

voltage dip, frequency varia�on, current

harmonic distor�on limits and the flicker

level are the key parameters to be measured

and controlled, rather than the sudden ac�ve

power varia�on.

generator performance and also opera�on.

Too much frequency varia�on will cause

sha� vibra�on and damage. Most of the

manufactures prescribe that the transient

electrical power varia�on for con�nuous

opera�on should be less than 25% of the

machine ra�ng

4. To determine whether it is possible to

run the cap�ve power plant and the plant

varying load without the grid support

5. To study the requirement of the dynamic

Var compensators like SVC or STATCOM and

designing the ra�ng and control ranges for

the same and

6. To study the system performance

improvement with the installa�on of the SVC

and compute the flicker levels before and

a�er installing the SVC

Figure 10 shows the voltage plot at 33 kV

rolling mill load bus without and with SVC. It

can be seen that with the help of SVC, the

voltage dip is curtailed to a large extent.

Figure 11 gives the voltage plot at 220 kV

bus. It is concluded that with the help of the

SVC, the 220 kV bus voltage varia�on is

within the acceptable limits.

Most of the steel plant loads are �me-

varying in nature and it is not possible to limit

the ac�ve power varia�on on the incoming

electrical torque cuts the load torque. Data

required to model the motor for star�ng

studies are obtained from the no load and

short circuit tests of the motor. When the

mul�ple motors are started at the plant bus

simultaneously, the motor star�ng current

and the star�ng �me may cause the tripping

of the incomer feeder or transformer over

current relay. In such cases, instantaneous

and over current relay se�ngs should be

properly co-ordinated with the motor

star�ng current and the star�ng �me to

avoid the nuisance trippings.

7. Cyclic Load Varia�on

Using the transient/dynamic stability

analysis program, it is also possible to

ascertain the effect of the varying load on the

system performance. Varying loads can be

represented as cyclic loads in most of the

so�ware tools by defining the �me period

and ac�ve and reac�ve power load at

different �me intervals. Rolling mill load

considered in the present case study has the

�me period of 200 seconds. The cyclic load

data is given in annexure. The data pertains

to typical hot strip mill having 5 roughing

passes and 7 finishing stands. Equal �me

interval is considered for each pass and

stand, even though in reality, the �me

interval will vary. The cyclic load varia�on

study is required to ascertain the following:

1. To determine the transient voltage dip at

the point of common coupling (PCC) and

compare this with the acceptable values

prescribed in the u�lity grid code

2. To determine the voltage varia�on at the

plant 33 kV bus and ascertain the effect of

this voltage varia�on on the other loads

3. To determine the voltage, frequency and

power varia�on at the generator terminal to

determine the effect of these on the Figure 10: 33 kV rolling mill bus voltage without and with SVC


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frequency protec�on would operate and

trip both the units, resul�ng in plant

total black out. On sensing the tripping

o f g r id l ines , i f the i s land ing

delay of 1 to 2 seconds and generator under

frequency protec�on limit is generally set at

47.5 Hz with a �me delay of 1 to 2 seconds.

Hence, it is quite certain that generator

6. Grid islanding

For major disturbances in the u�lity, cap�ve

power plant generator along with the

essen�al plant load should be disconnected

and their tripping should be minimized to

avoid total black out in the plant. Design of

the proper grid islanding scheme is done by

performing various transient simula�on

studies and arriving at the proper se�ngs for

the islanding relays and incorpora�ng the

required control func�ons. For the sample

system considered in this paper, whenever a

major grid disturbance occurs, the islanding

breaker trips. CPP unit 1 along with the

essen�al load is always protected from

tripping. If the grid survives, the CPP unit 2

along with the rolling mill load survives and if

grid dies, the CPP unit 2 collapses along with

the rolling mill load.

For the sample system considered, the

importance of segrega�ng one of the power

plants with the essen�al load from that of

the second unit and the varying load is

illustrated by considering a severe fault at the

grid (3 phase bus bar fault at grid bus). As a

consequence of the fault, it is assumed that

both the grid lines trip. Following cases are

simulated:

Case 1: No opera�on of the islanding breaker

and

Case 2: Tripping of the islanding breaker

within the plant Figure 12 shows the CPP

generator terminal voltage for both the

cases. It can be concluded that voltage

recovers and system is secure. However, it is

not just sufficient to look at the voltage plot,

but it is essen�al to observe the frequency

plot. Figure 13 shows the plot of generator 1

frequency for both the cases. In case 1,

machine frequency raises to almost 55 Hz

and in the reverse swing reaches to almost

46 Hz and then se�les at about 50.5 Hz.

Power system engineer without the

protec�on background may conclude that it

is stable opera�on as both voltage and

frequency are finally se�ling. However, it is

important to note that the turbine

mechanical over speed trip is at 55 Hz,

instantaneous and generator over frequency

protec�on is generally at 52.5 Hz with a �me

Figure 11: 220 kV plant bus voltage without and with SVC

Figure 12: Plant generator #1 terminal voltage for grid islanding cases

Figure 13: Frequency of plant generator #1 for grid islanding cases


Turbine & Governor Control Block Data

IEEE Type 1

Droop: 5%, T : 0.1 s, T : 0.03 s, T : 0.4 s1 2 3

K : 0.276 pu, K : 0.324 pu, K : 0.4 pu, 1 3 5

K : 0 pu7

K : 0 pu, K : 0 pu, K : 0 pu, K : 0 pu2 4 6 8

T : 0.26 s, T : 10 s, T : 0.5shp rh ip

Gpup: 0.1, Gpdown: -1, P : 1.05 pu, m ax

P : 0 pumin

AVR Data

IEEE Type 1

Tr: 0.05 s

K : 100, K : -0.05, K : 0.05a e f

Ta: 0.1 s, Te: 0.5 s, Tf: 0.5

Vse1: 0.06, Vse2: 0.3

Vrmax: 1, Vrmin: -1

Efdmax: 2.7, Efdmin: 0

Annexure – System Data

Plant Generator Data – 120MW Rated MVA: 141.5 MVA

Terminal Voltage: 11 kV

Iner�a constant: 3.1 MJ/MVA

X : 1.95 pu, X : 1.84 pud q

X' : 0.22 pu, X' : 0.38 pud q

X" : 0.17 pu, X" : 0.18 pud q

T ': 7.15 s, T ': 2.5 sd0 q0

T ": 0.039 s, T ": 0.15 sd0 q0

Transmission Line L1&L2 Data

Voltage Level: 220 kV

Length: 100 km

Z: 0.0748746+j 0.3992516 ohm/km

B/2: j1.466942e-006mho/km

Grid Data

Voltage Level: 220 kV

Iner�a constant: 1000 MJ/MVA on 100 MVA base

Fault level: 4000 MVA

Source X/R Ra�o: 20

Motor Data

MVA ra�ng: 4.6875

MW ra�ng: 3.6

Voltage ra�ng: 6.6 kV

Stator resistance: 0.00591 pu

Stator reactance: 0.11111 pu

Rotor resistance: 0.02565 pu

Rotor reactance: 0.11111 pu

Magne�zing reactance: 3.98938 pu

Iner�a constant: 1.4881 second

Running slip: 0.02

Star�ng method: Direct On Line (DOL)

Load Torque characteris�c: Propor�onal to square of the speed

breaker also trips within a �me delay of

about 2-3 cycles, generator unit 1 along with

the essen�al plant load survives. From

frequency plot in figure 13, it is concluded

that the generator frequency excursions are

well within the generator over and under

frequency relay se�ngs. On the similar lines,

if there is import of power at the islanding

breaker during the islanding, part of the

loads can be tripped to mi�gate the

overloading of the generator. The under

frequency load shedding se�ng should be

co-ordinated with the generator under

frequency tripping se�ng, so that the load

shedding occurs before the generator

tripping takes place.

9. Conclusions

In this ar�cle, the concept of transient

stability study is explained through a typical

case study. The methodology explained

star�ng from the data requirement, studies

to be performed etc. to the analysis of results

can be used by the beginners of the system

stability studies to move forward from the

usual steady state analysis like load flow to

gain exper�se in power system stability

studies.

10. References

[1.] PrabhaKundur, “Power System Stability and Control”, (Book), Tata McGraw Hill Educa�on, 1994.

[2.] R. Nagaraja, “Power System Stability Studies”, PRDC Newsle�er special issue on 'Power system studies', vol. 2, issue 1-3, January-September 2012, pp 12.


Newsletter

Table 1:Rolling mill load varia�on data, power factor = 0.707

From -second

To - second Laod in MW Remarks

0 10 0

10 20 10 Roughing pass 1

20 30 0


40 50 0


60 70 0


80 90 0


100 120 0

120 130 35 Finishing stand 1






180 190 35 Finishing stand 7 190 200 0

SVC control Block Schema�c

MiPower Free Programmable Block Realiza�on of SVC control


examples.

2.. Review of PSS

While analog PSS with the input signal based

on the integral of accelera�ng power is

available since eigh�es, the digital PSS

(labelled as PSS 2B) was proposed in the

nine�es. Subsequently, PSS4B was

introduced based on the work carried out at

IREQ, Canada. The advantages are be�er

performance at low frequencies of

oscilla�ons (0.1 -0.8 Hz). Kamwaet al(2005)

claim that in spite of usage of PSS for a long

�me, “ it may s�ll be one of the most

misunderstood and misused pieces of

generator control equipment. Following the

western U.S. interconnec�on blackouts in

1996, was found that key PSS's were either

out of service or poorly tuned. Even today,

a�er these problems have been fixed, large

disturbances tend to induce 0.2 Hz low

frequency oscilla�ons in the grid. In Brazil,

the north-south interconnec�on has given

rise to a new low-frequency inter-area mode

between 0.17 and 0.25 Hz, necessita�ng a

retuning of PSSs throughout the system.

Inter-area oscilla�ons have also been

reported on the UCTE / CENTREL

interconnec�on in Europe, at 0.36, 0.26, and

even 0.19 Hz. The recent 2003 blackout in

eastern Canada and the U.S. was equally

accompanied by severe 0.4-Hz oscilla�ons in

several post-con�ngency stages.”

require special control measures for their

mi�ga�on. Normally, damper(amor�sseur)

windings provided on the generator rotor are

adequate.

Mi�ga�on of local and inter-area modes are

a�empted using Power System Stabilizers

(PSS). PSS using speed or frequency,

electrical power signals have been used.

With sa�sfactory design, they are useful in

damping local mode oscilla�ons. The block

diagram of a PSS is shown in Figure 1.The

input signal to PSS can be derived from

speed, frequency or electrical power.

However, speed and frequency signals can

destabilize the torsional modes of the

turbine generator sha�. The power signal

can cause excessive Var modula�on during

mechanical power changes. Thus, a

composite signal that represents the integral

of accelera�ng power is used in PSS.

The development and applica�on of FACTS

controllers in AC transmission lines has made

it feasible to apply damping controllers in

these power electronic devices. Both shunt

FACTS controllers (SVC and STATCOM) and

series FACTS controllers(TCSC) are being

widely used for control of voltage and power

flow. In addi�on, it is feasible to modulate

the voltage and power flow based on

varia�ons inthe signals derived from local

measurements of voltage and power flow.

The control law and the op�mum loca�on of

these damping controllers can be obtained

from energy concepts, The methodology for

the design of damping controllers based on

FACTS will be presented in the paper with

1. Introduc�on

During normal opera�on of power systems,

the voltages and currents in the transmission

lines remain steady and vary slowly as the

power outputs of generators vary depending

on the changes in the load. The load

varia�ons are assumed to be slow. However,

even in steady state opera�on, there are

small disturbances present due to small,

random changes in the load. If the system is

small signal stable (steady state stable),

the transients due to perturba�ons in the

system decay and do not pose any problem.

However, if the opera�ng point(equilibrium

point) is not stable, then even small

perturba�ons in the system can lead to

spontaneous oscilla�ons that can grow and

lead to loss of synchronism. These

oscilla�ons are normally caused by

oscilla�ons of the generator rotors that have

frequencies in the range of 0.1 to 2.5 Hz. If

there are N generators in the system, the

number of frequencies are (N-1). The

frequencies of oscilla�ons depend on the

loading of generators and system

configura�on. The modes of oscilla�on

having frequencies in the range of 0.8 to 1.8

Hz are labelled as local modes and typically,

small number of generators in a specified

area, par�cipate in these oscilla�ons. The

modes of oscilla�on having frequencies in

the range of 0.1 to 0.5 Hz are labelled as

inter-area modes and several generators,

spread over a large area par�cipate. In

general, it can be said that as the frequency is

reduced, more number of generators

par�cipate. Conversely, less number of

generatorspar�cipate in oscilla�on having

higher frequencies. When only generators in

a power plant par�cipate, the modes of

oscilla�on are called as intra-plant modes.

Typically, the intra-plant modes do not

Technical Article

Low Frequency Oscillationsin Power Systemsand their MitigationK.R. Padiyar

Figure 1: Block diagram of PSS


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the two swing modes, with the controller are:

Th is c lear ly shows the ro le o f the controller in damping Mode 2.With mul�ple controllers there is a need for coordinated control, that is simultaneous tuning of all the control parameters.

5. Shunt FACTS Controller (STATCOM)The damping controller termed as SMC (Supplementary Modula�on Controller) associated with the STATCOM is designed to modulate the reac�ve current injected by the STATCOM. At bus j,

X�� = 0.0450, we get the closed loop e igenva lues for the swing modes , a�er installing the SMC at a STATCOM connected to bus 4 as,

and

The shunt FACTS controllers are not as effec�ve as series FACTS controllers as they are suscep�ble to the phenomenon of “Strong Resonance” or mode-coupling between the swing mode and an exciter mode. This is a generic phenomenon that is also observed in the tuning of PSS.

We have,

and

Figure 2 shows the damping controller for a series FACTS controller that includes a washout circuit. Tm can be chosen as 0.01s.

4. ExampleConsider the 3 machine system shown in Figure 3. The data is essen�ally same as in [4] except for the following m o d i fic a �o n s . A l l g e n e ra t o rs a r e equipped with sta�c exciters with KE = 200, TE = 0.05. A shunt susceptance o f 0 . 5 p u i s p ro v i d e d a t b u s 5 fo r voltage support. The loads are assumed to be constant impedance type and mechanical damping is assumed to be zero.The eigenvalues for the swing modes at the opera�ng point are calculated as,

The tuning of the two parameters associated with a damping controller (G and x��) was done using Sequen�al Linear Programming (SLP) op�miza�on technique to maximize the damping of M o d e 2 s u b j e c t t o t h e f o l l o w i n g constraints: (a) the damping ra�o of all the eigenvalues is greater than 0.02 and (b) the real part of all eigenvalues is less than -0.65. The op�mal values of the

thcontroller parameters are x = 0.3391 and G = 0.2698 for the damping controller in line 5 – 4 of figure 3. The eigenvalues for

3. Energy Based Damping Controllers using FACTSThe low frequency oscilla�ons are due to exchange of the kine�c energy stored in the generator rotors and the magne�c energy stored in the transmission lines. The natural damping arises from the mechanical damping encountered by the rotors and frequency dependent load characteris�cs. By providing a series connected FACTS controller (say, TCSC) in s er ies w i t h a t ra n s mis s io n l in e ( t y p i c a l l y, a �e l i n e b e t we e n t wo coherent groups of generators), it is possible to provide damping of the low frequency oscilla�ons of power flow in the line by modula�ng the capaci�ve reactance injected by the TCSC. The control s ignal is obtained from the difference in frequencies of two buses, one of them is the terminal bus of the line (say, k) and the other, a fic��ous bus such that the reactance between two buses is the sum of the net line reactance and a Thevenin reactance (which can be viewed as a tunable parameter). The increment in the power flow in the line is given by,

Where,B = P / X .k ko k

If we propose a control law given by,

Where,G is the propor�onal gain.k

From the network analogy we have iden�fied the appropriate control signal as,

which can be synthesized from the locally measured quan��es. I f the voltage magnitudes V and V are assumed to be i j

equal to unity and

Figure 2: Damping controller for a series FACTS Controller


6. References

[1] K.R.Padiyar, 'Power System Dynamics-Stability and Control', (Book), Second E d i �o n , B S P u b l i c a �o n s , 2 0 0 2 , Hyderabad

[2] I.Kamwa, R.Grondin and G.Trudel, 'IEEE PSS2B versus PSS4B: The limits of performance of modern power system stabilizers', IEEE Trans. on Power Systems, Vol. 20(2), 2005, pp.903-915

[3] K.R.Padiyar, 'FACTS Controllers in Power Transmission and Distribu�on' (Book) , New Age Interna�onal . Publishers, 2007, New Delhi

[4] P.M.Anderson and A.A. Fouad, 'Power System Control and Stability', (Book), Iowa State University Press, 1977, Ames, U.S.A.

[5] K . R . P a d i y a r a n d H . V . S a i Kumar, ' Inves�ga�ons of strong resonancein mul�-machine power s y s t e m s w i t h S T S T C O M s u p p l e m e n t a r y m o d u l a �o n c o n t r o l l e r ' , I E E E Tra n s . Po w e r Systems, Vol.21(2), 2006, pp.754-762.

Blood dona�on camp in PRDC Blood donors are special people!

Rotary Bangalore –TTK Blood Bank organized a Blood dona�on camp in PRDC on 16th April 2015. In all

there were 45 donors from PRDC, a great contribu�on for a social cause by any standard! Rotary

Bangalore were

so apprecia�ve of the event and even desired to have this as a biennial CSR ac�vity of

PRDC.

Figure 3: A three machine system (Anderson and Fouad)


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3. Sample System

The system under considera�on represents two area systems as shown in Figure 1, with Area 1 being genera�on rich supplying power to Area 2. Area 1 has 2500 MW of genera�on and nearly 1400 MW of load. Area 2 has 2000 MW of genera�on and around 3000 MW of load. Area 2(a) is cri�cal industrial zone requiring high reliability. The addi�onal load in Area 2 is supplied by Area 1 over two 400 kV single circuit lines. In Area 1, the governors of Gen 1, Gen 5 and Gen 9 are considered to be opera�ng in droop mode. The other governors of area 1 are in constant power mode. In area 2, all the governors are considered to be opera�ng in droop mode. MiPowerTM so�ware is u�lized to build the sample system and carrying out transient stability simula�ons [4].

Under the situa�ons when any one of the 400 kV line i.e. Line 1/Line 2 is not available, the two areas become vulnerable to stability problems under the con�ngency of another line outage. The following con�ngencies can be envisaged under which ac�on of SPS will be required to safeguard the study system.

2. Design aspects of SPS

Designing SPS for any system can be divided into various major ac�vi�es such as [2]:

System Study for the considered system

Developing Logical Solu�on

Design and Implementa�on of SPS

Periodic Review and Records

There are many issues which can impact

reliable power system opera�on. However,

the most common issue is typically the

heavily loaded transmission system.

The tripping of heavily loaded line is

o�en seen as the root cause of system

instability [3]. The understanding of this

issue is cri�cal in power community, since it

can lead to poten�al blackout scenarios. This

ar�cle highlights one such scenario on a

sample two area system. It also suggests the

logic to implement SPS in order to safeguard

the system to the extent possible and avoid

system blackout.

1. Overview

Power system protec�on is o�en limited either to equipment protec�on or adjacent faulty equipment in vicinity. The size and complexity of the power system makes it vulnerable and subject to collapse under cri�cal situa�ons such as power conges�on, frequency and voltage viola�ons, power swings, etc. In order to secure wide area opera�ons, different class of protec�on schemes are proposed which are popularly known as Special Protec�on Systems (SPS). Most commonly used protec�on measures for such SPS are genera�on rejec�on, load rejec�on, under frequency load shedding, system separa�on and their combina�on [1]. It is important to note that the response �me and the quantum of load/ genera�on balancing required are key indices for the successful opera�on of SPS.

SPS is intended to safeguard the power grid during un-planned outage (con�ngency) or system opera�ng condi�ons where power demand cannot be met. Implementa�on of such schemes involves many factors such as [2]:

Understanding the need to implement Special Protec�on System

Complete knowledge of the system for which scheme is to be applied

Iden�fying undesired yet possible con�ngency condi�ons

I n fo r m a �o n o f o v e ra l l sy s te m performance and responses through system studies

Detailed design and implementa�on plan for opera�on and restora�on

Reliable communica�on system

Technical Article

Enhancing Power System Stability and Control Using Special Protection SystemsNitesh Kumar. D and Faraz Zafar Khan

Figure 1: Typically two area system considered for study


system.

Further, in order to analyze whether

addi�onal load shedding in area 2 can help

safe guard the system, a case is studied by

tripping around 1500 MW of load in area 2

for case 2. The result of this case is shown in

Figure 6.

It can be inferred that for case 1, 1000 MW

of load shedding in Area 2 is sufficient to

maintain the frequency profile of the system.

However, for case 2 when the second outage

occurs before the system equilibrium is

a�ained following the fault and first outage,

the simple load shedding based approach

may not be adequate to safe guard the

Case 1 – One of the lines (Line 1 or Line 2) is

out of service due to fault or planned

maintenance, which results in complete

transfer of load on the other l ine.

Subsequently the system a�ains a new

steady state opera�on. A few minutes

later the other line is also tripped due to a

fault.

Case 2 - Both the lines i.e. Line 1 & Line 2

are lost in quick succession due some

disturbance. The outage of second line

happens before the system a�ains

new equilibrium point following first

disturbance. The typical disturbance

sequence can be, loss of first line due to

fau l t and the second due to load

encroachment occurring as a result of high

power swings.

The frequency profile of the two islands

for case 1 and case 2 without SPS are

shown in Figure 2 and Figure 3 respec�vely.

It can be observed from Figure 2 and Figure 3

that during either of the case, the system

cannot survive the disturbance and will result

in total blackout of the system. This suggests

the need for special protec�on system which

can act to restore the system to the

maximum extent possible. For this case, SPS

will facilitate taking intelligent decision so as

to maintain load genera�on balance. This

can be implemented in tradi�onal way by

having frequency based relays at selected

loca�ons or by using the modern day Wide

Area Measurement System (WAMS)

technology.

4. Implementa�on of

conven�onal frequency

based scheme

Frequency relays are placed in the Area 1 to

trip around 1000 MW of genera�on at

frequency of 52 Hz. In Area 2, the frequency

relays are placed such that around 100 MW

of load is shed at 48.5 Hz. The frequencies of

the two areas following islanding in the two

cases are shown in Figure 4 and Figure 5

respec�vely.

Figure 2: Frequency of Area 1 and Area 2 for case 1

Figure 3: Frequency of Area 1 and Area 2 for case 2

Figure 4: Frequency of Area 1 and Area 2 for case 1 with simple load shedding scheme


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stability through the detailed analysis of the system for various worst case scenarios. This ar�cle demonstrates simple logic for SPS. However, complex SPS can also be derived for saving the power systems from cri�cal opera�ng condi�ons.

6. Conclusions

The selec�on of SPS is aimed at securing the system as top priority followed by recovery of sub-systems. While designing SPS, it is important to assess the performance of the steady state as well as dynamic/transient

For this case, it can be inferred that the frequency remains below 47 Hz for considerable �me dura�on which will result in tripping of all the generators in the area 2 on under frequency se�ng. Hence it can be observed that with system islanding occurring in case 2, simple frequency based load shedding may not be adequate.

5. Implementa�on of Special

Protec�on System (SPS)

The logic based SPS is designed as shown in

Figure 7. A�er isola�on of Area 1 and Area 2

due to tripping of Line 1 and Line 2, the SPS

scheme will try to save maximum possible

load in the cri�cal Area 2(a) by crea�ng a sub

island. The scheme takes the inputs from

breaker status of Line 1 and Line 2. When the

breaker status of both Line 1 and Line 2 is

high (tripped), a signal will be sent to the

relay of Line 3 to isolate Area 2(a). Also

Signal will be sent to enable the frequency

relay of Area 1, Area 2 and Area 2(a). When

the frequency relays are enabled, tripping of

concerned load or genera�on will occur

when the frequency se�ng of the relay is

crossed.

The results for case 1 and case 2 islanding

with suggested SPS is given in Figure 8 and

Figure 9. For case 1, use of logic based SPS

results in crea�on of sub-island and shedding

of addi�onal load. However, for case 2 the

cri�cal load in Area 2(a) is saved.

In the suggested SPS shown in Figure 7,

addi�onal logic can be included to decide if

the sub-islanding is actually necessary or not.

This can save the extra load shedding which

occurred in Case 1. In all the simula�ons, the

load shedding is carried out in single stage.

However, the load shedding can be divided

into different stages. This will result in

minimum loss of load. The stage wise load

shedding can also be incorporated in the

logic of SPS, which makes the scheme more

effec�ve.

Figure 5: Frequency of Area 1 and Area 2 for case 2 with simple load shedding scheme

Figure 6: Frequency of Area 1 and Area 2 for case 2 with increased load shedding in Area 2

Figure 7: Logic of SPS designed for sample two area system


Protec�ve

Re l ay E n g i n e e rs , Te xa s A & M University, Mar 30 – Apr 1, 2004, pp.1-12.

[3] V.K. Agrawal, R.K. Porwal, Rajesh K u m a r, V i v e k P a n d e y a n d T. Muthukumar, “Deployment of System Protec�on Schemes for Enhancing Reliability of Power System”, Interna�onal Conference on Power System, IIT Madras, Dec 2011, pp. 1-6.

[4] User Manuals, MiPowerTM, 2012.

7. References

[1] P. M . A n d e r s o n a n d B . K . LeReverend, “Industry Experience with Special Protec�on Schemes”, I E E E Tr a n s a c �o n s o n P o w e r Systems, Vol. 11, No. 3, Aug 1996, pp.1166-1179.

[2] VahidMadani, Mark Adamiak and M a n i s h T h a ku r, “ D e s i g n a n d I m p l e m e n t a �o n o f W i d e AreaSpecial Protec�on Schemes”, 5 7 t h A n n u a l C o n fe r e n c e fo r

Also, it can be clearly observed that the damage to the system, in terms of load shed, depends on the response �me.

In fact there is exponen�al rela�onship of damage to the system with respect to the response �me. Design of fast ac�ng SPS could reduce the poten�al deteriora�on of the system in the cri�cal fault condi�ons.

PRDC received order from an interna�onal client “M/s. KhimjiRamdas LLC, Sultanate of Oman” for the supply of Distance relay lab set-up.

Distance Relay Lab-Setup for Oman

Figure 8: Frequency of Area 1 and Area 2 for case 1 with logic based SPS

Figure 9: Frequency of Area 1 and Area 2 for case 2 with logic based SPS


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assessment, find protec�ve device se�ngs, and apply the necessary remedy or enhancement to improve the system stability.

2. Case Study

The stability study is carried out for a steel plant with a combined capacity of 860,000 TPA of Sponge Iron, 300,000 TPA of Steel, and 60,000 TPA of Ferro Alloys and power genera�on facility of 170MW and is interconnected to u�lity at 132 kV level.

A. Network model forconsidera�on

For stability studies, the en�re industrial network from High Voltage (HV) to either Medium Voltage (MV) or Low Voltage (LV) level needs to be modeled for base case load flow and short circuit analysis. Dynamic modeling (sub transient or transient model) of plant cap�ve generators along with automa�c voltage regulators (AVR) & turbine governors need to be carried out. It is not required to model complete grid network for the stability studies, only grid substa�on model with its equivalent short circuit

therefore it is not advisable. For sensing such a condi�on, careful selec�on of islanding relay se�ngs is required. Also a�er islanding, load genera�on balance need to be maintained in the island formed. Thus transient stability study plays a vital role in islanding studies by establishing the CCT for industrial plant generators for external faults.

M/s PRDC has carried out a number of projects rela�ng to stability studies for industrial power systems. The calcula�on of CCT for industrial plant generators for a three phase fault both in u�lity and industrial plant usingMiPower™ so�ware is briefly described in this technical ar�cle.

The MiPower™ transient stability analysis program inves�gates the stability limits of a power system before, during and a�er system changes or disturbances. The program models dynamic characteris�cs of a power system, implements the user-defined events and ac�ons, solves the system network equa�on and machine differen�al equa�ons interac�vely to find out system and machine responses in �me domain with a user defined reference. From these responses, users can determine the system transient behaviour, make stability

1. Introduc�on

Technical Ar�cle Stability Studies for Industrial Power Systems

Maheedhar Patnala and T. Guru Charan Das When an industrial plant with cap�ve power genera�on is connected to u�lity, it may result in stability problems to the cap�ve generators in the plant due to transient disturbances such as three phase faults, loss of genera�on or loss of a large load etc., both in grid and plant. For a given disturbance, the longest fault dura�on which does not result in instability of the generators is referred to as the Cri�cal Clearing Time (CCT). The CCT for the cap�ve generators need to be calculated by conduc�ng transient stability for various disturbances. The most onerous abrupt change is usually a three-phase fault; a three-phase fault causes the power transfer through the line to be reduced to zero from the working condi�on.

During transient disturbances in the industrial plant (internal faults), the faulty sec�on isola�on needs to be carried out by protec�ve relay within CCT so as to avoid instability of the cap�ve generators. Thus transient stability study plays a vital role in relay coordina�on by establishing the CCT for industrial plant generators so as to co-ordinate the relay se�ngs in such a way that the protec�on relay gives the trip signal before cap�ve generators trip or become unstable for internal faults.

During transient disturbances in u�lity (external faults), industrial plant generators have to withstand this major disturbance or should island from gird for a permanent fault. The challenge for islanding system is to island from the grid within the CCT to protect industrial plant generator from tripping or becoming unstable.

Moreover, it has to ensure that the islanding is absolutely necessary i.e. islanding should not take place for every temporary disturbance in the grid. Frequent islanding of the system will reduce the reliability and

Technical Article

Stability Studies for Industrial Power SystemsMaheedharPatnala and T. Guru Charan Das

Figure 1: Network model for simula�ons


protec�ve system opera�ng �me is longer than the CCT of plant generators, system will become unstable.

Remedial Ac�on: The exis�ng protec�on

schemes are modified and a new unit protec�on schemes are recommended so as to isolate the faulty sec�on from the system within CCT of plant generators. Similar methodology is adopted and CCT at different loca�ons in the plant is calculated and is summarized in Table 2. For the corresponding loca�on of fault, the exis�ng protec�on system opera�ng �me is calculated by inspec�ng the exis�ng protec�on schemes and se�ngs. The maximum exis�ng protec�on system opera�ng �me for faults at different loca�ons is calculated and is summarized in Table 2.

bus is 700 ms.

Exis�ng protec�on system opera�ng �me calcula�on: For the corresponding loca�on of fault, exis�ng protec�on

system opera�ng �me is calculated by i n s p e c �n g t h e ex i s�n g p ro te c �o n

schemes and se�ngs.

In present case, the minimum IDMT opera�ng �me of the exis�ng protec�on system (i.e. overcurrent protec�on) for a fault at 33kV plant bus is 800 ms. As the

MVA shall be sufficient to carry out stability studies. For large induc�on machines (MV motors) the detailed dynamic representa�on is needed, however small induc�on machines (LV motors) are represented as a sta�c load (constant power).

B. Transient Stability Analysis

Various simula�on studies have to be conducted to determine the CCT for faults in industrial plant and u�lity. A case study for calcula�on of CCT is illustrated below.

S y s t e m c o n d i �o n : P l a n t c a p �v e generators are synchronized to grid. The plant load, genera�on and import from the grid are shown in the Table 1.

a. Internal Faults:

The transient disturbances with in the industrial plant at any voltage level in the plant are termed as internal faults to the industrial plant.

CCT Calcula�on for internal faults: The first s te p i s to s i m u l ate fo l l o w i n g sequence of the events in the transient stability program of MiPower™ so�ware.

Time t =1.0 s Se�ng a temporary

three phase fault at 33kV Plant Bus.

Time t =1.1 s Fault cleared.

The next step is to itera�vely change the

fault clearing �me and observe the swing curve of the plant generators. The Figures 2 and 3 illustrate the swing curve with different fault clearing �mes. It can be observed that the CCT of the plant generators for a three fault at 33kV plant

Figure 2: Swing curve for TG1, TG2 & TG3 - Fault clearing �me of 700ms



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Unstable case(s) [i.e. here CASE C] is

iden�fied from the stability analysis and

recommenda�ons are suggested so as to

maintain the stability of plant generators for

internal faults in the plant.

b. External Faults:

The transient disturbances in the u�lity grid

at transmission voltage level termed as

external faults to the industrial plant.

CCT Calcula�on: The following sequences of

the events are simulated in the transient

stability program of MiPower so�ware.

Time t =1.0 s Se�ng a temporary

three phase fault at 132 kV grid bus.

Time t =1.1 s Fault cleared.

The fault clearing �me is itera�vely changed

and the swing curve of the plant generators

is observed for assessing stability of plant

generators. The Figures 4 and 5 illustrate the

swing curve with different fault clearing �me.

It can be observed that CCT of the plant

generators for a three phase fault at grid bus

is 220 ms.

Exis�ng protec�on system opera�ng �me

calcula�on: For the corresponding loca�on

of faults in the u�lity grid, the exis�ng

protec�on system opera�ng �me is

calculated by inspec�ng the exis�ng

protec�on schemes and se�ngs. The

exis�ng protec�on se�ngs of distance

protec�on which is meant to take care for

faults in the u�lity grid are given in below

Table 3.

In present case, the minimum opera�ng

�me of the exis�ng protec�ve system for a

fault at 132kV Grid is 400ms on zone2 of

distance relay. As the protec�ve system

opera�ng �me is longer than the CCT of

plant generators, system will become

unstable.

Remedial Ac�on: New protec�on scheme

with under voltage in conjunc�on with

direc�onal overcurrent (logic and se�ng) is

proposed in the islanding relay so as to

isolate from the fault in the u�lity within CCT

of plant generators. The proposed scheme

(logic & se�ngs) are given in below Table 4.

Table 2:Summary for CCT for internal faults

Case Fault

Loca�on

Bus

voltage

(kV)

Cri�cal Clearing Time (s)

Opera�ng �me of exis�ng

protec�on system (s)

System

stability

CASE A 132kV Power

Plant Bus132 0.21 0.10 Stable

CASE B Auxiliary

Bus 132 0.21 0.10 Stable

CASE C 33kV

Plant

Bus 33 0.70 1.0 Unstable

CASE D 6.6kV Plant Bus

6.6 1.5 1.0 Stable

132kV




The above proposed scheme takes care of

islanding of the plant generators in case of

faults in the u�lity only. For disturbances in

the u�lity related to wide fluctua�ons in grid

frequency, frequency based islanding

scheme shall be used and its se�ngs need to

be coordinated with the plant generator

frequency protec�ons. The aspect of

frequency based islanding scheme is detailed

in the next sec�ons.

C. Disturbances in the u�lity

related to wide fluctua�ons in

grid frequency

Any wide fluctua�on in grid frequency for

higher dura�ons of �me will be treated as a

disturbance for which islanding of plant from

u�lity is necessary to avoid tripping of

generators on generator protec�on. The

exis�ng generator protec�on se�ngs are

given below in Table 5.

The frequency based islanding se�ngs are

to be coordinated with the generator

protec�ons and proposed islanding relay

se�ngs are given below in Table 6.

Under Frequency Case for Islanding:

For a frequency disturbance in u�lity where

grid frequency is falling with a rate of (-ve) 0.4

Hz/s, the islanding relay operated on

proposed under frequency scheme and

islanded the plant generators before the

generator under frequency protec�on

operates. The plant generator's frequency

response a�er islanding from u�lity is

illustrated in Figure 6.

Nega�ve dF/dT Case for Islanding: For a

frequency disturbance in u�lity where grid

frequency is falling with a rate of (-ve) 5 Hz/s,

the islanding relay operated on proposed

under frequency &dF/dT scheme and


generator under frequency protec�on



illustrated in Figure 7. Figure 6: Frequency response a�er islanding from u�lity on under frequency


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Over Frequency Case for Islanding: For a


frequency is increasing with a rate of (+ve)

0.4 Hz/s, the islanding relay operated on

proposed over frequency scheme and


generator over frequency protec�on



illustrated in Figure 8.Posi�ve dF/dT Case for Islanding: For a


frequency is increasing at a rate of (+ve) 5

Hz/s, the islanding relay operated on

proposed over frequency & posi�ve dF/dT

scheme and islanded the plant generators

before the generator over frequency

protec�on operates. The plant generator's

frequency response a�er islanding from

u�lity is illustrated in Figure 9.For all the cases of islanding from u�lity

illustrated above, a�er islanding load

shedding is not required as the in-plant

genera�on is always more than the load. It

should be noted that a load shedding is

necessary in case the in-plant load is more

than the genera�on during islanding. In that

case, the islanding se�ngs are to be

coordinated with under frequency based

load shedding and generator under

frequency protec�on.

3. Conclusion For a given disturbance, the longest fault

dura�on which does not result in instability

of the generators is referred as the cri�cal

clearing �me (CCT).The stability studies for industrial power

system are different from stability studies

carried out for transmission system as the

la�er studies deal with only establishing CCT

and checking whether the calculated CCT's

are within the range s�pulated by the grid

standards. Transient stability plays a vital role

in understanding the stability problems of

cap�ve generators when connected to u�lity

grid. The CCT for the cap�ve generators need

to be calculated by conduc�ng transient

stability for various disturbances. The stability

study for industrial power system was

discussed in this ar�cle right from modeling

to the significance of the results.

Figure 7: Frequency response a�er islanding from u�lity on under frequency & nega�ve dF/dT

Figure 8: Frequency response a�er islanding from u�lity on over frequency

Figure 9: Frequency response a�er islanding from u�lity on over frequency & posi�ve dF/dT


Events & AchievementsThe en�re industrial network from high

voltage (HV) to either Medium Voltage (MV)

or Low Voltage (LV) level needs to be

modeled and it is not required to model

complete grid network for the stability

studies; only grid substa�on model with its

equivalent short circuit MVA shall be

sufficient to carry out stability studies for

industrial power system.

It should be noted that a par�cular type of

solu�on suggested in one plant need not be

applicable for a different plant. Various

simula�on studies need to be conducted for

internal & external faults and CCT of the

cap�ve generators needs to be established

for faults at different loca�ons and

condi�ons. The complexity involves in

iden�fying unstable cases, and then in

revisi�ng the exis�ng protec�on schemes in

order to recommend new protec�on

schemes or modifica�on of exis�ng

protec�on scheme based on the outcome of

CCT based stability studies and the analysis

of lacunae in the exis�ng protec�on system.

4. References

[1] P ra b h a Ku n d u r, ' Po w e r Sy ste m Stabilityand Control' (Book), McGraw Hill Educa�on, 1994

[2] Central Electricity Authority, 'Manual On Transmission Planning Criteria', June 2013, New Delhi.

[3] Central Board of Irriga�on and Power, India, 'Manual On Protec�on Of Generators, Generator transformers And 220kV And 400 kV Networks', Publica�on No. 274, November 1999.


Newsletter

LED ligh�ng system at Alarnath Temple.

Replacement of exis�ng conven�onal

lights by LED fixtures along Grand Road

and Lord Jagannath temple surrounding.

Some of the important areas of Puri city

covered under the scheme are:

High Mast LED ligh�ng at Grand Road

where three chariots move all along

during RathYatra.

High mast and street ligh�ng system at

famous Sea Beach where almost

thousands of visitors gather every day.

Street Ligh�ng system along Puri - Konark

Marine drive.

PRDC has emerged as one of the major

u�lity consul�ng firms in India in providing

LED ligh�ng solu�ons using modern tools.

The company has provided LED ligh�ng

solu�on for six important areas of the Holy

city of Puri in Odisha. The responsibility of

PRDC included survey, design, prepara�on of

tender specifica�on, tender evalua�on, GTP

and drawings approval and supervision

during execu�on of work. Overall project

cost for LED ligh�ng scheme is Rs. 12 Crores.

PRDC being PMC for all the ligh�ng projects,

was responsible for ensuring quality job and

�mely comple�on.

LED ligh�ng is one of the revolu�onary

technologies over conven�onal ligh�ng

system in the recent years. It is widely

accepted due to its many advantages

such as long life �me, energy efficiency,

eco-friendliness, dimmability, instant ligh�ng

capability etc. Also, it has been proved that

LED ligh�ng is one of the best energy saving

drives accepted across the globe.

Apart from the LED Ligh�ng scheme, PRDC

was also entrusted by the Government of

Odisha to prepare the DPR for developing a

robust electrical distribu�on system to

provide uninterrupted power supply in Puri

city. During the world famous Nabakalebar

fes�val of Lord Jagannath, there is almost 30

lakhs foot fall of pilgrims in the holy city to

witness the grand event.

For the LED Ligh�ng scheme in Puri city,

more than 837 nos. of 80 wa�s, 300nos. of

160 wa�s and 340 nos. of 280 wa�s of LED

fixtures were used in 97 High mast towers as

well as street light poles. The ligh�ng designs

were carried out as per applicable IS and IEC

standards with the help of ligh�ng so�ware

tools. Fi�ngs and high mast poles were from

reputed suppliers.

PRDC Provides LED Lighting Solution for the Holy City of Puri

Figure 1: High mast LED lights at the Lord Jagannath Temple entrance

Figure 2: Puri Sea Beach LED Ligh�ng


PRDC Annual Day - April 2015


Newsletter

At PRDC, we conduct various training

programmes throughout the year. The

dura�on of the training programme

varies from one to four weeks.

One Week Training

W e c o n d u c t o n e w e e k t r a i n i n g

p ro g ra m m e o n M i Po w e r ™ . I t i s a

standard course.

MiPower Training Level 1

Level 1 is a training programme on basic

theory & simple problems (hands - on).

Level 1 Batch:

16thNovember to 20thNovember 2015

MiPower Training Level 2

Level 2 is a training programme which

consists of only hands-on and solving

own system problems, sor�ng out issues

and clarifica�ons.+

Level 2 Batch:

14thDecember to 18thDecember 2015

Short Term Training /Workshop

In addi�on to the above said programme

PRDC is also conduc�ng short term

training program and workshops to

i m p a r t k n o w l e d g e a n d p r a c �c a l

approach on specific topics, which are of

relevance to power engineers in day-to-

day works . Such t ra in ing not on ly

enhances their knowledge but also helps

to implement these techniques in their

rou�ne works. For short term and special

training programme, please contact our

marke�ng team at the following address:

marke�[email protected]

Our Expertise in Training

Upcoming Events


supply business) in the power sector by introducing mul�ple supply licensees so as to bring in further compe��on and efficiency in the distribu�on sector by giving choice to the consumers.

Source: pib.nic.in

Na�onal Smart Grid Mission

Government has approved the Na�onal Smart Grid Mission (NSGM) -an ins�tu�onal mechanism for planning, monitoring and implementa�on of policies and programs related to Smart Grid ac�vi�es. The total outlay for NSGM ac�vi�es for 12th Plan is Rs 980 crore with a budgetary support of Rs 338 crore. The major ac�vi�es envisaged under NSGM are development of smart grid, development of micro grids, consumer engagements and training & capacity building etc. NSGM entails implementa�on of a smart electrical grid based on state-of-the art technology in the fields of automa�on, communica�on and IT systems that can monitor and control power flows from points of genera�on to points of consump�on.

Source: pib.nic.in

India's solar installa�ons set to quadruple in two years

According to the Ministry of New and Renewable Energy (MNRE), India has installed solar capacity of 4,262 MW, of which 518 MW were built in the current financial year. MNRE expects 4,345 MW of fresh capacity to come up in 2015-16 (including the 518 MW achieved so far.) Further, going by the bids on the anvil, the government expects to add 10,859 MW in 2016-17 alone. The numbers add up to close to 19,000 MW by March 2017 compared with the previous target of 20,000 MW by 2022.

Source: Business line

Amendments to the Electricity Act

The Union Cabinet has approved the proposals for amendment in Electricity Act, 2003 on 10th December, 2014 as contained in the Electricity (Amendment) Bill 2014. The Electricity (Amendment) Bill, 2014 was introduced in the Lok Sabha on 19th December, 2014. This was referred to Parliamentary Standing Commi�ee on Energy and the commi�ee has submi�ed its report to the Parliament on 7th May, 2015. The amendments proposed in Electricity (Amendment) Bill, 2014 seeks to end the monopoly of power distribu�on companies by segrega�ng the carriage (distribu�on sector/network) from the content (electricity

Power Policy

The Government has revised the Na�onal Solar Mission target of Grid Connected Solar Power projects from 20,000 MW to 1,00,000 MW by 2022. The revised Na�onal Solar Mission is under implementa�on.

The Union Ministerof State (IC) forPower, Coal, New & Renewable Energy stated that the it is planned to achieve the revised target of 1,00,000 MW by se�ng up Distributed Roo�op Solar Projects and Medium & Large Scale Solar Projects, the break-up of which is is given in the Table.

Source: pib.nic.in

Na�onal Clean Energy Fund

In the year 2014-15, an amount of Rs 16,388.81 crore has been collected as coal cess for Na�onal Clean Energy Fund (NCEF). As per the budget es�mates, during 2015-16 an amount of Rs 13,118.04 crore will be collected as coal cess for NCEF. Na�onal Clean Energy Fund (NCEF) is created for funding research and innova�ve projects in clean energy technologies. Out of 44 projects recommended for NCEF support in renewable energy, 30 projects are awai�ng alloca�on of fund.

Source: pib.nic.in

Indian Power Sector Highlights


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Dr. R. Nagaraja is the founder and Managing

Director of M/s. Power Research &

Development Consultants Pvt. Ltd.,

Bangalore- one of the reputed Power System

Consultants in the country.

R. Nagaraja has done his B.E. in Electrical and

Electronics Engineering from Mysore

University (India) in 1986. He obtained his

M.E in 1988, specialized in Computer

Applica�ons to Power System and Drives

and Ph.D. Degree in the field of Energy

Management System from Indian Ins�tute of

Science (IISc). His specializa�ons are Power

System Analysis, Simula�on, Power

Engineering Educa�on and Power System

Protec�on. Dr. Nagaraja has authored several

technical papers and conducted a number of

workshops / conferences / seminars

throughout the country.

Dr. Nagaraja is the brain behind the

architecture, design and development of the

MiPower™ – Power system analysis so�ware

package widely used by Electric u�li�es,

Industries, Consultants and Engineering

colleges. Dr. Nagaraja has been involved in

the planning studies of State U�li�es and

Industries in India and abroad.

Prof K.R. Padiyar is associated with Indian

Ins�tute of Science, Bangalore since 1987,

where he is currently an Honorary Professor

in the Department of Electrical Engineering.

Prior to joining IISc, he was with Indian

Ins�tute of Technology, Kanpur. He obtained

M.E degree in 1964 from IISc and Ph.D

degree from University of Waterloo, Canada

in 1972. He has taught and lectured at

various Universi�es in Canada and USA.

He has authored over 200 papers and 4

Books including 'Power system dynamics,

stability and control'. His research interest are

in the areas of HVDC and FACTS, power

system stability and control. He was a

member of the Review commi�ee on the

Na�onal HVDC project. He is the recipient

of 1999 Prof. Rustom Choksi award for

excellence in research. He was ABB chair

professor (2001-03). He is fellow of Indian

na�onal academy of Engineering.

Faraz Zafar Khan is presently working as

Senior Engineer in Power Research and

Development Consultants Pvt. Ltd. He is also

pursuing his PhD under VTU in the research

area of “Advanced Protec�on and Analysis

Schemes for Transmission System”. He

completed M-Tech in Power system from

VNIT, Nagpur.

About the Authors

Dr. R. Nagaraja Prof K. R. Padiyar Faraz Zafar Khan

Nitesh Kumar D is presently working

as Engineer in Power Research and

Development Consultants Pvt. Ltd. He

completed BE in Electrical and Electronics

Engineering from VTU and is presently

pursuing M.Sc (Engg.) by research under VTU

in the research area of generator protec�on

enhancement. His area of interest includes

power system stability, automa�on &

control, Power system protec�on and PMU

applica�on.

Nitesh Kumar D


T. Guru Charan Das obtained his

gradua�on in Electrical Engineering from

Dayalbagh University in 1997. Therea�er, he

began his career as GET in DCM Shriram

group of Industries. His ini�al role was in

power plant opera�on and maintenance.

During this associa�on for 9 years he has

gained insights into power genera�on,

distribu�on and u�liza�on in Cement plants,

Fer�lizer plants, PVC plants, Tex�le plants,

Sugar plants, Dis�lleries and other chemical

processes and industries.

He then joined Bajaj group of industries in

2006 to look a�er Power plant projects and

BoP Engineering. He has successfully

contributed in adding 450 MW power

genera�ons to the group. His role involved

technical, commercial and project

management aspects of the Power projects.

His associa�on with PRDC began in 2010 and

is presently working as DGM-Power System

Consul�ng. He has 19 years of experience

and his area of interest is in Power Plant

engineering, Energy management, Industrial

plants and their related issues with Electrical

Power Systems' applica�on and opera�on

Maheedhar Patnala obtained B.Tech

(Electrical & Electronics) in 2003 and M.Tech

(Power Systems) in 2006 from Kaka�ya

University, Kothagudem and NIT, Warangal

respec�vely. His areas of interest include

power system simula�on studies especially

transient stability studies for industrial plant.

He joined in ABB Limited in 2006 as

Engineer-Consul�ng Department and then

joined in PRDC in 2012 as Team Lead-PSS

department. From 2006, he executed various

projects in the field of power system

Studies for industrial plant involving Load

flow & Short circuit analysis, Transient

stability analysis, Harmonic analysis, Islanding

and load shedding and relay coordina�on

studies.

T. Guru Charan Das Maheedhar Patnala

R.N.I No. KARENG/2013/51589

Printed & Published by : Dr. R. Nagaraja on behalf of Power Research & Development Consultants Pvt. Ltd.Printed at : M/s. Art Print, Dr. Modi Hospital Main, WOC Road, Bengaluru - 560 086. Cell : 98452 33516. Editor : M.M. Babu Narayanan

Power Research & Development Consultants Pvt. Ltd.# 5, 11th Cross, 2nd Stage, West of Chord Road,

Bengaluru - 560086. INDIA. Phone : (080) 4245 5555 / 2319 2209Website : www.prdcinfotech.com

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