hvdc and facts handouts - bvrit hyderabad

BVRIT HYDERABAD College of Engineering for Women

Department of Electrical and Electronics Engineering

Hand Out

Subject Name: Fundamentals of HVDC and FACTS Devices (HVDC & FACTS)

Prepared by: Dr. Chava Sunil Kumar, Professor, EEE

Year and Sem., Department: IV Year - II Sem, EEE

UNIT – I: Introduction

Important Points / Definitions:

Comparison of AC and DC transmission:

Advantages of HVDC over AC:

1) Technical Merits of HVDC: The advantages of a DC link over an AC link are:

• A DC link allows power transmission between AC networks with different frequencies or networks,

which cannot be synchronized, for other reasons.

• Inductive and capacitive parameters do not limit the transmission capacity or the maximum length of

a DC overhead line or cable. The conductor cross section is fully utilized because there is no skin

effect.

For a long cable connection, e.g. beyond 40 km, HVDC will in most cases offer the only

technical solution because of the high charging current of an AC cable. This is of particular interest for

transmission across open sea or into large cities where a DC cable may provide the only possible

solution.

1 A digital control system provides accurate and fast control of the active power flow.

2 Fast modulation of DC transmission power can be used to damp power oscillations in an AC grid

and thus improve the system stability.

2) Economic considerations:

For a given transmission task, feasibility studies are carried out before the final decision on

implementation of an HVAC or HVDC system can be taken. Fig.1 shows a typical cost comparison

curve between AC and DC transmission considering:

• AC vs. DC station terminal costs

• AC vs. DC line costs

• AC vs. DC capitalized value of losses

The DC curve is not as steep as the AC curve because of considerably lower line costs per

kilometer. For long AC lines the cost of intermediate reactive power compensation has to be taken into

account. The break-even distance is in the range of 500 to 800 km depending on a number of other

factors, like country-specific cost elements, interest rates for project financing, loss evaluation, cost of

right of way etc.

3) During bad weather conditions, the corona loss and radio interference are lower for a HVDC line

compared to that in an AC line of same voltage and same conductor size.

4) Due to the absence of inductance in DC, an HVDC line offers better voltage regulation. Also,

HVDC offers greater controllability compared to HVAC.

5) AC power grids are standardized for

to interconnect two power grids working at different frequencies with the help of an AC

interconnection. An HVDC link makes this possible.

6) Interference with nearby communication lines is lesser i

that for an HVAC line.

7) In longer distance HVAC transmission, short circuit current level in the receiving system is high.

An HVDC system does not contribute to the short circuit current of the interconnected AC system.

8) Power flow control is easy in HVDC link.

9) High reliability.

Disadvantages of HVDC transmission:

• Converter stations needed to connect to AC power grids are very expensive

substations are more complex than HVAC substations, not only in additiona

equipment, but also in more complicated control and regulating systems.

• In contrast to AC systems, designing and operating multi

• Converter substations generate current and voltage harmonics, while

accompanied by reactive power consumption. As a result, it is necessary to install expensive

filter-compensation units and reactive power compensation units.

• During short-circuits in the AC power systems close to connected HVDC substations, p

faults also occur in the HVDC transmission system for the duration of the short

• The number of substations within a modern multi

system can be no larger than six to eight, and large differences i

The larger the number of substations, the smaller may be the differences in their capacities.

• The high-frequency constituents found in direct current transmission systems can cause radio

noise in communications lines tha

• Grounding HVDC transmission involves a complex and difficult installation, as it is necessary to

construct a reliable and permanent contact to the Earth for proper operation and to eliminate the

possible creation of a dangerous “step voltage.”

Applications of HVDC transmission:

• Connecting remote generation

• Interconnecting grids

Fig 1: Total cost/distance

During bad weather conditions, the corona loss and radio interference are lower for a HVDC line

mpared to that in an AC line of same voltage and same conductor size.

Due to the absence of inductance in DC, an HVDC line offers better voltage regulation. Also,

HVDC offers greater controllability compared to HVAC.

AC power grids are standardized for 50 Hz in some countries and 60 Hz in other. It is impossible


interconnection. An HVDC link makes this possible.

Interference with nearby communication lines is lesser in the case of HVDC overhead line than

In longer distance HVAC transmission, short circuit current level in the receiving system is high.


Power flow control is easy in HVDC link.

Disadvantages of HVDC transmission:

Converter stations needed to connect to AC power grids are very expensive

substations are more complex than HVAC substations, not only in additiona

equipment, but also in more complicated control and regulating systems.

In contrast to AC systems, designing and operating multi-terminal HVDC systems is complex.

Converter substations generate current and voltage harmonics, while the conversi


compensation units and reactive power compensation units.

circuits in the AC power systems close to connected HVDC substations, p

faults also occur in the HVDC transmission system for the duration of the short

The number of substations within a modern multi-terminal HVDC transmission

system can be no larger than six to eight, and large differences in their capacities are not allowed.


frequency constituents found in direct current transmission systems can cause radio

noise in communications lines that are situated near the HVDC transmission line.

Grounding HVDC transmission involves a complex and difficult installation, as it is necessary to


tion of a dangerous “step voltage.”

Applications of HVDC transmission:

Connecting remote generation

During bad weather conditions, the corona loss and radio interference are lower for a HVDC line

Due to the absence of inductance in DC, an HVDC line offers better voltage regulation. Also,

50 Hz in some countries and 60 Hz in other. It is impossible


n the case of HVDC overhead line than

In longer distance HVAC transmission, short circuit current level in the receiving system is high.


Converter stations needed to connect to AC power grids are very expensive. Converter

substations are more complex than HVAC substations, not only in additional converting

terminal HVDC systems is complex.

the conversion process is


circuits in the AC power systems close to connected HVDC substations, power

faults also occur in the HVDC transmission system for the duration of the short-circuit.

terminal HVDC transmission

n their capacities are not allowed.


frequency constituents found in direct current transmission systems can cause radio

t are situated near the HVDC transmission line.

Grounding HVDC transmission involves a complex and difficult installation, as it is necessary to


• Connecting offshore wind

• Power from shore

• Dc links in ac grids

• City-center in feed

• Connecting remote loads

Types of DC link: For connecting two networks or system, various types of HVDC links are used.

are classified into three types. These links are explained below: Monopolar link:

Bipolar link:

Homopolar link:

Typical layout of HVDC system:

The HVDC system has the following main components.

• Converter Station

• Converter Unit

• Converter Valves

Connecting offshore wind

connecting two networks or system, various types of HVDC links are used.

are classified into three types. These links are explained below:

Fig 2: Monopolar DC link

Fig 3: bipolar DC link

Fig 3: Homopolar DC link

Typical layout of HVDC system:

The HVDC system has the following main components.

connecting two networks or system, various types of HVDC links are used. HVDC links

• Converter Transformers

• Filters

• Reactive Power Source

• Smoothing Reactor

• HVDC System Pole

Fig 4: Schematic diagram of typical HVDC converter station Analysis of Gratez circuit:

Fig 7:

Let the instantaneous line – to – neutral source voltages be ea = Em Cos( ωt+60º )

eb = Em Cos( ωt-60º )

ec = Em Cos( ωt-180º) Then the line-to-line voltages are

chematic diagram of typical HVDC converter station

Fig 7: Circuit diagram for Graetz Bridge

neutral source voltages be

line voltages are

chematic diagram of typical HVDC converter station

eac = ea-ec = √3EmCos( ωt+30º )

eba = eb-ea = √3EmCos( ωt-90º )

ecb = ec-eb = √3EmCos( ωt+150º )

Fig 8: Thyristor voltage waveforms (

Fig 10: Thyristor voltage waveforms (with delay

In this case, the magnitude of the direct voltage output is given by the equation

º )

Fig 8: Thyristor voltage waveforms (α=0)

Fig 9: dc output waveforms (α=0)

Fig 10: Thyristor voltage waveforms (with delay α)

In this case, the magnitude of the direct voltage output is given by the equation

Commutation angle (overlap angle):

With both the delay angle and comm

determined from equation

Current Waveforms:

Fig 11: Thyristor current waveforms

Commutation angle (overlap angle):

With both the delay angle and commutation being present, the magnitude of the direct voltage may be

Fig 11: Thyristor current waveforms

utation being present, the magnitude of the direct voltage may be

If commutation angle is not considered, we can easily calculate the r.m.s. value of the AC

current on the transformer secondary Is as in equation.

Usually harmonic filters are provided on the AC system, so that only the fundamental

components need to be supplied

shown that the fundamental component is given as follows, resulting in equation

Inversion:

Fig 12: Thyristor voltage waveforms for inversion

Fig 13: Direct voltage waveform & thyristor current waveform

Fill in the blanks / choose the Best:

1. Most commonly used DC link

A) Bipolar dc link B) Monopole link

2. High frequency filters are used to suppress

A) Low frequency currents

C) Both 1& 2

3. A 12-pulse bridge is preferred in HVDC because

A) It eliminates certain harmonics

C) Series connection of converters on DC side is better


current on the transformer secondary Is as in equation.


to be supplied / absorbed from the AC system. From Fourier analysis, it can be

e fundamental component is given as follows, resulting in equation

Fig 12: Thyristor voltage waveforms for inversion



ommonly used DC link

B) Monopole link C) Homopolar link D) None of these

High frequency filters are used to suppress

B) High frequency currents

D) None of the above

pulse bridge is preferred in HVDC because

A) It eliminates certain harmonics B) It results in better power factor

C) Series connection of converters on DC side is better D) None of the above



absorbed from the AC system. From Fourier analysis, it can be

e fundamental component is given as follows, resulting in equation


[ a ]

D) None of these

[ b ]

[ a ]

D) None of the above

4. Classification of DC links [ d ]

A) Bipolar dc link B) Monopole link C) Homopolar link D) All of the above

5. Which DC link has one conductor [ b ]

A) Bipolar dc link B) Monopole link C) Homopolar link D) None of the above

6. The power loss in an AC system is __________ more than a DC system

[ A ]

A) 33% B) 20% C) 10% D) None of the above

7. A back to back HVDC link can be advantageous compared to AC primarily because [ c ]

A) It is cheaper B) Of stability consideration

C) Of controlled power flow D) All of the above

8. IGBT stands for Insulated Gate Bipolar Transistor.

9. The value voltage rating is specified in terms of Peak inverse voltage.

10. The converter transformers are designed to with stand DC voltage stress.

11. The back to back DC links are designed for Low DC Voltage levels.

12. Which type of DC links are most commonly use for submarine cables Monopolar DC link.

13. The transient reliability can be defined as Number of times HVDC system performed as

designed/number of recordable AC faults.

14. SIL of 100KV cable is [ c ]

a) 400 MW b) 400KW c) 250 MW d) 250 KW

15. The phase difference in voltage of the transformers in 12 pulse converter is [ c ]

a) 600

b) 900

c) 300 d) 45

0

16. The optimum value of q which results in maximum utilization is [ d ]

a) 2 b) 6 c) 1 d) 3

17. At α= 900, the following conditions prevail [ a ]

a) pf=0, Q is only consumed b) pf=1, P is only consumed

c) pf=0.7, P and Q are consumed d) None

18. Which among these HVDC projects are committed in India [ d ]

a) Rihand-Delhi HVDC b) Vindhyachal back to back only

c) Chandrapur only d) All the above

19. A 12 pulse bridge is preferred in HVDC because [ a ]

a) It eliminates certain harmonics b) it results in better pf

c) Series connection of converters on DC side is better d) None

20. Modern HVDC systems are all

a) 3 pulse converters

c) 12 pulse converters

Questions

1. What is the need of static converter in HVDC systems and how is it configured?

2. Differentiate between 6-pulse and 12

3. a) List out Converter Station Equipment and describe about them in detail.

b) Explain Modem trends and planning of HVDC Transmission System.

4. a) Explain the Operation of 6

form and derive the Expression for output Voltage?

b) List out the factors that decide the Converter configuration

UNIT – II: Converter & HVDC System Control


Control characteristics of converter:

The complete characteristic of

equipped with C.C. control and the C.E.A. control. This is shown in figure for a single convertor.

Compounding of Convertors:

Figure shows a system of 2 convertors, connected by a hvdc link. Both convertors are provided

with CEA and CC control so that either can work as a rectifier or an invertor. The compounded

characteristics are shown in figure.

Modern HVDC systems are all

b) 6 pulse converters

d) 24 pulse converters

What is the need of static converter in HVDC systems and how is it configured?

pulse and 12 - pulse converters?

a) List out Converter Station Equipment and describe about them in detail.

b) Explain Modem trends and planning of HVDC Transmission System.

a) Explain the Operation of 6 - Pulse Converter with neat circuit diagram. Sketch the wave

form and derive the Expression for output Voltage?

factors that decide the Converter configuration.

II: Converter & HVDC System Control


f converter:

The complete characteristic of each convertor has the negative voltage


a system of 2 convertors, connected by a hvdc link. Both convertors are provided


characteristics are shown in figure.

[ c ]

What is the need of static converter in HVDC systems and how is it configured?

circuit diagram. Sketch the wave

II: Converter & HVDC System Control

each convertor has the negative voltage characteristic and


a system of 2 convertors, connected by a hvdc link. Both convertors are provided


The margin setting Idm betwe

usually kept at about 10% to 20% of the current setting. The setting of the convertor operating as

rectifier is kept higher than the setting of that as invertor by the margin setting Idm.

The usual operating point for power transfer is the intersection of the CC control of the

rectifier and the CEA control of the inverter. (For comparison, the characteristics of convertor B have

been drawn inverted). It must also be ensured

the convertor operating in the rectification mode is higher than the C.E.A. characteristic of the

inverter, as Vo of the two ends are not necessarily equal.

With convertor A operating as rectifier, a

current under all circumstances will remain within the upper limit (I

That is, the system direct current will not change by more than I

reversing the margin setting Idm, that is making the setting of convertor B to exceed that of A, power

flow can be automatically reversed. Convertor B will then operate as a rectifier and A as an inverter.

The reversal of power occurs as a result

The margin setting Idm between the current setting Ids for the invertor and for the rectifier is





been drawn inverted). It must also be ensured by proper tap changing that the N.V. characteristic of


inverter, as Vo of the two ends are not necessarily equal.

With convertor A operating as rectifier, and convertor B operating as inverter, the steady state

current under all circumstances will remain within the upper limit (Ids + Idm) and the lower limit Ids.

That is, the system direct current will not change by more than Idm under all operating condition

, that is making the setting of convertor B to exceed that of A, power

reversed. Convertor B will then operate as a rectifier and A as an inverter.

The reversal of power occurs as a result of the reversal of polarity of the voltage.

en the current setting Ids for the invertor and for the rectifier is





by proper tap changing that the N.V. characteristic of


nd convertor B operating as inverter, the steady state

) and the lower limit Ids.

under all operating conditions. By

, that is making the setting of convertor B to exceed that of A, power

reversed. Convertor B will then operate as a rectifier and A as an inverter.

of the reversal of polarity of the voltage.

Compounding of Convertors:

The convertor operating equations for voltage V

It is useful to draw the convertor chart in per unit. For this purpose th

base voltage is the maximum direct voltage output Vdo. There is no such natural current base. Thus it

is convenient to select the constant appearing in equation for current as the base quantity.

Fill in the blanks / choose the B

1. The HVDC converter

A) Does not consume reactive power B) Consumes as much reactive power as real power

C) Consumes 50% of the real power

2. Fault on two terminal DC link is removed by

A) Breakers on DC side

C) Current control of converters

3. The size of Ac filter in a terminal station varies between

A) 15 to 60% of DC power rating

C) 5 to 40 % of DC power rating

4. VDCOL stands for Voltage dependent current order limit

The convertor operating equations for voltage Vd and current Id are expressed as follows.

It is useful to draw the convertor chart in per unit. For this purpose the natural selection for the





C) Consumes 50% of the real power D) None of the above

Fault on two terminal DC link is removed by

B) Breakers on AC side

C) Current control of converters D) None of the above

The size of Ac filter in a terminal station varies between

A) 15 to 60% of DC power rating B) 10 to 50% of Dc power rating

ating D) 1 to 15% of the power rating

e dependent current order limit.

are expressed as follows.

e natural selection for the



[ c ]


[c ]

[a ]

5. High pass filters also called as Damped filters.

6. Units of angular frequency Radians/second.

7. EPC stands for Equivalent pulse control.

8. Under normal conditions, rectifier operates at ______ control and inverter at ____ control. [ b ]

a) CEA, CC b) CC, CEA c) CV, CC d) CC, CV

9. The factors considered in designing protection system in converters are ____ [ d ]

a) Selectivity b) Sensitivity c) Reliability d) all the above

10. Main disadvantage of HVDC-VSC schemes is [ a ]

a) Both P and Q can be controlled b) does not require dc filters

c) Can be used for very high power> 1500MW d) all the above

11. The frequency of the ripple in output voltage of the converter is determined by pulse number.

12. Current margin is typically about 10 % of rated current in current controller.

13. The single tuned filters are designed to filter out characteristic harmonics of single frequency.

14. The major design objective of AC filters is to reduce telephonic interference.

15. The harmonics in AC current of the order h=np±1.

16. To eliminate residual harmonics, 12 pulse converters are to be operated at equidistant control in

steady state.

17. The cost of the filter is minimum when reactive power supplied by the filter is given by Qf*=(B/A)1/2 .

Questions

1. What do you understand from the term system hierarchy control?

2. Why is 'control' required in HVDC systems? : Draw the converter control characteristics

3. a)Describe firing angle control in HVDC converters.

b) Explain with the help of control characteristics how the constant current control and constant

extinction angle are used to maintain the constant power flow in the HVDC Link

4. a) Discuss the mechanism involved in starting of DC link.

b) Detail the details about current control scheme used in HVDC converters.

UNIT – III: Harmonics, Filters, Reactive Power Control and Power Flow Analysis

in AC / DC Systems


HARMONIC FILTERS:

The filter arrangements on the AC side of an HVDC converter station have two main duties:

• To absorb harmonic currents generated by the HVDC converter and thus to reduce the impact of

the harmonics on the connected AC systems, like AC voltage distortion and telephone

interference.

• To supply reactive power for compensating the demand of the converter station.

Design Criteria for AC Filters:

Reactive Power Requirements: The reactive power consumption of an HVDC converter depends on the active power, the

transformer reactance and the control angle. It increases with increasing active power. A common

requirement to a converter station is full compensation or overcompensation at rated load. In addition,

a reactive band for the load and voltage range and the permitted voltage step during bank switching

must be determined. These factors will determine the size and number of filter and shunt capacitor

banks.

Harmonic Performance Requirements: HVDC converter stations generate characteristic and non-characteristic harmonic currents. For

a twelve-pulse converter, the characteristic harmonics are of the order n = (12 * k) ± 1 (k = 1,2,3...).

These are the harmonic components that are generated even during ideal conditions, i.e. ideal

smoothing of the direct current, symmetrical AC voltages, transformer impedance and firing angles.

The characteristic harmonic components are the ones with the highest current level, but other

components may also be of importance. The third harmonic, which is mainly caused by the negative

sequence component of the AC system, will in many cases require filtering.

The purpose of the filter circuit is to provide sufficiently low impedances for the relevant harmonic

components in order to reduce the harmonic voltages to an acceptable level. The acceptance criteria

for the harmonic distortion depend on local conditions and regulations. A commonly used criterion for

all harmonic components up to the 49th order is as follows: Dn individual harmonic voltage distortion

of order n in percent of the fundamental AC busbar voltage (typical limit 1%) Drms total geometric

sum of individual voltage distortion Dn (typical limit 2%)

Network Impedance: The distortion level on the AC busbar depends on the grid impedance as well as the filter

impedance. An open circuit model of the grid for all harmonics is not on the safe side. Parallel

resonance between the filter impedance and the grid impedance may create unacceptable amplification

of harmonic components for which the filters are not tuned. For this reason, an a

model of the grid for all relevant harmonics is required in order to optimize the filter design.

There are basically two methods to include the network impedance in the filter calculations:

• to calculate impedance vectors for all relevant

• to assume locus area for the impedance vectors

The modelling of a complete AC network with all its components is very complex and time

consuming. For this reason, the locus method is very often used. It is based on a limited n

measurements or calculations. Different locus areas for different harmonics or bands are often

determined to give a more precise base for the harmonic performance calculation.

DC Filter Circuits: Harmonic voltages which occur on the DC side of a c

are superimposed on the direct current in the transmission line. These alternating currents of higher

frequencies can create interference in neighbouring telephone systems despite limitation by smoothing

reactors. DC filter circuits, which are connected in parallel to the station poles, are an effective tool for

combating these problems. The configuration of the DC filters very strongly resembles the filters on

the AC side of the HVDC station. There are several types

filters with or without the high-

utilized in a converter station.

Design Criteria for DC Filter Circuits:

The interference voltage induced on the

equation:

The equivalent disturbing current combines all harmonic currents with the aid of weighting

factors to a single interference current. With respect to telephone interference, it is the equival

the sum of all harmonic currents. It

between the HVDC and telephone lines:

Operating mode of the HVDC system (bipolar or monopolar with metallic or ground return)

Specific ground resistance at point x the intensity of interference currents is strongly dependent on the

operating condition of the HVDC. In monopolar operation, telephone interference is significantly

stronger than in bipolar operation.

AC-DC Power Flow:

Most of the solution techn

approaches. The sequential and the unified (or simultaneous) solution methods. The sequential

solution methods, AC and DC system equations are solved separately in each iteration until the

terminal conditions of converters are satisfied. Because of modular programming, the sequential

methods are generally easy to implement and simple to incorporate various control specifications. In


amental AC busbar voltage (typical limit 1%) Drms total geometric

sum of individual voltage distortion Dn (typical limit 2%)

The distortion level on the AC busbar depends on the grid impedance as well as the filter

cuit model of the grid for all harmonics is not on the safe side. Parallel


of harmonic components for which the filters are not tuned. For this reason, an a



to calculate impedance vectors for all relevant harmonics and grid conditions

to assume locus area for the impedance vectors

The modelling of a complete AC network with all its components is very complex and time

consuming. For this reason, the locus method is very often used. It is based on a limited n



Harmonic voltages which occur on the DC side of a converter station cause AC currents which



C filter circuits, which are connected in parallel to the station poles, are an effective tool for


the AC side of the HVDC station. There are several types of filter design. Single and multiple

-pass feature are common. One or several types of DC filter can be

Design Criteria for DC Filter Circuits: The interference voltage induced on the telephone line can be characterized by the following


factors to a single interference current. With respect to telephone interference, it is the equival

the sum of all harmonic currents. It also encompasses the factors which determine the coupling

between the HVDC and telephone lines:


t point x the intensity of interference currents is strongly dependent on the


stronger than in bipolar operation.

Most of the solution techniques for AC/DC power flow are divide

. The sequential and the unified (or simultaneous) solution methods. The sequential


conditions of converters are satisfied. Because of modular programming, the sequential



amental AC busbar voltage (typical limit 1%) Drms total geometric

The distortion level on the AC busbar depends on the grid impedance as well as the filter

cuit model of the grid for all harmonics is not on the safe side. Parallel


of harmonic components for which the filters are not tuned. For this reason, an adequate impedance



harmonics and grid conditions

The modelling of a complete AC network with all its components is very complex and time-

consuming. For this reason, the locus method is very often used. It is based on a limited number of



onverter station cause AC currents which



C filter circuits, which are connected in parallel to the station poles, are an effective tool for


of filter design. Single and multiple-tuned

pass feature are common. One or several types of DC filter can be

telephone line can be characterized by the following


factors to a single interference current. With respect to telephone interference, it is the equivalent to

also encompasses the factors which determine the coupling


t point x the intensity of interference currents is strongly dependent on the


iques for AC/DC power flow are divided into two different

. The sequential and the unified (or simultaneous) solution methods. The sequential


conditions of converters are satisfied. Because of modular programming, the sequential


the unified methods, the AC as well as DC system equations are combined together with the residual

equations, describing the rectifier terminal behaviors, in one set of equations to be solved

simultaneously. The unified methods, with their better computing efficiency and convergence, seem

more suitable than the sequential methods for use in industrial AC/DC power systems, even if they

might be more complicated to program. The Gauss-Seidel (G-S), the Newton-Raphson (N-R), and the

fast decoupled N-R methods may be used to solve the power flow problems in AC/DC systems as they

do in the pure AC systems. The G-S method generally needs accelerating factors to improve the

iteration process because of its slow rate of convergence. The N-R method, with its powerful

convergence characteristics, appears to be the most attractive technique in solving the AC/DC power

flows.

For solving the load flow problem of an A.C. system in which one or more HVDC links are

present, either of the following two approaches are followed; a. Simultaneous solution technique b.

Sequential solution technique In simultaneous solution technique, the equations pertaining to the A.C.

system and the equations pertaining to the DC system are solved together. In the sequential method,

the AC and DC systems are solved separately and the coupling between the AC and DC system in

accomplished by injecting an equivalent amount of real and reactive power at the terminal AC buses.

In other words, for an HVDC link existing between buses ‘i’ and ‘j’ of an AC system (rectifier at bus

‘i’ and inverter at bus ’j’), the effect of the DC link in incorporated into the AC system by injections P

(R) DCi and Q (R) DCi at the rectifier bus ’i’ and P (I) DCj and Q (I) DCj at bus ’j’ (the super scripts

’R’ and ’I’ denote the rectifier and inverter respectively). Therefore the net injected power at bus ’i’

and ’j’ are: P total i = PACi + P (R) DCi; QT otal i = QACi + Q (R) DCi; P T otal j = PACj + P (I)

DCj ; QT otal j = QACj + Q (I) DCj . With these net injected powers the AC system is again solved

and subsequently, the equivalent injected powers (P (R) DCi, Q(R) DCi, P(I) DCj , Q(I) DCi) and the

total injected powers (P T otal i , QT otal i , PT otal j , QT otal j ) are updated. This process of

alternately solving AC and DC system quantities is continued till the changes in AC system and DC

system quantities between two consecutive iterations become less then a threshold value. Although

simultaneous technique gives the solution of the system without any to and fro switching between the

AC and DC systems, the sequential solution technique is actually quite easy to implement as we will

see later. Now let as look at the equations of the DC system.

DC system model For deriving a suitable model of a HVDC system for steady state operation,

few basic assumptions are adopted as described below; a. The three A.C. voltages at the terminal bus

bar are balanced and sinusoidal. b. The converter operation is perfectly balanced. c. The direct current

and voltages are smooth. d. The converter transformer is lossless and the magnetizing admittance is

ignored. With the above assumptions, the equivalent circuit of the converter (either rectifier or

inverter) is shown in Fig. In this figure, the notations are as follows;

Sequential AC-DC load flow:

Simultaneous Method:


1. Interline power flow controller comprises a number of ____________ compensators. [ a ]

a) Static Synchronous series b) Static synchronous shunt c) Passive d) None

2. Var compensation is used for _____________ compensation at mid point. [ c ]

a) Resistances b) Power c) Voltage d) None

3. With midpoint compensation reactive power in line becomes _________ times the uncompensated

power. [ a ]

a) Four b) One by fourth c) Zero d) Double

4. Shunt connected switched reactors are to minimize overvoltage.

5. With midpoint compensation transmitted power becomes double times the uncompensated Power.

6. TCR and TSR is shunt Type of compensation.

7. In a converter, the angle control is _____while tap changer control is__________ [ b ]

a) discrete, continuous b) continuous, discrete c) both discrete d) both continuous

8. The other name of simultaneous solution methodology in DC/AC power flow analysis is [ c ]

a) Sequential b) alternating c) unified d) combinatorial

9. FACTS can help in [ d ]

a) Enhancing controllability b) Increasing power transfer capability

c) Increasing system security d) all the above

10. Shunt compensator is functionally a controlled_______________ source [ d ]

a) Voltage b) Current c) Voltage & current d) Reactive current

11. GCS is a switch in shunt with____________ [ b ]

a) Reactor b) Capacitor c) both (a) and (b) d) none

12. Function of GTO controlled series capacitor source is [ c ]

a) Synchronous timing b) Generation of pulses c) both (a) and (b) d) none

13. STATCOM provides shunt type of compensation.

14. The commutation failures in inverters can be avoided by using forced type of commutation.

15. Most common type SVS scheme used in reactive compensation is variable impedance type.

Questions

1. Define harmonics List the causes for harmonics.

2. What is Individual Phase control Scheme? What is the effect of harmonics on IPC?

3. a)What do you understand from the term reactive power? Explain the causes of reactive power

absorbed by HVDC converter substation.

b) Write in brief about AC and DC filters used HVDC systems.

4. a) Explain in detail how interaction between AC and DC systems takes place with the help of neat-diagrams

where ever necessary.

b) Distinguish between simultaneous method and sequential method with appropriate diagrams

in power flow analysis.

UNIT – IV: Introduction to FACTS


FACTS ie., Flexible AC transmission system incorporate power electronic based st

controllers to control power (both active and reactive power needed ) and enhance power transfer

capability of the AC lines . Let Bus 1and Bus represent two AC systems where in power is to be

transmitted from 1 to 2through a line of impedance r+jx.

By changing the effective value of ‘x’ ,the power transmitted can be increased or dec

Further it modifies reactive power needed.

• Increase or decrease of ‘x’ will change value.

• Maximum power that can be transmitted is obtained when

• The FACTS technology is not a single high

Which can be applied individually or in coordination with others to control one or more of the

interrelated system parameters?

• The parameters that govern the operation of transmission system including.

� Series impedance

� Shunt impedance

� Current

� Voltage

� Phase and

� Damping of oscillations at various frequencies below the required system frequency.

• The FACTS controllers can enable a line to car

• In flexible (or) controllable AC systems, the controllable parameters are Control of line reactance.

• Control of phase angle δ when it is not large(which controls the active power flow)

• Injecting voltage in series with line and at 90°phase with line current i

power in series. This will control active power flow.

• Injecting voltage in series with line but at variable phase

reactive power flow.

• Controlling the magnitude of either V1or V2.

• Controlling or variation of line reactance with a series controller and regulating the voltage with a

shunt controller. This can control both active and reactive power.

BASIC TYPES OF FACTS CONTROLLERS:

In general FACTS controllers can be divided into four categories.

• Series controllers

• Shunt controllers

• Combined Series- series controllers

IV: Introduction to FACTS – Static Shunt Compensators


FACTS ie., Flexible AC transmission system incorporate power electronic based st



transmitted from 1 to 2through a line of impedance r+jx.

changing the effective value of ‘x’ ,the power transmitted can be increased or dec

Further it modifies reactive power needed.

Increase or decrease of ‘x’ will change value.

Maximum power that can be transmitted is obtained when δ= (provided are fix

The FACTS technology is not a single high-power controller but rather a collection of controllers.


interrelated system parameters?

rs that govern the operation of transmission system including.

Damping of oscillations at various frequencies below the required system frequency.

The FACTS controllers can enable a line to carry power closer to its thermal rating.

In flexible (or) controllable AC systems, the controllable parameters are Control of line reactance.

δ when it is not large(which controls the active power flow)

Injecting voltage in series with line and at 90°phase with line current i.e. Injection of reactive

power in series. This will control active power flow.

n series with line but at variable phase angle. This will control both active &

Controlling the magnitude of either V1or V2.

Controlling or variation of line reactance with a series controller and regulating the voltage with a

ntroller. This can control both active and reactive power.

BASIC TYPES OF FACTS CONTROLLERS: In general FACTS controllers can be divided into four categories.

series controllers

Static Shunt Compensators

FACTS ie., Flexible AC transmission system incorporate power electronic based static



changing the effective value of ‘x’ ,the power transmitted can be increased or decreased.

= (provided are fixed).

power controller but rather a collection of controllers.


Damping of oscillations at various frequencies below the required system frequency.

ry power closer to its thermal rating.

In flexible (or) controllable AC systems, the controllable parameters are Control of line reactance.

when it is not large(which controls the active power flow)

e. Injection of reactive

This will control both active &

Controlling or variation of line reactance with a series controller and regulating the voltage with a

• Combined series- shunt controllers Figure shows general symbol for a FACTS controller.

SERIES CONTROLLER:

Figure shows series type FACTS controller. • A series controller is variable impedance like a capacitor an inductor or a power electronic

switched device of variable source with either mains frequency or sub harmonic frequency. Series

controller injects voltage in series with a line.

• As long as the voltage is in phase quadrature with the line current, the series controller only

supplies or consumes variable re

• Any other phase relationship will involve handling of real power as well.

SHUNT CONTROLLER:

• A shunt controller can be variable impedance, variable source or combination of both.

• Shunt controller inject current into the line (system) at the point of connection.

• As long as the injected current is in quadrature with line voltage, shunt controller only supplies or

consumes variable reactive power.

• Any other phase relationship will involve handling o

Fig: shows series

This could be a combination of separate series controllers which are controlled in a co

ordinate manner, in multi-line transmission systems. ➢ Series controllers provide independen

real power among the lines via the power link. ➢ The real power transfer capability of unified series controller ,referred to as “Inter Line Power

Flow controller”, makes it possible to balanthere by maximize the utilization of transmission system.

controllers

Figure shows general symbol for a FACTS controller.

Figure shows series type FACTS controller.

A series controller is variable impedance like a capacitor an inductor or a power electronic

variable source with either mains frequency or sub harmonic frequency. Series

controller injects voltage in series with a line.

As long as the voltage is in phase quadrature with the line current, the series controller only

supplies or consumes variable reactive power.

Any other phase relationship will involve handling of real power as well.

Fig: shows shunt FACTS controller

A shunt controller can be variable impedance, variable source or combination of both.

r inject current into the line (system) at the point of connection.

As long as the injected current is in quadrature with line voltage, shunt controller only supplies or

consumes variable reactive power.

Any other phase relationship will involve handling of real power as well.

Fig: shows series-series FACTS controller

This could be a combination of separate series controllers which are controlled in a co

line transmission systems.

Series controllers provide independent series reactive compensation for each line but also transfer real power among the lines via the power link.

The real power transfer capability of unified series controller ,referred to as “Inter Line Power Flow controller”, makes it possible to balance both real & reactive power flow in the lines and there by maximize the utilization of transmission system.

A series controller is variable impedance like a capacitor an inductor or a power electronic

variable source with either mains frequency or sub harmonic frequency. Series

As long as the voltage is in phase quadrature with the line current, the series controller only

A shunt controller can be variable impedance, variable source or combination of both.

r inject current into the line (system) at the point of connection.

As long as the injected current is in quadrature with line voltage, shunt controller only supplies or

This could be a combination of separate series controllers which are controlled in a co -

t series reactive compensation for each line but also transfer

The real power transfer capability of unified series controller ,referred to as “Inter Line Power ce both real & reactive power flow in the lines and

Note: The term “Unified” means that the DC terminals

together for real power transfer.

SHUNT CONNECTED CONTROLLERS:

(i) Static Synchronous Compensators (STATCOM

shunt connected static var compensator whose capacitive or inductive output current can be controlled

independent of the AC system voltage.

based on a voltage sourced or current sourced converter.

Figure shows a simple one

and a current sourced converter.

system harmonics.

(ii) Static Var Compensator (SVC):

is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameter

of the electrical power system.

This is a general term for a thyristor controlled or thyristor switched reactor and/or thyristor switched

capacitor or combination. SVC is based on thyristors without gate turn

separate equipment for leading and lagging vars, the thyristor controlled or thyristor switched reactor

for absorbing reactive power on thyristor switched capacitor for supplying the reactive power.

(iii) Thyristor Controlled Reactor (TCR):

effective reactance is varied in a continuous manner by partial

valve. subset of SVC in which conduction time and hence, current in a shunt reactor is controlled by a

thyristor based ac switch with firing angle control.

(iv) Thyristor Switched Reactor (TSR):

reactance is varied in a stepwise manner by full

TSR is another subset of SVC. TSR is made up of several shunt connected inductors which are

switched in and out by thyristor switches without any firing angle controls in order to achieve the

required step changes in the reactive power consumed from the system. Use of thyristor

without firing angle control results in lower cost and losses, but without a continuous control.

(v) Thyristor Switched Capacitor (TSC):

effective reactance is varied in a stepwise manner by full

valve. TSC is also a subset of SVC in which thyristor based ac switches are used to switch in and out

shunt capacitors units, in order to achieve the required step change in the reactive power supplied to

The term “Unified” means that the DC terminals of all controller converters are

TED CONTROLLERS: Static Synchronous Compensators (STATCOM): A static synchronous generator operated as a


independent of the AC system voltage. STATCOM is one of the key FACTS controllers. It can be

based on a voltage sourced or current sourced converter.

Figure shows a simple one-line diagram of STATCOM based on a voltage sourced converter

and a current sourced converter. STATCOM can be designed to also act as an active filter to absorb

Static Var Compensator (SVC): A shunt connected static var generator or absorber whose output

is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameter


capacitor or combination. SVC is based on thyristors without gate turn-off capability. It includes

t for leading and lagging vars, the thyristor controlled or thyristor switched reactor


Thyristor Controlled Reactor (TCR): A shunt connected thyristor controlled inductor whose

effective reactance is varied in a continuous manner by partial-conduction control of the thyristor

subset of SVC in which conduction time and hence, current in a shunt reactor is controlled by a

ch with firing angle control.

Thyristor Switched Reactor (TSR): A shunt connected thyristor-switched inductor whose effective

reactance is varied in a stepwise manner by full-or zero-conduction operation of the thyristor valve.

f SVC. TSR is made up of several shunt connected inductors which are


required step changes in the reactive power consumed from the system. Use of thyristor


Thyristor Switched Capacitor (TSC): A shunt connected thyristor-switched capacitor whose

effective reactance is varied in a stepwise manner by full-or zero-conduction operation of the thyristor

TSC is also a subset of SVC in which thyristor based ac switches are used to switch in and out


of all controller converters are all connected

): A static synchronous generator operated as a


one of the key FACTS controllers. It can be

line diagram of STATCOM based on a voltage sourced converter

o also act as an active filter to absorb

A shunt connected static var generator or absorber whose output

is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters


off capability. It includes

t for leading and lagging vars, the thyristor controlled or thyristor switched reactor


controlled inductor whose

conduction control of the thyristor

subset of SVC in which conduction time and hence, current in a shunt reactor is controlled by a

switched inductor whose effective

conduction operation of the thyristor valve.

f SVC. TSR is made up of several shunt connected inductors which are


required step changes in the reactive power consumed from the system. Use of thyristor switches


switched capacitor whose

conduction operation of the thyristor

TSC is also a subset of SVC in which thyristor based ac switches are used to switch in and out


the system. Unlike shunt reactors, shunt capacitors cannot be switched continuously with variable

firing angle control.

(vi) Static VAR System (SVS): A combination of different static and mechanically-switched VAR

compensators whose outputs are coordinated.


1. Series compensation is highly effective in controlling ____________ in line. [ d ]

a) Resistances b) Angle c) Phase d) Power flow

2. SSSC is ______________ type of compensator. [ b ]

a) Shunt b) Series c) Both d) None

3. In TSSC a capacitor is inserted into line by line the thyristor valve at _______ crossing of current.

[ b ]

a) Peak b) Average c) Zero d) Negative

4. DC voltage should be ______________ always for proper compensation. [ a ]

a) Constant b) varying c) negative d) None

5. TSSC is Series type of compensation.

6. Series capacitive compensation is to decrease the overall series effective impedance.

7. PLL is employed for synchronizing of circuit.

8. VSI uses Capacitor as energy source.

9. Series controllers inject _________ when they are connected in the line [ a ]

a) voltage b) current c) both voltage & current d) either (a) or (b)

10. At which firing angle, the TCSC helps in limiting fault current [ a ]

a) 900 b) 1800 c) between 900 and 1800 d) 00

11. The equation and range for degree of compensation, k can be expressed as [ a ]

a) Xc/X, 0≤k<1 b) X/Xc, 0≤k<1 c) Xc/X, k<1 d) X/Xc, 0<k<1

12. An electronic interface between energy source and converter is Chopper.

13. The function of reactor in TSC is to limit surge currents.

14. TSC-TCR purpose is to minimize standby losses.

15. Voltage stability decreases with inductive loads and increases with capacitive loads.

16. SVS exchanges both real and reactive power with transmission system.

17. Expression for real power in terms of V, X, sinδ is P0=V2/X sinδ.

Questions

1. How are FACTS controllers make transmission flexible?

2. What is the best place to locate STATCOM in transmission system? Why so?

3. a)List out the advantages of FACTS controllers used in transmission system in brief.

b) Explain how SVC and STATCOM, functions under dynamic situation for shun

Compensation along with appropriate diagrams and characteristics?

4. a)What are the parameters controlled during series and shunt compensation

b) Enumerate the various methods available fo

UNIT – V: Static Series Compensators


COMBINED SERIES –SHUNT

Figures shows combinations of series and shunt controllers,

manner.

➢ The combined shunt and series controllers inject current into the system with the shunt part of the

controller and voltage in series in the line with series part of controller.

When shunt and series controllers are unified .there c

and shunt controllers via the power link.

SERIES CONNECTED CONTROLLERS

Static Synchronous Series Compensator (SSSC):

an external electric energy source as a seri

and controllable independently of, the line current for the purpose of increasing or decreasing the

overall reactive voltage drop across the

SSSC may include transiently rated energy storage or energy absorbing devices to enhance the

dynamic behavior of the power system by additional temporary real power compensation, to increase

or decrease momentarily, the overall real (resistive) vo

SSSC is one of the most important FACTS controllers. It is like a STATCOM, except that the output

ac voltage is in series with the line. It can be based on a voltage

converter. Battery-storage or superconducting magnetic storage can also be connected to a series

controller to inject a voltage vector of variable angle in series with the line.

How are FACTS controllers make transmission flexible?

What is the best place to locate STATCOM in transmission system? Why so?

List out the advantages of FACTS controllers used in transmission system in brief.

how SVC and STATCOM, functions under dynamic situation for shun

Compensation along with appropriate diagrams and characteristics?

What are the parameters controlled during series and shunt compensation?

b) Enumerate the various methods available for controllable VAR generation.

V: Static Series Compensators – Combined Compensators


SHUNT CONTROLLERS: Figures shows combinations of series and shunt controllers, which are controlled in a co

The combined shunt and series controllers inject current into the system with the shunt part of the

controller and voltage in series in the line with series part of controller.

When shunt and series controllers are unified .there can be a real power exchange between the series

and shunt controllers via the power link.

SERIES CONNECTED CONTROLLERS Static Synchronous Series Compensator (SSSC): A static synchronous generator

an external electric energy source as a series compensator whose output voltage is quadrature with,


overall reactive voltage drop across the line and thereby controlling the transmitted electric po



or decrease momentarily, the overall real (resistive) voltage drop across the line.


ac voltage is in series with the line. It can be based on a voltage-sourced converter or current

ge or superconducting magnetic storage can also be connected to a series

controller to inject a voltage vector of variable angle in series with the line.

List out the advantages of FACTS controllers used in transmission system in brief.

how SVC and STATCOM, functions under dynamic situation for shun

r controllable VAR generation.

Combined Compensators

trolled in a co-ordinate

The combined shunt and series controllers inject current into the system with the shunt part of the

an be a real power exchange between the series

A static synchronous generator operated without

es compensator whose output voltage is quadrature with,


line and thereby controlling the transmitted electric power. The



ltage drop across the line.


sourced converter or current- sourced

ge or superconducting magnetic storage can also be connected to a series

(i) Interline Power Flow Controller (IPFC):

Series Compensators which are coupled via a common dc link to facilitate bi

power between the ac terminals of the SSSCs, and are controlled to provide independent reactive

compensation for the adjustment of real power flow in each

of reactive power flow among the lines. The IPFC structure may also include a STATCOM, coupled

to the IPFC’s common dc link, to provide shunt reactive compensation and supply or absorb the

overall real power deficit of the combined SSSCs.

(ii) Thyristor Controlled Series Capacitor (TCSC

consists of a series capacitor bank shunted by a thyristor

smoothly variable series capacitive reac

The TCSC is based on thyristors without the gate turn

and like an SSSC, it is a very important FACTS Controller. A variable reactor such as a Thyristor

Controlled Reactor (TCR) is connected across

(iii) Thyristor-Switched Series Capacitor (TSSC):

of a series capacitor bank shunted by a thyristor

series capacitive reactance. Instead of co

switching inductors at firing angle of 90 degrees or 180 degrees but without firing angle control, could

reduce cost and losses of the Controller. It is reasonable to arrange one of the modules to

thyristor control, while others could be thyristor switched.

(iv) Thyristor-Controlled Series Reactor (TCSR):

consists of a series reactor shunted by a thyristor controlled reactor in order to provide a smoothly

variable series inductive reactance. When the firing angle of the thyristor controlled reactor is 180

degrees, it stops conducting, and the uncontrolled reactor acts as a fault current limiter. As the angle

decreases below 180 degrees, the net inductance decreases until firing angle of 90 degrees, when the

net inductance is the parallel combination of the two reactors. As for the T

single large unit or several smaller series units.

(v) Thyristor-Switched Series Reactor (TSSR

a series reactor shunted by a thyristor

control of series inductive reactance. This is a complement of TCSR, but with thyristor switches fully

on or off (without firing angle control) to achieve a combination of stepped series inductance.


1. In switching converter type series compensators injected voltage is in _______ with the line

current.

a) Phase b) opposite phase

2. In UPFC series converter provides controllable_____ and _____ in series with the line.

a) Real and reactive power b) voltage and phase angle

Interline Power Flow Controller (IPFC): The combination of two or more Static synchronous

Series Compensators which are coupled via a common dc link to facilitate bi-directional flow of real


compensation for the adjustment of real power flow in each line and maintain the desired distribution


IPFC’s common dc link, to provide shunt reactive compensation and supply or absorb the

it of the combined SSSCs.

Thyristor Controlled Series Capacitor (TCSC): A capacitive reactance compensator which

consists of a series capacitor bank shunted by a thyristor-controlled reactor in

smoothly variable series capacitive reactance.

The TCSC is based on thyristors without the gate turn-off capability. It is an alternative to SSSC above

and like an SSSC, it is a very important FACTS Controller. A variable reactor such as a Thyristor

Controlled Reactor (TCR) is connected across a series capacitor.

Switched Series Capacitor (TSSC): A capacitive reactance compensator which consists

of a series capacitor bank shunted by a thyristor-switched reactor to provide a stepwise control of

series capacitive reactance. Instead of continuous control of capacitive impedance, this approach of


reduce cost and losses of the Controller. It is reasonable to arrange one of the modules to

thyristor control, while others could be thyristor switched.

Controlled Series Reactor (TCSR): An inductive reactance

shunted by a thyristor controlled reactor in order to provide a smoothly

iable series inductive reactance. When the firing angle of the thyristor controlled reactor is 180

ng, and the uncontrolled reactor acts as a fault current limiter. As the angle


net inductance is the parallel combination of the two reactors. As for the TCSC, the TCSR may be a

single large unit or several smaller series units.

Switched Series Reactor (TSSR): An inductive reactance compensator which consists of

a series reactor shunted by a thyristor-controlled switched reactor in order to provide




itching converter type series compensators injected voltage is in _______ with the line

b) opposite phase c) Quadrature d) None

In UPFC series converter provides controllable_____ and _____ in series with the line.

eal and reactive power b) voltage and phase angle c) voltage and current

The combination of two or more Static synchronous

directional flow of real


line and maintain the desired distribution


IPFC’s common dc link, to provide shunt reactive compensation and supply or absorb the

): A capacitive reactance compensator which

controlled reactor in order to provide a

off capability. It is an alternative to SSSC above

and like an SSSC, it is a very important FACTS Controller. A variable reactor such as a Thyristor-

A capacitive reactance compensator which consists

switched reactor to provide a stepwise control of

ntinuous control of capacitive impedance, this approach of


reduce cost and losses of the Controller. It is reasonable to arrange one of the modules to have

compensator which

shunted by a thyristor controlled reactor in order to provide a smoothly

iable series inductive reactance. When the firing angle of the thyristor controlled reactor is 180

ng, and the uncontrolled reactor acts as a fault current limiter. As the angle


CSC, the TCSR may be a

): An inductive reactance compensator which consists of

controlled switched reactor in order to provide a stepwise



itching converter type series compensators injected voltage is in _______ with the line

[ c ]

In UPFC series converter provides controllable_____ and _____ in series with the line. [ b ]

c) voltage and current d)None

3. Converters mostly employed in FACTS controllers are____________ . [ a ]

a) VSC b)CSC c) VSS d)None

4. In switching converter type series compensators Voltage is injected into the line.

5. In UPFC shunt converter provides real power support to series converter.

6. Switching converter type Var generator operation is similar to Alternator.

7. Convergence in unified method is faster than alternating method.

8. Example of combined series-series controllers is [ a ]

a) UPFC b) IPFC c) SVC d) VSC

9. The transmitted power can be increased to maximum at small transmission angles.

10. SSSC and TCSC can maintain maximum compensation voltage with decreasing line current.

Questions

1. What do you understand from the term compensation? List out the objectives of series compensation?

2. Which of the two devices-characteristics combined in UPFC?

3. a) Distinguish between TCSC and SSSC.

b) Explain the functioning of static series synchronous compensator (SSSC)

4. What is UPFC? Why is it called unified? How active and reactive powers are controlled

independently with UPFC.

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