chapter 3 ps, psm, tms

8/17/2019 Chapter 3 Ps, Psm, Tms

1/32

1

Primary and backup protectionNeed of coordination

Properties of protection systems

Overload and overcurrent

2

• Three essential properties of the protective

relaying schemes are:

sensitivity

selectivity and

speed

• These are not always the properties of the

relays but are properties of correct relaysetting and protection schemes

application.

Properties of protection schemes


2/32

3

Pickup and Sensitivity• Pick up (plug setting) → Minimum operating current of the relay.

• Lower the pick up of the relay, higher will be the sensitivity.

• Higher the sensitivity, fault currents of lower magnitudes can also be

detected.

– Primary operating current, P.O.C. = Pickup*CTR

– % Sensitivity = (P.O.C./Fault current)*100 (S = 1/ PSM)

4

Sensitivity V/s Thermal capacity

• In case of static relays and numerical relays: only

thermal capability of relay imposes restriction on

the choice of the lowest setting.

• For example, SPAJ 140C, of ABB/7SJ600 of Asea

Brown / Micom P120 of Areva make relays have

thermal capacity of 100*In for 1 second, where, In

is relay rated current.

• In case of electro-mechanical relay: (1) lower the

setting of the relay, more will be the burden on theCT resulting into large exciting current drawn by

CT and (2) more heating of the relay element.


3/32

5

• Consider a relay connected to 1000 / 1 CT. Let thefault current be 20 kA. Assume thermal capacity of

relay be 100 A for 1second.

• If relay is set for 10% , relay current at fault =

20,000 / (1000 x 0.1) = 200 IN

• I2 t Criteria: (200IN)2 (t) = (100IN)

2 x 1 sec

t = 0.25 sec

• If operating time of the relay is less than 0.25 sec,

the 10% pick up is permissible, otherwise relaymay get damaged.

Sensitivity V/s Thermal capacity

6

SelectivityIt refers to the selective tripping of the

protective gears and also called as the

discrimination.

The three methods to achieve the

discrimination are:

A. Discrimination by Time,

B. Discrimination by current andC. Discrimination both by Time & Current


4/32

7

Discrimination by Time

8

DMT relay (Definite Minimum Time)(50/2) is a goodexample of achieving discrimination by time. If thecurrent exceeds the set value, operating time isindependent of current magnitude.

A. Discrimination by Time (50/2)


5/32

9

• Assume the discrimination time between successiverelays, in fig.1, is say 0.3 sec.

• let the fault current be above the pick up values for allthe relays. For fault on MCC – 1 outgoing feeder, fuseoperates in 10 miliSec, relay R7 operates in 0.31 secand relay R6 operates in 0.61 sec. All upstream relaysare grades accordingly.

• The disadvantage of using DMT relays is that theoperating time of the upstream relays will be very high.

• The fault closest to the source takes longest time toclear.

• The advantage of using DMT relays is that the operatingtime is well defined for variable source operatingconditions.

Discrimination by Time (50/2)

10

B. Discrimination by current (50 or 50N)• Applicable only when substantial difference between the

fault current magnitudes exists at a given location (L1)

for the fault on the two ends (F1 and F2) of the

equipments.


6/32

11

• The impedance of the equipment shall besubstantial that will create the above differencelike transformer or long cable.

• For illustration consider a fault on LT side of transformer TR2 in fig.1.

• The fault current is 39,227 A on 415 V side andthe reflected current on 6.6 kV side is 2467 A.

• If the fault is on 6.6 kV side, the fault current is16kA. By setting the pick up current for relay R4above 2467 A, the relay R4 will not pick up for fault on the LT side but will pick up for the fault

on HT side.• The disadvantage is that discrimination is

obtained but no back up is ensured.

Discrimination by current (50 or 50N)

12

C. Discrimination both by time and current (51/ 51N)

• IDMT (Inverse Definite Minimum Time) (51 and51N) relays are used to obtain discrimination byboth time & current.

• The operating time of IDMT relay is inverselyproportional to current magnitude.

• Even for highest current, time for operation is notinstantaneous but a minimum time.

• For the same fault current and specified pick up,relay operating time can be varied by adjustingtime dial.


7/32

13

Discrimination both by time and current: NI characteristics

14


8/32

15

Discrimination both by time and current

16

• Assume the discrimination time between successive relaysis say 0.3 sec.

• Let the fault current be above the pick up values for all therelays.

• For fault on MCC – 1 outgoing feeder(F1):

– fuse operates in 10 msec, relay R7 operates in 0.32 secand relay R6 operates in 0.63 sec and relay R4operates in 0.94 sec.

• For fault on HT side(F2):

– of transformer TR2, the fault current is 16 kA and relayR4 operates in 0.73 sec.

• Reduced operating time of relay R4 for 6.6kV faults resultin reduced operating time of upstream relays R2 & R3 for HT fault.

Discrimination both by time and current


9/32

17

IDMT(51)• The advantage gained by using IDMT relay is that:

– With the same pick up and time dial setting:

• lower time of operation for near end fault and higher

operating times for far end fault inherently achieved.

– In case of same fault current magnitude along the

system:

• desired operating time can be achieved in IDMT

relays by adjusting Pick up (PS) & time dial.

DMT(50/2)

• In case of difference in fault current magnitudes at one

point along the system: – DMT relays are superior to the IDMT relays.

DMT and IDMT compared

18

Speed: Fault clearing time and Critical Clearing

Time

• If the fault clearing time is less than 100 msec,it is considered as high speed tripping.

• High speed tripping minimizes the damage tothe equipments, increases stability margin for synchronous machines and avoids unwarrantedtripping of voltage sensitive loads.

• Critical Clearing Time (CCT) is the minimumtime before which fault has to be cleared.

• Typically it varies between 200 msec to 1second and depends upon location of fault.

• System becomes unstable, when fault clearingtime is more than the CCT.


10/32

19

Speed

20

Speed

• Speed without selectivity leads to poor co-ordination.

Small

fault clearing time


11/32

21

Radial Power Systems

• Radial Power System is a system in which power flowsin the direction from the distribution substation to an

individual customer.

• Radial system looks like a branch of a tree with a main

line connected to a series of a smaller circuit.

• From the smaller circuit, the circuit will branch off to

contain the customer need. Radial system will have one

source of power or a group of power sources in the

same area.

• Power failure, short circuit, down the power line will

cause disruption to the system and the system cannot be

restored until the fault is fixed.

22

Methods to achieve high speed tripping

1.Unit protection: (Generator, transformer, bus, feeder,

motor).

Protection is provided to trip instantaneously for faults

only within the unit under protection. No co-ordination

with external protection is required.

Examples of unit protection are: bus differential

protection, feeder pilot wire protection, transformer or

motor differential protection.

2.Directional protections for multi source / non redialsystems.


12/32

23

• Speed must be weighed against economy.• In LT distribution networks, loads are connected

at radial end of system, fault clearance time is

shorter and hence need of speedy clearance is

not critical.

• Unit Protection in LT system are generally not

employed.

• In generating plants, M.V. & H.V. systems high

speed tripping is essential to maintain system

stability and voltage stability.

Unit protection

24

Example of Unit protection


13/32

25

DISCRIMINATION TIME

The time margin between the settings of these relays must take intoaccount three factors:

• The operating time of the circuit breakers. Modern medium voltagebreakers are rated for 5 cycle interrupting time, and the time allowanceis traditionally 0.1 second.

• Overtravel, which is the tendency for a relay to continue to time after the fault current is interrupted by a downstream circuit breakers.Overtravel is a natural characteristic of the inertia of electromechanicalinduction disk relays, and while exact values vary widely, thetraditional allowance is 0.1 seconds. Most static analog and digitalrelays are designed to have no practical overtravel. If the backup relayhas an overtravel tendency, it’s time delay must be long enough toaccount for that overtravel.

• Margin to account for imponderables such as the uncertainties inthe magnitude of fault current, inaccuracies in instrument transformers,manufacturing and calibration variations in relays, etc. The amount of

margin allowed depends upon how much risk the relay engineer iswilling to assume; typical values range from 0.1 to 0.3 seconds.

• The total “coordinating time margin” is the sum of these three factorsand ranges from 0.2 to 0.5 seconds; a 0.3 second margin is oftentaken as a reasonable compromise between the objectives of speedand security.

26

DISCRIMINATION TIME (CO-ORDINATION INTERVAL) - Speed

• This refers to the time interval between the operation of two

adjacent breakers or fuse and breaker.


14/32

27

Circuit Breaker Interrupting Time

• The circuit breaker interrupting the fault musthave completely interrupted the current before

the discriminating relay ceases to be energized.

• The time taken is dependent on the type of

circuit breaker used and the fault current to be

interrupted. Manufacturers normally provide the

fault interrupting time at rated interrupting

capacity and this value is invariably used in the

calculation of grading margin.

28

Circuit breaker operating time


15/32

29

Relay Timing Error

• All relays have errors in their timingcompared to the ideal characteristic asdefined in IEC 60255.

• For a relay specified to IEC 60255, a relayerror index is quoted that determines themaximum timing error of the relay.

• The timing error must be taken intoaccount when determining the grading

margin.

30

Overshoot• When the relay is de-energized, operation may continue

for a little longer until any stored energy has beendissipated. For example, an induction disc relay will havestored kinetic energy in the motion of the disc; staticrelay circuits may have energy stored in capacitors.

• Relay design is directed to minimizing and absorbingthese energies, but some allowance is usuallynecessary.

• The overshoot time is defined as the difference betweenthe operating time of a relay at a specified value of inputcurrent and the maximum duration of input current,which when suddenly reduced below the relay operatinglevel, is insufficient to cause relay operation.


16/32

31

CT Errors

• Current transformers have phase and ratioerrors due to the exciting current required tomagnetize their cores.

• The result is that the CT secondary current is notan identical scaled replica of the primary current.

• This leads to errors in the operation of relays,especially in the time of operation.

• CT errors are not relevant when independentdefinite-time delay over current relays are being

considered.

32

Final Margin• After the above allowances have been

made, the discriminating relay must just

fail to complete its operation.

• Some extra allowance, or safety margin, is

required to ensure that relay operation

does not occur.


17/32

33

Co-ordination interval shall incorporate the following

time periods:

A. Interrupting time of downstream breaker:

Approximately 100 mSec. (Though the actual operating time is muchsmaller, say 50 mSec).

B. Relay error factor:

Refers to negative or positive errors in operating time of the upstream+ Downstream relays / fuse involved in grading.

For Co-ordination time between Fuse & Breaker = 0.4t

For Co-ordination time between Breaker & Breaker = 0.25t

Where, t = operating time of Downstream Fuse / Relay

C. Overshoot time of upstream relay:

Operating time more than set value due to contact over travel etc. it isabout 50msec for electro-mechanical relays. This is not relevant for modern numerical relays.

D. Safety Margin:

Refers to the extra allowance to ensure a satisfactory gap betweenoperating time of two breakers or breaker and fuse. It can be about100msec for electro-mechanical relays. If numerical relays are used for both upstream and downstream, this can be reduced, to even 50msec.

34

EMPIRICAL FORMULAS FOR CO-ORDINATION TIME

For Fuse – Breaker IDMT (Conventional Relays)/ Fuse – IDMT/INST - IDMT relay/fuse-DMT

Discrimination Time = 0.4t + 0.15, where

Fuse operating time = t

Downstream Inst. Relay Error / Fusing Factor = 0.4t

Upstream Relay overshoot Time = 50 m Sec.

Safety Margin = 100 m Sec.

Interrupting time of Downstream Breaker = 0 m Sec (N.A.)

For Breaker – Breaker (Conventional Relays) / IDMT - IDMTrelays

Discrimination Time = 0.25t + 0.25,where

Downstream Relay operating Time = tRelay Error Factor = 0.25t

Upstream Relay overshoot Time = 50 m Sec.

Safety Margin = 100 m Sec.

Interrupting time of Downstream Breaker = 100 m Sec.


18/32

35

Present Scenario for Discrimination Time Formula

Following changes can be done in discrimination timeformula:

a) Relay error factor for numerical relays : 0.1t (instead of 0.25t)

b) Overshoot time : Static Relay = 50 mSec: Numerical Relay = 0 mSec

c) Modern VCB / SF6 CB operating time:50 mSec, (insteadof 100msec)

d) Safety margin can be reduced (from 100msec.) to50mSec, if relays are accurately set.

36

IDMT NI Characteristics• The IDMT relay

operates with a

fixed time for PSM

> 20.

• Also the relay

characteristic is

defined for PSM >

2.

• Relay has no

definedcharacteristic for

PSM < 2.


19/32

37

Overload and over current

• Over load withstand capacity of equipment in generalvarious from – many seconds to minutes. For Example,

a)Generator overload capacity

b)Transformer overload capability

Time (Sec) 120 60 30 10

Stator current (%) 116 130 154 226

Time (Minutes) 120 80 45 20 10 2

Current (%) 130 145 160 175 200 300

38

Overload and over current• Over current is short circuit current and fault has to be removed within 1

sec, where as overload can be sustained in minutes / hours. Hence, over

current relay with any characteristic cannot be used for over load

protection.


20/322

39


40


• If over current persists for more than 1 sec, it will result in loss of

synchronism (angle instability) or motor stalling (Voltage instability).

• Conceptually over current relays cannot be used for over load protection.


21/322

41

Setting DMT Relays

Primary Operating Currento Must lie above maximum running load current

and largest drive starting current by safety

margin.

o Max. Running Load current includes motor full

load current. Hence, it is subtracted.

o Must lie below the lowest through fault current.

o Relevant for generally used motor with DOL

starting.

42

IF > P.O.C. > (IRL – IFLM + ISTM)

Where,

IF = Minimum fault current relay to sense.

P.O.C = Desired Primary Load Current of relay

IRL = Max. Running Load Current

IFLM = Highest Rating Drive Full Load Current

ISTM = Highest Rating Drive Starting Current

Remark: The First Comparison IF > P.O.C is generally

satisfied in most of the cases, since fault currentmagnitude is generally very high. The only critical case in

which this comparison is important is when source fault

level is low.

Setting DMT Relays


22/322

43

Desired Pick Upo PICK UP ≥ Primary Operating

Current (P.O.C) / C.T. Ratio

DMT relay – independent of fault

current, hence, plug setting

multiplier applicable for IDMT

relays not relevant for DMT

relays.

I > ISETIf ISET = 5000A

I = 5001A or I = 20000A,

operating time is same.

Setting DMT Relays

44

DMT relay calculations

Example of DMT Relays:Electromechanical relays: CTU, CAG + VTT. All numerical relays have

inbuilt DMT feature.


23/322

45

Setting IDMT Relays

Primary Operating Current

• P.O.C Must lie above (Maximum running load current -

Highest drive full load current of motor + Largest drive

starting current).

• Max. running load current includes motor full load current

of started motor. Hence, it is subtracted.

• P.O.C Must lie below the lowest through fault current.

Formula: IF > P.O.C. >= IRL – IFLM + ISTM

Where, P.O.C. = Desired primary operating current

IRL = Max. running load current

ISTM = Highest drive starting current of motor

IFLM =Highest drive full load current of motor

46

Desired Relay Pick up – PS (Plug Setting)

Ratio of primary Operating Current of relay to C.T. Ratio

(C.T.R.)

PS = P.O.C.

C.T.R.

= (IRL – IFLM + ISTM)

C.T.R.

Selected pick up setting:

Select the next higher available step.

Actual Primary operating current (P.O.C.)

Actual P.O.C. = Selected Pick up x C.T.R.Plug Setting Multiplier – PSM

P.S.M = Fault Current

Actual P.O.C.

Setting IDMT Relays


24/322

47

Desired relay operating time t1

t1 = t + td

where, t = Downstream Relay / Fuse Operating Time

td = Discrimination Time.

Desired Time Dial Set Point – TMS (TIME MULTIPLIER

SETTING)

Desired TMS Setting =

Desired Relay Operating Time t1 (OT)/ Relay Operating

Time @ Selected PSM and TMS 1.0 (OT1)

Desired Relay Operating Time t1 = Desired TMS setting*

Relay Operating time @ selected PSM and TMS = 1.0

Selected Time Dial (TMS) SettingNearest Higher Time Dial setting selected.

Setting IDMT Relays

48

IDMT Setting: Plug Setting (PS),

Plug Setting Multiplier (PSM) and

Time Multiplier Setting (TMS)

PS : Plug Setting (Current Setting /Pickup Setting)= (IRL - IFLM + ISTM) / CTR

PS = (583 - 145 + 869) /1600

= 0.816Set P.S. =0.9APrimary Operating Current (P.O.C)

=C.T.R. x P.S.

=1600 x 0.9 = 1440 APlug Setting Multiplier (P.S.M.)= Fault Current / Actual P.O.C.= 38872 / 1440 = 26.99PSM will be used to find operatingtime as per next slide.

0.4

Notation: PS(TMS)


25/322

49

Desired operating Time t1:Downstream fuse blow – off time t = 0.01 Sec

Co- ordination interval td = 0.4 t + 0.15

= 0.154 Sec

Desired operating time t1 = t + td

= 0.01 + 0.154

= 0.164 Sec

Relay R7 : Location : Incomer of MCC-1(415 V)

50

Relay R7 : Location : Incomer of MCC-1(415 V)

Choose normal inverse characteristic.

TMS = Required Time Setting = OT/OT1

= Desired Operating Time / Operating Time @ TMS = 1.0

OT ⇒ From Coordination Requirement

OT1 ⇒ From Equation or Standard Graphs

Operating time @ PSM > 20 & time dial 1.0 = Operating Time @ PSM =

20 (For PSM>20, Take PSM = 20) & time dial 1.0:

Operating Time of R7 (OT1)

= 0.14/ (PSM)0.02 -1 = 2.267

(For NI @ Time dial 1.0)

TMS = Desired Operating Time t1

Operating Time @ TMS 1.0 & PSM 20

= 0.164

2.267

= 0.072

Set Time Dial at 0.08. [Time Dial =0.05 – 1.00, Step = 0.01]

Actual operating Time = 0.181 Sec for fault. [2.267x 0.08]


26/322

51

IDMT Setting: Time dial V/s operating time

From graph:

OT1 = 2.267

OT

0.08

52

Time dial V/s operating time

IDMT Relay Equation/IECequation

OT = TMS * β / (PSM)α -1.0

OT1 = β / (PSM)α -1.0

OT: Operating Time in sec

TMS: Time Multiplier Setting

PSM: Plug Setting Multiplier

IDMT Characteristics ß αNormal Inverse (NI) 0.14 0.02

Very Inverse (VI) 13.5 1.0

Extremely Inverse (EI) 80.0 2.0

Long Time Inverse (LI) 120.0 1.0


27/322

53

54

IDMT relay typesExtremely inverse (EI)

Generally used to back up

fuse

Very Inverse (VI)

Preferred on upstream

side of transformer

Normal inverse (NI)

If in doubt, use NI

Used in majority of

applicationsLong time inverse (LI)

To protect NGR


28/322

55

IDMT relay types

There are a number of characteristics in common use. Each

of these exists to address specific application needs.Following is a list of the most common characteristicstogether with their usual applications and also the codenumber which GE uses to identify each curve shape:

• Inverse medium time (51) - best suited for applicationswhere the variations in the magnitude of fault current arerelated primarily to switching of sources on the system, suchas in paper mill systems with a number of smallhydroelectric generators which are switched on and off depending on water conditions.

• Very inverse medium time (53) - best suited for generalapplications where the variations in the magnitude of faultcurrent are primarily determined by system impedance andfault location. This relay characteristic is the best choice for most industrial and commercial applications.

56

IDMT relay types• Inverse medium-long time (57) - best suited for

applications as backup ground fault protection on complexlow-resistance grounded medium voltage systems.

• Inverse long time (66) - best suited for overload andlocked rotor protection of motors.

Extremely inverse medium time (77) - best suited for application on utility residential distribution circuits whereselectivity with distribution fuse cutouts and reclosers is arequirement, and where “cold load pickup” is aconsideration.

Inverse short time (95) - best suited for backup ground fault

protection applications on solidly grounded low voltagesystems where the feeders have instantaneous groundfault protection.


29/322

57

58

Very Inverse (VI): CDG13 Operating Time @ PSM = 10 & TMS =

1.0 => 1.5 sec

Operating Time

= TMS x 13.5 / [ ( PSM – 1.0) ]

2 < PSM < 20

Used on H.V. Side of transformer to co-ordinate with NI Characteristic relay onL.V. side.

Very inverse characteristic is usefulwhere substantial reduction in faultcurrent occurs due to large impedance of protected object, e.g. on upstream side

of transformer. Advantage of NI and VI combination is

that for L.T faults, operating timeincreases to coordinate with downstreamfaults and for HT faults, operating time isminimum to clear faults within CCT.


30/323

59

Extremely Inverse (EI)

Application of EI characteristics Fuse – Relay Co-ordination :

Depending on cable size, cable length and fuse

rating, co-ordination between fuse and certain

characteristics of relays may not be achievable.

Sending end fault level : 35 kA (say).

Ground fault occurs at motor end.

60

Example : For 300 sq.mm cable (Motor Rating – 110 kW,DOL) Point ‘R’ : Length > 108 m, fault current reduces below

5kA Point ‘Q’ : Length > 184 m, fault current reduces below

3kA Point ‘P’ : Length > 564 m, fault current reduces below

1kA Depending on fuse rating, the fault at receiving end may

be cleared in much more than normal operating time of 10 to 20 milliseconds.

Fig shown is with fault impedance zero. If fault

impedance is also considered, fault current will stillfurther go down. Co-ordination between fuse and upstream relay become

critical.



31/323

61

62


32/32

63

64

Breaking capacity of contactor

Rating AC3 AC4

100A 6 Ith 8 Ith

Over load relay shall trip

Contactor, in case of switch – fuse – contactor module.

Contactor again!, in case of MCCB – contactor module.

Too frequent opening of MCCB not good for its life.


chapter 3 ps, psm, tms

Documents