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PROTECTION ANDINTERLOCKING SCHEME OF

MV SWITCHGEAR

BY MURTAZA HUSSAINSR ENGR, SWE


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SWITCHGEAR IEC/IS

BHEL Manufacture Switchgear as per IEC:62271-100-

2001, IS:3427:1997/IEC:298:1990, IS:13118:1991/IEC:56:1987

IEC:62271-100-2001 High voltage alternating current circuit breaker.

IS:3427:1997/IEC:298:1990 AC Metal enclosed switchgear and Controlgearfor rated voltage above 1 kV & upto & including 52 kV.

IS:13118:1991/IEC:56:1987 Specification for High Voltage Alternatingcurrent circuit breaker.


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A MECHANICAL SWITCHING DEVICE CAPABLE OF MAKING,

CARRYING, AND BREAKING CURRENTS UNDER NORMAL CIRCUIT

CONDITIONS AND ALSO MAKING, CARRYING FOR A SPECIFIED TIME

AND BREAKING CURRENTS UNDER SPECIFIED ABNORMAL CIRCUIT

CONDITIONS SUCH AS THOSE OF SHORT CIRCUIT.

SWITCHGEAR

A GENERAL TERM COVERING SWITCHING DEVICES AND THEIR

COMBONATION WITH ASSOCIATED CONTROL, MEASUREING,PROTECTIVE AND REGULATING EQUIPMENT, ALSO ASSEMBLIES

OF SUCH DEVICES AND EQUIPMENT WITH ASSOCIATED

INTERCONNECTIONS, ACCESSORIES, ENCLOSURE AND

SUPPORTING STRUCTURES.

CIRCUIT BREAKER


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CHAMBERS IN SWITCHGEAR

Instrument Chamber

(Relay/meters/switchesetc.)

HT CHAMBER

LT CHAMBER

Breaker Chamber

Busbar Chamber

CT/PT Chamber


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LOCATION OF VARIOUS MAJOR

COMPONENTS IN SWITCHGEAR

Circuit Breaker

Current Transformer

Instruments

Potential Transformer

Surge Suppressor

Busbar

INSERT PICTURE


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SWITCHGEAR IN POWER SYSTEM CAN ACT AS :

Line PT

Bus PT

Bus Coupler

Plant Feeder / Outgoing feeder

Transformer Feeder

Motor Feeder

TIE Feeder

Incomer Feeder


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Incomer Feeder: Switchgear Panel intended for supply power to the Switchboard.

TIE Feeder: Switchgear panel which connects the two same voltage levelswitchboard. Power can flow in either direction TIE feeder.

Motor Feeder: Switchgear panel employed for feeding the motor.

Plant / Outgoing Feeder: Switchgear panel employed for supply power to otherswitchboard.

Bus PT: Switchgear panel having voltage transformer and used for thedetection of bus voltage.

Feeder/Line PT: Switchgear panel having voltage transformer and used forthe detection of feeder/line side voltage.

Transformer Feeder: Switchgear panel employed for feeding the transformer.


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TIE INCOMER

LINE PT

BUSP

T

TRAF

OFDR

MOTOR

FDR

Typical Power Plant Single Line Diagram (SLD) (PART)

OUTGOING/PLANT FDR


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SWITCHGEAR INTERLOCK SCHEME

The major functions of switchgears are protection, control andfacilitating the maintenance of the electrical network including the

switchgear itself.Control and inter-locking schemes constitute a very important aspectof medium voltage switchgears. The switching operation involves avariety of control and inter-locking schemes.

Following are the variety of schemes which are being used:

Automatic supply transfer schemes.

Synchronizing schemes and

Alarm schemes,

Voltage selection schemes;

Trip circuit supervision schemes;

Tripping schemes;

Closing Scheme;

Safety Interlocks & schemes using position limit switches;


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Alarm schemes,



Tripping schemes;

Closing Scheme;



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SAFETY INTERLOCKS:

i) The VCB truck cannot be racked in or out unless circuit

breaker is in 'Open' condition.

ii) The VCB truck can not be racked in unless secondaryPlug & socket are engaged.

iii) The circuit breaker closing operation is not possible

Unless secondary plug & socket are engaged.

iv) The secondary plug and socket can not be disengaged When theVCB truck is in 'Service or any Intermediate position

between these two positions.

v) The circuit breaker closing operation is not possible unless thetruck is in 'Service' or 'Test' position.

vi) The interlock mechanism cannot be operated unless the circuitbreaker is in 'Open' condition.


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IX) Earthing Truck (Test to Service) Limit Switch

VIII) Provision for Earthing

Earthing feature & Interlock (operation of Earthing module)a) FEB Feeder Earthing Breakerb) BEB Bus Bar Earthing Breaker

VII) Inter changeability of trucks of different current ratings are notpossible.


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Alarm schemes,



Tripping schemes;

Closing Scheme;



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Alarm schemes,



Tripping schemes;

Closing Scheme;



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CLOSING CIRCUIT :

The closing circuit

consists of fuses, control

switch, anti pumping

device, spring chargedlimit switch & closing coil.

Closing command is

executed by control switch

through breaker NC

contact when spring is

charged. All auxiliaryswitch contacts position

changes i.e. NO contact

closes and NC contact

opens. The CB can beclosed manually by green

coloured manual closeknob provided in the

mechanism box.


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ANTI

PUMPING :

Anti pumpingdevice prevent theCB from gettingrepeated closingand trippingimpulses when a

continuous closingcommand is givenbefore the trippingimpulse iswithdrawn.


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Alarm schemes,



Tripping schemes;

Closing Scheme;



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Alarm schemes,



Tripping schemes;

Closing Scheme;



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Tripping Schemes

Shunt Tripping Schemes

Series Tripping Schemes


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SHUNT TRIPPING CIRCUIT :

The tripping circuit consists of fuses, control switch, protective relay & tripping coil. Breaker can beopened intentionally by control switch & on fault, breaker gets tripping command from relay. All aux.

switches will restore their original positions i.e. NO will open and NC will close.Note :The tripping spring gets charged while the closing spring isdischarged.


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Series Tripping Schemes

Using Relays

Using Summation CT

Using Motor Protection Circuit Breaker (MPCB)

Using Time Limit Fuses


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PANEL ILLUMINATION :

40W filament lamp is provided

inside the instrument panel. Thedoor operated panel illumination

lamp gets automatically lighted

on opening the door.

3 PIN SOCKET& SWITCH:

5/15 Amps, 240 V, 5 Pin socket

with piano switch is also providedon the panel for hand lamp.


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ANTI CONDENSATION :

Two tubular heaters with

thermostat and piano switch

are provided for anti

condensation in breaker

chamber and CT chamber.


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Alarm schemes,



Tripping schemes;

Closing Scheme;



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Alarm schemes,



Tripping schemes;

Closing Scheme;



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Trip Circuit Supervision Schemes

The Trip circuit extends beyond the protection relay and othercomponents such as fuses, relay contacts, switches etc requiresconsiderable amount of circuit breaker wiring with intermediateterminal boards. These interconnections coupled with theimportance of the circuit, results in the requirement to monitor theintegrity of the circuit .


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Alarm schemes,



Tripping schemes;

Closing Scheme;



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Alarm schemes,



Tripping schemes;

Closing Scheme;



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Voltage Selection Schemes

Voltage signals to instruments and meters mounted on switchgear panels

are derived from the potential transformer (PTs). These PTs are either Busconnected or Feeder connected. Incase of fault any source feeder,arrangement should be made in such a way that PT signal should beavailable to meters and instruments

Need for Voltage Selection scheme ?


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Synchronizing schemes and Alarm schemes,



Tripping schemes;

Closing Scheme;




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Tripping schemes;

Closing Scheme;



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Alarm Schemes

Alarm Cancellation Scheme Alarm Annunciation Scheme



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Tripping schemes;

Closing Scheme;




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Tripping schemes;

Closing Scheme;



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c) The phase difference of the two supplies must be within acceptable limits.

Synchronizing SchemesTo bring new bus (source) into the switchboard when old one is running

and shifting to new one.

To meet synchronizism that means two AC supplies are correctly paralleledfollowing condition should be satisfied.

A check synchronizing relay is used to prevent inter-connection of two badlysynchronized supplied. Its dual purpose is to Safeguard manualsynchronizing.

Methods adopted for synchronization are :

Manual Synchronization

By Check Synchronizing Relays

a) The voltages of the two supplies must be within acceptable limits.

b) The frequencies of the two supplies must be within acceptable limits.


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AUTO SYNCHRONIZING



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Tripping schemes;

Closing Scheme;




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Tripping schemes;

Closing Scheme;



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Automatic Supply Transfer SchemesRequirement of Automatic Bus Transfer Scheme ?

Unit Switchgear

Station Switchgear


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Modes of Bus Transfer

A. Manual Bus Transfer

i) Without Voltage Interruption

ii) With Voltage Interruption

a) Slow changeover

b) Fast changeover

B. Automatic Bus Transfer (under fault condition) with Voltage Interruption

a) Slow changeover

b) Fast changeover

El i l I l k


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Electrical Interlockin Closing Circuit

Electrical Interlockin Tripping Circuit

Electrical Interlocking scheme is guided by the logic diagram.

Protection Schemes for Medium Voltage Switchgear


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Protection Schemes for Medium Voltage Switchgear

Importance of Protection System in Electrical System ?

5-S Principles

Security : Protective system should be reliable so that security of supply is ensured.

Sensitivity : Protective system should be able to sense minimum value of faultcurrent, thereby reducing the consequent damage.

Speed : Protective system should be able to isolate fault in the shortest possibletime.

Selectivity : Protective system should be able to select and trip only the nearestcircuit breaker.

Stability : Protective system should not operate for external faults.

FAULT : It is defined as any abnormal condition, which causes reduction in thebasic insulation level strength of system.

FAULT DETECTION : POSITIVE, NEGATIVE & ZERO Phase sequence

component of system.

PROTECTION SCHEMES IN MV SWITCHGEAR


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PROTECTION SCHEMES IN MV SWITCHGEAR

Non Directional Over Current for Phase Faults (50/51)

Non Directional Over Current for Earth Fault (50N/51N)

Directional Over Current for Phase Faults (67)

Directional Over Current for Earth Fault (67N)


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UNIT PROTECTION SCHEME

- Pilot Wire Protection Scheme

- Bus Differential Protection Scheme

- Motor/ Transformer Differential Protection Scheme

- Restricted Earth Protection Scheme

In Unit Protection sections of power system are protected individually asa complete unit without reference to other section.

Some of the Unit scheme which MV Switchgear employed

For the protection of CABLE connecting two Feeder

For the protection of BUSBAR

For the protection of MOTOR/ TRANSFORMER WINDINGS

For the protection of TRANSFORMER WINDINGS


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TRANSFORMER PROTECTION

Non- Directional Over Current and Earth Fault Protection(50/51/50N/51N)

Sensitive Earth Fault Protection (50N/2)

Differential Protection (87T)

Restricted Earth Fault Protection (64R)

Incipient Faults (49/63TX)


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MOTOR PROTECTION

Wide range of A.C Motors

Motor characteristics due to various duties

All Motor needs protection and choice should

be independent of a type of motor & loadconnected.

O OT CT O


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NEED FOR PROTECTION

Allowing operation under normal conditions.

Quick isolation from supply under abnormalconditions.

Averting damage to the motor & drivenmechanism.

Enhancement of life of motor.

MOTOR PROTECTION


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MOTOR PROTECTIONA. MOTOR INDUCED

B. LOAD INDUCED

C. ENVIRONMENT INDUCED

D. SOURCE OR SYSTEM INDUCED

E. OPERATION AND APPLICATION INDUCED

1. INSULATION FAILURE

2. BEARING FAILURE

3. MECHANICAL FAILURE

4. LOSS OF FIELD (SYNCHRONOUS MOTOR)

1. OVERLOAD/ UNDERLOAD

2. JAMMING

3. HIGH INERTIA

1. HIGH AMBIENT TEMPERATURE2. HIGH CONTAMINATED LEVEL-BLOCKED VENTILATION

3. COLD, DAMP AMBIENT TEMPERATURE

1. PHASE FAILURE

2. OVER VOLTAGE/UNDER VOLTAGE

3. PHASE REVERSAL

4. OUT-OF-STEP

1. SYNCHRONIZING, CLOSING OR RECLOSING OUT OF PHASE

2. HIGH DUTY CYCLE

3. JOGGING

MOTOR PROTECTION


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MOTOR PROTECTION

Thermal Over Load protection (49)

Single phasing/Negative Phase Sequence Protection (46)

Short-circuits between phases or between phase and earth in themotor winding or its connections. (50/51)

Partial or complete collapse of voltage (27)

Locked rotor (51S)

Start or Stall Protection(48/51LR)

Earth Fault Protection (50N)

Loss-Of-Load Protection (37)

Out of Step Protection (46)

RTD/BTD Protection (26)

Limitation of the number of start, Time between start (66)


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EXPECTATIONS FROM MODERN MOTOR PROTECTIONRELAYS:

1. The design of a modern motor protection relay must beadequate to cater for the protection needs of any one of the vastrange of motor designs in service and many of designs havingno permissible allowance for overload.

2. The relay should ideally be matched with the motorcharacteristics and be capable of close sustained overloadprotection, a wide range of relay adjustment id desirabletogether with good accuracy & low thermal overshoot.

3. Relay curves should take into account the extremes of zero pre-fault current known as the COLD condition & full rated currentpre-fault known as HOT condition.


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PROTECTION AGAINST SWITCHING SURGES


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PROTECTION AGAINST SWITCHING SURGES:

What is an Electrical Surge ?

External surge & Internal surge

Atmospheric Lightning cause External surge

Switching action of devices cause internal surge

CAUSES OF SURGE GENERATION:

Normal Switching On of a stationary motor.

Normal switching Off of a stationary motor.

Switching a Stalled motor or one running upto speed.

PROTECTION AGAINST


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PROTECTION AGAINST

SWITCHING SURGES:

SURGE PROTECTION DEVICES:These devices limits the over voltages in electrical system to the specified protection

level, principally lower than the withstand voltage of equipment. a/ C-R type surge suppressors.

b/ ZnO type surge arrestors.

IOCL SPECIFIC SCHEMES


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IOCL SPECIFIC SCHEMES

BUS DIFFERENTIAL

MOTOR RE-ACCELERATION

PILOT WIRE PROTECTION

TWO OUT OF THREE BREAKER SCHEME

MOTOR DIFFERENTIAL


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THANKYOU


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Differential relay for bus bar protection can be implemented in one of the following three ways:1.Sample by sample comparison. 2.Comparison of current phasors. 3.High impedance busdifferential relay The main difficulty in bus differential protection is that significant differential


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differential relay. The main difficulty in bus differential protection is that significant differentialcurrent may appear due to saturation of CT on external fault. When a CT saturates, its secondarycurrent is not scaled replica of primary current. Therefore, sum of CT secondary current is notequal to sum of primary currents even though primary CT currents sum to zero.

This causes a differential relay to operate on even external faults, leading to maloperation of bus

protection scheme. This compromises security and is not acceptable. While the percentagedifferential can provide security against normal CT errors due to mismatch of CT turns ratio andmagnetization current; it is not adequate to handle severe CT saturation problem. So the relevantquestions to be asked now are: (1)How was this problem handled in the past, i.e. in the era priorto numerical relays? (2)How do numerical relays cope with this problem? High Impedance BusDifferential Relay This approach has been the most successful with traditional electromechanical and solid state relay. It is based upon the following ingenious and innovative thinking.If you cannot beat CT saturation, exploit it! In fact this is now a well accepted principle in theory ofsystematic innovation, also known as TRIZ (a Russian acronym), that one innovative way to

problem solving is to exploit the harm:

If you cannot undo the harm, stretch the harm to the extreme and then exploit it to youradvantage".Recall that when a CT core saturates, it behaves more like an air core device. The couplingbetween the primary and secondary winding is negligible. The impedance now offered by the CTas seen from the CT secondary terminals is very low and it equals the impedance of the CTsecondary winding. The CT is no more a current source with high impedance shunt. Rather, it is aplain low impedance path. Thus, if we increase the impedance of the relay element which was tocarry the differential current significantly, then sum of all the CT secondary currents (except forthe saturated CT) will be diverted into the low impedance path of saturated CT's secondary.Therefore, differential current would be negligible and hence protection system will not operate(See fig 38.6).Thus, now saturation of CT itself is responsible for saving a false operation.

In contrast, numerical relays offer a low impedance path. Hence, thisscheme of differential bus bar protection cannot be emulated with


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scheme of differential bus bar protection cannot be emulated withnumerical relays. Therefore, with numerical relays the busbar protectionhas to be very fast. i.e preferably decision making has to be completedbefore the CT saturates. Recall that saturation of CT is primarily aconsequence of DC offset current. The time for CT core saturation also

depends upon time constant (L/R) of transmission line. If the protectionsystem could reach trip decision before the onset of CT core saturation,then it would be reliable. Hence, numerical relaying based bus barprotection is expected to operate in quarter of a cycle. Development ofsuch protection scheme requires ingenuity because of the well knownspeed vs accuracy conflict. Non linear % Differential Characteristics Ifthe CT core saturation factor could be discounted for, then we could

use constant % differential characteristic for bus bar differentialprotection. We model a CT as scaled current source due totransformation ratio in parallel with magnetizing impedance (Norton'sequivalent). However, the magnetizing impedance itself is nonlinear. Itis large when CT core is not saturated and small when CT core issaturated. The current in this branch directly contributes to thedifferential current.

This suggests that % differential characteristics should be modified tohave higher slopes to take care of CT saturation A fast protection


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have higher slopes to take care of CT saturation. A fast protectionscheme can be devised by instantaneous sample based differentialprotection scheme. In contrast, a phasor summation scheme will beinherently slower as correct phasor estimates will have to wait until themoving window is totally populated with post fault current samples. Oneway out of this imbroglio is to use a smaller data window (e.g. 3 samplewindow). On the other hand, the comparison scheme basedcomputation of instantaneous samples can be error prone due to noisetransient related problem. To obtain reliability, it is necessary thatconsistent differential current should be obtained. A transient monitorfunction can be used to check that. A transient counter is initialized tozero. If a fault is detected due to presence of differential current, thencounter is incremented. Conversely, if counter is greater than zero, andno fault is detected (small enough differential current magnitude) thencounter is decremented. If the counter crosses a preset threshold value,trip decision is implemented. This scheme will not trip on transient.

However, in addition to internal faults, it will also trip on external fault.For this purpose, the differential protection relay also has to have aninbuilt feature to detect CT saturation. One way to detect CT coresaturation is based on measuring current change in consecutivesamples with the expected sinusoidal signal model. A change muchbeyond the expected change in sinusoidal model indicates CT coresaturation. Many more innovative schemes can be thought out to detectCT saturation which is beyond the scope of this lecture.


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NEED OF STABILIZING RESISTOR & METROSIL


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IN HIGH IMPEDENCE CIRCUIT

Stabilizing resistor are used to Limit the heavy fault current to safevalue for relays

Metrosil are used to limit voltage drop across the relays

HIGH & LOW IMPEDENCE CIRCUITS


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HIGH & LOW IMPEDENCE CIRCUITS

The high impedance protection is "more sensitive" compared tolow impedance protection. Apart from that high impedanceprotection is faster that of low impedance protection. But this is atthe cost of high CT requirements like same CT ratio and high CTknee point voltage requirement. More over to make theprotection stable for thro fault, we need to consider stabilising

resistor and metrosil. Another alternative to high impendence differential protection is

using low impedance protection. It involves comparatively lesscomplexity than that of High impedance protection as it does notrequire any external component like stabilising resistor / metrosil.Here the thro fault stability is achieved thro biasing technique.

When compared to high impedance low impedance will be bitslower, but the CT requirement is less as it can be used withdifference CT ratios with 5P class of CT's and comparativelyless Vk requirement.

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