PROTECTION AND

INTERLOCKING SCHEME OF

MV SWITCHGEAR

BY MURTAZA HUSSAIN

SR ENGR, SWE

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 Controlgear

for rated voltage above 1 kV & upto & including 52 kV.

IS:13118:1991/IEC:56:1987 – Specification for High Voltage Alternating

current circuit breaker.

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

CHAMBERS IN SWITCHGEAR

Instrument Chamber

(Relay/meters/switches etc.)

HT CHAMBER

LT CHAMBER

Breaker Chamber

Busbar Chamber

CT/PT Chamber

LOCATION OF VARIOUS MAJOR

COMPONENTS IN SWITCHGEAR

Circuit Breaker

Current Transformer

Instruments

Potential Transformer

Surge Suppressor

Busbar

INSERT PICTURE

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

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

TIE Feeder: Switchgear panel which connects the two same voltage level

switchboard. 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 other switchboard.

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

Feeder/Line PT: Switchgear panel having voltage transformer and used for

the detection of feeder/line side voltage.

Transformer Feeder: Switchgear panel employed for feeding the transformer.

TIE INCOMER

LINE PT

BU

S P

T

TR

AF

O F

DR

MO

TO

R F

DR

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

OUTGOING/

PLANT FDR

SWITCHGEAR INTERLOCK SCHEME

The major functions of switchgears are protection, control and facilitating the maintenance of the electrical network including the switchgear itself.

Control and inter-locking schemes constitute a very important aspect of medium voltage switchgears. The switching operation involves a variety 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;

SWITCHGEAR INTERLOCK SCHEME

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;

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 secondary

Plug & 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 the

VCB truck is in 'Service‘ or any Intermediate position

between these two positions.

v) The circuit breaker closing operation is not possible unless the

truck is in 'Service' or 'Test' position.

vi) The interlock mechanism cannot be operated unless the circuit

breaker is in 'Open' condition.

IX) Earthing Truck (Test to Service) Limit Switch

VIII) Provision for Earthing

Earthing feature & Interlock (operation of Earthing module)

a) FEB – Feeder Earthing Breaker

b) BEB – Bus Bar Earthing Breaker

VII) Inter changeability of trucks of different current ratings are not

possible.

SWITCHGEAR INTERLOCK SCHEME

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;

SWITCHGEAR INTERLOCK SCHEME

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;

CLOSING CIRCUIT :

The closing circuit consists of fuses, control switch, anti pumping device, spring charged limit switch & closing coil. Closing command is executed by control switch through breaker NC contact when spring is charged. All auxiliary switch contacts position changes i.e. NO contact closes and NC contact opens. The CB can be closed manually by green coloured manual close knob provided in the mechanism box.

ANTI PUMPING : Anti pumping device prevent the CB from getting repeated closing and tripping impulses when a continuous closing command is given before the tripping impulse is withdrawn.

SWITCHGEAR INTERLOCK SCHEME

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;

SWITCHGEAR INTERLOCK SCHEME

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;

Tripping Schemes

Shunt Tripping Schemes

Series Tripping Schemes

SHUNT TRIPPING CIRCUIT :

The tripping circuit consists of fuses, control switch, protective relay & tripping coil. Breaker can be opened 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 is

discharged.

Series Tripping Schemes

Using Relays

Using Summation CT

Using Motor Protection Circuit Breaker (MPCB)

Using Time Limit Fuses

PANEL ILLUMINATION :

40W filament lamp is provided inside the instrument panel. The door 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 provided on the panel for hand lamp.

ANTI CONDENSATION :

Two tubular heaters with thermostat and piano switch are provided for anti condensation in breaker chamber and CT chamber.

SWITCHGEAR INTERLOCK SCHEME

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;

SWITCHGEAR INTERLOCK SCHEME

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;

Trip Circuit Supervision Schemes

The Trip circuit extends beyond the protection relay and other

components such as fuses, relay contacts, switches etc requires

considerable amount of circuit breaker wiring with intermediate

terminal boards. These interconnections coupled with the

importance of the circuit, results in the requirement to monitor the

integrity of the circuit .

SWITCHGEAR INTERLOCK SCHEME

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;

SWITCHGEAR INTERLOCK SCHEME

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;

Voltage Selection Schemes

Voltage signals to instruments and meters mounted on switchgear panels

are derived from the potential transformer (PTs). These PTs are either Bus

connected or Feeder connected. Incase of fault any source feeder,

arrangement should be made in such a way that PT signal should be

available to meters and instruments

Need for Voltage Selection scheme ?

SWITCHGEAR INTERLOCK SCHEME

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;

SWITCHGEAR INTERLOCK SCHEME

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;

Alarm Schemes

Alarm Cancellation Scheme

Alarm Annunciation Scheme

SWITCHGEAR INTERLOCK SCHEME

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;

SWITCHGEAR INTERLOCK SCHEME

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;

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

Synchronizing Schemes To 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 paralleled

following condition should be satisfied.

A check synchronizing relay is used to prevent inter-connection of two badly

synchronized supplied. Its dual purpose is to Safeguard manual

synchronizing.

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.

AUTO SYNCHRONIZING

SWITCHGEAR INTERLOCK SCHEME

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;

SWITCHGEAR INTERLOCK SCHEME

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;

Automatic Supply Transfer Schemes

Requirement of Automatic Bus Transfer Scheme ?

• Unit Switchgear

• Station Switchgear

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

Electrical Interlock

in Closing Circuit

Electrical Interlock

in Tripping Circuit

Electrical Interlocking scheme is guided by the logic diagram.

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 fault

current, thereby reducing the consequent damage.

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

time.

Selectivity : Protective system should be able to select and trip only the nearest

circuit breaker.

Stability : Protective system should not operate for external faults.

FAULT : It is defined as any abnormal condition, which causes reduction in the

basic insulation level strength of system.

FAULT DETECTION : POSITIVE, NEGATIVE & ZERO Phase sequence

component of system.

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)

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 as

a 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

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)

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 & load

connected.

NEED FOR PROTECTION

Allowing operation under normal conditions.

Quick isolation from supply under abnormal conditions.

Averting damage to the motor & driven mechanism.

Enhancement of life of motor.

MOTOR PROTECTION A. 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 TEMPERATURE

2. 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

• Thermal Over Load protection (49)

• Single phasing/ Negative Phase Sequence Protection (46)

• Short-circuits between phases or between phase and earth in the

motor 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)

EXPECTATIONS FROM MODERN MOTOR PROTECTION RELAYS:

1. The design of a modern motor protection relay must be adequate to cater for the protection needs of any one of the vast range of motor designs in service and many of designs having no permissible allowance for overload.

2. The relay should ideally be matched with the motor characteristics and be capable of close sustained overload protection, a wide range of relay adjustment id desirable together 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 current pre-fault known as ‘HOT’ condition.

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

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

BUS DIFFERENTIAL

MOTOR RE-ACCELERATION

PILOT WIRE PROTECTION

TWO OUT OF THREE BREAKER SCHEME

MOTOR DIFFERENTIAL

THANK

YOU

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 bus differential relay. The main difficulty in bus differential protection is that significant differential current may appear due to saturation of CT on external fault. When a CT saturates, its secondary current is not scaled replica of primary current. Therefore, sum of CT secondary current is not equal 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 percentage differential can provide security against normal CT errors due to mismatch of CT turns ratio and magnetization current; it is not adequate to handle severe CT saturation problem. So the relevant questions to be asked now are: (1)How was this problem handled in the past, i.e. in the era prior to numerical relays? (2)How do numerical relays cope with this problem? High Impedance Bus Differential Relay This approach has been the most successful with traditional electro mechanical 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 of systematic 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 your advantage". Recall that when a CT core saturates, it behaves more like an air core device. The coupling between the primary and secondary winding is negligible. The impedance now offered by the CT as seen from the CT secondary terminals is very low and it equals the impedance of the CT secondary winding. The CT is no more a current source with high impedance shunt. Rather, it is a plain low impedance path. Thus, if we increase the impedance of the relay element which was to carry the differential current significantly, then sum of all the CT secondary currents (except for the 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, this scheme of differential bus bar protection cannot be emulated with numerical relays. Therefore, with numerical relays the busbar protection has to be very fast. i.e preferably decision making has to be completed before the CT saturates. Recall that saturation of CT is primarily a consequence of DC offset current. The time for CT core saturation also depends upon time constant (L/R) of transmission line. If the protection system could reach trip decision before the onset of CT core saturation, then it would be reliable. Hence, numerical relaying based bus bar protection is expected to operate in quarter of a cycle. Development of such protection scheme requires ingenuity because of the well known speed vs accuracy conflict. Non linear % Differential Characteristics If the CT core saturation factor could be discounted for, then we could use constant % differential characteristic for bus bar differential protection. We model a CT as scaled current source due to transformation ratio in parallel with magnetizing impedance (Norton's equivalent). However, the magnetizing impedance itself is nonlinear. It is large when CT core is not saturated and small when CT core is saturated. The current in this branch directly contributes to the differential current.

This suggests that % differential characteristics should be modified to have higher slopes to take care of CT saturation. A fast protection scheme can be devised by instantaneous sample based differential protection scheme. In contrast, a phasor summation scheme will be inherently slower as correct phasor estimates will have to wait until the moving window is totally populated with post fault current samples. One way out of this imbroglio is to use a smaller data window (e.g. 3 sample window). On the other hand, the comparison scheme based computation of instantaneous samples can be error prone due to noise transient related problem. To obtain reliability, it is necessary that consistent differential current should be obtained. A transient monitor function can be used to check that. A transient counter is initialized to zero. If a fault is detected due to presence of differential current, then counter is incremented. Conversely, if counter is greater than zero, and no fault is detected (small enough differential current magnitude) then counter 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 an inbuilt feature to detect CT saturation. One way to detect CT core saturation is based on measuring current change in consecutive samples with the expected sinusoidal signal model. A change much beyond the expected change in sinusoidal model indicates CT core saturation. Many more innovative schemes can be thought out to detect CT saturation which is beyond the scope of this lecture.

NEED OF STABILIZING RESISTOR & METROSIL

IN HIGH IMPEDENCE CIRCUIT

• Stabilizing resistor are used to Limit the heavy fault current to safe

value for relays

• Metrosil are used to limit voltage drop across the relays

HIGH & LOW IMPEDENCE CIRCUITS

The high impedance protection is "more sensitive" compared to low impedance protection. Apart from that high impedance protection is faster that of low impedance protection. But this is at the cost of high CT requirements like same CT ratio and high CT knee point voltage requirement. More over to make the protection 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 less complexity than that of High impedance protection as it does not require 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 bit slower, but the CT requirement is less as it can be used with difference CT ratios with 5P class of CT's and comparatively less Vk requirement.