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1.0 Line Protection Requirement

a Limit voltage variation as per grid code

b Unbalance in the phase angle and magnitude of voltage at the interconnection point

beyond the limits prescribed.

c All Users shall take all possible measures to ensure that the grid frequency always

remains within the 49.7 50.2 Hz band.

d

All distribution licensees / STU shall provide automatic under-frequency and df/dt

relays for (1) load shedding in their respective systems, to arrest frequency decline

that could result in a collapse/disintegration of the grid, as per the plan separately

finalized by the SLDC and shall ensure its effective application to prevent cascade

tripping of generating units in case of any contingency and (2) to effect islanding.

e

All distribution licensees, STU shall ensure that the above under-frequency and df/dt

load shedding/islanding schemes are always functional. The provisions regarding

under frequency and df/dt relays of relevant CEA Regulations shall be complied with.

SLDC shall carry out periodic inspection of the under frequency and df/dt relays and

maintain proper records of the inspection. SLDC shall decide and intimate the action

required by distribution licensee and STUs to get required load relief from under

frequency and df/dt relays. All distribution licensee and STUs shall abide by these

decisions. SLDC shall keep a comparative record of expected load relief and actual load

relief obtained in Real time system operation.

2.0 132 kV and 66 kV Transmission Lines Requirement

a

A single distance protection scheme, which shall be numeric, with four independent

zones (three forward and one reverse), non-switched (with separate measurement for

all phase to phase and phase to ground faults) fast acting distance protection scheme

with permissible inter-trip at remote end (in case of zone-2 fault). The scheme shall

have inbuilt features of power swing blocking, fault recorder, disturbance recorder,event logger, relevant communication ports, single and three phase auto re-closing

(with deadline charging and synchro- check facility), Local Breaker Backup (LBB), VT

fail, broken conductor and sufficient LEDs to display faulty phases and zones.

Maximum operating time of relay on fault should not exceed 50 ms.

For back-up protection, a numerical directional IDMTL over current (for each phase)

and earth fault relay shall be provided.

For short transmission lines, appropriate alternative protection schemes may be

adopted such as pilot wire protection schemes

3.0 Transformer Requirements

a The following minimum protections should be provided for transformers:

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b

All power transformers shall be provided with numeric fast acting differential, REF,

open delta (Neutral Displacement Relay) and over-fluxing relays. In addition, there

shall be back up IDMTL over current and earth fault protection. For parallel operation,

such back up protection shall have inter-tripping of both HV and LV breakers. For

protection against heavy short circuits, the over current relays should incorporate a

high set instantaneous element. In addition to electrical protection, transformer own

protection viz. buchholz, OLTC oil surge, gas operated relays, winding temperature

protection, oil temperature protection, PRV relay shall be provided for alarm and tripfunctions.

c

For 132 kV, 66 kV and 33 kV class transformers of capacity above 5 MVA, the

protection shall be same as mentioned above except over-fluxing, REF and PRV relays.

However, REF shall also be provided for transformers of capacity equal to or more

than 20 MVA.

d In addition to electrical protection, gas operated relays, winding temperature

protection and oil temperature protection shall be provided.

4.0 Sub-Station Protection

a

All circuit breakers installed in the sub-station from 11 kV to 132 kV level should clear

the faults in two and half cycles and above 132 kV level in two cycle. The total time of

clearing fault including main protection relay shall not exceed that specified as At

generating stations, line faults should be cleared at the generating station end within

the critical clearing time so that the generators remain in synchronism.