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Schedule Grid Standards for Operation and Maintenance of Transmission Lines

January 25, 2021

1. Frequency

The standard frequency of system operation is 50 Hz and all efforts shall be made to operate at frequency close to nominal as possible. The frequency shall not be allowed to go beyond the range 49.0 to 50.5 Hz, except during the transient period accompanying tripping or connection of load.


2. Voltage

The steady state voltage shall be maintained within/the, limits given below: Nominal System Voltage kV rms Voltage Variation


765 +/- 3%

400 +/- 3%

220 +/- 5%

132 and below +/- 10%

Temporary over voltage due to sudden load reject Nominal System Voltage kVrms Phase to Neutral Voltage kV peak For the voltage level below 132 kV, the voltage variation limits as given in (2) above shall be decided by the State Commission in the respective State Grid Code.


3. Voltage Unbalance

The maximum permissible values of voltage unbalance shall be as under: Nominal System Voltage kVrms Voltage Unbalance % 765 and 400

i)Bulk consumers shall ensure balanced load during operation.


ii)Low Voltage Single phase loads shall be balanced periodically at the distribution transformer by Distribution licensees.


4. Protection Standards

i) The Transmission Licensee and Users shall provide standard protection systems having the required reliability, selectivity, speed and sensitivity to isolate the faulty equipment and protect all components from any type of faults, within the specified fault clearance time. Protection coordination shall be done by the RFC.


ii) Fault Clearance Time:

The maximum fault clearance times are as given below: Nominal System Voltage kVrms Maximum Time ( in milliseconds)the event of non clearance of the fault by a circuit breaker within the time limit prescribed in 4


(ii) above the Breaker Fail Protection shall initiate tripping of all other breakers in the concerned bus-section to clear the fault in next 200 milliseconds. 237


5. Criteria for System Security

The following minimum security criterion shall be followed for operation and maintenance planning of the elements of the grid: The Grid System shall be capable of withstanding one of the following contingencies without experiencing loss of stability:


(a) Outage of one single largest generating unit of the system or

(b) Outage of a 132 kV Double circuit line or

(c) Outage of a 220 kV Double circuit line or

(d) Outage of a 400 kV Single circuit line or

(e) Outage of a 400 kV Single circuit line with series compensation or

(f) Outage of 765 kV Single circuit line without series compensation or

(g) Outage of one pole of HVDC Bipolar line or

(h) Outage of an Interconnecting Transformer


6. Transient Stability

Under any one of the following contingencies the system shall remain stable and sustain integrity (i.e., no generator shall lose synchronism and no part shall get isolated from the rest of the system):


a)Tripping of a single largest generating unit or


b)Transient ground fault in one phase of a 765 kV Single Circuit Line close to the bus or


c)A sustained single phase to ground fault in 400 kV single circuit line followed by 3 pole opening ofthe faulted line or


d)A sustained fault in one circuit of a 400 kV Double Circuit Line when both circuits were in service in the pre-contingency period or


e)A transient single phase to ground fault in one circuit of a 400 kV Double Circuit Line when the second circuit is already under outage or


f) A three-phase sustained fault in a 220 kV or 132 kV or


g)A sustained fault in one pole of HVDC bipolar in a HVDC Converter Station.


7. Harmonics

The voltage wave-form quality shall be maintained at all points in the Grid.

i) In this Standard Harmonic Content limits are stipulated as f the harmonic of n*1 order (corresponding frequency is 50 x n Hz)


ii) Maximum Limits of total Harmonic Distortion .