Busbar Protection Calculation

A typical 132kV double bus generating station is made up of two 100MVA generators and associated step-up transformers, providing power to the high voltage system, by means of four overhead transmission lines. If the main bus is sectionalised with a bus section circuit breaker and the reserve bus is split by bus section isolators, calculate for a high impedance circulating current scheme having 4 zones and an overall check feature, as follows :-

1.    The stability level of the busbar protection.

2.    The discriminating and check features primary fault settings.

3.    The value of stabilising resistor required with the MCAG14 relay whose current setting range is 0.2 – 0.8A and has a burden of 1VA at the current setting.

4.    The supervision relay primary setting, using a VTX31 with a voltage setting range of 2 – 14 volts. Assume the relay burden at the setting is 2 milli VA.

5.    Determine the CT knee point voltage requirements.

Given that the switchgear rating is 3500MVA, the system voltage is 132kV solidly earthed, the maximum pilot resistance is 4 ohms and that the non-linear resistors to be connected across the buswires to limit the peak volts follow the expression V=900I^0.25. The current transformers are of ratio 500/1 amp, have a secondary internal resistance of 0.7 ohms and have a magnetising impedance of 2000 ohms.

The layout of the station is a shown in the diagram.

Solution:

1.    The stability level of the busbar protection is governed by the switchgear rating which in this instance is
 = 15300 Amps

2.    The primary operating current of busbar protection is normally between 2% and 10% of the stability level i.e. between 306 amps and 1530 amps. Where 30% of the minimum fault current is more than the full load current of the largest circuit, the primary operating current should be made more than the full load current i.e. 130% of full load current.

Full load current =

                                  = 438 Amps

\ Primary operating current    = 1.3 x 438

                                                      = 569 Amps

Discriminating Feature

Relay voltage setting     =                                           =
                                          = 144 volts

Magnetising current taken by each C.T. at 144 volts
                                          =
                                          = 0.072 Amps

Metrosil r.m.s. current at 144V is given by:

 = = 1.36mA
In practice the metrosil current is considered to be negligible and is ignored.

To obtain a primary operating current of 569 amps the relay current setting must be calculated as follows :-

Primary operating current        = CT ratio (I_RELAY + n.I_CT)

Where n = maximum number of CT’s per zone (i.e. 5)

\ I_RELAY    === 0.778A

The nearest available setting is 0.8A.

This gives an actual primary operating current of :-

Primary operating current        = 500 (0.8 + 5 x 0.072)

                                                      = 500 x 1.16

                                                      = 580 Amps

Check Feature

Relay setting voltage                 = 144 volts

Relay setting current (I_s)           = 0.8 Amps

Maximum number of circuits   = 6

Primary operating current        = 500 (0.8 + 6 x 0.072)

                                                      = 500 x 1.232

                                                      = 616 Amps

3.    Ohmic value of stabilising resistor (R_ST)
      RST    =                   =                   = 178.44 ohms

4.    Voltage setting of the supervision relay Vsp = I (R // Zm)

R = Rst + Rr = resistance of stabilising resistor and relay

Zm = parallel impedance of CT magnetising impedances assuming 1 CT is open circuit

I = supervision operating current

Whenever possible the supervision primary operating current should not be more than 25 amps or 10% of the smallest circuit, whichever is the greater.

For the discriminating zone there are 5 CT’s. Therefore assuming one CT is open circuit :-

      Zm      =

                        = 500 ohms
     R         = Rst + Rr = 178.44 +                = 180 ohms

Assuming I = 25 amps
      V         =

                  = 6.62V

For the check zone there are 6 CT’s. Therefore assuming one CT is open circuit
      Zm      =                  = 400 ohms
      V         =                  = 6.2V

5.    Knee point voltage of the current transformers should not be less than twice the relay setting voltage.
      V_K        = 2 x 144
                  = 288 volts