In starting situation we front to unbalance and zero sequence secondary currents due to unequal three ct characteristics (CT magnetizing current and ratio error), possible unequal CT core saturation and unequal CTs secondary circuit impedance.
Generally there are some recommendations in this regards as following:
To provide earth-fault protection, it is recommended the core-balance CT solution whatever type of network earthing is used. This method allows for the detection of small resistive values of earth fault current.
Furthermore, an earth fault unit supplied from the residual connection of three line CTs as mentioned above carries the risk of mal-operation during starting. In order to obtain stability the pick-up must not be set below 0.15 to 0.2 In of the CTs, which is often too high compared to the maximum earth-fault current.
Most LV systems fall into solidly earthed category, for reasons of personnel safety. Two types of earth fault protection are commonly found – depending on the sensitivity required.
For applications where a sensitivity of > 20% of motor continuous rated current is acceptable, conventional earth fault protection using the residual CT connection. A lower limit is imposed on the setting by possible load unbalance and/or (for HV systems) system capacitive currents. Care must be taken to ensure that the relay does not operate from the spill current resulting from unequal CT saturation during motor starting, where the high currents involved will almost certainly saturate the motor CT’s.
It is common to use a stabilizing resistor in series with the relay, with the value being calculated using the formula:
Ist = starting current referred to CT secondary
I0 = relay earth fault setting (A)
Rstab = stabilizing resistor value (ohms)
Rct = d.c. resistance of CT secondary (ohms)
Rl = CT single lead restistance (ohms)
k = CT connection factor (= 1 for star pt at CT= 2 for star pt at relay)
Rr = relay input restistance (ohms)
The effect of the stabilizing resistor is to increase the effective setting of the relay under these conditions, and hence delay tripping. When a stabilizing resistor is used, the tripping characteristic should normally be instantaneous. An alternative technique, avoiding the use of a stabilizing resistor is to use a definite time delay characteristic. The time delay used will normally have to be found by trial and error, as it must be long enough to prevent mal-operation during a motor start, but short enough to provide effective protection in case of a fault. On a solidly grounded system, the ground fault protection can be provided by a time overcurrent relay residually connected in the phase CT circuit. Inverse or short time relay characteristic curves are recommended. Residually connected instantaneous relays are not advised. During motor starting and when the motor backfeeds into a fault, differences in the individual phase CTs and their connected circuits will lead to unequal saturation of the phase operation of residually connected relays. Time overcurrent relays can override the backfeed conditions, but must be set above the error currents during starting.
A typical pickup setting for the time overcurrent ground relay on a motor feeder is 50 to 80% of the phase relay setting. If a more sensitive relay setting is required, it is necessary to use a core-balance CT. This is a ring type CT, through which all phases of the supply to the motor are passed, plus the neutral on a four-wire system. The turns ratio of the CT is no longer related to the normal line current expected to flow, so can be chosen to optimize the pickup current required. Magnetizing current requirements are also reduced, with only a single CT core to be magnetized instead of three, thus enabling low settings to be used.
In low resistance earthed method, the value of resistance is chosen to limit the fault current to a few hundred amps – values of 200A-400A being typical. With a residual connection of line CT’s, the minimum sensitivity possible is about 10% of CT rated primary current, due to the possibility of CT saturation during starting. For a core-balance CT, the sensitivity that is possible using a simple non-directional earth fault relay element is limited to three times the steady-state charging current of the feeder. The setting should not be greater than about 30% of the minimum earth fault current expected. Other than this, the considerations in respect of settings and time delays are as for solidly earthed systems.
In some HV systems, high resistance earthing is used to limit the earth fault current to a few amps. In this case, the system capacitive charging current will normally prevent conventional sensitive earth fault protection being applied, as the magnitude of the charging current will be comparable with the earth fault current in the event of a fault. The solution is to use a sensitive directional earth fault relay. A core balance CT is used in conjunction with a VT measuring the residual voltage of the system, with a relay characteristic angle setting of
+45° (see Chapter 9 for details). The VT must be suitable for the relay and therefore the relay manufacturer should be consulted over suitable types – some relays require that the VT must be able to carry residual flux and this rules out use of a 3-limb, 3-phase VT. A setting of 125% of the single phase capacitive charging current for the whole system is possible using this method. The time delay used is not critical but must be fast enough to disconnect equipment rapidly in the event of a second earth fault occurring immediately after the first. Minimal damage is caused by the first fault, but the second effectively removes the current limiting resistance from the fault path leading to very large fault currents. An alternative technique using residual voltage detection is also possible, and is described in the next section.