In many cases it is difficult to achieve selective earth-fault protection based on the magnitude of residual current only. To obtain a selective earth-fault protection scheme, it is necessary to take the phase angle of Io into account. This is done by comparing the phase angle of the operating and polarizing quantity.
Relay characteristic angle
The Characteristic angle setting, also known as Relay Characteristic Angle (RCA), Relay Base Angle or Maximum Torque Angle (MTA), is used in the "Phase angle" mode to turn the directional characteristic if the expected fault current angle does not coincide with the polarizing quantity to produce the maximum torque. That is, RCA is the angle between the maximum torque line and polarizing quantity. If the polarizing quantity is in phase with the maximum torque line, RCA is 0 degrees. The angle is positive if the operating current lags the polarizing quantity and negative if it leads the polarizing quantity.
Example 1
The "Phase angle" mode is selected, compensated network (φRCA = 0 deg)
=> Characteristic angle = 0 deg
Example 2
The "Phase angle" mode is selected, solidly earthed network (φRCA = +60 deg)
=> Characteristic angle = +60 deg
Example 3
The "Phase angle" mode is selected, isolated network (φRCA = -90 deg)
=> Characteristic angle = -90 deg
Directional earth-fault protection in an isolated neutral network
In isolated networks, there is no intentional connection between the system neutral point and earth. The only connection is through the phase-to-earth capacitances (C 0) of phases and leakage resistances (R 0). This means that the residual current is mainly capacitive and has a phase shift of -90 degrees compared to the polarizing voltage. Consequently, the relay characteristic angle (RCA) should be set to -90 degrees and the operation criteria to "IoSin" or "Phase angle". The width of the operating sector in the phase angle criteria can be selected with the settings Min forward angle, Max forward angle, Min reverse angle or Max reverse angle. Figure 4 illustrates a simplified equivalent circuit for an unearthed network with an earth fault in phase C.
Directional earth-fault protection in a compensated network
In compensated networks, the capacitive fault current and the inductive resonance coil current compensate each other. The protection cannot be based on the reactive current measurement, since the current of the compensation coil would disturb the operation of the protection relays. In this case, the selectivity is based on the measurement of the active current component. The magnitude of this component is often small and must be increased by means of a parallel resistor in the compensation equipment. When measuring the resistive part of the residual current, the Operation mode should be set to "Phase angle" and the relay characteristic angle (RCA) should be set to 0 degrees. Alternatively, the Operation mode can be set to "IoCos". Figure 5 illustrates a simplified equivalent circuit for a compensated network with an earth fault in phase C.
The Petersen coil or the earthing resistor may be temporarily out of operation. To keep the protection scheme selective, it is necessary to update the characteristic angle setting accordingly. This can be done with an auxiliary input in the relay which receives a signal from an auxiliary switch of the disconnector of the Petersen coil in compensated networks. As a result the characteristic angle is set automatically to suit the earthing method used. The RCA_CTL input can be used to change the operation criteria as described in Table 1 and Table 2.
| Operation mode setting: | RCA_CTL = FALSE | RCA_CTL = TRUE |
|---|---|---|
| Iosin | Actual operation mode: Iosin | Actual operation mode: Iocos |
| Iocos | Actual operation mode: Iocos | Actual operation mode: Iosin |
| Characteristic angle setting | RCA_CTL = FALSE | RCA_CTL = TRUE |
|---|---|---|
| -90° | φ RCA = -90° | φ RCA = 0° |
| 0° | φ RCA = 0° | φ RCA = -90° |
Use of the extended phase angle characteristic
The traditional method of adapting the directional earth-fault protection function to the prevailing neutral earthing conditions is done with the Characteristic angle setting. In an unearthed network, Characteristic angle is set to -90 degrees and in a compensated network Characteristic angle is set to 0 degrees. In case the earthing method of the network is temporarily changed from compensated to unearthed due to the disconnection of the arc suppression coil, the Characteristic angle setting should be modified correspondingly. This can be done using the setting groups or the RCA_CTL input. Alternatively, the operating sector of the directional earth-fault protection function can be extended to cover the operating sectors of both neutral earthing principles. Such characteristic is valid for both unearthed and compensated network and does not require any modification in case the neutral earthing changes temporarily from the unearthed to compensated network or vice versa.
The extended phase angle characteristic is created by entering a value of over 90 degrees for the Min forward angle setting; a typical value is 170 degrees (Min reverse angle in case Directional mode is set to "Reverse"). The Max forward angle setting should be set to cover the possible measurement inaccuracies of current and voltage transformers; a typical value is 80 degrees (Max reverse angle in case Directional mode is set to "Reverse").