Protection Riddle No.18 - Generator Stator Earth-Fault Protection at Low Frequency Injection

Why is injected voltage chosen to be 20Hz and not 5Hz or 10 Hz.??

#1
Sat, October 11, 2008 - 13:32
The relay measurement unit shall be blocked out of 10-40 HZ, because in this frequency range a zero voltage can also be generated by generators starting up or slowing down.

The 100-% stator earth fault protection detects earth faults in the stator windings of generators which are connected with the network via a unit transformer. This protection function, which works with an injected 20 Hz voltage, is independent of the system-frequency displacement voltage appearing in earth faults, and detects earth faults in all windings including the machine star point. The measuring principle used is not influenced at all by the generator operating mode and allows to perform measurements even with the generator standing still. The two measuring principles used – measurement of the displacement voltage and evaluation of the measured quantities at an injected 20 Hz voltage – allow  to implement reliable protection concepts that complement one another.
Figure 1  shows the basic protection principle. An external low-frequency alternating voltage source (20 Hz) injects into the generator star point a voltage of max. 1 % of the rated generator voltage. If an earth fault occurs in the generator star point, the 20 Hz voltage drives a current through the fault resistance. From the driving voltage and the fault current, the protective relay determines the fault resistance. The protection principle described here also detects earth faults at the generator terminals, including
connected components such as voltage transformers.

Fig.1

To implement the above concept, some additional equipment is required. Figure 2 shows that a 20 Hz generator generates a square-wave voltage with an amplitude of approx. 25 V. This square-wave voltage is fed via a band pass into the loading resistor of the earthing or neutral transformer. The band pass serves for rounding the square wave voltage and for storing energy. The 20 Hz resistance of the band pass is approx. 8 ohm . The band pass has also a protection function. If the load resistor carries the full displacement voltage in case of a terminal-to-earth fault, the higher series resistance of the band pass protects the 20 Hz generator from high feedback currents.
The driving 20 Hz voltage is picked up directly at the loading resistor via a voltage divider.
In addition, the 20 Hz current flow is measured via a miniature CT. Both quantities (USEF and ISEF) are fed to the protection device. The voltage to be injected into the generator star point depends on the driving 20 Hz voltage (voltage divider: load resistor and band pass), and on the transformation ratio of the neutral or earthing transformer.
To prevent the secondary load resistance from becoming too small (it should be > 0.5 ohmwhere possible), a high secondary rated voltage should be chosen for the earthing or neutral transformer. 500 V has proven to be a good value. The same measuring principle can also be used with a primary loading resistor. The 20 Hz voltage is connected in this case via a voltage transformer, and the star point current is directly measured.

Fig.2

From the two measured quantities USEF and ISEF in Figure 2, the 20 Hz current and voltage vectors are calculated, and from the resulting complex impedance the ohmic fault resistance is determined. This method eliminates disturbances caused by the stator earth capacitance, and ensures a high sensitivity. The measuring accuracy is further increased by using mean current and voltage values obtained over several cycles for calculating the resistance.
The model takes into account a transfer resistance RPS that may be present at the neutral, earthing or voltage transformer. Other error factors are taken into account in the angle error.
In addition to the determination of the earth resistance, the protection function features an earth current stage which processes the current r.m.s. value and thus takes into account all frequencies. It is used as a backup stage and covers approx. 80 to 90 % of the protection zone.
A monitoring circuit checks the coupled external 20 Hz voltage and the 20 Hz current and detects by evaluating them a failure of the 20 Hz generator or of the 20 Hz connection. In case of a failure, the resistance determination is blocked. The earth current stage remains active.
The evaluation of the earth resistance measurement is blocked between 10 Hz and 40 Hz, because in this frequency range a zero voltage can also be generated by generators starting up or slowing down. Such a zero voltage would then superimpose the connected 20 Hz voltage, causing measurement errors and over functioning.
The resistance measurement function is active with frequencies below 10 Hz (i.e. at standstill) and above 40 Hz. The earth current measurement is active over the entire range.

#2
Thu, July 9th, 2009 - 11:51
20 Hz is totally independent source for fault impedance measurement . 20 Hz is neither harmonic nor subharmonic of 50Hz.

#3
Thu, July 9th, 2009 - 14:40
why 20 HZ?
Why 10,25,30,40,... HZ cannot be suitable?