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As you know there are several schemes for 100% generator ground fault. The Sub-

harmonic Injection Scheme as an alternative to third harmonic 100% schemes was

developed by a European manufacturer and is widely used in Europe and other

countries outside the U.S. Sensitivity of this scheme is constant over the entire

stator winding. It can provide complete ground fault protection during startup,

shutdown and even on turning gear and alternator dead condition. The scheme isnormally taken out of service when the machine is offline for personnel safety,

because the injected voltage is typically over 100 V. This scheme injects a low

frequency sub-harmonic into the generator stator windings. The injected frequency

is 15-20 Hz. The signal is injected across the neutral grounding transformer neutral.

The load that is presented to the injector in this scheme is the line-to-neutral

capacitance of the generator windings, associated bus/cable that connects the

generator to the GSU and the delta winding of the GSU and auxiliary transformer.

The use of a low frequency sub-harmonic makes this capacitive reactance high

impedance. Thus, the KVA size of the injection transformer is reduced over what it

would be if fundamental frequency were used. Under normal conditions, a small

level of changing current will flow at the sub-harmonic frequency. When a ground

fault occurs anywhere in the winding of the generator or its associated bus work,

the capacitance is shorted in that phase and higher current flows which is detected

by an over-current relay. The scheme has the added advantage in that it can detect

a stator ground fault in an off-line generator prior to it being put in-service.

The major problem with this scheme when it was developed in the 1960s and 1970s

was that it was implemented with electronics that were very expensive. The injector

and the filters were the main costs. As a result, not many U.S. users thought that it

was not worth the high cost to protect the last 5-10% of the generator stator

winding. The advent of digital technology has helped to reduce the schemes costs

and many U.S. users are giving this scheme a second look.

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, whichworks 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

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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 thetransformation 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.

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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

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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.

20 Hz is totally independent source for fault impedance measurement . 20 Hz is

neither harmonic nor subharmonic of 50Hz.