Earth fault relay

Earth fault relays are voltage relays for the detection of earth faults in isolated or compensated power networks . The displacement voltage in three-phase systems when an earth fault occurs is used to excite them . An earth fault is characterized by the reduced voltage between the earth-faulted conductor to earth and, on the other hand, the increased voltage of the unaffected conductor.

With a full earth fault , the earth-touching conductor carries practically no voltage to earth, the healthy conductors of the three-phase system carry the higher triangular voltage to earth. A displacement voltage occurs between the neutral point of the healthy three-phase system and the earth potential . This voltage is obtained with the help of a so-called open delta connection from three 100/3 V voltage converter secondary windings, which results in (3-1) × 100/3 = 66 V in the event of a ground fault in any phase, and in the event of a full ground fault. The relay is designed for 0 - 66 V corresponding to 0 - Un / 1.732 V.

Dimensioning

The response value of the earth fault relay is set between 20% and 30% of the nominal voltage. A setting value below 20% is not usual, since network asymmetries (due to unbalanced load) cause displacement voltages that can easily reach 10% to 15%. Despite mostly lower response voltages, earth fault relays are designed for the full displacement voltage. In order to prevent transient earth faults from responding, a timer with command times of up to 3 seconds can be added to the relay as required. The displacement voltage occurs everywhere in the galvanically connected network and regardless of the fault location. For this reason, permanent earth fault monitoring is required for every network for safety reasons. A ground fault network can continue to operate for up to two hours. During this time, the search for the fault must be carried out by means of so-called containment circuits or troubleshooting circuits . In doing so, the power supply units or branches are switched off and on again one after the other until the part of the system with a ground fault has been detected. This method is very complex and carries the risk of expansion to a double earth fault through a secondary earth fault.

Detection or location of the earth fault

Selective earth fault detection

A selective earth fault detection is intended to enable the deliberate disconnection of an earth-connected line. This brings significant advantages in the operational control of networks.

The total zero current and displacement voltage, from which the earth fault direction is determined, serve as measurement criteria. Due to the capacities and inductances of the network, the occurrence of an earth fault is not a sudden phenomenon, but a balancing process that takes place in three stages:

Discharge process: The voltage collapses at the point of failure. The capacitance of the conductor concerned is discharged via the fault location. This is done with frequencies from 500 to 100,000 Hz .
Charging process: The increase in voltage in the healthy conductors to earth results in additional charging of these capacities via the transformer windings at frequencies between 70 and 4000 Hz.
Compensation process: When using an earth-fault suppressor, the transition to a stationary process takes place at mains frequency .

Wattmetric method

The wattmetric method for recording earth fault directions waits for this equalization process before the relay is triggered.

With wattmetric detection in an isolated network, a capacitive earth fault current flows in the event of a ground fault, which is much larger in the faulty feeder than that in the faultless feeder. The earth fault direction relay is used here. The direction of the capacitive reactive power is measured with a relay. ${\ displaystyle \ sin \ phi}$

The detection of the earth fault direction in a network operated with compensation is much more problematic. Here the power distribution is much less favorable than in an isolated network. As a result of the compensation, only a residual watt current in the order of magnitude of up to 5% of the capacitive earth fault current flows through the fault location. This so-called active current is used to measure the direction of power. It is made up of:

the active component of the earth fault current
the leakage current of the insulation
the currents from harmonics in the network

Accordingly, a relay is used for measurement here. The tilting line of the relay is now in relation to the displacement voltage in such a way that the angle errors of the current and voltage transformers can falsely influence the directional decision. This is why relays are used in digital protection technology that mask angles in the range from 80 ° to 89 °. In the case of the analog directional earth fault relay, the unbalance current of the output to be protected is supplied via current transformers and the neutral point earth voltage via the winding of the voltage transformer. If an earth fault occurs, the relay compares the polarity of the equalizing current and the displacement voltage. If the earth fault is in the tripping direction, the relay responds. The disadvantage of the analog directional earth fault relay is that after an earth fault has been displayed and saved, the relay is blocked and no further earth faults are detected. The blocking can be canceled by hand or by resetting automatically after the earth fault has been cleared. ${\ displaystyle \ cos \ phi}$ ${\ displaystyle en}$

In order to clearly determine a ground fault in the compensated network, the method of increasing the residual active current is used. For this purpose, the secondary winding of the earth-fault reactor ( Petersen coil ) is loaded with a resistor in such a way that a directional earth-fault relay can make a clear decision through this increased active current flow to the fault location. With this application of the principle, it must be ensured that the transient situation of the occurrence of the earth fault is completed. Then the automatic sequence of increasing the residual active current begins. This process can be repeated as required, whereby it must be ensured that the connected resistor in the secondary winding of the earth fault reactor is mostly only designed for short-term operation and is therefore subject to a certain cooling time.

In networks with earth fault compensation, it is possible to operate a short-term low-resistance star point earthing in parallel (usually with KE, KZE, or rarely abbreviated to KNOSPE), in which about 150 ms after the earth-fault relay has responded, the earth-fault extinguishing coil with a single-pole circuit breaker, to which an approx. 25- until a 50-ohm resistor is connected in series, which means that the protective relays of the affected feeder respond from the resulting current and lead to shutdown. The current of approx. 500 A (30 kV network) is supplied by means of a single pole. Current transformer detected. This method has the advantage that when a conductor or voltage funnel comes into contact with it, there are greatly reduced electrical accidents due to voltage compared to previous methods.

Detection of transient earth faults

In addition to the wattmetric method of selective earth fault detection, transient earth fault detection can be used. The equalization processes in the event of the occurrence of an earth fault are evaluated. Total zero current and zero sequence voltage form the excitation criteria for the earth fault direction, since at the beginning of the process, the polarity comparison of these two oscillations provides evidence of the direction. Same polarity usually always means one direction. This process, which takes place in the first half-wave of the oscillation after the occurrence of the fault, can only be recorded by means of electronic relays.

Fault location by initiating a double earth fault

Every earth fault in an isolated or compensated network harbors the risk of expansion to a double earth fault . When the double earth fault occurs, one of the two fault locations is triggered by the respective protective relay , as this is an almost two-pole short circuit . Therefore - depending on the system configuration - there is the possibility of deliberately initiating a second earth fault. With a suitable time delay of the protection of the second earth fault point, the protection of the first earth fault point is activated and tripping occurs. The earth fault is thus located. An automatic system is required for this process, which determines exactly in which conductor the first earth fault occurred and then initiates a second earth fault in another conductor. A separate circuit breaker with three individually switchable poles is required for this.

literature

A. Varduhn, W. Nell: Handbook of electrical engineering. Fachbuchverlag, Leipzig 1952.
L. Schauer, A. Reissmann: Operation of electrical systems. 5th, through. Edition. German publishing house for basic industry, Leipzig 1986.