Apr 30, 2017

Earth fault protection - How to test correctly?


ELCOME Dear friends of protection and control engineering!

In our new three-part series of technical papers, we devote ourselves intensively and extensively to the issue of earth-fault protection. Ground fault protection systems may not be one of the most complicated protective functions, but in practice there are always problems. There are many possible sources of error when it comes to verifying the correct directional decision. This and the two following articles provide simple tips on how to avoid errors when checking the ground fault direction detection.

In our first part, we provide you with the basics for recording the measured values and the resulting protection methods.

Part 1: Measurement basics

Earth faults occur in isolated or compensated networks. Due to the high star-point impedance, single-pole conductor-to-earth faults do not lead to an effectively flowing short circuit, but to an earth fault. However, the fault current flowing in this way is, as a rule, much lower than that of a short circuit. Let's compare the typical magnitudes of different states:

- Short-circuit current: in the range of kA

- Load current: in the range of 100 A

- Earth-fault current: in the range of 10 A

This means that in the event of a short circuit, the fault location can be found quite simply because of the immensely high fault current. In case of an earth fault, however, the residual current is usually still far below the load current. As a result, the fault location is much more difficult to determine.

The voltages here acting different. The phase-to-ground voltage of the faulty phase becomes almost zero, but the conductor-to-conductor voltages remain unchanged. This increases the phase-to-phase voltages of the fault-free phases by a factor of 3 (see figure below: left picture = faulty-free operation | right picture = earth-fault in phase 1).

At the same time, this also causes a displacement voltage between the star point of the feeding transformer and the ground. It is easy to see from the voltages that there is an earth fault. The ground fault location, however, is difficult to find because of the low ground fault current.

The purpose of the earth-fault protection is therefore to correctly display and report the direction of the earth fault at the respective measuring point. For this purpose, two processes are used particularly frequently in Europe: the "transient earth fault method" and the "wattmetric method". In order to distinguish these from each other more closely, we first have to consider the current and voltage curves during an earth fault.

Through the error entry, the network "jumps" from one state to another. This change in state causes a transient phenomenon, which results in high-frequency currents and voltages with partly high amplitudes. After this transient oscillation has subsided, the network is in a state of stationary ground fault, where fault currents and voltages with operating frequency can be measured.

Transient earth-fault methods use the transient process to determine the fault direction by means of the current and voltage curves at the beginning of the ground fault. Wattmetric methods, however, can not process these high-frequency signals and instead use the stationary current and voltage values.

In order to be able to carry out a test, one must, of course, also know which currents and voltages are measured. There are three possibilities for currents:

- Measuring the three phase currents. Here the earth fault current is calculated from the sum of the three phase currents.

- Holmgreen connection: Here, the three phase currents are connected after the respective measurement and once again given to a measuring input (see picture).

- Split core current transformer: Here an additional transformer is installed, which encloses all three primary phases. As a result, only the sum of all three phases is measured and a lower ratio can be used. This increases the measuring accuracy. However, it is also important to ensure that the cable shield is grounded back through the split core current transformer (see picture).

There are also different possibilities for voltage measurement:

- Measurement of the three conductor-to-earth voltages: Here, the displacement voltage is calculated from the three conductor-to-earth voltages.

- Measurement at the open delta winding: Here, the displacement voltage is determined directly by the measurement. As a rule, this voltage is 1,73 times higher in the case of an earth fault than the phase-earth voltage in the fault-free network

- Both together: some protection devices use both the phase-to-phase voltages as well as the voltage of the open delta winding for their earth-fault protection function.

Since the measurement methods are frequently single-phase, the danger for errors increases considerably. The simple exchange of two connections leads to a phase rotation of 180 ° and thus to a directional decision in the wrong direction. Therefore, particular attention should be paid to the polarity test of all converter connections during commissioning. This can be ensured either by the classic "battery test", or by using the polarity checker from OMICRON. Also again and again a mistake: the winding test of the converter.

How the "battery test" is performed have already been described in this article.

Ok, the first part ends with the fundamentals of the earth fault and the measurement of currents and voltages. In our second part, we continue with the test of transient earth fault relays.

HEARTfelt Greetings Alexander Muth

Safety Notice

Failure to observe the following points can result in death, serious injury or material damage!

Hazardous voltages may be present when carrying out the tests and checks described in this manual. The safety rules and regulations regarding electrical systems must be strictly observed at all times. The generator must always be shut down when working on the primary system; appropriate grounding and short-circuiting facilities are to be provided at the respective workplaces. When carrying out a primary check on a turbine set, take care to ensure that no overheating of the turbine occurs.

The work described in this manual may only be carried out by qualified personnel, who must be conversant with the relevant safety regulations and safety measures as well as the warnings in the manuals provided by the suppliers of the various components. The contents of this manual 10

must not construed as work instructions. All statements in this manual must be carefully considered in light of the safety rules and regulations. The information presented in this manual does not claim to be complete.