Jan 17, 2023

Why harmonic method for ground fault protection?


elcome dear friends of protection, control and electrical engineering. Why do we need selective ground fault location and how can we implement it using the harmonic method? In our new two-part article, you will learn everything you need to know. In today's first part we ask the question why, i.e. what is the harmonic method needed for in the first place, and also show you the practical implementation. In the second part, we will go into detail about how it works and the theory that goes along with it.


As always, we start with the question of why. In our meshed and predominantly discharged 110 kV networks as well as in complex medium-voltage structures, the currently most reliable ground fault location method is the so-called transient method. Due to continuously optimized protection algorithms, which were largely developed by Dr. Gernot Druml, we succeed in correctly detecting the feeder affected by the earth fault with the transient transient method.

The problem with the transient method

If we have detected a faulty feeder using the transient method, we consistently carry out targeted switching operations in practice to selectively disconnect the faulty feeder. When we have caught the earth fault, the zero-sequence voltage immediately drops to a normal operating level. This is a sure sign for us that we have caught the ground fault with the switching action we have taken. If the zero- sequence voltage does not drop as expected, this would be a sure sign that the earth fault is still present. If you now start to check the transient relay displays again when the ground fault is present, you have already lost. Since nothing is perfect in our world, there is also a major limitation to the transient method. Due to the inherent operation of this protection function, no switching actions can accompany the measurement to isolate the fault. Thus, with the transient method, it is only possible to evaluate the direction of the earth fault in its moment of occurrence. The transient method is now no longer able to provide further measurement results within the scope of switching operations.

The wattmetric method as solution?

In very simple radially arranged beam networks, the wattmetric earth fault location method is now frequently used as a supplement. This well-tried measuring method provides useful results and is also suitable for permanent measurement in the context of search circuits. The above-mentioned restriction of a simple network structure applies here and, in general, the mode of operation and effect must be considered separately for isolated and cleared networks. In isloated networks the sin-phi method is used, which in practice often meets suitable excitation ratios. Strictly speaking, this is a directional reactive power protection in the zero-sequence system.
Compensated nets, on the other hand, are monitored using the cosphi method and have a particularly high sensitivity to angular errors. This is because the amplitudes of the residual current used for evaluation may be very small and can be deflected by much larger reactive components. In addition, transducer errors become more apparent at very small currents, and other effects, such as circular currents due to phase splitting, often lead to false readings. We therefore conclude: In complex meshed networks and with very small residual currents, the cosphi method quickly reaches its limits and no longer leads to reliable directional decisions.

So, why now?

Now the question arises: Can we nevertheless realize a permanent earth fault direction detection for the evaluation of a stationary pending earth fault? The clear answer here is: "YES we can". This is where the harmonic method comes into play. With the harmonic method it is possible to permanently display the earth fault directions of the feeders and thus to monitor the results of the initiated switching operations. In doing so, the harmonic method provides much more accurate results than the conventional line-frequency wattmetric earth fault location method and also has several advantages. We will take a look at why this is so and what these advantages are in the second part of our series of articles.

The practical structure

In this part, we will now take a look at how the function is set up in practice. The basic structure is as follows: Each feeder has an earth fault relay, which permanently monitors its own feeder for earth fault excitation and earth fault direction.

The secondary measured variables required for this are recorded classically via current and voltage transformers. The common secondary zero-sequence voltage is required at each feeder relay, which is usually obtained via the delta windings of the 3-phase voltage transformers connected in open delta. For this purpose, a ring line is established at the da-dn circuit of the voltage transformers and connected to the protective relays in the feeders.

Furthermore, we need the zero-sequence current, which we measure via suitable
cable converters in each of the outgoing feeders.

All those who are familiar with the classical wattmetric earth fault method can already see the analogy here, because the measurements for harmonic method and wattmetric method are constructed identically. The earth fault locating relay EOR-3D of the company A. Eberle shown in the figure is even able to combine and prioritize several protection functions such as wattmetric method, earth fault transient method and harmonic method in one device.

Important to know: In the case of the harmonic method, the zero-sequence currents can also be detected using the Holmgreen circuit, but in the case of the wattmetric earth-fault location method, it is mandatory to use cable conversion transformers.

Why this is so, how the harmonic method works in detail and what else to look out for we will look at in the next article.

Warm wishes, thank you and see you soon.


Read our second part as well

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