Name / Gernot DRUML / Session / 3
Company / A.Eberle GmbH / Block / 3
Address / Aalener Str. 32 / Question n° / Paper 50
D-90441 Nürnberg / Language used on the floor / English
Phone / +49-911-7230928 / Accompanying visuals on file ? / YES
e-mail /
A NEW DIRECTIONAL TRANSIENT RELAY FOR
HIGH OHMIC EARTH FAULTS
Statistics show that earth faults constitute a large portion of grid faults. Conventional relays are designed only for low ohmic earth faults under stationary conditions. They cannot handle high ohmic earth faults, which occur especially in rural networks with overhead lines, or intermittent earth faults in compensated cable networks. As a consequence the earth fault is very often not recognized or the wrong feeder is selected to be healthy. This increases tremendous the time for the localization of the earth fault. On the other side the effective protection of the networks became more and more important in the competitive markets.
In this paper, a new algorithm for detecting also high ohmic earth faults up to some kOhm and its benefits are explained in detail.
The Requirements for atransient-relay with all these features are:
•Detection of high ohmic earthfaults up to some kOhm
•Detection of restriking faults
•Detection of intermittent faults
•Directional detection during the previous items
•Independence from the moment of ignition
In the following minutes I will present the following Items:
•The transient behaviour of an earthfault
•The new qu - algorithm for detection of high ohmic and intermittent faults
•The qu-algorithm applied on different types of earth faults:
• low ohmic earthfault
•high ohmic earthfault
•intermittent faults
•At the final I will show some results and pictures from field tests
To explain the behavior of a single pole earth fault, three different processes can be superposed. All three processes are starting at the same time, but their duration is different.
It can be distinguished between the following processes:
discharge of the faulty line over the earth (a => b )
charging of the two healthy lines over the earth (c)
stationary state of the earth fault
In the zero-sequence system the discharge of the faulty phase delivers a damped high frequency zero sequence current of above 10 kHz. The zero-sequence current depends essential on the capacity of the line, the resistance at the fault location and on the moment of ignition.
On the other side the charge of the healthy phases is independent of the moment of ignition.
The resulting zero-sequence current of the charge process essentially depends on:
capacity of the healthy phases
resistance at the fault location
and is independent of the time of ignition.
This leads to the following simplified equivalent circuit for a network with three feeders and one fault in feeder three.
The equation shows, that the zero-sequence voltage is proportional to the charge of a healthy feeder, respectively to the integral of the zero-sequence current flowing in a healthy feeder.
This relation is not valid for a faulty feeder. This behaviour can be used to identify a faulty feeder
In the left picture the zero-sequence voltage and the zero-sequence current of the two healthy feeders are shown. In the right picture the values for the faulty feeder are shown.
The charge q of each feeder can be calculated by integration of the zero-sequence current beginning from one zero-point of the zero-sequence voltage. Applying the charge of the feeder on one axis and the zero-sequence voltage to the other axis results in the following qu-diagram.
It can be recognized, that the two healthy feeders deliver straight lines with a positive rise. The curve of the faulty line has a negative rise at the starting point and is not a straight line. Using various methods for pattern recognition the two different types of feeders can be distinguished.
In case of a high ohmic fault, the rise of the zero-sequence voltage is very slow. The trigger of the relay occurs very late. The decision of the direction depending on the sign of the zero-sequence current compared with the sign of the zero-sequence voltage at the moment of trigger is not more valid. Using digital relays it is possible to go back in the history and to apply the qu-algorithm. The following picture shows the result. A distinguish between the faulty feeder and the healthy feeders is obvious.
In case of a restriking fault the charge process is very short. During the healthy period the zero-sequence current is not zero and the zero-sequence voltage is above the trigger level. This results to erroneous detection of the direction in conventional relays. Using the qu-algorithm a clear distinguish between the healthy feeder and the faulty feeder is again possible.
The next picture shows the real measurement of a high ohmic fault. The zero-sequence voltage has reached only 50%. Applying the qu-algorithm resulted to a correct indication of the faulty feeder.
The next picture shows a simulated break down ofa cable in a well compensated 350 A mixed network.The wattmetric current was about 10 A.The arc was very small and restriking with a period of about 0,3 s.
The next picture shows the same fault with an overcompensation of 30 A. The arc is still restriking.
On the left side the damage of the cable after the small current with a duration of about one minute is shown. On the right side the damage after the high current also with a duration of about one minute is shown.
One result was, that the damage also for a large network of 380 A is very small, if the network is well tuned.
Another result was, that the restriking error can take very long time, as the damage is very small.
During the field tests a restriking fault was also detected in case of a broken line touching the arm of a concrete pole.
The next picture shows a broken line on ground at an overcompensation of 100 A
In all cases the application of the qu-algorithm was successful.
Summary
•Detection of high ohmic transients is now possible
•Direction of the earthfault can be detected also during restriking and intermittent earthfaults
•The sensitivity can be selected direct as function of the capacitive current Ice
•Restriking faults can occur at
⇨cable networks
⇨poles made of concrete
⇨configurations with an air gap
⇨configurations with time-invariant resistance
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