Distance Protection: The Early Developments

Author: Walter Schossig, Germany

First Improvements

Many patents were granted in the first years of distance protection technology development. Examples of patents from 1908 up to the 1920's include those of Kuhlmann,K.; Wecken,W.; Chrichton,L.N.; Meyer,G.J., Ackermann,P. and Biermanns,J. Kesselring,Fr. combined the protection and directional relays in one box, the "N-relay". Cohn,,A. proposed to use bimetal strips, saturation transformers and other elements. Kesselring,Fr. further developed the N-relay, the voltage and directional elements from 1924 up to 1927.
The Norwegian Wideröe,R. was granted 41 German and 2 American patents in the years 1928-1932 when he worked with AEG; during this time N. Jacobsens Elektriske Verksted (NJEV) was granted 10 patents in Norway. See Figures 14,16.

Short-Circuit Tests for Relay Usage in Grids

Ackermann,P conducted short circuit tests on Shawinig Water and Power Co's 50kV transmission network in 1920. He observed that time relays with overcurrent tripping devices did not trip due to the low short circuit currents in small machine applications. The impact of arc resistance was not considered at this time. He observed the reduction of the current levels during the short circuit but explained it as a decrease of the initial short- circuit current up to the level of sustained short-circuit current.

The Preuß. Kraftwerke Oberweser A.G. Cassel 60kV network was fully equipped with V&H voltage drop relays after short circuit tests were carried out. See Figure 5. Other utilities also performed short circuit tests to collect information about the performance of relays and the network under fault conditions. One observation was that short circuit currents could be smaller than the nominal currents. In 1924 Dr. M. Schleicher described the impact of arc resistance on impedance relays. Arc resistance in their 110kV network was investigated by the German utility "Bayernwerke" in 1926/27. The proposal to use the reactance to estimate the distance to the fault was subsequently discarded.

Following significant investigations and network tests with 30 relays, a Norwegian interconnection company "Samkjöringen" (translation in English is 'Cooperation') decided in 1936 to use distance relays to protect their main lines in Eastern Norway. They used Dr. Wideröe, NJEV protection scheme. The Viennese ELIN AG relays subsequently acquired the commercial rights to this protection scheme. The application of an AEG distance relay in a network model was shown at a fair in 1924.

The advantages of distance protection relays were demonstrated by statistics from Elektrowerke AG. They had 43 disturbances on their 100kV network in 1924. Most of them originated in the medium voltage network. The relays in the medium voltage (overcurrent and directional relays) did not work properly 32 times and the number of trips was 3 times higher that they should have been. Replacement of the relays with distance protection resulted in only 2 relay misoperations during the 27 disturbances in 1927.

Six-, Three-, Two- and One-Relay-Schemes

When distance protection was introduced it became obvious that in a case of a double earth fault (base point of fault in different phases of different systems) different measuring values had to be used. Biermanns proposed a scheme in 1924.

The voltage coils were on the phase to phase voltage for short circuit faults and were on the phase to earth voltage for earth faults. He introduced the changeover of measuring circuits with zero-sequence current in the summation current circuit as is used today. The name "zero sequence startup" is used incorrectly - in this application it refers to a changeover of measuring values. O. Mayr proposed a similar scheme in 1924. The commonly used schemes for resistance-dependent protection are described briefly. The following assumptions were made:

  • there is no changeover in the current circuit
  • in isolated or compensated circuits there are opportunities to reduce the number of CTs required, thereby reducing the number of relays required
  • it uses startup overcurrent

The six relay circuit detects each and every phase-phase fault and phase-earth fault with separate measurement elements. In the case of a double earth fault (where a zero sequence voltage or current occurs) the three relay circuit (Fig. 15) uses the voltage change between phases rather than the phase-earth voltage. The two relay circuit uses two measurement elements only with two current transformers. A changeover is possible with the zero-sequence voltage only. A further simplification is possible with one relay circuit, also known as one relay impedance protection. Only one measuring element is necessary and the changeover is dependent on the zero-sequence voltage. The one relay circuit (using 3 current transformers with measuring value changeover in the case of a zero sequence current) is the default solution in the medium and high voltage applications. Subject to phase or zero sequence current startup, the measuring element is connected via interposing relays to the currents and voltages (in accordance with directional elements).

To avoid a changeover of the currents, current proportional values are obtained from interposing transformers or shunts.

Due to the reduced control time and redundancy, six relays circuits are only used in the EHV grid applications.

Practical experience has shown that easy changeout of the scheme is appreciated since it would allow a change of transformer or line lengths settings on site. The Biermanns relays allowed on site changes of rate of rise with a ratio 1:2 of winding groups connected in series or in parallel. To change the characteristic of an N-relay; the cam disc, the bi-metal strip or the saturation transformer had to be changed. Every new characteristic curve had to be calibrated with a number of measuring points.

Due to the aforementioned results of the short circuit tests, new characteristics were developed, e.g. characteristics that during nominal voltages required double the nominal current value to initiate a trip but in the case of very low voltage (short-circuit!) a current level of 30% of the nominal value was sufficient to trip the relay.

Between 1925-1927 the German utility Bayernwerk in collaboration with vendors S&H, AEG and BBC carried out 70 short circuit tests to study the behavior of the distance protection schemes.

Requirement for Fast Impedance Protection

At the end of the 1920s calls for shorter tripping times (less than 2s) grew. This was necessary to prevent the network getting out of step with generators, dynamotors and motors. The breaking power of the oil circuit breakers increased allowing shorter tripping times. The operating time of resistance relays grew proportionally with the distance from the fault location to the relay. The rate of rise of the characteristic had to be considered carefully to avoid protection overlaps along the length of line. The fault locator was born out of the fact that consideration of the operating time of the definite characteristic of the protection relay allowed the fault location to be identified.

The goal for the 60 and 110kV networks was to achieve the shortest tripping time over the entire length of line. For this reason, the more commonly used continuous time characteristics were abandoned in favor of the new step or mixed characteristics.

Further steps in the development of distance protection will be covered in the next magazine issue.

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Power. Flexible. Easergy.
BeijingSifang June 2016