Distance Protection: The Early Developments

Author: Walter Schossig, Germany

Introduction

The Zoned Voltage Drop Protection
At the beginning of the 20th century, effective protection coordination using overcurrent or directional overcurrent relays was no longer sufficient due to high clearing times and operation with spur lines and rings circuits. Distance protection became the most important protection technology.

Krämer, Chr., F&G proposed a protection device with the basic structure of a distance relay in 1904. The patent claim definition in DRP 174 218 by Felten & Guilleaume-Lahmeyer-Werke AG (F&G) was: "A relay for automatically switching off an alternating current if the current is higher than a nominal value. A series connected coil and a coil connected in parallel exert a force on a rotatable disk. The intention is that the closing time of the auxiliary contact is determined by the current exceeding the nominal value and the associated voltage drop.." See Figure 1.

AEG (Allgemeine Elektrizitäts-Gesellschaft) and the company Dr. Paul Meyer AG made great contributions to distance protection. Both were the first German companies to put distance protection into operation around the same time in 1923/1924 and contributed several patents prior to that. A patent (by inventor Kuhlmann,K.) was granted to AEG on the 23rd April 1908. The inventor proposed a Ferraris disk, driven by current. A special voltage magnet worked as a brake and the operating time was dependent on distance.

Another Kuhlmann patent was for a distance protection device that worked as a balance-beam relay dependent on undervoltage and overcurrent with a Ferraris anchor and rotating armature (Patent: DRP 214 164). The balance-beam is a mechanical device so the phase angle between the current and voltage had no impact. Thus the impedance circle, typical for electromechanical relays, was born.

The next invention was by Wecken,W.(Patent: DRP 248 466) and was the basic for selective voltage drop protection. The relays that existed at that time were used for spur lines with single infeeds. Wecken,W. proposed the ring operation in 1912 and suggested using voltage drop relays to protect the ring. A directional element to determine the direction of the power during short circuits was not available. Meyer,G.,J. developed a current and voltage dependent relay for ring operation one year later. Although this relay worked without a directional element just as other similar devices at that time, Meyer's patent (DRP 269 759) discussed the fact that the direction of power should be the value being controlled. The directional elements were not taken into consideration for ring operation or with parallel lines because only one oil circuit breaker was used in the substations at the time and therefore it was not necessary to use a directional element. Today, of course, we use circuit breakers in both directions to maintain supply. (Fig. 2, 3, 6)

The operating principle of such a voltage drop device is described as follows. Four driving cores beat against a drum exerting a force. The coils of the cores are connected with a resistance to the voltage. If the voltage is the nominal value, the arbor is lifted by a silk cord. Under fault conditions, the voltage decreases, the arbor will be "coiled up" with an associated speed creating a distance dependent operating time.

A metal-filament lamp connected in series and adjustable resistances allow a change of the characteristics (straight or warped). (Fig. 7)

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