Introduction

Distance protection became the most important protection technology in the twentieth century

This article discusses the next phase in distance protection development

From continuous to multi-zone characteristics

First publications and first relays for distance protection were covered in the last issue. The requirement of the utilities was a decrease of the tripping time to a value less than 2 s. To achieve this they skipped the distance-to-fault depending continuous tripping characteristic and changed to cascaded (multi-step) or mixed characteristics. The distance relays provided by BBC and Siemens in 1928 still used the continuous characteristic. Stoecklin J. proposed and BBC developed a Relay that used the crossed-coil-ohmmeter (known from measuring devices). It was patented for selective protection. The base time of this relay was 0.5 to 1 second, which increases with the distance to the fault up to five second.

The device consisted of three mechanically united main parts. The impedance startup started a timing mechanism, while an ohmmeter limited the relay's time. The timing element -clockwork with manual winding - measured the time and operated exactly. It disburdens the current system; the result was a well working device with small power consumption, even with low currents. The clockwork stored approximately

100 seconds operating time - equal to 50 operations of the device. Only after this, a manual raise was necessary - an issue that was welcomed by operating staff at this time because it requires a systematic check of the relays. The Ohmmeter functioned as the directional element as well, eliminating the need for special reverse-power relays. A flag showed an operation of the relay and a slave pointer the distance of the fault. For resetting, a winding up of the clock up to a stop position was necessary- pointer and clock came back into normal position.

Impedance protection of Siemens was put into operation with the 50-kV-ring Bleicherode-Huepstedt-Muehlhausen- Langensalza (Germany) in 1929 . (Figure 6). In the same year distance protection was used for the first time in the 28-kV-grid of Vienna (Austria). To prevent out-of-step of generators and motors, a change from continuous to multi-step time characteristics was observed in the next 10 years. A fast tripping time of less than 0,3 s was achieved with balance beam electromechanical elements. Therefore, these relays had their own name - "fast distance protection".

At the same time "express impedance relays" for use in medium voltage grids were developed. Their advantage was the use of a step time characteristics (Figure 3). They were able to protect 70% of the length of line with an operating time of 0,3 s . Neugebauer,H. and Geise,Fr., Siemens, proposed an express impedance relay in 1932. It was the first distance relay in an economical single plate housing per end of line. Fast distance relays were used to achieve short tripping times in the EHV-grid (solid earthed star point). Usually they had three measuring elements (in the English-speaking countries up to six). Single-pole autorecloser with definite 3-phase trip was possible now.

In the medium voltage, the grids had an isolated star point. Petersen,W. invented the earth-fault neutralization in 1917. Since then, especially in the German-speaking countries, compensated grids are quite common. The capacitive earth fault current is compensated by the inductive current and continuing operation of the grid is possible. Fast distance relays with only one measuring element were sufficient to detect 2-and 3-phase short circuit faults.

The distance protection in Europe was the most often used protection technology on mashed or parallel-operated high voltage grids. When the short-circuit power in the grid became higher, the requirement for fast tripping on the whole line length became important. Ackermann already showed a proposal for a step protection in 1920/21. This was used in Siemens reactance relays in 1930, in the Oerlikon-Minimum-

Impedance-Protection and the newer distance relays of Westinghouse Co. and General Electric Co.

AEG developed their first fast distance relay in 1934 (SD1). It uses pure three-step characteristics; fast tripping times of 0.3 up to 0.4 s were achieved. As an under-impedance protection it uses two balanced beams, which were set up to different lengths of the line. Additionally it consists of a 3-step timing element and an iron-cored dynamometer as a directional element. Startup was realized with built-in overcurrent elements or - in a separate housing - with under-impedance elements. The right housing consists of measuring elements and the directional element with a tapped voltage-matching transformer (for impedance setup). The other two devices contained the startup, the choice of measuring values and the three-step timing relay. For the detection of two-phase to earth fault the SD1 used for the first time the sum current and a change to the phase-earth voltage for the measurement of the impedance. The one-system protection relay required the right choice of measured values. Special auxiliary relays, with strong contacts were necessary. The SD1 was already equipped with HF-channel to realize a directional comparison protection. For the medium voltage, the less complex SD2 was provided.