Protection History - Generations of Protection

Author: Walter Schossig, Germany

Dry Disk Rectifiers

Even if mentioned already in an AEG patent from 1925, dry disk rectifiers have been used in protection in 1937 for the first time. Crystal detectors as known from radios have been too sensitive in case of shock. That’s why SSW preferred copper/copper oxygen rectifiers, AEG worked with Selenium rectifiers.
FIigure 1 shows an underlayed clapper-type armature relay B37 produced by Reyrolle for AC operation utilizing DC relays with rectifiers. It was used for zero current indication and worked with nominal voltages of 63.5; 110 or 190 V AC. In case of 10% of the nominal value the power consumption was 0.06 VA, the operating time was 30 ms.

Until 1937 electromechanical distance relays or relays utilizing Ferraris principle have been used. These devices reached almost the limit of their capabilities and no further improvements have been possible. Nevertheless, there have been wishes and requirements of the users because of more and more complicated distribution networks. Reduced dependability of the devices from the current and angle of the measurement system, bigger short circuit currents, detection of failures in short cables in municipalities or industrial grids are just examples. Another requirement was that the operating time should be reduced from 400 ms to 100 ms to avoid out of step of generators. To fulfill these requirements was not that easy. Bigger short circuit currents caused saturation and dependencies from the current. The torque increases with the square of the current which causes huge stress for the bearing, vibrations could occur, and as a result the contacts have not been safe anymore. The requirement to measure even short distances caused the demand for sufficient torques even in case of low voltages- another reason for the very high power consumption. To reduce the operating times was not possible with the relays used.

Due to cost reduction reasons three-relay-schemes have not been used anymore, nevertheless there still was the wish to detect doubled earth faults selectively. The widely used single-relay-scheme required to switch over currents and voltages. In the current path this was done with interruption-free relays. Figure 2 shows the selection of measured values to measure the impedance. (U/I . directional element U * I in RZ4, SSW, in case of doubled earth fault RS). So the time was right for a new approach.

Distance relays with dry disk rectifiers
The impedance elements have been estimated as the most important and with space for improvements and so the rectifiers have been used here for the first time in 1937. The torque increased no longer quadratically but linear and no saturation occurred. A system working well under all electrical circumstances was possible now. Operating times could be divided in half because of the reduced mass of moving parts. The low power consumption of the rectifiers was another advantage. The characteristics of the relays could be visualized linearly now what was much easier for the operator. Figure 7 shows the measurement system of such a fast distance relay (SD4 of AEG, used from 1937 up to the end of world war II). The experiences have been quite good, so the principle became widely used.

With the reduced power consumption of rectifiers there was no need any more for a direct connection of the current. It was sufficient to use the voltage drop on a 4-pole-shunt. The selection of the currents occurred with switchover-contacts. Voltages and currents could be selected in parallel. The contacts could be adjusted in that manner, so that the current path was interrupted before the voltage path and was closed in the other order in a short time. This made the distance system very stable, there have been no contacts in the CT-circuit which was another advantage. The switchover as introduced here is still used today in distance protection. Figure 6 showed the solution of EAW (RD7, 1952).

Introduction of Moving Coil Relays
Bringing a coil into the air gap allowed high sensitivity. Figure 10 shows a directional relay.
Also for the overcurrent startup and measuring parts new elements have been developed. In the 1930s high-quality ring shaped gap magnets have been used in speakers. During the war other systems with reduced size came out. The experiences were very good. A new rectifier scheme to be used in railway applications was developed by AEG in 1943- the distance protection SD4M. The rectifiers operated as a non-linear element in parallel to the moving coil. Due to this the torque even in case of short circuits was not that huge thus improving the reliability. Depending from the use case devices with one or two measurement values were used (Figures 3, 8).

The main measurement transformer delivers the measurement values current and voltage M. In the entrance-network E they are transformed to useable values. Here we find resistors and interposing transformers, which are also used for galvanic separation. The rectifier is realized as a Graetz-bridge or as a diac with tap. The rectified value is connected to the moving coil measurement, where additional resistors are available for fine tuning. The contact of the moving coil is connected with a contactor (for tripping and indication). In the relay with the two measurements the two Graetz-Bridges are connected antiparallel and the moving coil measurement in between (electrical balance beam). With additional elements there is not just the comparison of the measurement values possible but even the calculation of product and ratio. The later used ring form is shown in Figures 5 and 9.
A modified Graetz scheme is shown in Figure11.

Two selenium rectifiers are connected in “Poleck”-Scheme which means that two of the rectifiers in a conventional bridge are replaced by resistors. This means that the dependability of the performance from the current decreases. The grading is done with capacitances C. A typical example a in SD4 (EAW) and c in RK4 (Siemens).
William K. Sonnemann, Westinghouse, presented in AIEE in 1942 a similar scheme for a new Single-Phase-to-Ground Fault-Detecting Relay (Figure14).
Sonnemann reported in 1961 about the usage of diodes in tripping circuits for the first time (Westinghouse TRB tripping diodes, (Figure15). AEG’s SD4M (Figure12) was launched in 1951.

Further developments have been caused by an official recommendation of the organization of the German utilities (VDEW) in 1951 to utilize DC measurement more. A compact setup containing the rectifiers in the bottom of the socle is shown Figure13. The usage of rectifier as directional element of energy is shown in Figure17. This setup allows a comparison of voltage and current values.

A simplified scheme to detect directions was developed by SSW in their conductance relay R1KZ7 in 1970 (Figure 16). The ring modulator can be seen in Figure 19. At one side there is the voltage proportional to current, the other voltage is proportional to the voltage. Through the single diodes different currents are flowing. The resulting current passes through the bridge relay N. In case of reverse direction of current also the current in the bridge relay weakens. Also in this case a phase shift of 90° between current and voltage causes a zero current in the bridge relay so that there is no torque to detect direction. To adapt the line angle an additional inductance is connected (45°). The sensitivity for the direction reached is < 1 ‰ of nominal voltage at nominal current.

To support high sensitive moving coil distance relays their impedance- and directional system was equipped with a system to increase the pressure of the contacts (SD14, 1954, Figure 21).
When the moving coil contact K closes, the current iE passes through auxiliary relay E. The current is delivered by saturation transformer S and rectified. The low voltage and the small resistance R causes an additional current iV , passing through the rectifier on the right hand side and the coil D. Since current iV got the same direction as the original bridge current iB , the related torque is amplified. The circuit is calculated in such a manner, that iV is much bigger than iB. As a result the contacts work bounce free and robust even at the limits of the measurement range.

If the switching power (5 W or 8 VA) of the moving coil relay (Figures 20, 18) was not sufficient, an additional relay with more power had to take over the switching. The R1V16 (Figure 23) of S & H combining the moving coil relay D (Figure 22) and an auxiliary relay allowed apparent power of up to 1000 VA. Additionally the lock increased the lifetime of the relay.

Rectifiers in Line Differential Protection
1938 produced Westinghouse the development of Edwin L. Harder the high speed type HCB (Figure 28) pilot wire current differential relay. Figure 24 shows the alternating current pilot wire scheme using HCB Relays. To transmit the pilot signals only two pilot wires are needed.
The Russian line differential protection Д3Л presented in 1962 is shown Figure 25.

The Soviet line differential RDL / (page 70), presented in 1949 could protect lines up to 20 km utilizing two auxiliary wires. The picture shows the selenium rectifiers on the left hand side and in the middle and on the right hand side a glow lamp. Saturation transformers have been used to limit the voltage at rectifiers and wires.

Rectifiers have been used in pilot-relay D.M.W, The English Electric Company Limited, EE, Stafford, UK, (Figure 33). The polarized relay is shown in Figure 30, the line to be protected could be up to 20 miles long.
The static distance relay developed by EE in 1960 used diodes as comparators. In EE’s differential relay DMH (Figure 26 and 31) the tripping element with realized using Graetz bridge. Also in rotor earth fault relays with DC injection method rectifiers have been used.

SIEMENS fault locator (Figure 27) was presented in 1959. A capacitance stored short circuit currents and voltages. The discharge was used to estimate the fault location. This function was triggered by the distance relay.
After lots of investigations in rectifier technology BBC presented the 3-phase- overcurrent relay I21, the capacitance unbalance relay I22, the voltage unbalance relay UXS21, differential relay D2, minimum impedance relay Z31 and the 2-stage- load unbalance relay IG25 (Figure 32) in 1966.

Rectifier relays have been faster than mechanical relays (because of the decreased mass) and have been widely used in all line-, transformer-, generator protection systems as well as automation before electronics started.
A typical example is EAW’s distance relay RD11 (Figure 29) with moving coil relays 1 and 2 (1961). In a relay combination one RD11 was used for one phase, additional devices have been used to realize a 6-phase-system startup and directional elements.

Additional electronic elements will be covered in the next series.


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