History Protection - Generations of Protection

Author: Walter Schossig, Germany

Direct Axis Current Comparison Protection
Differential Protection was covered in the 2008’s summer and autumn editions (Merz and Price and others). Additionally,  we will describe mechanical solutions now. Reyrolle started with line protection series SOLKOR (= “solid cores”) in 1904. First SOLKOR relays have been line differential relays. Three currents have been connected “magnetically” (Figure 4) or with mixing transformers. Comparison lines have been rated for high voltage to earth (5, later 15 kV). Figure 2 shows an example used in 15 kV; on the left hand side is an auxiliary box which was mounted on the rear of the protection table.

Reyrolle’s solution for Merz-Price protection utilized sensitive clapper-type relays (Figure 6), type W was used for lines and generators. The principle was that of a drive-hammer. The anchor was operated by a magnet. This unblocks the hammer so this could operate the relay. Settings could be changed with a spring using a knurl.

The induction relay TJG was produced as transformer differential (Figure 10).
A German version of Charles Herman Merz’ and Bernard Price’s patent was bought by AEG in 1907. Karl Kuhlmann, AEG, improved security and sensitivity with new patents (DRP 206388, 1907; DRP208470-1908 and DRP 228989-1909).  Secondary windings of the current transformers are now no longer connected in opposite direction, but in series. In auxiliary wires there was a current even in the case of undamaged main line (Figure 5).

In addition to electrical solutions still electro-magnetic have been used in 1920 (Figure 3).
Only the difference between the input and output current was used to detect a fault. Georg Stark of AEG developed a stabilized differential relay in 1930.  The “Quotient-differential-relay” worked electromechanically in three phases. It was consisting of two identical magnetic systems. Their moving anchor on a common axle operated against each other. The upper system a worked with a fault current, while the lower one b (stabilizing system) with operating current. In the case of a fault the release contact f was closed (Figure 11).

Induction relay with braking winding -156 was produced in the Soviet Union (Figure 9). It consists of a aluminum disc 3, controlled by operating system 1 and braking system 2. The startup value of the relay was controlled by spring 4 . The smallest setting possible was 2 A. Lever 8 was responsible for delay in the case of inrush.
ChEAZ’s differential relay -561 (1951) with interposing transformer is shown in Figure 7 and Figure 8.

The starting value should be set not too low due to inrush currents when switching on transformers, normally 30 % of transformer’s nominal value with a delay of 1 up to 2 seconds. Magnetizing current could be used. In series with differential relay there was a power relay (Figure 1). This provided a very sensitive differential protection allowing permanent supervision to detect e.g. oxidation.

S & H’s watt metrical relay (Figure 14) was working with electro dynamic principle. Contact on axle could be used for indication or tripping. The device visualized iron losses in operation. This allowed finding out, how big the disturbance was. Nominal have been known and marked in red at scale. This device was very sensitive. Tests showed that it worked already if 0.5% of all windings have been involved. Another advantage was the stability in case of inrush currents, because in such a case there is only a reactive power.


A setup according to Siemens’ & Halske’s principle combining three separate current relay to a single one is shown in Figure 15.
Power consumption of magnetic circuit was 0.15 VA only, which allowed usage with small power transformers. The devices needed less space and had a single contact only. The current coils have been star connected (Figure 17).

A Schweitzer and Conrad Rotating-Armature Overcurrent Multiple-Circuit Sensitive Relay Type P (Pilot wire, 1920) is shown in Figure 12.  At the same time S & C’s differential relay type D (Figure 20) and GE’s PD-3 came out.
Westinghouse’s principle for three winding protection CA-4 is shown in Figure 13.
The induction principle shown in Figure 21, and Figure 22 shows induction type phase balance relay.

Cross-Differential Protection
Transmission lines are realized as double-circuit lines very often to increase reliability. This required special protection systems. Overcurrent relays have been used as comparison protection (transverse current). This protection recognized unbalances (“balance protection”) and was called because of the design of secondary coil also “octagonal protection”. This protection detected also broken wires. To detect affected line current direction relays have been used. Two or more parallel lines had to be connected to the same busbar. Scheme of AEG’s transverse current protection and SSW’s octagonal protection have been equivalent in general.  AEG built directional element with differential one, SSW used overcurrent relay and sensitive directional element.

The differential relay of AEG (Figure 24) consists of Ferraris disc D and 3 driving cores. The outer one (C) have been connected to voltage, and in the middle B with two current coils. Contacts E and F of DC circuits have been normally open. Disc D contained an isolating element J.
The rotating direction was chosen in such a manner, that the affected line was switched off at first. Figure 16 shows a scheme for parallel operated lines.
Currents of the same phases have been compared. If currents of more than 2 parallel lines have been compared they called it polygonal protection.

Power. Flexible. Easergy.
BeijingSifang June 2016