70PA

C h

isto

ry

IB-2 Single-Phase Power-Directional Relay, Connected to Current Transformers of Two Parallel 3-Phase Feeders, for Selective Ground-Protection; Along With Unbalanced-Current Protection by PD-3 Relays, and Overcurrent Protection by IA-201 Relays, GE, approximately 1920

Current Relays

Type IB-2 Relay

Over-Current Element

Auxiliary switches open when oil circuit breaker is open

Directional Element

Time Reset Locking Relays

Current Transformers

Potential Transformer

Grounding Resistor

D-C Control Bus

OilCircuitBreaker

Ti Neutral of Transformer

A-C Control Bus

Fuse

Fuse

Current Differential Relays

History is the tutor of life

PAC.SEPTEMBER.2012

71

by Walter Schossig, Germany

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-differen-tial-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

We started describing generations of protection in the last issue of the magazine. The idea is to describe generations with typical examples and thus we started with overcurrent protection. Long times for clearing faults (up to several seconds) caused huge problems up to destructions of substations. New solutions with shorter fault clearance times and improved selectivity need to be found.

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.

BiographyWalter Schossig

(VDE) was born

in Arnsdorf (now

Czech Republic) in

1941. He studied

electrical engi-

neering in Zittau

(Germany), and

joined a utility in

the former Eastern

Germany. After the

German reunion

the utility was

renamed as TEAG,

now E.ON Thuer-

inger Energie AG in

Erfurt. There he re-

ceived his Masters

degree and worked

as a protection

engineer until his

retirement. He was

a member of many

study groups and

associations. He is

an active member

of the working

group “Medium

Voltage Relaying”

at the German

VDE. He is the

author of several

papers, guidelines

and the book

“Netzschutztechnik

[Power System Pro-

tection]”. He works

on a chronicle

about the history

of electricity sup-

ply, with emphasis

on protection and

control.

Protection

The Beginning of Electromechanical Differential Protection

Generations Protection

History

P S

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72

PAC.SEPTEMBER.2012

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 ProtectionTransmission 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.

2 Solkor, 15-kV Reyrolle (appr. 1960)

4 Solkor scheme , Reyrolle(Currents connected “magnetically” or with mixing transformers)

5 Differential scheme, AEG, 1908

(Differential relay -561 with transformer )

1 Power differential relay, AEG (appr. 1920)

6 Clapper type relay (W, Reyrolle, approximately 1960 )

7 ChEAZ, SU, 1951 ,

3 Differential protection with induction balance relay

Figure 11: a Upper magnet

system

b Lower magnet

system

c Arrow

d Scale

e Spring

f Contact

g Coil

h Anchor

i Indicating disc

k Rotating anchor

PAC.SEPTEMBER.2012

73

Currents of the same phases have been compared. If currents of more than 2 parallel lines have been compared they called it polygonal protection.

Octagonal ProtectionThe “Selective relay system” for 66 kV ring system of

Duquesne Light Company (US) was presented at AIEE in 1923 (Figure 23). The short-circuit of the lines is secured by the use of cross-connected reverse-power Westinghouse Type CR relays. The ground protection is affected by the means of balanced-current Westinghouse Type CD relays connected in the neutral of the balanced lines. These lines are balanced in pairs in every case, except at Junction Park, where no balance is made, and where straight reverse power protection with interlocked overload relays for directional ground protection is used.

A scheme of an octagonal protection made by SSW is shown on Figure 18. The big relay is the sensitive directional relay. If one of the lines is out of service, the device works as a simple overcurrent protection

V & H chose another solution. They introduced an unbalance protection (Figure 19). Two limbs of a three-leg coil have been equipped with reactor-couples, one of them connected with line 1, and the other one with line 2. In a normal operation their action was compensated. In the case of a fault voltage decreases, the direction of the fault was inverted and moves the anchor.

Polygonal ProtectionFor more than two parallel lines the SSW’s polygonal

protection was a solution. Current transformers of 4 lines have been connected in series (Figure 25). The common points of I and II; II and III and so on have been connected via

8 Differential relay -561 scheme, ChEAZ, Soviet Union, 1951

Octagonal and unbalance protection are other solutions.

12 Multiple-circuit sensitive relay P, S & C, appr. 1920

9 Induction relay -156, Soviet Union, 1948

10 TJG, Reyrolle, appr. 1960 11 Differential relay QS1, AEG, 1930

Figure 9: 1 Operating

system

2 Breaking

system

3 Aluminum disc

4 Spring

8 Lever

Three winding transformer

Restraining winding

Operating winding

Type CA-4 relay

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PAC.SEPTEMBER.2012

case when balance contacts are open. The secondary windings of current transformers are connected in a series with auxiliary contacts S. In case of a fault in e.g. line 1 secondary current used tripping coils and auxiliary contacts S. The tripping circuit breaker opened S and closed B - the current transformer and the relay was bypassed, and the secondary circuit was restored.

To avoid tripping in the case of small differences of currents, the relays have been operated with bias voltage.

To detect single phase-to-earth faults, single-phase directional relays IB-Y2 by GE have been used (Figure 27).

Long cables have been protected with voltage differential protection, the currents transformers with shunts need to be tuned.

In the case of cable differential the lines itself became a burden. To achieve equivalent load, diagonal connection was realized with a 3rd line. In the case of a 3-phase system this would cause too many leads and thus mixing transformers have been used (Figure 26). This protection was very fast, but tripping times varied.

overcurrent relays. In case of normal conditions the current was zero. In the case of fault at line I there was a current in relays 1 and 4. The current in 2 and 3 was zero. With a smart scheme of operating current of an oil circuit breaker, they achieved that only a single, and selective breaker was tripped without any directional relays (Figure 32).

Figure 30 shows connections for protecting two parallel 3-phase lines against unbalancing with GE’ relay type IK-107.

Unbalance protection could be used for more than 3 parallel lines as well (Figure 29 with GE’s IK-104).

Line Differential ProtectionW. Wilson published in 1924 in the magazine „The

Electrician“ a system which was successfully tested in England (Figure 31). As some other differential protection systems, it is based on a comparison of the currents connected. It was equipped with an additional restraining to ensure that the fault current to load current ratio reaches a certain level. How this was solved for 3 lines can be seen in Figure 31.

Relays have been equipped with similar coils (operating coil AS, and restraining coil VS). Both operated on an arm, in the

16 Connection diagram differential relay AEG, approximately 1920

15 Three phase differential relay, S&H, approximately 1930

Line differential protection was introduced

in 1924.

18 Octagonal protection, SSW, approxi-mately 1920

13 Scheme CA-4, Westinghouse

14 Watt-metric differ-ential relay, (S & H, appr. 1920)

17 Three phase differential relay, S&H,appr. 1930

Figure 21: Induction principle

Differential Current

Pallet Switches

3 Phase Bus

Type CR Relays

Type CD Compensator

Type CD Relays

Trip Coils

Ground

Interlocking Relays

PAC.SEPTEMBER.2012

75

Differential for regulating transformersDifferential protection as proposed by Merz and Price

became essential for protection of generators, lines and transformers. The disadvantage of all common schemes was that in case of faults outside the protection zone, huge fault currents could cause false trips. Current differential protection used for generators and short lines required fine tuning of current transformers and their overcurrent behavior. The burden needs to be distributed equally, and long lines required compensation resistances. For transformers, different ratios and behaviors became an issue.

This was especially valid for regulating transformers because the setup was valid for a certain ratio only.

The starting values of most differential relays was set up by intrinsic angular momentum (spring or weight).

Dr. Hermann Schulze proposed in the German magazine ETZ in 1929 a solution for differential relays in the case of changing transformer ratios. The arbor a and the changing torque utilizing b allowed setup (Figure 34). A sensitive transformer differential relay was introduced considering high- and low-voltage-side voltages as shown in Figure 33.

BEWAG in Berlin developed “differential watt protection” supervising iron losses of regulating transformers. It consists of 2 wattmeters, connected to a arbor and operating in different directions.

An US solution is shown in Figure 28.In the case of a busbar short circuits, Merz-Price differential

protection was also used (See page 70). The idea is quite simple:

In case of normal operation the sum of currents is zero In case of busbar fault, there is more power into the

substation than out of it In case of differential, all secondary windings are

connected in parallel (maximum current relays) In case of a fault, all circuit breakers of line- and

transformer-feeders will trip

The electromechanical distance protection and its beginning will be covered in next issue.

walter.schossig@pacw.org www.walter-schossig.de

21 Induction-type ratio-differential relay for generator and transformer

23 Diagram of connection of 66 kV Line Relays, DLC, 1923

24 Scheme of differential relay (AEG, appr. 1920)

19 Unbalance relay, S & H, appr. 1920

20 Current-differ-ential relay D, S & C,

22 Induction type phase balance relay

25 Polygonal protection, SSW, appr. 1920

(appr. 1960)

li = current normalre= current in case of short circuit line 1

a Interposing transformerb Inductive resistances

c Moving-iron relays with quicksilver latching relay

a Arbor differential relayb Ferrari system for correction of regulation and load

c Tripping contacts for

oil circuit breaker

Trip Coil

Oil Circuit Breaker

Oil Circuit Breaker Trip

Coil

Polarity Marks

Polarity Marks

Reverse PowerRelay

Time Limit OverloadRelays

Potential Transformers

Current Transformers

Tripping Reactors

Operating Bus

Fuse

Bus

Other Phases Similar

Voltage Relay

Loading Resistor

Star - SeriesConnectedPottential

Transformers

Circuit-ClosingOver-CurrentTime Relays

CurrentTransf.

OilCircuitBreaker

AuxiliaryRelays

Open only when operating with one Line

PolarityMarks

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PAC.SEPTEMBER.2012

31 Parallel lines protection, Wilson, UK, 1924

27 IB-Y2, GESingle-phase directional relay, approximately 1920

32 Tripping elements, SSW, appr. 1920

29 IK-104, GE, protection connections -

26 RN22, SiemensLine differential protectionapproximately 1955

28 Typical large regulating transformer protection

33 Scheme of sensitive differential relay

30 Comparison protection IK-107, GE, 1920

34 Scheme for regulating transformers

against unbalanced power in 3 or more parallel incoming, outgoing or tie lines