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PAC World magazine : History Protection - Generations of Protection

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Home . September 2012 Issue . History . History Protection - Generations of Protection

History Protection - Generations of ProtectionAuthor: Walter Schossig, Germany

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

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

The induction relay TJG was produced as transformer differential (Figure10).A German version of Charles Herman Merz’ and Bernard Price’s patent wasbought by AEG in 1907. Karl Kuhlmann, AEG, improved security andsensitivity with new patents (DRP 206388, 1907; DRP208470-1908 and DRP228989-1909). Secondary windings of the current transformers are now nolonger connected in opposite direction, but in series. In auxiliary wires therewas a current even in the case of undamaged main line (Figure 5).

In addition to electrical solutions still electro-magnetic have been used in1920 (Figure 3).Only the difference between the input and output current was used to detecta fault. Georg Stark of AEG developed a stabilized differential relay in1930. The “Quotient-differential-relay” worked electromechanically in threephases. It was consisting of two identical magnetic systems. Their movinganchor on a common axle operated against each other. The upper system aworked with a fault current, while the lower one b (stabilizing system) withoperating 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 ofa aluminum disc 3, controlled by operating system 1 and braking system 2. The startup value of the

relay was controlled by spring 4 . Thesmallest setting possible was 2 A. Lever 8was responsible for delay in the case ofinrush.ChEAZ’s differential relay -561 (1951) withinterposing transformer is shown in Figure 7and Figure 8.

The starting value should be set not toolow due to inrush currents when switchingon transformers, normally 30 % oftransformer’s nominal value with a delay of1 up to 2 seconds. Magnetizing current

could be used. In series withdifferential relay there was apower relay (Figure 1). Thisprovided a very sensitivedifferential protection allowingpermanent supervision to detecte.g. oxidation.

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S & H’s watt metrical relay (Figure 14) was working with electro dynamic principle. Contact on axlecould be used for indication or tripping. The device visualized iron losses in operation. This allowedfinding out, how big thedisturbance was.Nominal have beenknown and marked inred at scale. Thisdevice was verysensitive. Tests showedthat it worked already if0.5% of all windingshave been involved.Another advantage wasthe stability in case ofinrush currents,because in such a casethere is only a reactivepower.

A setup according toSiemens’ & Halske’sprinciple combiningthree separate currentrelay to a single one isshown in Figure 15.Power consumption ofmagnetic circuit was0.15 VA only, whichallowed usage with small power transformers. The devicesneeded less space and had a single contact only. Thecurrent coils have been star connected (Figure 17).

A Schweitzer and Conrad Rotating-Armature OvercurrentMultiple-Circuit Sensitive Relay Type P (Pilot wire, 1920)is shown in Figure 12. At the same time S & C’sdifferential relay type D (Figure 20) and GE’s PD-3 cameout. Westinghouse’s principle for three winding protection CA-4 is shown in Figure 13.The induction principle shown in Figure 21, and Figure 22shows 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 ascomparison protection (transverse current).This protection recognized unbalances(“balance protection”) and was calledbecause of the design of secondary coilalso “octagonal protection”. This protectiondetected also broken wires. To detectaffected line current direction relays havebeen used. Two or more parallel lines hadto be connected to the same busbar.Scheme of AEG’s transverse currentprotection and SSW’s octagonal protectionhave been equivalent in general. AEG builtdirectional element with differential one,SSW used overcurrent relay and sensitivedirectional element.

The differential relay of AEG (Figure 24)consists of Ferraris disc D and 3 drivingcores. The outer one (C) have been



connected to voltage, and in the middle Bwith two current coils. Contacts E and F ofDC circuits have been normally open. DiscD contained an isolating element J.The rotating direction was chosen in such amanner, that the affected line was switchedoff at first. Figure 16 shows a scheme forparallel operated lines.Currents of the same phases have been compared. If currents of more than 2 parallel lines havebeen compared they called it polygonal protection.

Octagonal Protection

The “Selective relay system” for 66 kV ring system of Duquesne LightCompany (US) was presented at AIEE in 1923 (Figure 23). The short-circuitof the lines is secured by the use of cross-connected reverse-powerWestinghouse Type CR relays. The ground protection is affected by themeans of balanced-current Westinghouse Type CD relays connected in theneutral of the balanced lines. These lines are balanced in pairs in every case,except at Junction Park, where no balance is made, and where straightreverse power protection with interlocked overload relays for directionalground 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 ofservice, 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. Ina normal operation their action was compensated. In the case of a faultvoltage decreases, the direction of the fault was inverted and moves theanchor.

Polygonal Protection For more than two parallel lines the SSW’s polygonal protection was asolution. 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 beenconnected via overcurrent relays. In case of normal conditions the currentwas 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 ofan oil circuit breaker, they achieved that only a single, and selective breaker



was tripped without any directional relays (Figure 32).

Figure 30 showsconnections forprotecting twoparallel 3-phaselines againstunbalancing withGE’ relay type IK-107.Unbalance

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

Line Differential Protection W. Wilson published in 1924 in themagazine „The Electrician“ a system whichwas successfully tested in England (Figure31). As some other differential protectionsystems, it is based on a comparison ofthe currents connected. It was equippedwith an additional restraining to ensure thatthe fault current to load current ratio reaches a certain level. How this was solved for 3 lines can beseen in Figure 31.

Relays have been equipped with similarcoils (operating coil AS, and restraining coilVS). Both operated on an arm, in the casewhen balance contacts are open. Thesecondary windings of current transformersare connected in a series with auxiliarycontacts S. In case of a fault in e.g. line 1secondary current used tripping coils andauxiliary contacts S. The tripping circuitbreaker opened S and closed B - thecurrent transformer and the relay wasbypassed, and the secondary circuit wasrestored.

To avoid tripping in the case of small differences of currents, the relays have been operated withbias voltage.To detect single phase-to-earth faults,single-phase directional relays IB-Y2by GE have been used (Figure 27).

Long cables have been protected withvoltage differential protection, thecurrents transformers with shunts needto be tuned.In the case of cable differential thelines itself became a burden. Toachieve equivalent load, diagonalconnection was realized with a 3rdline. In the case of a 3-phase systemthis would cause too many leads andthus mixing transformers have beenused (Figure 26). This protection wasvery fast, but tripping times varied.

Differential for regulating transformers Differential protection as proposed byMerz and Price became essential forprotection of generators, lines andtransformers. The disadvantage of allcommon schemes was that in case offaults outside the protection zone, huge



fault currents could cause false trips.Current differential protection used forgenerators and short lines required finetuning of current transformers and theirovercurrent behavior. The burden needsto be distributed equally, and long linesrequired compensation resistances. Fortransformers, different ratios andbehaviors became an issue.

This was especially valid for regulatingtransformers because the setup was validfor a certain ratio only.

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

Dr. Hermann Schulze proposed in the German magazine ETZ in 1929 a solution for differentialrelays in the case of changing transformer ratios. The arbor a and the changing torque utilizing ballowed 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 wattprotection” supervising iron losses of regulatingtransformers. It consists of 2 wattmeters,connected to a arbor and operating in differentdirections.An US solution is shown in Figure 28.

In the case of a busbar short circuits, Merz-Price differential protection was also used (Seepage 70). The idea is quite simple:

In case of normal operation the sum of currentsis zeroIn case of busbar fault, there is more power intothe substation than out of itIn 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.

[email protected] www.walter-schossig.de

Biography

Walter Schossig (VDE) was born in Arnsdorf (now Czech Republic) in 1941. He studied electricalengineering in Zittau (Germany), and joined a utility in the former Eastern Germany. After theGerman reunion the utility was renamed as TEAG, now E.ON Thueringer Energie AG in Erfurt. Therehe received 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 andthe book “Netzschutztechnik [Power System Protection]”. He works on a chronicle about the historyof electricity supply, with emphasis on protection and control.

Sidebars:Octagonal and unbalance protection are other solutions.

Line differential protection was introduced in 1924.

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