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WINTER.2009

by Walter Schossig
71 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 Thueringer Energie AG in
Erfurt. There he received his Masters degree and worked as a protection
Biography
Walter Schossig
(VDE) was born in Arnsdorf (now
Czech Republic) in
1941. He studied electrical engi-
Transformers and its Protection
According to the patents of Károly Zipernowski, Miksa Déri and Ottó Bláth, the first transformers were produced in 1885 by the company Ganz & Co.. They were small alternating current ring-transformers or shell-form transformers. The magnetic circuit was closed jointless. The patentees used the word "transformer" for the first time.
Five years later Dolivo-Dobrowolsky invented the
3-phase-transformer. A new, improved A.C.-system for
"3-times diametric voltage" was his intention. A paper published in the German ETZ in 1891 "Transmission of force with alternating currents in different phases [rotating current]" includes the first usage of the German word "Drehstrom" for
"rotating current". This paper has been translated into different languages and since then the term „rotary currents“ has been accepted. To use oil for isolation purposes was proposed by Schwinburne in 1889. The company "Maschinenfabrik
Oerlikon" (Switzerland) delivered in 1889 the first transformers for the utility EW Reichenhall (Germany).
With the new century several companies started to produce high power and high voltage transformers. Siemens-
Schuckertwerke transformers with 12500 kVA (shell-form) and Westinghouse's 100 kV are examples of leading edge transformers at this time. With the invention of transformers, the development of transmission grids could start. Rapidly increasing demand for power forced this development in the
1920's. Huge transmission grids have been connected, the amplitudes of the short-circuit current reached substantial values, several failures in windings occurred. Due to the dynamic impact of the initial symmetrical short-circuit currents windings, arresters and bushing broke down.
Short-circuit proof windings have been developed later.
First Protection Devices for Transformers
The lack of protection devices resulted in fires and blackouts. The fuse, invented by Blathy,O.T. (Germany) and the American Wurts, A., in 1890 ("cell fuse") allowed fast interruption of the short circuit. At first the fusible link was sufficient for the protection of lines, generators and transformers. It starts operating if the current at the location of a fault was higher than the nominal values. This works fine in case of small nominal values. With the increasing nominal values of power this was not sufficient anymore, leading to the development of tripping devices and relays.
The first switchgears have been "air-arm-"; mercury- and tube-breakers. First oil circuit breakers with fuses have been proposed in 1895. Brown,C.E.L., BBC, proposed in the 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 Protection]”. He works on a chronicle about the history of electricity supply, with emphasis on protection and control.
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1
Thermostat or quicksilver remote thermometer
(left) or
Platinum-resistance
Thermometer (right), alongside Protective pipe company’s headquarters "Porta Volta" in Milan in 1897 to put the 5 kV circuit breaker directly into an oil drum. This test was performed successfully, and so a new 16 kV breaker was built for Paderno in the same year.
Kalamazoo's survey in 1901 showed the predominance of oil breakers. The first 2 kV oil-circuit breakers (50 A) with direct release was produced in 1902 by S & H. Brown, C.E.L.,
BBC, applied in 1902 for a patent on current-dependent timing relay (D.R.P. Nr. 143556). The heating of the transformer was supervised with thermal relays - a good base.
Thermal Relays
To utilize transformers efficiently, short overloads have to be accepted (up to a multiple of nominal values). To achieve this permanent supervision of the heating of the transformer is necessary in order to avoid aging of the windings and their isolation. The German utility Ostpreußenwerk tested thermal relays (v. Wiarda) with transformers in 1928. The idea was to clarify how the oil absorbs the heating and releases it.
In 1930 V.M.Montsinger investigated the behavior of isolating material at higher temperatures. He demonstrated the coherence of the aging of paper-oil isolation systems. The rating life will be cut in half if the temperature of the asset increases with an amount of 8 Kelvin above the maximum operating temperature ("8-K-formula"). To avoid exceeding the temperature of 115 °C in supervised substations, these devices are set up with a value of 80 °C ( warning ) and 90 °C
( trip ) (Fig. 3).
Due to difficulties in measuring the temperature of the windings directly, a thermal model emulates the winding temperature. This thermal relay is outside the transformer and closes a contact at a certain level of temperature. Bimetals are used for thermal replicas of motors, generators and transformer windings. An example is a bimetal relay produced by SSW in 1932 (Fig. 2).
Another example is OERLIKON's Limitherm- Relays (Type
BIT, 1950) (Fig. 4) which is equipped with a bimetal tripping device in a "thermal block". This device allows delay times between 15 and 80 minutes. Due to safety reasons the delay time was selected smaller to ensure that the temperature of the
3
overload rated load Residual
Lifetime new actual age expected lifetime winding is not going to reach a critical value. The calculated temperature was higher than the real one. A compensating winding considers ambient temperature.
The thermal models could be used to protect against overload. The difference to previously mentioned thermal relays was especially that these devices are dipped into the isolation oil - now the functionality depends on the temperature of the oil. The higher the temperature, the earlier the device will trip. Of course this takes into account the changes of temperature of oil - at lower temperatures a higher load is possible. An advantage of these thermal models was that it only considers the difference of temperature between winding and oil, but not between oil and air. The thermal replica of the winding was mounted on protective pipes that have been dipped into the oil (Fig.1 - Thermal Models with
Thermostat or Quicksilver Remote Thermometer (bottom) or with Platinum-Resistance Thermomenter (up), alongside
Protective Pipe ).
Measurement and supervision was the task of a thermostat, a resistive element with measuring instrument for measurement of temperature or the quicksilver remote thermometer. The thermostat was used for annunciation of an increased winding temperature. Well known are "stick thermometers", system "Horn".
Oil-air cooling systems have been equipped with oil flow controllers that immediately detect the failure of an oil pump.
2
4
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This was necessary because the rough walls of the tank do not get even the capability to purge no-load losses for a longer time. Without a circulating pump, these transformers had to be switched off as fast as possible. Very important stations have been equipped with two circulating pumps for safety reasons.
They have been connected in parallel with stop valves.
A typical example for overload protection for oil transformers is the thermal relay RN1-CIT (Fig. 7) by
SPRECHER ENERGIE (1992). This device was a combination of statical overcurrent protection with immediate tripping and thermal overload. The part "T" contains a special circuit that models warming and cooling of the transformer using the voltage proportional to the current. It contains two delay times which could be set up in such a manner that the behavior of the transformer could be modeled. Now the transformer was safely protected against overheating. The short time delay
(5 min) was for high overcurrents; the longer one for small overcurrents (temperature of oil). The nominal currents of the current transformers had to be adapted to the nominal current of the transformer. This device could be used in small stations without batteries as well. It could be supplied by AC with its tripping capacitor and trip with the measurement transformer current. Mechanical bi-stable indicators showed the indication even in the case of loss of power supply.
Buchholz Protection and Relays for Supervision of
Oil
Using oil for transformer isolation was an important milestone in the development of transformers. Implementing expansion tanks (since 1910) decreases the aging of transformer oil. These devices were at first mounted on the wall (Fig.5) and later above the tank. Change of volume in case of change of load or change of temperature could be adjusted.
Since it was possible to adapt the overcurrent protection to local needs, there still remains the disadvantage that at the fault location the current has to be bigger than the nominal current of the apparatus.
Additionally, for selectivity reasons, the time delay was very long near the source and in some complicated cases the system was not usable. Overcurrent protection was only used for assets, where the impact of electric arcs was limited
- for instance at transmission lines. This is different for transformers. The material selected is not robust and the value of the asset is very high.
If a transformer is separated from the grid in case of a thunderstorm, it does not show on the outside if it is damaged or not. Until the 1920's it was the decision of the operator whether to switch the asset on after a failure. This was more a decision depending on the character of the operator and less on his knowledge. If he was a careful guy, he would take the transformer out of service and start opening and disassembling it. After two or three days he would learn that the transformer is OK or damaged. This wastes a lot of time if the transformer was without damage. Some brave engineers decided to switch on the transformer without approval - it could happen that the transformer explodes.
Max Buchholz (Fig.6), while working in the
Elektrizitätsamt“ Kassel (Germany) later „Preussische
Kraftwerke AG“ examined transformer damages. He figured out that the big heat of the arc destroys insulation material and delivers gas. What to do with this important, but rudimental awareness was probably unclear to Buchholz at this time. Some say that an experience in the bath tub was helpful for him. He performed the first experiments in his son's aquarium. The idea was to lead the gas bubbles under the transformer cover to an appropriate place. There the quality and quantity of the gas can be estimated. After a lot of trials he found the solution. The gas could be collected with a light inclination of the cover. A disposed pipe should lead the gas to the expansion tank. Here
5
6
7
8
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Buchholz received his first patent
DRP 386629, in 1921.
9
its color could be observed. In case of an explosion the huge amount of gas produces a blast wave. Color and Quantity of the gas could be estimated outside the expansion tank, it could be checked if it is flammable or not. This was sufficient to decide what happened in the transformer. Buchholz received his first patent in 1921(DRP 386629)- and his name is the name of the device until today.
The Buchholz protection is the first device that does not detect the difference of a current, voltage or power from a certain level - this device uses mechanical action. Now the changes in the quality of oil could be detected easily and very early. The company Max Buchholz AG was founded in Kassel and later Elektrokustos AG was established in Zurich. These two companies have been responsible for the sales outside
Germany. The main business was the production under license at AEG and Siemens in Berlin and at Micafil in Zurich for BBC.
Figures 8; 9 and 10 show Buchhol- relays made in the 1920's.
The Buchholz relay ( some non German speaking countries use the name „Buchholtz“ or „Buchhulz “) was produced in 3 varieties (1, 2 and 3 inches). This was a possibility to diversify the price according to the size of the transformer.
In the mid 20's the lower floater was realized in such a manner that even in case of strong flow the floater moves the connected contact. Experience had shown that in case of serious failures, the time from creation of the gas bubbles until reaching the relay was too long to limit the danger of destroying the transformer. In the mid 30's the lower floater was connected to a flow flap to achieve a higher sensitivity on flow. Tests performed by AEG with the BEWAG (Berlin) showed that the startup speed was 100 cm/s. After 1945
Buchholz relays with small height have been developed and standardized in DIN 42566 in 1961.
In 1934 Konrad Täuber proposed to implement a throttle control in the pipe between the tank and the expansion tank.
If the temperature of the gas increases, the increase of pressure could be measured and a warning or tripping provided. A simple principle of the Täuberprotection is shown in Fig. 11.
This differential pressure measuring device measured the dynamic pressure (due to flow of oil) and the static pressure
10
11
2
3
1
1 Measuring Orifice 2 Pressure Chamber 3 Differential Pressure
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(due to oil on the installation location). The Esti-cartridge should be mentioned. It consists of a small crystal ball with a mandrel. Apart from a small gas bubble the ball is filled with a liquid which expands during a raise in temperature and compresses the gas bubble until the inner overpressure blasts the ball at a certain level of temperature and trips a contact.
This device was built into a thermometer case.
After transformer explosions in the USA Dann, Walter.M. und Hill, Leland,H., at Westinghouse (US Patent 1605026) developed a pop valve for rise of pressure in 1927.
After false tripping of the Buchholz relay during earth tremors or start of oil circulating pumps several further developments started. Aigner (Germany) reported a new development in 1960- a shock-proof Buchholz device (up to
1 g). Reliability discussions in the 1960's proposed redundant
Buchholz relays (in series). Failures above the transformer cap should be detected by fast differential protection. At the Hannover Fair 1998 an " electronic Buchholz protection " was presented by the University of Hannover, Messko and
SIEMENS (Fig. 12). Huge transformers are equipped with further Buchholz relays, e.g. for bushings (Fig. 13).
Differential Protection
The last issues of PAC World covered in detail the
"differential protection story". To mention again the first application in South Africa at Victoria Falls and Transvaal
Power Co. Ltd. in 1908 ( plant Brakpan 6 MW and 3x3,75 and 2x4,5-MVA-Transformers, 40/10 and 2/10 kV )
1909/10 ( Simmerpan, 18 MW and 3x3,75 and 4x4,5-MVAtransformers, 40/10 and 2/10,5 kV ); 1911 ( Rosherville, 68
MW and 5x12,5 and 2x4-MVA-transformers, 5/42 and
20/42 kV ) and 1912 ( Vereeniging, 44 MW and 2x12,5 and
4x9-MVA-transformers, 5/42 kV ).
Petersen coils have been used for zero sequence current compensation since 1930 (Fig. 14). The special case of a differential protection of a Scott-circuit transformer is shown in Figure 15
Residual current elimination during the grounding of the transformer's star point was realized with interposing transformers with delta windings or with a filter in numerical relays. The disadvantage of this solution was a reduced sensitivity for single phase short circuit current by a value of
2/3. Transformer failures are more critical because the startup value decrease is not linear (Fig. 16).
A solution for this issue was the Restricted Earthfault
Protection (REF) that allows a more sensitive setup. In
English speaking countries the high-impedance principle for measurement is quite popular. This is not valid for the
German speaking countries where REF and low-impedance principle do not play a major role. One of the reasons is the use of Petersen coils in the neutral-point connection in the grids with voltages less then 110 kV. Due to this, the unbalanced residual current is quite small.
In 1992 SPRECHER Energie developed a static differential relay RN1-DT (Fig. 17) that allows usage without interposing transformers for adaptation of transformers ratio and vector group (except for YNyn0 and YNyn6 solid earthed).
12
14
13
15
ALSTOM transformer, with Buchholz main tank, diverter switch and
Bushings.
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16
The 1941 AIEE Transactions paper "Prolonged Inrush
Currents with Parallel Transformers Affect Differential
Relay" by Hayward,C.D. discusses the "sympathetic inrush" phenomena. This happens very seldom – and maybe due to this the experts sometimes have no idea what happened when sometimes during switching on of one transformer another transformer (in operation) trips.
Frame Ground Protection
Differential protection does not work well in grids with isolated star point or utilizing Petersen coils due to the small currents in case of an earth fault. The isolated assembly of a transformer and a current transformer with connected overcurrent protection, is known as a frame ground protection (See
Figure 18).
In 1947 the French EdF decided to avoid using differential relays for transformer protection. Frame ground protection combined with Buchholz was selected as the solution. This was discarded later on.
German Rail for instance still uses as default solution for protection of their 110/15-kV-transformers (16,7 Hz) Buchholz- , differential and frame ground protection relays.
Overcurrent and Distance Protection
Overcurrent and later more and more distance protection is used as a backup protection for the Buchholz and the differential protection, as a busbar protection or as a backup protection of a line protection on the lower and higher voltage winding. In 1934 Walter,M., AEG, proposed to extend the overcurrent protection with a high-current stage and created a fast backup protection for a big part of the transformer. This is also possible with a distance protection on the higher-voltage winding.
In several countries distance protection with raised tripping time is also used to utilize a busbar protection in transformer feeders. This is a fast backup protection for faults on the line as well. Magyar Tröszt Budapest (Hungary) developed in
1974 a stand-alone backup protection AZT. This overcurrent protection was located directly on the transformer – that is why the connecting wires are very short. Redundancy was guaranteed as far as possible by connecting to the measuring core of the current transformer and to a second coil of the circuit breaker.
The power supply of the relay and the tripping was realized with the higher-voltage current transformer using energy stored in a capacitor. The operating time was dependent on the pre-load and the type of failure.
Transformer Protection Applications
The application of transformer protection depends on multiple factors and has changed significantly through the years with the changes of both transformer and protection technologies. It is also affected by the philosophy of the users, the importance of the substation where the equipment is located and the available resources.
Different national and international industry organizations had produced guidelines on the application of transformer protection which will be covered together with the protection of unit transformers in power plants in a later issue of PAC
World.
walter.schossig@pacw.org
www.walter-schossig.de
17
18
2
3
1 Current Transformer 2 Relay
1
3 Ground
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