Protection History

Authors: Walter Schossig, Germany, and Thomas Schossig, OMICRON electronics GmbH, Austria

Test Technique: Introduction

Protection History
Test Technique: Introduction

by Walter Schossig, Germany, and Thomas Schossig, OMICRON electronics GmbH, Austria

This article opens the series covering measurement and testing technique. All steps from putting into operation till operation will be described in general manner. Later primary and secondary testing of relays and measuring transformers will be covered. Tests performed in laboratories as well as grid finalize the article.

Checking by Measuring Current, Voltage, and Resistance as well as Insulation Strength

When generation and transmission of electrical energy was used widely at the end of 19th and the beginning of 20th century vendors as well as owners took care of safety. At first the isolation should be checked during commissioning and in operation. The voltmeter was invented by Professor William E. Ayrton and John Perry in London in 1881 as well as the "Ammeter"/ "Amperemeter". Also the title was formed by them. (Figure 1)
One year later Ayrton presented a real Ohmmeter- with moving magnet and 2 coils. In 1889 the patent for Ohmmeter and manual generator (Figure 2) was issued to Ayrton and Perry.
Already in 1886 Siemens & Halske (S & H) presented testing apparatus.  The device (Figure 3) consists of small magnetic inductor, operated by crank handle. The Galvanometer got a scale divided in resistance values already.

 

Two separate power stations have been connected for the first time in Berlin (Markgrafenstrasse and Mauerstrasse) on 19th of November 1887.

The engineers have been quite skeptical what would happen but since they managed everything fine it worked properly. For the synchro check until the 1950s voltage measurement devices have been used (to measure the difference of voltages) or bulbs (bright scheme, dark scheme or combined bright and dark scheme).

One method is illustrated in Figure 4 at the Glasgow International Exhibition.  The "Synchroscope" worked with bulbs. (Figure 5).
Electrifying the cities reached a first climax in 1888. The company Goolden & Trotter produced dynamos at this time. The employee Sidney Evershed was responsible for all installations within their facilities. To guarantee safety he was looking for an easy to use and safe method to measure the quality of isolation. At this time this could be only checked in laboratories utilizing low voltages. It was obvious for Evershed that checking the isolation with low voltages will not deliver reliable results. To guarantee the level of isolation, nominal voltage or even higher magnitudes shall be used. Another feature important for him was local operation and direct reading of values.

So his "Ohmer" was the first Ohmmeter at all over the word visualizing values directly (Figure 7, Figure 8).  The device contains small permanent magnet in the middle of two coils. One coil was connected to the power supply of the device. The other coil was connected with the voltage source for the isolation under test. This makes the measurement independent from the voltage.
The power supply was realized with small crank handle generators (as used for phone dialing at this time). The first Mega-Ohmmeter was born. The principle was accurate and reliable and was patented by Evershed in 1889. It was launched as "Megger"- Figure 10 shows further development according to US Patent Specification US400728 in 1932.
In the same year at AEG (Germany) K. Wilkens and G. Benischke developed isolation tester (110 V, Figure 6). The sensitivity to measure resistances up to 10 Mω had to be quite high.
To do this they took an induction measurement system operating like an excited dynamo meter.

The magnetic system was powered via built-in transformer from the grid. The transformer delivered measurement voltage as well
Donald Macadie, General Post Office Telephone, London, applied for a patent in 1922 at the British patent office. His "Avometer" (Figure 9) was the first portable multifunctional measuring device.

To rate the isolation resistance of high voltage systems (up to 250 V) Austria's ETV (Elektrotechnischer Verein) published in 1888 safety rules – later called "Vienna formulas":
 W = K * E / J
Also in Germany a formula was defined (1896, "Verbandsformel"):
 W = 10,000  ω + 106  ω

Where:
W            Isolation resistance in ω
K             Coefficient for isolation level (K = 5,000)
E             Nominal voltage in V
J              maximum current in A
n             number of bulbs.  10 bulbs have been assumed as equivalent for single arc lamp single motor or other consumer.

The first earth fault tester for permanent installation was used by Weinhold in 1885, with bulbs for the same voltage as in the plant in series. The earth connection was in the middle between the bulbs (Figure 12).

If the red light went out the earth fault is in the plus pole connection, in case of blue within the minus and when all lights are out there must be an interruption or short circuit.
British Standardization Rules for Electrical Machinery, (B.E.S.A., UK), published specification 168 in 1926. For plants utilizing over 10.000 V insulation resistance tests have been required. The insulation resistance in Mega ohms should be not less than:
(Rated pressure in Volts)/ (1.000 + Rated output in kVA).
Additional voltage tests ("pressure tests") with 2 x Un + 1000V  (duration 1 min for machines with over 1 h.p.), preferable of sine wave form were defined. Protection against "over-pressures" have been described as necessary.

"Current- and voltage phasors", system Uppenborn, have been developed at Siemens Schuckert (SSW), Nuremberg, between 1881 and 1884. These devices allowed reading of currents and voltages directly at the electrical lightning system (Figure 11). A disk made of iron in front of an electromagnet is stored eccentrically on edges. Counterforce is gravity. The iron body will move within the field that electromagnetical  force and gravity are balanced. So a certain current causes a certain position. This makes scaling possible, and the calibration of the devices was done in an empiric way.

SSW produced in 1881 an isolation tester with "needle-galvanoscope" (Figure 13) and came with an additional battery consisting of dry elements. The moving needle visualized the current through the isolation resistance. The scale shows the resistance.
Within battery systems it is necessary to know if they are charging or discharging. For this application SSW's Koepsel developed the current pointer as shown in Figure 16.

At the World's Columbian Exposition 1893 in Chicago Hartmann & Brown presented an entire laboratory (see title) and have been awarded for 8 products (Figure 14).
The portable voltmeter by H&B (Figure 15), with 3 feeds in housing made of brass had a design that was very close to the alarm clocks at this time.
For the first high voltage line mainly static voltmeters have been used. A. Imhof described in Bulletin SEV (CH) in 1919 a measurement system for high voltage up to 45 kV by Trüb, Täuber & Co. with capacitive voltage transformers.
The same system was used in Figure 18. Measuring up to 140 kV was possible because 95% of voltage was over the capacitance - only 6 kV left over for the voltmeter.

First Grid Models
One of the first real-time-power-models was developed by Thomas Alva Edison in his laboratories at Menlo Park, NY. One model was for his first larger grid, the 110 V DC grid, known today as New York City Pearl Street Station (1882). As known there was a development towards AC grids which made the models more complex and influenced the entire industry.
Electromechanical models have been used for a very long time. The company Ganz and Co. presented at Vienna's electro technical exhibition in 1883 an AC distribution system.

While preparing the first 3-phase AC transmission (Lauffen a.N. to Frankfurt/Main in Germany) there have been huge doubts within German industry as well the government.
Oskar von Miller cooperated with Maschinenfabrik Oerlikon (MFO) and AEG.
C. E. L. Brown performed practical investigations. At MFO's facilities in autumn 1890 the first trial was installed (see title). In Figure 17 we see on the left C. E. L. Brown, on to the right Walter Boveri and in the middle Oskar von Miller.

After a visit of the German authorities and proper results the launch could start and on 25th of August in 1891 the first 3-phase transmission (30 kV) was put in operation.
Figure 23 shows the center in Lauffen. On the switch panel are the voltage measurement for the 3 phases, voltage and current measurement for the excitation system and current measurement in the primary line. Fuses (with adapted leads), protection relays (relaying magnets tripping on minimal or maximum current and interrupted the excitation current of generator).
To measure the current the devices have been mounted directly on busbar's potential at the first time- and they have been large enough reading the values from safe distance as well.  (Figure 22).

R. F. SCHUCHARDT and E. 0. SCHWEITZER contributed a paper to A.I.I.E. in 1911 about the experiences in Chicago. The operation of the Fisk Street Station with its pioneer 5000-kW turbo-generators showed an increasing severity in the disturbances (due to apparatus and cable break-downs) with the growth of the stations. Therefore, apparently due to the increase in generator capacity. From 9800 kW in October 1903, the totalcapacity in high voltage-alternators on the system of the then Chicago Edison Company amounted to total capacity of the three large stations to about 250,000 kW.
In tracing the various steps in the evolution of the engineering with respect to provisions for safe-guarding the generating apparatus, the writers proposed testing scheme (Figure 21).

Cable
Even when taking care while storing and mounting cables, errors could occur.  A "huge portable device for testing the isolation" (Model No. 509b- displayed in Figure 24) was produced by Hartmann & Braun (H&B) in 1894 in Frankfurt am Main. The solid housing made of oak contained the galvanometer. It could be locked by a key. It was equipped with 3 feeds and could be folded. Resistances from 10.000 Ohm - 15 MOhm could be measured with the 14 kg device.

A carriage with 2 wheels for transporting the galvanometer and the 90-Volts-measuring battery was possible. With an easy to build up tent the cable measurements carriage was ready (H&B; Figure 20). The tent protected the measurement engineer and allowed access to the devices in the back when the carriage was open. A special tripod, adapted to the carriage made fix installation possible. All devices have been inside the carriage.

Figure 19 illustrates the application of carriage made by Felten & Guilleaume Actien-Gesellschaft (F & G), Mühlheim am Rhein, Germany in 1909. The picture was taken in Leipzig, Connewitz substation.
Special developments of the dynamometric measurement devices for testing laboratories and acceptance tests were delivered by Siemens in 1912. Capsuled with metal they have been small and handsome.

Transformers and Isolators
In 1886, William Stanley first successfully demonstrated the use of the transformer in an alternating current system to provide electric lights for the Town of Great Barrington, Massachusetts.
The testing of transformers built at the Morning-side plant of the Stanley Electric Manufacturing Company took place in Building 1, the main transformer assembly building, which was built in 1901. In 1907, when the plant became the Pittsfield Works of the General Electric Company, adjoining Building 2 was completed and the testing operation expanded into this new area. Similarly, two years later, Building 3 was added and the testing operation expanded again.

Primary equipment as transformers, circuit breakers and measurement transformers as well as isolators were tested at the manufacturer's facilities as well as on site regarding the electric strength.
The German VVDE published guideline for "construction, testing and application of AC high voltage devices" in 1914. The recommendation was 2 time nominal voltage plus 10 kV.
The recommendation published by VDEW Germany (1920) covering proposed test voltage for transformers and in-house isolators (1920) can be found in Table 1.  To limit test current voltage transformers have been tested with 100 Hz.

Testing Protection Devices
Even doing the best while commissioning and maintaining substations disturbances and problems occurred. Developments of testing devices and improving the technology will be covered in the next issues.

walter.schossig@pacw.org        www.walter-schossig.de
thomas.schossig@omicronenergy.com     

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BeijingSifang June 2016