Protection History

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

First Guidelines for Testing Protection Relays and the Beginning of stationary Relay Test Sets.

The community of German utilities (VDEW) published a “relay book” in 1930 (Figure 1). “The relay shall be designed with love- this is the task of the designer. If this is true this is just more valid for the leading engineer putting into operation the entire plant. He should not believe in what the vendors are promising but try this out with own trials. Doing this he will learn and understand the limits of the device.” On another side “The relay personnel shall be educated very well …. Additionally, when testing the relay the engineer should show more joy and passion than in normal daily business.

Minimum requirements are detailed knowledge on operation principles of the relays as well as the test set.” The authors would like to confirm this statement.
The common variants for relay testing as defined in 1929 are shown in Figure 3.

Creation of Testing Guidelines for Protection Relays in Germany
The German magazine ETZ published some rules related to protection in 1927 for the first time. Two years later the rules “Regeln für die Konstruktion, Prüfung und Verwendung von Wechselstrom-Hochspannungsgeräten REH“ (Rules for construction, testing and application of AC high voltage apparatus) were published. They defined:

Tripping Error
This describes the ratio of

For primary tripping devices, this should be less than ±7,5%, for secondary relays maximum ± 5 %.
The devices common at this time, especially the primary tripping devices, caused higher errors. Retesting was estimated as essential.

Time Error
The time error for primary release devices shall be less than 0.5 sec (8 sec) or 1 sec (> 8 sec). For secondary relays the error shall be less than 0.4 sec. In case of dependent characteristics there have been no definitions.

Tripping Current Ratio
Since this was defined for electromechanical devices, they have been thinking about relay armature devices. In case of 1.4 times overcurrent the mechanism operates. Higher currents keep the device tripped. Decreasing the current now opens the mechanism at a certain value. When this happens, it shall be measured and the ratio to be calculated. The value shall be 1.33 at the maximum.

Tripping Time Ratio
It was essential, that the devices turn back to startup position before the time was over - for instance in case that the fault disappears. The moving parts of the devices shall stop immediately if the overcurrent stops. So, they defined the ratio between the tripping times set up and the time that it takes in case of overcurrent to reset. The demand for this ratio was 1.5.

Decrease of Voltage Release
Devices must stay in switched on status if the voltage decreases to 60%. If the voltage goes down to 35% it should trip.
The experts in Germany discussed in 1936, to extend the common test period of 4 weeks. Three-phase test sets have been demanded,
Also, V. H. Todd’s book “Protective Relays” (Figure 2) describes testing practically:
"After making all connections so the load, or overload, can be quickly applied, the time may be determined with a stop watch. First carefully set the rheostat to give the desired current, at which current it is desired to take the time; disconnect and let it fully reset.
Then quicklyapply the load and press the crown to start the watch; both at the same instant. Press again to stop the watch when the contacts close.

The watch hand indicates in fifths of a second the time required to close at that particular current. A third press resets the watch for the next trial. If impossible to apply the load quickly, quite close result can sometimes be obtained by building up the load and holding down the plunger by hand, releasing it and snapping the watch at the same instant. By varying the load and the time settings, the time of the various combinations can be obtained."
In testing the directional element, both voltage and current, should be supplied to see that the relay functions correctly on reversal of current. In many cases, it is desirable to step the voltage down to 1 or 2 per cent of normal and put on a heavy overload at low power factor and see if operation is satisfactory.

Stationary Test Setups
The N relays have been described in one of the first parts of this history series. To test their characteristic, current had to be injected. To do this a small CT was used with a rheostat, ampere meter and clock. If the relay needed to be calibrated with voltage and current, big transportable test equipment was used.
The picture on page 70 shows such a setup. It was made by the company Dr. Paul Meyer Ag in the 1920 and could be used for even complex tests.
It allowed calibration of relays as well as the connected transformers. It consists of “calibration desk,” regulating transformer and power transformer to power the grid transformer. The chronograph recorded the tripping values automatically. On the left-hand side, an insulated CT (60 kV) can be seen.
The test equipment was connected with 3 phase connector 7f to the station transformer.

All three phases are necessary for the phase shifter. The device under test is connected via plugs 7a and 7b with calibration set. The 7 poles on plug 7b make the connections unmistakable.
Also, the currents and voltageconnections cannot be mixed because of special plugs. Also, the different connections between regulating transformer, high current CT and relay transformer are marked. To test a resistance dependent relay the high current CT  5 will be connected with regulating transformer 6 close the relay transformer 4. The high current transformer is equipped with air cooling. Thus, currents of 750, 1500 and 3000 A have been possible. To check operating time the recording device 18 was used. Every phase angle could be defined on a phase controller. Via changing the direction of voltage the direction of energy could be set.  See Table 1.

To transport, as well as to put in operation these test devices is extremely time consuming. Because of that, such equipment was being used mainly stationary at vendors as well as utilities. Vendors used it for final testing, utilities have been working with them for test after buying, services and modifications. Also, extended tests in own labs have been performed.
An AEG publication (1931) shows the testing lab for N-relays (Figure 4). Twelve relays have been calibrated at the same time and compared with normal device.

At first the voltage scale was calibrated as common for measurement devices. After that the voltage stays connected with a certain value, the current will be applied (for instance 20 A).
The different tripping times of the relay are marked on a troammel, rotating quite quickly. 1 mm of circumference is equal to 0.05 s - this makes differences from the nominal value visible.
Adjusting the discs for the volts makes calibration possible. This will be done until the requested tolerance of ± 0.1 s is reached.
They claim “calibrating 12 devices at the same time makes evenness possible which is for selective protection even more important than as the exact values and times.

An definitive reference book in the US was published by A.F.Hamdi (The New York Edison Company) in 1926. The “Relay Handbook “(Figure 5) shows stationary test setup for single induction type overcurrent relays (see also Figure 9 in December 2017 issue of the magazine).
AEG presented in 1931 a series of bench tables. There was the “one field table” for testing current relays (AC) and voltage relays (AC and DC).
Under the table a power transformer is mounted. A drawer with key stored the tools.
On the table, there was enough space for the device under test and measurement devices. On the backside, a switch panel was mounted. Switches, plugs, voltage measurement with range changing device, voltage divider and a power plug have been available. Cables and the resistance have been on the back.
The rheostat for the current circuit is mounted on the front side, other resistances and can be added easily. See Table 2.
Figure 6 shows a single field test table by AEG.

The “two field variant” (Figure 7) is in one hand as the apparatus described already. The second field contains the phase transformer with an adjustable secondary winding (phase shifter), The voltage can be set between zero and 110 V and the phase shift will be adjusted via handwheel. To avoid manual calculation there is a scale showing angle and cos directly.
The two variants described be “standard variant”. Additional special tables have been available as well:
Three-phase voltage testing tables as one-field variant for measurements of 3-phase voltage relay, phase interruption relays and earth fault relays
 Three-phase current testing table- realized as one-field variant for differential relays
 Two-field tables (Figure 8) with fixed installed and wiretapping possibility. This could be used for testing distance relays. The operating time could be measured via flexible resistances and different phase angles between voltage and current.  See Table 3.
The testing tables could be used for fixed operation as well as portable (with wheels). Another table produced by S&H at the end of 1920 is described at the end. The apparatus consists of fix operating equipment and measurement table.

All switches, control devices and measurement instruments are mounted. All plugs for connecting instruments, counter and relays are available. The voltage circuit is realized as 3 phase.
The controller for the voltage consists of three potentiometers, connected in a star. A phase controller, equipped with 6-phase switch allowed to set up every phase shift. The rotating iron measurement could visualize even voltages as low as 10% of nominal voltage.
An iron wire lamp made this possible. A red light indicated operation additionally.  See Table 4.     


Walter Schossig (VDE) was born in Arnsdorf (now Czech Republic) in 1941. He studied electrical engineering in Zittau (Germany), and joined a utility in the former Eastern Germany.  After the German reunion the utility was renamed as TEAG, Thueringer Energie AG in Erfurt. There he received his Master’s 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 Protection]”. He works on a chronicle about the history of electricity supply, with emphasis on protection and control.

Thomas Schossig (IEEE) received his master’s degree in Electrical Engineering at the Technical University of Ilmenau (Germany) in 1998. He worked as a project engineer for control systems and as a team leader for protective relaying at VA TECH SAT in Germany from 1998 until 2005.
In 2006 he joined OMICRON as a product manager for substation communication products. He is author of several papers and a member of standardization WGs.

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