First Generator Protection Devices

The first protection devices were fuses in series with knife switches. Before that, there were fuses at the generator only, so in case of a failure the whole grid (very small at this time) was without power. After a short time it became clear that not having a three-phase trip was a disadvantage. That's why from 1885 to1890 the first automatic circuit breakers were developed. One of the first automatic circuit breakers in Germany was made by Hermann Meyer, S & H, 1886 with the "biggest machine circuit breaker at this time" (knife switch) combined with an electromagnetic undercurrent relay (startup at 10% of nominal or reverse current). This was a combination of minimum- and reverse-current-automats (early relays), used for parallel operating DC generators.

After this the first "triumvirate"-automats were developed: minimum-automat; reverse-current-automat and maximum-automat (Figure 4) as fundamental components of the DC-generation and -distribution at this time together with batteries in power stations. The task of these devices was the switching-off during low currents, wrong direction of currents or overcurrents.

The tripping was accomplished electromechanically. Several types were created. The first industrial transmission of current in Switzerland took place in 1887 with commissioning of the DC-interconnection Kriegstetten-Solothurn ( 8 km, 2500 V, 50 PS, = 70 %) by Oerlikon. Figure 5 shows the wooden switchboard, above the interface-device ( 3 wires with a diameter of 6 mm, one of them as spare). On the left and on the right the automatic short-circuiters are arranged; they are shorting the field magnets in case of a fault. They were used to replace the lead fuses.

?Switchboards"

In the first years of high-current technology (1880- 1890) a "switchyard" consisted of a DC-dynamo connected to a switchboard. A board made of planks. Wood was considered a good insulator at this time. In Figure 6 you can see the planks and the attached instruments, main switch and four knife-switches with stripe fuse. The word "board" (in German "Brett") is still used for terminal boxes of generators and motors - "terminal board" in English. A mock up circuit is sometimes called a "breadboard" ("Brettschaltung").

Automatic Overcurrent Devices

In 1885 American engineers equipped the knife switches with overcurrent magnets (Figure 1). A click holds the knife in position "On"; replaceable-devices made of coal took the lightning arc, additional extinguishing possibilities were not prepared. C.E.L. Brown, OERLIKON, built a maximal automat as shown in Figure 7 (open position) in 1888.

In case of a generator overcurrent condition, the automat starts up and shorts the excitation winding.

The knife falls down and connects the moving contacts. This approach solves the challenge of interrupting high currents in case of high self-induction voltage.

Later overcurrent automats had more force. The knife closed with a clamped spring (during switching on or with a weight). If the magnet tripping device operated it moved the switch into the Off-position. The smallest starting current possible was 130% of the nominal current, in normal case they have been set with a value of 150%.

Automatic Undercurrent Devices

The task of automatic undercurrent devices was to switch off parallel operating generators in case of a decreasing generator current to avoid a feedback current from the battery to the generator. The moment of switching off depends on the prior magnetizing current (magnetization). The higher the preload, the lower the startup value.

The set point was 15% in 1882, later 5%. An unpleasant -even dangerous- behavior of some automatic undercurrent devices was a malfunction caused by rapidly decreasing currents with fast change of direction of currents due to the magnetic inertia.

Reverse Current Automatic Devices

Due to the uncertainty of some automatic undercurrent devices reverse current automats were used instead of them. In 1892 they could start with 20% of the nominal value.

The tripping magnet was equipped with a current-and a voltage-coil. During normal operation both coils are working against each other ("differential coil"), in case of reverse current the switch trips (Figure 9).

Under Voltage Tripping Device

Breakdown and recovery of voltage, a daily event in early times, required the usage of devices with under voltage tripping elements. At first "automatic circuit-breakers" (Figure 17) were used.

Later the devices were combined with other functions for switches. A solution for two-phase short-circuit protection with undervoltage trip was presented by V&H in the 1920's (Figure 8).

Primary tripping devices, were mounted on the breaker operate mechanism with a lever. The voltage tripping device is supplied by an external voltage transformer (Figure 12). When the voltage transformer operates in the direction of infeed it is guaranteed that the circuit breaker could be switched on only in case of existing voltage.

The company Dr. Paul Meyer introduced a three phase zero voltage tripping device in 1912

"Trip Free Mechanism"

The trip free mechanism, developed in 1900 was the main progress for automatic circuit-breakers. This was a special design of the switch, ensuring that it fully opened before it was enabled to be to closed again. Figure 10 shows such a trip free mechanism. It required a current in coil 1 before the switch could be operated.

If these devices were not available, the approach was to avoid disabling of the automatic circuit-breakers in case of a trip (e.g. switch onto fault): Then, the automatic circuit-breaker and the lever switch are connected in series.

When switching, the automatic circuit-breaker was first operated and then the lever switch was opened. This caused some failures due to wrong order of operation, this was reported for instance from the United States. That's why Sharpstein, S.H. required "a trip free mechanism" in 1899, realized by Emmett and Hewlett, GEC.