PAC history
70
History is the tutor of life
Flywheel Generator in Frankfurt/Main, Germany, BBC, 1894
PAC.SPRING.2009
by Walter Schossig
Protection
71
History
DC Generator, Kriegstetten (CH) Power Station,
Oerlikon, 1886.
Biography
Generators are important and expensive
assets.
Generator
Protection
The Beginning
Introduction
In the beginning of electrical power supply (1880-1890) the
importance of switching devices was minor. Small units for
illumination of a house and commercial building had been put
into operation. Knife switches, strip fuses and a simple bulb
for supervision of voltage were sufficient for dynamos with a
power of 15…25 kW.
Edison,T.A. started running the first central station and first
public electric utility in New York (Pearl Street) on September
4th, 1882. At first he used three and later six 150 kW, 110
VDC, 1200 rpm "Jumbo Generators" (due to their large
size). He developed a complicated lighting system with lines,
distribution boards, fuses, breakers and metering devices. The
first public power station in Germany with six steam machines
(150 HP each) and 12 dynamos with a total power of 540 kW
(110 VDC with two-wire connection) was put into operation
on May10th,1885 (Berlin, Markgrafenstrasse 44).
The first switching devices were more or less provisional
ones due to lack of experience and limited knowledge. They
have were in an empirically-manual manner and were suitable
for single tasks. Such an example was the "general interrupter"
in the Berlin power station which would enable an emergency
cutout of the whole grid. Figure 2 shows such a makeshift
construction (nominal current approximately 2000 A and
2x110 VDC. Unlocking the four clamps lets the copper ear
plate fall down and interrupt the circuit.
At this time DC was mostly used. Due to this and the
inductance of the machines the control and extinguishing
of the lightning arc when switch off occurred, was difficult
even at low voltages (65…110 VDC). Developing motors
and generators was a challenging task anyway- so nobody
was really interested in the so called "auxiliary devices" low voltage circuit breakers. The initial switching devices
originated from laboratories (mercury bowls) or from the
telegraph industry (switches and push buttons). Because these
devices did not meet heavy-current requirements, engineers
avoided them or tried to reduce the breaking capacity with
de-excitation of generators.
At the time of introduction of alternating current, oil
was estimated as the optimal extinguishing agent. The
pioneer of alternating current, Ferranti, was the first at
using oil for that purpose. He plunged the agile part of the
PAC.SPRING.2009
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,
now E.ON Thueringer Energie AG in
Erfurt. There he 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
and the book
“Netzschutztechnik
[Power System Protection]”. He works
on a chronicle
about the history
of electricity supply, with emphasis
on protection and
control.
PAC history
72
1
Maximal
automat, USA,
3
Ferranti-oil-fuse (1894) into the oil after opening the contact.
Results of a short circuit in a unprotected
The first high-voltage device working completely in oil was
machine (appr. 1930)
developed by C.L.E. Brown (one of the founders of Brown,
Boveri & Cie) in Baden/Switzerland in 1897. He proposed
at the headquarters Porta Volta in Milan to put an air-circuit
breaker (5 kV) in a barrel of oil for testing purposes. This was
successful, and a new 16-kV-breaker was developed in the
same year for Paderno (with an oil hutch made of glass). Later,
steel plate hutches were used.
One could now protect the generator by switching it off
with a circuit breaker and de-exciting it in case of a certain
smell, electrical arc, or loud noises. Previously, this was not
possible due to the long reaction time (Figure 3).
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 2500 V, 50 PS, = 70 %) by Oerlikon. Figure 5 shows the
were developed. One of the first automatic circuit breakers in wooden switchboard, above the interface-device ( 3 wires
Germany was made by Hermann Meyer, S & H, 1886 with with a diameter of 6 mm, one of them as spare). On the left
the "biggest machine circuit breaker at this time" (knife switch) and on the right the automatic short-circuiters are arranged;
combined with an electromagnetic undercurrent relay (startup they are shorting the field magnets in case of a fault. They were
at 10% of nominal or reverse current). This was a combination used to replace the lead fuses.
„Switchboards“
of minimum- and reverse-current-automats (early relays),
used for parallel operating DC generators.
In the first years of high-current technology (1880- 1890)
After this the first "triumvirate"-automats were a "switchyard" consisted of a DC-dynamo connected to a
developed: minimum-automat; reverse-current-automat and switchboard. A board made of planks. Wood was considered
maximum-automat (Figure 4) as fundamental components a good insulator at this time. In Figure 6 you can see the
of the DC-generation and -distribution at this time together planks and the attached instruments, main switch and four
with batteries in power stations. The task of these devices was knife-switches with stripe fuse. The word "board" (in German
the switching-off during low currents, wrong direction of "Brett") is still used for terminal boxes of generators and motors
currents or overcurrents.
- “terminal board“ in English. A mock up circuit is sometimes
The tripping was accomplished electromechanically. called a "breadboard" ("Brettschaltung").
Automatic Overcurrent Devices
Several types were created. The first industrial transmission of
current in Switzerland took place in 1887 with commissioning
In 1885 American engineers equipped the knife switches
of the DC-interconnection Kriegstetten-Solothurn ( 8 km, with overcurrent magnets (Figure 1). A click holds the knife
2 "General-Interrupter" in power station
Markgrafenstrasse, Berlin, 1885
(approximately 1885)
4 Automats, open, hand-drive, S & H, 1892
1) Maximal-Automat, Nr. 5998; 2) Reverse-Current-Automat Nr. 5084; 3) Minimal
Automat Nr. 5969, Startup 15 % of Nominal Current
1
PAC.SPRING.2009
2
3
73
The trip free mechanism
developed in 1900,
was the main progress
for automatic
circuit-breakers.
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
6 Switchboard“ of a DC-generator,
Schuckert, 1890
7 1888,
OERLIKON
5 Wooden switchboard in control center
Kriegstetten, Oerlikon, 1886
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PAC history
74
9 Principle of
Reverse Switch
10
Köckner,
Free tripping
8 Oil Circuit Breaker, V&H, appr. 1920
Breaker with:
Primary Tripping
Device and
Secondary
Undervoltage
Tripping Device
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.
11
Generator-Switchboard, AEG, Vienna
"Jubiläums-Gewerbeausstellung", 1888
(approximately 1900)
PAC.SPRING.2009
Increase in demand on power
and transmission of energy
over long distances was
the reason to build bigger
generators.
Use of Measurement Transformers
Increase in demand on power and transmission of energy
over long distances was the reason to build bigger generators
or to operate generators and grids in parallel. This required
the use of measuring transformers. It seems that the first
voltage transformer had been used in the "Californian Light
Company" in San Francisco in 1879. The current transformer
was invented by Benischke,G. in 1898. And since 1900 relays
were designed to use current transformers.
Emmet,W.L.R. und Hewlett,H.E., GEC, built in 1901 oil
circuit breakers with two tripping coils, directly connected to
the current transformers to open the locking mechanism.
Figure 16 shows an "automatic machine tripping device"
with an overcurrent relay supplied by current transformers
(used by AEG in power stations in 1905). The figure shows
the solution with auxiliary power supply. But this breaker was
manufactured with tripping coils supplied by two parallel
operating current transformers.
It was a logical conclusion to change the measuring devices
in such a manner that the indicator also operates the contact.
Nevertheless this solution was unsatisfying. The breaking
capacity was poor (6 VA at 220 V) and the electric arc held the
contacts together.
Since the same measuring principles have been used
the first relays have been quite similar to measuring devices
(especially the round form Figure 19).
12
Voltage decrease tripping device, V&H,
appr. 1920
75
The Swedish Company ASEA delivered the first induction
relay for a water station of the Swedish Rail (16 2/3 Hz) in
1912. To cut one's own path was the decision of Voigt &
Haeffner (V&H). Vogelsang, M. developed an overvoltage
relay with timing element in 1902. They decided to do it
like that because they did not produce current transformers
themselves. Additionally Vogelsang developed an "oil circuit
breaker with fuse". To change the fuse it was at first necessary
to switch off the breaker before opening the cap.
The history of measuring transformers will be covered in a
later issue of PACWorld.
Overcurrent Protection
The main element of generator protection was overcurrent
protection from the beginning. It protected the generator
of inner damages, and was also the backup protection for all
further assets such as transformers and lines. Smaller machines
were equipped with direct overcurrent release, connected
with a lever (Figure 13; Figure 15). The first stand-alone
electromechanical relay was designed in1904. Figure 18
shows the first time-overcurrent relay made by ASEA, type
TCB, manufactured in 1905. ASEA's (now ABB) first relay of
induction type was delivered in 1912 to a hydropower station
in the north of Sweden, built to deliver power at 16 2/3 Hz to
the railway from Kiruna to Luleå, which was built to transport
iron ore. This was the first electrified railway in Sweden.
Overload Protection
The use of thermal relays for protection of generators
was introduced exclusively by BBC in Europe and was very
successful.
German recommendations required generators with a
nominal power of more than 5000 kVA to use six resistancethermometers or thermo elements in the stator to supervise
the temperature of winding. Once they were installed it was
very difficult to reach them again- that's why they very often
were not changed after damage. So it was the decision to
supervise the temperature with thermal relays. These devices
are equipped with thermo elements (heating relays with
current proportional to main current delivered an image of
temperature of the machine- Figure 14.
The outer insulating mat "O", was working as a protection
against thermal radiation. It encloses the source of heat, the
measuring element and the heat storage "P". The heating
element – a band made from a resistive material- heated the
measuring column and the heat material storage, consisting
of changeable measuring boards. The thermal time constant
of the relays could be changed with the numbers of boards
in 6 stages between 20 and 110 minutes. The upper scale
was a display of the temperature to allow a later estimation
of temperature. Additionally the relays were equipped with a
tripping device.
Later single pole overcurrent relays with a setting of 1.1 In
and 10 s have been used for indicating overload.
Short Circuit Protection
To use overcurrent relays between generator and busbar
in case of failures inside the generator was active only if other
machines were able to deliver short circuit currents. The
13 Independent direct release HB,
BBC, 1943
14 Thermal image of ST-Relays (BBC)
The main
element of
generator
protection
was a
overcurrent
O
P
from the
beginning.
15
16
Thermal relays
Maximum On-Off
HT, BBC, 1943
switch, AEG, 1905
17
"Automat",
Berlin, 1891
"Automat" with Undervoltage Tripping Device,
Gebrüder Naglo
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PAC history
76
18
First time-overcurrent relay, TCB,
ASEA, 1905
power of another machine had to be as big as the power of the
machine to be protected. In case of its own failure the generator
delivered a huge short circuit current but this could not be
detected with such a setup. That's why overcurrent relays
have been installed in the neutral point of the generator. This
setup was the only possibility if the generator was the single
source in the grid. Figure 21 shows a three-phase overcurrent
protection "S", manufactured by SIEMENS in 1936.
Connecting generators and grids in parallel increased the
reliability of power supply but on the other hand created
19 TOC,
AEG, 1916
20 Thermal
21
Overcurrent
overcurrent (ST,BBC) protection S, Siemens
Pl Nr 69036G, 1914 &
Pl Nr 69007
PAC.SPRING.2009
unmanageable short-circuit currents in case of failure.
Sometimes the unreasonable guaranties for small voltage drops
in the machine required the use of coils for the limitation of the
current from generators with high short circuit power (hard
machines). Later "soft" generators were used, equipped with
a huge short-circuit reactance. The Swiss grid reported good
experiences with the usage of a cutback of excitation to achieve
a limitation of a sustained short-circuit current. All big power
stations have been equipped with such devices. The automatic
decrease of excitation could limit the steady-state short-circuit
current to a certain value (as 1.4*Inom). After switching off
the short circuit the voltages recovers. Out of step machines
could be "catched". The current level in case of a short circuit
sometimes reached the level of overload current.
Nevertheless, in case of a malfunction of the line
protection the generator overcurrent protection should trip.
Solowjew,L.E., USSR, proposed in 1932 to use "undervoltage
supervision". This allowed a more sensitive setup of the
overcurrent startup. An overcurrent relay produced by BBC in
the 1960's is shown in Figure 22.
The overcurrent setting must be over the highest possible
operating current. Since these devices started up with a fault in
the grid already the operating time must be the biggest in the
grid. If several generators are working in parallel all overcurrent
relays had the same operating time. In case of terminal
short-circuit of a generator there was no selectivity anymore.
This was only possible if the faulty generator could be tripped
faster than the other ones.
Further solutions such as reverse power protection,
differential protection, interwinding fault protection and
earthfault protection will be covered in a later issue of this
magazine.
walter.schossig@pacw.org
www.walter-schossig.de
22 Overcurrent Relays S1 without Housing,
BBC, appr. 1960