PAC history
70
Reverse-Power
Leakage Suppression
Winding for Differential
CTs, Bütow, 1925
History is the tutor of life
Generator Protection, AEG
PAC.SUMMER.2009
by Walter Schossig
Protection
71
History
Special protection functions have been developed for bigger generators.
Biography
Generator
Protection
Overvoltage, Differential, Turn-to-Turn-Fault
The first developments in generator protection
have been discussed in the last issue of PACW. Since the
machines became bigger special protection functions have
been developed and will be discussed in this article.
Reverse-Power Protection
In the first years a reverse power has been indicated by
an annunciation only. H&B produced a reverse-current and
direction-of-current indicator in 1894 (Fig. 1). The rotating
red disk in front of white plate showed the irregularity.
Directional relays have been used to distinguish between
short-circuits at busbar or in feeders and failures in the
generator. They could detect if the current flows from the
generator into the grid or in reverse direction. These relays
used current transformers in the generator circuit breakers;
this location was the border where the overcurrent should trip
without time delay.
A combined overcurrent and reverse-power relay
for generators was shown by AEG in 1903 (Fig. 5). An
aluminum-disk was driven by a magnetic three-leg core. The
outer legs have been excited by the voltage, the middle one
by current. At normal direction of current even in case of a
huge overcurrent the relay is delayed, in case of reverse current
it operates more or less immediately. In 1920 generators
were equipped with at least two-phase or better three-phase
reverse-current tripping device with sensitive setup: the relays
should only trip in case of internal faults. Backup protection
was realized by high-current tripping devices with a long
delay. In case of tripping of the generator circuit-breaker the
generator had to be de-excited. This avoids fire in the winding
of the generators.
Only one power relay was used at this time because the
engineers thought that only in case of a failure a power in that
direction could occur. Such a "reverse power" is possible in case
of power swing, bad synchronization or during a short circuit
with up to 15% of the nominal power of the generator. Such
reverse relays should not endanger normal operation. The
setting should be above the value mentioned or with a time
longer than the power swing (1.5 s). Of course in that case the
efficiency of the protection was quite poor.
The clearing time was long (for a generator protection)
and the relay operates only in case of a terminal short-circuit
because the voltage collapses and an active power of more than
10% could not be measured anymore. This "dead zone" could
be avoided particularly with a directional relay used in a 30°- or
PAC.SUMMER.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
Reversecurrent & direction-of-current
indication
V&H,
1894
2
Direction of
power relays
RR2, AEG
5
50°-scheme. Now the relay starts up even in case of inductive
Combined overcurrent and reverse-power
reactive currents during unequal excitation.
relay, AEG, 1903
On Ascension Day 1924 a disaster occurred in a steam
station in Erfurt (Germany) during the taking of a generator
out of service. The bolt of the trip valve was full of salt and
could not interrupt completely the steam supply. Now the
rotor was accelerated and the new installed generator was
destroyed completely.
In the 1920s AEG developed the RR2 power relays. See
Fig. 2. They consist of two induction driving elements with
a common Ferraris-disk. Springs hold them in the middle
position. The driving elements work in Aaron-circuit. An
arm moved according to the amount and direction of power.
It was more or less a wattmeter with a contact. The switching
capacity was poor and an auxiliary relays was necessary.
Reverse power protection was later used for protection of
steam turbines.
The turbine operates as a synchronous motor and it could
be damaged.
Circuit and view of a 2-pole reverse-power protection
CG90c (BBC) is shown in Fig. 3 and Fig. 10.
To avoid a tripping of the protection in case of turbine blade
salt deposits, the tripping signal is active only if the valve 2 is
closed (Fig.4).
ZPA produced the reverse-power relay GSCT12 (Fig. 9)
in the early 1970s. For measuring a ferrodynamic relay SW
in Figure 8 was used. An advantage of this device was the
sensitivity for harmonics because it trips on the mean value
of the products of voltage and current. A torque was produced
A definite time reverse power relay, type WCG, produced
only in case of equal fundamental or harmonic.
by GEC in 1988 is shown in Fig. 11.
Differential Protection
These relays could be used for earth-fault detection too.
The successor was the static relay GSCT12X in 1981.
Effective short-circuit protection became possible with the
BBC produced a static PPX110/111 (Fig. 6) in the 1970s. introduction of differential protection. First developments
This relay was used for supervision and tripping of generators, and the usage for transformer and line protection have been
but it also could detect if a generator still receives energy in case covered in the last issues of this magazine. The most common
of a leak valve. Another usage was for huge changes of load basic connections in the 1930s are shown in Fig. 15.
which could cause an out-of-step of the generator. All these
Unlike transformer differential the same transformers
(type, construction, ratio) could be used in star point,
conditions could be supervised and evaluated with a counter.
3
Reverse-power
protection CG90c
BBC, 1943
approximately 1925
PAC.SUMMER.2009
4 Circuit of reverse- 6 Power relays PPX110-1
power protection
BBC, 1968
BBC, 1978
7 Differential current transformer
AEG, 1927
73
Effective and fast shortcircuit protection
became possible with the
introduction of differential
relays.
matching transformers and tap changers were not needed.
Due to missing no-load current a more sensitive setup was
possible.
The differential relay produced by AEG in 1925 (DR in
Fig. 14) worked without timing element. It operates with
Ferraris' principle and tripped with a time delay for small
currents and instantaneously with large currents. According
to the requirements from the customers two- and three-phase
devices with a time range of 1 up to 6 seconds have been
produced.
An appropriate circuit was the magnetic differential
(Byrd-transformer as shown in Fig. 15c).
An iron core was connected at the beginning and the end of
every leg. The secondary winding was connected to differential
relays (in that case sensitive overcurrent relays). The neutral
point is created beyond the transformer group. Other devices
such as overcurrent relays and measuring devices (not shown
in the figure) could be connected too.
Conventional differential schemes use six CTs transforming
the nominal current of the transformer (e.g. 1000 A) to 5 A.
Here the winding D was connected in a special manner to
achieve highest sensitivity. The impedance of the winding was
selected the same as the relay's. Instead of a ratio 1000 A/5 A
= 200, ratio of 25 has been used and so the sensitivity (or the
safety against disturbances) was 8 times higher.
If every iron core got only one winding, false currents
occurred even in case of equal primary currents due to
8 Reverse-power
9 Reverse-power
wiring diagram, ZPA, 1976
ZPA, 1976
relays GSCT12-S1
non-symmetrical configuration of the conductors. Dr. W.
Bütow proposed in 1925 (DRP 456202 and 480371) a
leakage suppression winding.
The iron core (shown on the spread) carried 18 coils,
couples connected in series. These groups have been connected
in parallel to the differential relays (clamp D). In case of one
electromagnetic force bigger than the other one (because it
was near to a primary conductor and that’s why in a stronger
field) the equalizing current flows to the coil with the smaller
electromagnetic force.
The magnetic field of the equalizing current superimposes
the field of the primary currents, the magnetic flux in all
cross-sections of the core was equal as long as the primary
currents are equal. The flux has been “moved” from a stronger
magnetic point in the core to a lighter magnetized one. With
such CTs differential currents as low as 0.1 % of the nominal
current could be safely detected.
For instance the four 30000 kVA generators in the
Vermuntwerk (Austria) and the two 40000 kVA generators
in the pump-storage power station Herdecke (Germany) have
been equipped with such a protection.
These CTs never became popular because the customers
preferred standard transformers.
In connection with stator earth fault protection BBC
recommended in 1945 a simplified differential protection
using single pole differential relays (Fig. 16). In case of
phase-to-phase short circuit this protection was quite fast,
while during two phase-to-earth faults it operates only in
some cases.
BBC introduced their TG generator differential relays in
1943. Figure 18 shows the further development TG3. The
10 Circuit of Reverse-Power Protection
BBC, 1943
11
Definite
time reverse
power relay WCG,
GEC, 1988
relays GSCT-S1
PAC.SUMMER.2009
PAC history
74
14 Simple differential protection, AEG,
1925
EM - Excitation Machine; FA - Field Surpression; RR - Reverse Relay;
S - Oil-Breaker; Sp - connected to Voltage Transformer; St - Current Transformer;
UMZ - definite time.overcurrent relays; DR single phase differential
12
Interturn
short-circuit
protection
RA2c with
Chain of reactors b
(Siemens, 1936)
C
T
S
R
a
b
UV W
connection for generators in delta-connection and the required
circuit for primary and secondary transformers are shown in
Figure 17.
In the 1960s ASEA produced the differential protection
RYDHA with high-impedance-stabilization. The differential
measuring elements have been equipped with big series
impedance working as a surge voltage protector. Choosing a
suitable operating point could avoid undesired tripping due
to saturation of current transformers without balanced-beam
relays or delays. Operating time was 15 ms (without the
tripping time of the auxiliary relays). Primary pickup-value
was 2% of the nominal current of the CT. The device was a
Interturn faults require the
immediate switch-off of the
generator in order to prevent
further damage.
3-pole one, for every phase an overcurrent relays RRID works
as a differential element with rectifier and a series impedance.
The supply of the relays was with silicon –rectifiers. The
RRIDs had no possibility for setup, nevertheless different
values could be achieved by changeover of the connectors.
In 1965 Oerlikon proposed a solution for coarse and fine
differential protection (Fig. 21). According to the rules in
Switzerland, Austria and Germany current transformers in
high voltage switchgears have to be earthed on the secondary
side. That is why the interposing transformers 8 were realized
in an wye/delta-circuit. In 1989 AEG produced the static
generator protection SQG. The choice of the characteristic
curve of the error-current stabilization was performed
by solder bridges. (Fig. 19). High impedance differential
protection such as the FAC produced by GEC in 1988 is quite
popular in the Anglo-Saxon language area (Fig. 20).
Turn-to-Turn Fault Protection
In case of interturn faults it is essential to switch off
the generator immediately due to local overload caused by
equalizing currents in the windings, especially in case of
several conductors in a single slot.
In transformers such a failure could be detected by the
Buchholz-protection, however this is not that easy to detect
in generators. The faulty line operates as a primary winding of
a transformer with short-circuited secondary winding. If the
phase of the transformer is equipped with 500 windings and
two of them are short-circuited the current is 250 times higher
than the normal current flowing through this phase (leackage
0
13
Interturn
fault protection
15
Basic Connections of generatordifferential, 1936
(R. Bauch, SSW)
16
BBC, 1945
G
S
R
PAC.SUMMER.2009
17
Simplified
Differential protecdifferential protection tion for generators with:
Delta-Connection, BBC, 1952
75
18
is not considered). It is not possible to detect this fault by a
Differential Relays TG3, BBC, 1952
differential protection because the currents at the beginning
and at the end of the winding are equal.
B. Bauch (SSW) patented in 1925 (DRP 432837) the
circuit shown in Fig. 13. The generator to be protected (G) is
connected to the “auxilliary inductor” (S). This is an image
of the generator and consists of a transformer with a primary
winding in star connected to the neutral of the generator.
The neutral points are connected via the relays R. At the
beginning these relays have been simple overcurrent devices.
The inductor was equipped with a delta connection. In case
of a turn-to-turn fault the phase-voltage decreased. The star
point of the generator moves, the star point of the inductor
moves with the impact of the delta-winding into the triangle
of the voltages. Since a third harmonic current flows in the
connection of the neutrals , R. Bauch used the wattmetric
relay R in Fig. 13 with two coupled systems connected to the
sinusoidal voltages U12 and U23.
Siemens used a circuit for interturn short-circuit protection
in 1936. The RA2 (c) worked with a chain of reactors (b). See
Generator differential protection,
Fig. 12. Details and characteristic of frequency (current limiting AEG, 1989
as a function of the frequency) is shown in Fig. 24, it was used
to keep off the third harmonic. A combined differential- and
interturn short-circuit protection (5 and 6) for generators with
two parallel windings 1 is shown in Fig. 26. Interturn faults
cause equalizing currents between the neutral points, flowing
though the equalizing winding 4 to the relays 6.
In the 1950s SSW used the circuit shown in Fig. 25. The
open delta winding was used for the interturn short-circuit
protection, connected to moving-coil relays with a rectifier
and a filter network (for the 3rd harmonic). The secondary was
realized as a wye connection. Measuring devices and relays
have been connected to the supporting coil.
For generators with two windings instead of the coils the
"double phantom circuit" was used (Fig. 28).
Over-Voltage Protection
An increase of voltage was dangerous especially at hydro
generators since it may result in huge increases of speed.
High impedance
Coarse and fine
In 1936 Siemens produced an “increase-of-voltage-relay”
differential protec- differential protec(RV5, Fig. 23). It worked properly for increases up to 200% of
tion, GEC, 1988
tion, Oerlikon, 1965
the nominal voltage. In 1984 Siemens produced a static relay
with two stages 7RE21-Z1 (Fig. 29).
When the short-circuit currents in the high voltage
grids became bigger this caused especially problems in
effectively grounded systems due to high fault currents for
phase-to-ground faults. A limitation was possible with
isolation of different neutral points of transformers. This
became common at unit transformers. In case of opening the
circuit breaker between the unit and the grounded grid (e.g. in
case of load-shedding) dangerous over-voltages could occur.
The first nuclear power stations in Switzerland (Beznau I and
II which NOK put into operation in 1969 and 1971) have
been equipped with a star-point breaker developed by AEG (4
in Fig. 27).
The effective power of both power stations together was
700 MW. Generators operated as one unit (1 and 2); four
19
20
21
PAC.SUMMER.2009
The choice
of the
characteristic
curve is
performed by
solder bridges.
22
Chain of
reactors for:
Interturn Short-Circuit
Protection RA2,
Siemens, 1936
PAC history
76
24
Circuit and characteristic of frequency,
Siemens
26
Combined differential and interturn
short-circuit protection
for generators with parallel windings, BBC, 1945
7
a
b
5
6
2
120
Penetrability of current%
100
80
1
60
8
40
3
20
0
0
20
40
60
80
50
a
Supporting Reactance
100 120 140 160 180
150
b Relays
three-phase transformers (220 MVA, 15,5/250 kV) supplied
into the 220-kV-grid. Neutral points on the high side have
been protected by lightning arresters (3) and could be earthed
by neutral earthing switch (4). They have been tripped at the
same time with the circuit breaker. With their operating time
of 19 ms they have been earthed before the contact separation
of the circuit breaker (Fig. 27).
Earth fault protection and other devices for generator
protection will be covered in a later issue of PAC World.
walter.schossig@pacw.org
www.walter-schossig.de
23Increase
of voltage
relay RV5
4
Hz
5 & 6 - A combined differential- and interturn short-circuit protection
1 - Generators with two parallel windings 4 - Equalizing winding 6 - Relays
27 Neutral earthing Switch, AEG, 1969
1
2
3
1 & 2 - Generator and Transformer
4 - Neutral earthing switch
25 Interturn short circuit protection, SSW, 28 Double phantom
um 1950
circuit for detection of
5
4
3 - Lightning arresters
5 - Circuit breaker
29 Increase of
voltage relay 7RE21
Interturn short-circuits for genera- Siemens, 1984
tors with parallel windings per Ph.
Siemens, 1936
G
G Generator
PAC.SUMMER.2009
S
S Open delta winding