PAC history
70
History is the tutor of life
(Moving the rotor Koepchenwerk, RWE, 1988)
PAC.MARCH.2010
by Walter Schossig, Germany
Protection
71
History
(Moving the rotor of a 265-MVA- generator
(station Waldeck II)
Biography
Generator
Protection
Rotor Earth Fault Protection
Failures in the rotor, caused by low exciting
voltage do not occur that often and single earth faults are
not that dangerous. But it was obvious, that in case of a second
breakdown of isolation, the turn-to-turn-fault exerts a force
on the axle. The detection of turn-to-turn faults is difficult.
With a low exciting voltage they occur in case of operation of
the machine only. Centrifugal forces and heating utilize the
winding mechanically and thermally.
This explains why a turn-to-turn fault occurs during
special load conditions and not if the generator was out of
service. Measurements are difficult due to a low resistance of
the rotor winding. Only a careful assembly of the winding was
a sufficient protection because the isolation was aged by the
de-excitation.
Earth Fault Protection
Max Pohontsch, Berlin, patented “Earth Fault Supervision
for DC Circuits” in 1928 (DRP 457323). See Figure 3.
Since rotor earth faults are not that dangerous, a simple
indication was considered sufficient. One terminal of the
DC circuit, supplied by a battery or a dynamo, was earthed
via a series connection with a measurement device or an
overcurrent relay, a capacity as DC barrier and the secondary
side of a small transformer. The primary side was connected
to a stable AC, e.g. the station transformer. AEG proposed a
simple scheme (Figure 4) in 1940. The whole rotor circuit was
shifted to an electrical potential of 30 up to 50 V (using AC
and a capacitor). In case of normal operation the current was
small, while in case of an earth fault the magnitude increases
and could be measured or trip the relay.
Rotor Earth Fault Protection SSW ("Siemens-SchuckertWerke") measures the currents with a current transformer
and a moving coil instrument operated with a rectifier (Figure
5). The setup range was between 10 and 50 mA and alerts if
the resistance to earth was less than 1000 Ohms. Auxiliary AC
voltage of 30…50 V was connected with a voltage divider to a
DC generator.
The auxiliary voltage had to be limited to avoid a danger
for the staff e.g. during checking of the brushings. That is why
Bütow used a wattmetric devices (for measurement or relaying)
as shown in Figure 1. The secondary side of the transformer
was earthed with a voltage coil (shown with dashed lines in
the circuit). The DC circuit to be supervised was connected
with the resistance R (200 Ω) and a capacity C (20 μF) on one
tap of the transformer. The voltage between the tap and the
PAC.MARCH.2010
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 Earth fault
supervision
(Bütow, 1932)
earth was appr. 40 V. The fixed coil with 5000 windings was
connected with a capacity of 4 μF and a resistance in parallel
to the secondary winding of the transformer (voltage 120 V).
With the capacity and the resistance in parallel the current
could be set up. The current was constant as long as the
primary voltage of the transformer was constant. The current
delivered the field for the electro dynamic device (required
170 mA). BBC produced a rotor earth fault device RBV in
1947 Figure 13. A scheme used in the United States in 1948
is shown in Figure 12.
In the 50s of the last century rotor earth fault detection
with alternating measurement of the voltage of the plus and
the minus pole to earth (Figure 7) was the common method
used.
Rudolf Ulbricht (GDR) proposed in 1951 a relay
implementation of this method (Patent 5278 Figure 8). The
idea was used in the REG5 of EAW. BBC produced the rotor
earth fault protection as shown in Figure 11.
If the generators have not been excited by rotating machines
but by the more and more used static exciting systems, the
protection was not sensitive enough. That is why improving
the system or development of other systems were required.
The reason was that the rectifier exciting systems caused
a ripple in the rotor current. This ripple caused in the
capacitance C in the excitation circuit currents with harmonics
which were responsible for false tripping. To avoid this, a
Ferraris measuring element has been used as rotor earth fault
protection PUM20. Polarized by the generator voltage, it
was stable against the ripple of the rectifier exciting currents.
This allows the detection of earth faults with earth contact
resistance of up to 1000 Ohms. Utilizing the PUM20, BBC
delivered the system “Compatrol” (Figure 14).
OERLIKON (CH) also used in the 1960s capacitors for
blocking. They provided the AC in the bridge circuit (Figure
6). The zone of protection was also 100% of the whole rotor
circuit; the winding itself and all coupled circuits as brushings
2 Rotor earth 3 Earth Fault Supervision,
fault protection REGL (AEG)
PAC.MARCH.2010
Pohontsch, 1928
4 Rotor earth fault protection, AEG, 1940
1. Voltage Transformer
2. Capacitor
3. Earth Fault Relay
G
2
3
1
G
_
~
and (if existing) an excitation system. Leopold Ferschl and
Franz Hofer at SSW in Vienna patented in 1964 the circuit as
shown in Figure 15 (AR 235933).
With a Ferraris system the CG30 relay (BBC) also detects
contact resistances of up to 1000 Ohms (1965, Figure 19).
Further developments have been the PUM201 (1968) and
the indication device ZUsw (1970) with ballast YZ/B2.
First it was just used to indicate the rotor earth faults and
afterwards, during the next maintenance the problem is being
fixed.
In the 1970 the first implementations have been used
for tripping too. The introduction of electronics allowed a
wider noise ratio and increased the tripping safety. SIEMENS
5 Rotor earth fault protection, SSW
73
Rotor earth fault detection
with alternating +/- pole to
earth voltage measurement
was common in the 50s.
7 Earth fault detection, 8 Rotor earth fault
1950
protection, 1951
Erarth Fault Detection with
Alternating Measurement if
the Voltages
Ulbricht-Principle
produced an electronic rotor fault protection in 1972.
Harmonics that occurred in thyristor controlled stations
(150…300 Hz at 500 V) had no impact on the sensitivity
anymore. The startup value could be set between 0.1 and
10 kΩ and detected earth faults without a “dead zone”. See
Figure 9.
Westinghouse produced the Field Ground Detection Relay
DGF in 1979 (Figure 17 and Figure 18).
Rotor earth fault protection 7RU21 produced by SIEMENS
in 1984 is shown in Figure 21.
For big units and static excitation systems with huge
harmonic distortion values the two-stages system 7UR22
with measurement of the resistance was provided (Figure 22).
To measure failures with high impedances a DC voltage
has been used between the excitation circuit and the earth.
The current IE was the measure for the contact resistance RE
(Figure16). The DC was synchronized with a low-frequency
clock with changing polarity.
AEG produced the static rotor earth fault protection SLG in
1992 (Figure 29).
Figure 25 and Figure 27 show the circuit and view of the
MRSU 04 by GEC Alsthom (1993). It worked with frequency
and voltage (4,75 Hz, 24 V).
The principle of the 7UM62 (SIEMENS) is shown in Figure
26, while the principle of REG216/316*4 with injection
unit REX010 and injection transformer unit with auxiliary
contactor REX011 (ABB) is in Figure 20.
R
r
R
~
T
V
V
V
9 Electronic rotor earth fault protection, 1972
Figure 9:
Schematics of
the Electronic
Rotor Earth
Fault Protection,
SIEMENS, 1972
6 Rotor earth fault protection, OERLIKON 10 Rotor earth relay REG5 11 Rotor earth fault
OERLIKON, CH, 1966
EAW, 1966
protection, BBC, 1940
PAC.MARCH.2010
PAC history
74
14
Rotor earth fault protection system
„Compatrol“, BBC, appr. 1960
Double earth faults in the
rotor are very
dangerous .
16 Principle
7UR22, Siemens, 1984
12
Fieldground detector schemes
(USA, 1948)
NARI (China) uses an injection scheme according to
Figures 28 and 32. The relay injects a square wave voltage
between one end or the two ends of rotor windings and the
rotor shaft, measures the earthing current and calculates the
earthing resistance, performs rotor single point earth fault
protection. During the dual-end injection, it can calculate the
position of rotor earth fault.
For example, on 12th September 2007, in LongTan
hydro-power plant, Guangxi of PRC, No.1 when a Generator’s
rotor earth fault occurred, the relay alarmed properly and
13
Rotor
earth fault
device RBV
indicated correctly that the earth fault position was lying at the
negative end of the rotor.
Rotor double earth fault protection
Double earth faults in the rotor are very dangerous. The
fluxes in the rotor became different and radial forces occur,
causing dangerous vibrations which could destroy the
generator. Harms, Berlin, demonstrated in 1949, that at a
4-pole-rotor the bypass of one pole could cause a magnetic
force on one side as high as 4-times the weight of the rotor.
Especially the limitation of generation in Eastern Germany
15 Circuit of Ferschl and Hofer, SSW, 1964 17 Field ground detection relay DGF
Westinghouse, 1979
(BBC, 1947)
Figure 2
Figure 1
PAC.MARCH.2010
Westinghouse
produced the
Field Ground
Detection Relay
DGF in 1979.
75
required an operation of generators during a single fault.
Rudolf Ulbricht, GDR, presented in 1951 his solution under
Patent 5278. See Figure 30.
Precondition was a solid rotor earth fault. In the bridge
connection the resistance R was defined in such a manner,
that the double-earth-fault relay was without a current
(mA-pointer). In case of a second earth fault, the relay A trips,
switches off the generator and de-excites the machine. The
bridge current was taken from the axle (“Welle”) directly. The
relay itself was a sensitive polarized DC-device with contacts
on both sides because the bridge current could be positive or
negative (depending on the 2nd earth fault). If everything was
dimensioned very well, double earth faults within 10…15%
of the winding could be detected.
The sensitivity depends on the exciting voltage. The
mA-pointer indicates starting failures already at their
beginning. The device also detects earth faults in other
equipments connected. The same scheme has been used in
Western Germany, the United States and in the Soviet Union.
In normal cases this protection was not a part of the standard
configuration; it has been used after a rotor earth fault. The
tuning was done during operation. Figure 23 shows the REG6
of EAW.
During the commissioning of the first 500-MW-unit
in Eastern Germany (Hagenwerder, 1974) and in later
installations (as Jänschwalde 1981) rotor double earth fault
protection built in the Soviet Union was used, (the device
K3P-2Y4). See Figures 24 and 31. The sensitive measuring
element has been two anti-parallel connected polarized relays
∏P1 and ∏P2. It was used as a built-in or mobile device (for
several generators).
In most countries double earth fault protection was not an
issue; in case of an earth fault the generator is no longer used.
19 CG30, BBC, 1965
20 Two pole rotor connection
Figure 20:
Two pole rotor
connection,
REG216/316*4
with REX010/
REX011, ABB
The history of generator protection will be continued.
walter.schossig@pacw.org
www.walter-schossig.de
18 Field ground detection relay DGF
Westinghouse, 1979
21 7UR21
(Siemens, 1984)
22 7UR22
(Siemens, 1984)
Linear resistor
Moving coil
connections
DC Contact
making
milliameter
Adjustable
stationary
contacts
Moving
contacts
PAC.MARCH.2010
PAC history
76
25
MRSU 04; GEC, 1993
29 Static relay SLG,
AEG, 1992
26 Connection Diagram 7UM62, Siemens
Ie
G
3~
UH
CE
RM Earth fault resistance
23 REG6,
EAW, 1956
RE
RV Coupling resistor
27 MRSU 04; GEC, 1993
UM
UH Auxiliary voltage
Ucontrol
RM
UM
Digital
protection
(7UM62)
calculation
of RE
UM Measuring shunt
CE Rotor capacitance
30 Scheme of Ulbricht, 1951
31 K3P-2, USSR,
1974
24 K3P-2,
USSR, 1974
28 Dual-end injection scheme, NARI, 2007 32Single-end injection scheme, NARI, 2007
PAC.MARCH.2010