PAC history
70
History is the tutor of life
Hydro Power Station Spichra, Thuringia (Germany)
with 3-phase-syncro generator 1250 kVA, 5250 V, 1925 (Picture 1998)
PAC.AUTUMN.2009
by Walter Schossig, Germany
Protection
71
History
Limiting the Damages
Biography
Generator
Protection
Stator Ground-Fault Protection of Generators
The first protection devices for overload, short-circuit, overand undervoltage, as well as differential and reverse-power
protection were described in the last issues of PAC World.
Another possibility to limit damages was made available by
the introduction of ground-fault protection.
The magnitude of the expected fault current depends on
the type of star point grounding of the generator. If the neutral
is earthed, fault currents are as huge as short-circuit currents.
In case of isolated neutral point, only earth-fault currents or
even residual currents will occur. The magnitude also depends
on the location of the fault in the winding.
One of the first forms of supervision of earth faults in
DC power stations was realized with bulbs (Figure 3). If the
red light turned off, an earth-fault or an interruption in the
connection to the plus-pole was the reason. If the blue light
turned off - the minus pole was involved. If both lamps were
off - a short circuit or an interruption occurred.
If the isolation of a winding became faulty, an arc to the
earthed ground of the machine occurred. The overcurrent
circuit breaker tripped at a value between 1.5 and 2-times
of the nominal current of the machine. Due to that, an arc
could develop at huge currents without a trip of the main
circuit breaker. Very often due to strong air flow caused by
the machine the arc remained for a long time and the breaker
tripped too late.
Emil Neumann proposed in his patent DRP 314415 in
1919 to isolate the machine against earth and to realize the
earthing with a certain connection, (Figure 4) a resistance,
a reactor or a current transformer. In case of a failure in the
isolation in the isolated machine a current flows between the
winding and the cabinet, causing a voltage on the clamp. Even
a small problem in the isolation caused a small current and a
small voltage that could be used for indications or alarms. In
case of a further increase of the current, the generator would be
switched off. In case of a separately excited machine, the patent
describes the usage of a resistance in the excitation.
Different approaches have been developed in Europe
and the United States for neutral grounding in AC grids.
Solid or low-resistance grounding was not that popular in
Europe due to the large earth-fault currents and even in case
of fast earth fault protection the stator lamination could
melt (“core-burning”). Isolated or high resistance grounding
was the preferred choice. Dr. Piloty, AEG, described in 1929
different approaches regarding neutral earthing and proposed
PAC.AUTUMN.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 Frame-leakage protection
RW7e
(Siemens, 1938)
2 Annular
Core Current
Transformers
4
recommendations for the estimation of grounding resistances.
Housing earth fault protection, Neumann,
He mentioned the advantage of voltage dependent resistances
1919
at this time.
Wattmetric earth fault protection of generators was
difficult, especially in the case of small earth fault currents, and
if more than 30 % of the winding should be protected. Thus
C B
solutions had been needed to increase the fault current to
A
allow a measurement with full neutral-displacement voltage,
but avoiding core-burning. The common schemes are shown
L
in Figure 5.
SIEMENS preferred scheme a) BBC contactors as b). These
R
contactors switched on a bigger resistance depending on the
neutral voltage.
In 1924 Walter Bütow, AEG, proposed to increase the
N M
G
sensitivity of the earth fault protection in the neutral of the
generator with voltage and current depending resistances
(“Iron –Resistance”, Figure 6, DRP 458137). It is also
D
known as „Bütow-Protection“. Figure 7 shows a generator
protection board with the resistances on top. The number of
used lamps was such that for the selected zone of protection
e
90% of the earth-fault currents had been less than 5 A to avoid
core-burning.
The self-supervision was realized with a built-in 15). The generators were mounted on concrete fundaments
pushbutton (Figure 7 below lamps). Pressing the button with big earth-contact resistance often occurring. They were
caused a flash on of all lamps. Another possibility had been connected with the high-voltage earth (protective earthing)
carbon-pile regulators, making possible a characteristic such and in this way with the cable jackets of the grid.
as c).
This earth connection had been equipped with an
Courtin, AEG, proposes a realization of generator stator wattmetric relay in an inductive parallel connection.
ground-fault protection with iron ribbon-core resistances
If the earth-fault current of the grid connected was not
(Figure 9). The generator protection developed in the early sufficient to trip the frame-leakage protection, a ground-fault
stages was sensitive and stable, nevertheless false tripping reactor was connected to the busbar. The secondary
sometimes occurred. They spoke about a “general disturbance delta-winding was under load with a resistance. The reactor
in the grid” and started huge investigations and developments and the coil were required only once for all generators
in the 1920s.
connected to the same busbar (Figure 8).
One proposal for a cheap and selective protection for small
To realize a frame-leakage protection with constant
machines was made by H.Seel in Leipzig in 1962 (Figure relay-voltage and linear-depending torsional relays Siemens
3 Earth fault supervision scheme
(USSR, 1937)
5 Limitation of earth fault currents
iE
iE
U0
U0
iE
U0
U2
G
U1
iE
a)
b)
iE
c)
iE
G - Generator
PAC.AUTUMN.2009
- Lamps
U0 100%
a - Constant
Resistance:
U0 100%
b - Tapped
U0 100%
c - Ferrum-Hydrogen (FeH)
73
Wattmetric relays were
introduced in the 1940s
for protection of generators.
developed a “selector relay scheme” for star connected
windings (Figure 13). In case of a failure, the frame-leakage
relay received a constant alternating current instead of the
varying zero sequence voltage. The selector d delivered to the
relays a voltage, which was dependent on the faulty phase over
the transformer i. To increase the sensitivity, Siemens replaced
the permanent magnetic field of a moving coil with a stronger
electrically produced DC-field. Wattmetric relays with current
and voltage paths were used (Figure 1).
Direction-of-power relays with error current stabilization
were used in the Soviet Union in 1933-1945. The first
realizations of these relays protected 92, 87 or 74 % of the
length of the stator winding (values valid for generators with a
nominal voltage of 10.5, 6.3 and 3.15 kV. The sensitivity was
selected with the precondition that the end of the protection
area near the generator neutral was 500 Volts. This value was
not sufficient to supply a lightning arc. The zero sequence
currents were created in a Holmgreen – a scheme that caused
high error currents depending on the magnetic characteristic
of the measurement transformer. That is why bushing-type
current transformers for cables with a clamp-on core were
developed in the USSR in 1937 (Figure 2).
After the Second World War, the Soviets developed
and used bushing-type current transformers with auxiliary
magnetization (Figure 12). Such a transformer consists
of identical iron cores. Due to the long rectangular shape
of the core it was possible to pull several three-conductor
7 Generator protection Bütow, AEG,
approximately 1940
Lamps were
used to
determine
the selected
zone of
protection.
8 Stabilized differential & frame-leakage
protection for generator busbar operation
Siemens
9 Stator
ground-fault protection Courtin, AEG, 1934
Realization
Scheme
6 Wattmetric earth-fault detection using FeH-Resistances and a Holmgreenscheme (1930)
TR
EW
G
S
NSp
E
R
N
NST
NT
ZT
k 28426
ER - Earth Fault Relays
SpR - Voltage Relays
EW - Iron Ribbon-Core Resistance
NSp - Neutral Voltage Transformer
NT - Neutral Transformer Resistance
NST - Neutral Current Transformer
PAC.AUTUMN.2009
PAC history
74
12
Bushing-type current transformers
with auxiliary magnetization, USSR
10
Directional earth-fault
relay R1W3
(Siemens,
approximately 1950)
cables through one core. This could be done in a row or as a
chessboard (see Figure 12).
The auxiliary winding consists of two sections, supplied by
an independent AC-source (as a generator voltage transformer).
Since the two sections were connected in series, (the excited
magnetic fluxes are opposing), this setup compensates the
electromagnetic forces.
Using ferromagnetic alloys with big magnetic permanence
(as Permalloy) further increased the sensitivity and decreased
11
Stator
ground-fault
relay
13 Selector relay scheme, Siemens,
approximately 1930
Bushing-type current
transformers
allowed the development of
new protection schemes.
error currents. Using these bushing-type current transformers
allows operating stator-ground fault protection with currents
up to 5 A. Such small currents required sensitive current relays
with low consumption of auxiliaries. In 1948/49 engineers in
the Soviet Union started a series of tests with high sensitive
current earth fault-relays. They were developed for generators
with cable connections (Figure 14). The toroidal-core current
transformers are made of Permalloy. One transformer was
used for one cable. In case of several cables, the transformers
were connected in parallel. To connect them in series was
also discussed. However, in that case the disadvantage was
large inductions in the core in case of unsymmetrical primary
values. The current relay itself was connected via an amplifier
to the current transformer, so relays with normal friction loss
could be used. The sensitivity alternates between 2.5 and 5 for
ten to twelve cables. The whole developments in the USSR
were driven by Prof. Dr. G. I. Atabekov (Moscow).
Siemens used high sensitive moving-coils directional
relays R1W3 in the 1950s (Figure 10). They were connected
to cable-type current transformers 60/1 A and were supplied
with the line-to-line voltage and the displacement voltage.
Utilizing cable-type transformers was no longer possible
with large generators and infeeds with busbars. Stabilization
and adjustment of CTs allowed usage of Holmgreen schemes.
This scheme consists of three current transformers with a
ratio of 100. The secondary sides are connected in parallel.
The precondition was an adjustment of the three transformers
14 Stator ground-fault protection, USSR,
1949
Stator
Ground-Fault
Protection
with
Toroidal-Core
Current
Transformers
and
Amplifieres
(RERZG, EAW, 1959)
PAC.AUTUMN.2009
75
with equal characteristics to avoid false tripping of the
earth-fault protection.
An auxiliary resistance was connected to increase the
sensitivity of the wattmetric earth fault protection during
startup of the generator. Using wattmetric stator ground-fault
protection required the vector of the displacement voltage.
In case of sustained ground fault and during transients, very
often false tripping occurred. Investigations of these false
trippings were very difficult, because the behavior of the
relay during stationary conditions was perfectly fine. Another
disadvantage of the wattmetric relays was the decreasing
torque with the square of earth-current and -voltage. To gain
linear dependencies special methods were developed, such as
voltage-stability, selection of voltage or selecting a single phase
of an artificial three-phase system.
A pure current relay was much simpler. If this relay
would use the fault current from a core-balanced CT without
consideration of voltage, it would false trip. Bruno Westphal,
Siemens, used a simple compensation scheme in 1950. A
small interposing transformer was used in the neutral return
system. The secondary side supplies a rectifier connected
with the genuine earth-fault relays. A zero sequence current
caused a trip, while the compensation current caused a
torque in blocking direction. In case of a generator fault the
compensation was weak (Figure 18).
The Swiss “Maschinenfabrik Oerlikon” patented a scheme
for sum current transformers with common magnet core.
This scheme used the principle of pre-magnetization to
increase sensitivity (Figure 17). The core consists of two
parts. The winding 9 is excited in such a manner that the
fluxes in the main winding are compensated. The current
source for excitation of winding 9 is the tertiary winding of
the three-phase reactor 6. In this reactor there is only current
in case of an earth-fault. An advantage of this scheme was
that in case of a fault the sensitivity is increased due to an
improvement in the magnetic characteristic of the CT. The
scheme is stable in case of no earth faults.
EAW produced wattmetric directional earth-fault relays
RERZG in 1959 (Figure 11).
To minimize the neutral point transformer f3 (Figure 21)
in case of an earth-fault detected by (e2), the FeH-resistance is
disconnected via a relay’s c1 after 2 seconds. The fault inside
the generator is cleared by e1 with a delay of 0.5 seconds. The
buttons b1 … bn were used for testing of the FeH lamps
(Figure 16).
Westinghouse presented the directional negative sequence
relay for ground protection CRG in 1974. Figure 20 shows
the discriminating element. Blackburn,J.L. proposed another
Westinghouse solution in 1977 (Figure 19). The CWC as
presented in 1973 is shown in Figure 22.
A special scheme to connect stator ground fault relay
presented by BBC shows Figure 25. To receive the zero
sequence current the transformer T6 is equipped with
additional secondary windings 3p-3q in every phase on
the short-circuited core 3k-3l. The reference voltage is the
phase-phase voltage of the busbar voltage transformers.
15 Stator ground-fault protection, Seel, 1962
16 Baretter Lamps FeH
Baretter lamps
change their
resistance as
a function of
their heating.
17 Stator ground 18 False current stabilization,
fault
Westphal
(Oerlikon, 1953)
(Siemens, approximately 1950)
+
PAC.AUTUMN.2009
PAC history
76
21 Scheme RERZG and REG5, EAW, 1959
a1
23 SEG, AEG, 1989
x
RERZG
f1
e1
G
3~
f2
e2
f5
g1
f4
c1
f3
f4 and f5
8/0,15 A
U > REG5
L1
N
b1
5x
b2
bn
4x10x
5x FeH
e2 Earth fault detection b1...bn buttons for testing FeH lamps
19
Blackburn' s
proposal
(Westinghouse, 1977)
A generator ground stator protection with third
harmonic was developed by Westinghouse in 1981 (Voltage
Comparator, type DGSH, Figure 24). The CV-8 (Figure 26)
filtered the 3rd harmonics and reached a sensitivity of 8%.
The stator ground-fault relays SEG presented by AEG in
1993 used the earth-fault current or the unbalanced residual
current for the estimation of the direction of earth-fault
(Figure 23). The adaptation of the current was utilized by the
coupling measuring W01 or WE115.
W01 / WE115 Coupling measuring the current
24 Generator ground stator protection
DGSH, Westinghouse, 1981
Generator ground stator protection with third harmonic
25 Relays
DPX113, BBC
(Scheme for Connection of Stator
Ground-Fault, 1978)
Stator ground-fault protection for generators with
unit-transformers and other apparatus will be discussed in a
later article.
walter.schossig@pacw.org
www.walter-schossig.de
20
Directional
negative sequence relay
for ground
protection
22 External schematic of the ground differ- 26 Scheme CV-8,
ential protection CWC, Westinghouse, 1973
(CRG, Westinghouse,
1974)
PAC.AUTUMN.2009
Westinghouse, 1987