Download Sensitive Stator and Rotor Earth Fault Protection at Hydro Generators

Sensitive Stator and Rotor Earth Fault Protection at Hydro Generators
Dr. Hans-Joachim Herrmann
Siemens AG, PTD EA13
Diego Gao
Siemens Power Automation Ltd.(SPA), TS
Nuremberg, Germany
Nanjing, P.R.China
Large hydro power plants are designed in a unit connection. The neutral point of the generator is
isolated or high ohmic earthed. The probability of a phase to earth fault - the stator earth fault - is
much higher than the other fault types. To detect this type of fault within a generator is one of the
important protection tasks. This contribution focuses on the stator earth fault protection in general
and highlights a principle which covers 100 % of the stator winding.
By virtue of the galvanic isolation of the rotor, an earth fault in the rotor leads to very small fault currents and doesn’t damage the rotor. If a second earth fault occurs at another location in the field
system, the current will be flowing through the affected turns. The field current from a large machine
can be high causing serious damage on the rotor and the exciter. If a large part of the field-winding
is short-circuited, the flux may result in a mechanical force which is strong on one pole but weak on
the opposite one. The result is an unbalanced force which in large machines may be of the order of
50-100 tons. This causes violent vibrations which may damage bearings or even displace the rotor
to damage the stator. Therefore it is important to design the protection as sensitive as possible.
Keywords: Stator earth fault protection, 100% protection range, 20-Hz-injection method, rotor earth
fault protection, sensitivity, numerical protection
1. Stator Earth Fault Protection
1.1 Basics of stator earth fault protection
Figure 1 shows a typical design of a unit connection with the components generator, step-up transformer and auxiliary transformer. Depending on the plant configuration, a generator circuit breaker
may be additionally available. The possible location of an earth fault is marked by the circle in figure
1. Transient overvoltages involved with isolation ageing lead to earth faults predominantly on the
terminal side of the generator. Mechanical problems can also cause isolation damage and an earth
fault arises out of that. The fault location can be anywhere in the stator winding, and an earth fault
close to the neutral point of the generator is possible, too.
To minimize the damage of the stator core and winding the fault current is normally limited up to 10
A and in worst case situations up to 20 A (see figure 2). The fault current is determined by the sum
of the earth capacitances on the generator side and the load resistor.
Possible region of
earth faults
Step-up
transformer
Auxiliary
transformer
Figure 1:
Typical plant design
Figure 2: Influence of fault current and duration on
damage of a stator core
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In praxis there exist two ways of calculating the necessary load resistance RL. In literature [1] the
argumentation starts with the possibility of transient overvoltages during intermittent earth faults at
isolated generator star points. A grounding of the star point only serves for limiting these transient
overvoltages, but a compromise must be found between the permitted fault current and the allowed
transient overvoltage. The general design rule describes equation (1).
RL 
with:
1
(1)
ω  CE
CE
sum of the earth capacitances
According [1] this design limits the transient overvoltage in the healthy phases to a maximum of 2.5
times of the phase to earth voltage.
Another argumentation in literature [2] highlights the risk of an overfunction by the stator earth fault
protection due to an earth fault on the high voltage side of the step-up transformer. The displacement voltage on HV-side can lead to a disturbance voltage on generator side via the capacitive
voltage divider according figure 3.
CCoup.
U0, distr.
Figure 3:
CE
U0,HV
Disturbance from the high voltage side
The load resistor on the generator side minimizes the disturbances influence of the displacement
voltage from the high voltage side and determines the protective range (80%, 90% or 95%) which
can be covered.
For typical unit connections both calculation methods lead to nearly identical load resistances, determined by the given earth and coupling capacitances.
Example:
90 % protection range: K = 0.1;
Generator voltage: UN;G = 20 kV; Network voltage: UN,Net = 550 kV ; fN = 50Hz
CE = 1.4 F/phase; Ccoup = 10 nF
1
Method 1:
RL 
Method 2:
IC  Fearth 
ω  CE

1
Ω  758 
2  π  50  3  1,4  10-6
U N,Net
3
 ω Ccoup  0.8 
550  103
2  π  50 10 10-9 A  0.8 A
3
1 U N;G
1
20  103
K
0,1
2
3 2
3   721 
RL 
IC
0,8
The factor Fearth =1 is used for isolated networks. For safety reasons the factor 0.8 (possibility of
open transformer star point) is recommended in solidly earthed networks. Normally the actual displacement voltage will be lower and the required load resistor may be higher than calculated.
2
The final discussion to this topic is the location of the load resistor. Normally a secondary resistor
with a transformer is used. One way is the connection on the neutral side of the generator with a
neutral transformer. This cost optimized solution lives with the risk of failures on the high voltage
side of the neutral transformer due to transient overvoltages, e.g. lightning overvoltages. Earthing of
the generator is then possible. Therefore this application requires unconditionally a 100% stator
earth fault protection.
A different preferred solution is an absolutely free neutral point of the generator. Nothing is connected on generator star point and this minimizes the additional risk of unintentional earthing of the
star point. The secondary load resistor is connected on a five-limb earthing transformer which is located on the generator main leads (see chapter 1.3, figure 5).
1.2 Protection based on the displacement voltage
The measurement of the displacement voltage on the generator side is the standard application of
stator earth fault protection at unit connection schemes. Due to harmonics (especially the 3 rd harmonics) and the requirement for a high sensitivity the measurement of the fundamental frequency
should be used. With a correct selection of the load resistor a protective range of 90% or 95% can
be reached. There exist no limitations from the numerical relay. An accurate measurement of a low
voltage is always possible.
In applications at pump storage power stations the protection should cover a wide frequency operating range, especially at starting conditions with U/f = const. Only the frequency tracking guarantees
the same accuracy over a wide frequency band. In [3] this function is active from 11Hz.
1.3 100%-stator-earth-fault protection with 20-Hz-injection method
Due to the risk of an earth fault close to the generator star point a secure protection principle is required and shall be based on a different measuring method to avoid common mode failures. A further requirement is the independence from the operating conditions of the generator. It was found
out that the risk of an earth fault during a startup process or after refurbishment is sometimes higher
than at continuous operation.
During the construction of the large power plants in Germany in the seventies the utilities demanded
a secure measuring principle. The injection principle based on the 20-Hz-frequency was developed
and within the numerical technology permanently improved.
The basic principle is explained in figure 4. A 20-Hz-voltage with a low magnitude (< 1% of rated
generator voltage) operates as an electromotive force (e.m.f) on the neutral point of the generator.
In case of an earth fault a closed loop exists and a 20-Hz-fault current flows. The detection of the
earth fault is independent of the fault location in the stator winding. A further feature is the activation
of the protection before a plant goes into service. The 20-Hz-generator must be switched on and
then the protection becomes automatically active.
max.
200 V
20 Hz
RE
I
Figure 4:
Basic idea of 20-Hz-injection method
3
Figure 5 represents a more detailed drawing with the necessary additional accessories. The band
pass smoothes the 20-Hz-square voltage and additionally protects the 20-Hz-generator in the case
of an earth fault with a full displacement voltage. The 20-Hz-e.m.f. on the load side results from the
voltage divider: load resistor and 20-Hz-series resistance of the band pass (8). Therefore a design
requirement for the load resistor exists. The resistor shall be higher than 0.5 (worst case 0.3). In
this case the secondary injected 20-Hz-voltage is close to (0.5/8.5)*25 V  1.5 V. The requirement for a higher secondary load resistor can be easy fulfilled, if a higher secondary voltage on the
neutral transformer is used or the primary voltage is the same as the phase to earth voltage of the
generator (saturation degree shall be according the rated generator voltage). A reduced transformer
ratio by the factor of 3 automatically leads to a 3 times higher load resistor.
Earthing transformer
Low ohmic
Bandpass
(8 at 20 Hz)
20-Hz-Generator
(appr. 25 V)
a
DC or AC
RL
b
Blocking
400A
5A
Relay
U
a
Neutral
transformer
Figure 5:
I
b
Practical design of the 20-Hz-injection method
The numerical protection samples the injected voltages and measured current. With FIR-filters the
20-Hz-components are calculated and further on a special “failure replica” for determination of the
fault resistance is used. The advantage is a higher sensitivity against the pure 20-Hz-current measurement. Especially at large hydro generators, due to the larger stator earth capacitances, a higher
capacitive 20-Hz-current will be measured under fault free conditions. In that case there exists no
safety margin for a reliable setting of a 20-Hz-current stage. The used model in [3] eliminates the
disturbance influence of the earth capacitances. The literature gives additional setting recommendations and describes the commissioning procedure.
1.4 Application on different plant conditions
A standard protection philosophy for large power plants is the design of two protection groups with
independent principles for the same protection task. At least the displacement voltage should be
available in one group and in the other group the 20-Hz-injection method.
Both principles shall be additionally controlled by a frequency tracking. In a transient operating
range of a generator between 10 Hz and 40 Hz a low zero sequence frequency close to 20 Hz is
possible from the generator side. Both signals, one from the generator and other from the 20-Hzgenerator, can superimpose like a power swing and lead to an overfunction of the 20-Hz-protection
(unwanted pickup). Therefore the 20-Hz-protection will internally be blocked temporarily. This feature is very important at pump storage power stations during startup (may be also via starting with a
static frequency converter) and also during braking.
Figure 6 summarizes both, the redundancy and the frequency operating range.
Finally the performance of the 100% stator earth fault protection will be explained with an example.
During commissioning of a large hydro power plant an additional device for insulation measurement
was installed. This device failed temporarily and caused a transient stator earth fault. Figure 7
shows the stored fault record in the protection relay. The transient fault conditions can clearly be
recognized and the binary traces on the bottom show the pickup and the trip by the protection.
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Frequency operating range
0Hz
10Hz
40Hz
70Hz
100%-Stator earth fault protection with 20 Hz injection
100 % Stator earth fault protection with 20-Hz- injection
90% Stator earth fault protection with displacement voltage
90% Stator earth fault protection with displacement voltage
Figure 6: Redundancy and frequency operating range
Figure 7: Fault record of a transient stator earth fault
200
200
150
R in Ohm
REm
k
SES100Aus
k
100
86
GAnr
k
60
50
0
0
100
200
300
400
k ta
t in ms
100
500
600
700
687.5
Resistance
Trip
Pickup
Figure 8:
Offline calculation of the earth fault resistance form the fault record
With this fault record the measured fault resistance could be calculated offline and figure 8 shows
the result. The measured resistance was for a certain time below the threshold which is marked by
the dotted lines (pickup at 60 and dropout at 86 - these are secondary setting values).
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2. Rotor Earth Fault Protection
2.1 Protection principles
As mentioned in the introduction a sensitive rotor earth fault protection is also required for large
generators. Two protective stages, one for alarm only and one for trip, are necessary due to the risk
of a double earth fault and the possibility of big damages on the generator side.
From the measurement point of view there exist two criteria for limitation of the sensitivity. One is
the rotor earth capacitance and the other comes from the excitation system as harmonics in a wide
bandwidth. These influences affect the accuracy of the protection.
The excitation system is isolated and in the case of a single rotor earth fault there is no closed loop.
For this reason an external source is necessary. One wide spread application uses the generator
voltage from the secondary side of the voltage transformers itself as an AC-source. The connection
to the rotor circuit is realized via coupling capacitors. For this coupling, there exist in practice different designs. One option is shown in figure 9. The rotor earth capacitances cause a leakage current
and limit the sensitivity if only a current measuring method is used. A fault model consisting of fault
resistance and rotor earth capacitance eliminates one disturbance source and yields a higher sensitivity. With the measured voltage and current the fault resistance can be calculated and sensitivity
up to 30k can be reached. However disturbances from the excitation side limit this advantage in
most applications. Practical settings for an alarm stage are therefore around 10 to 15 k.
Rotor
RCoup CCoup
from
secondary
side of VT
CE
RE
U
Connection on
earthing brush
Figure 9:
Relay
I
Connection principle of the AC injection method
The alternative solution - low frequency square wave principle (LF-principle) - unites both, the elimination of the rotor earth capacitances and the attenuation of disturbance influence from excitation
equipment side. At low disturbances a sensitivity up to 80k can be reached without any problems.
For detailed discussions see the next chapter.
A parallel operation of both principles which were explained reduces the sensitivity on both sides,
because the principles influence each other. The AC principle measures the coupling resistors from
the LF-principle in parallel with the fault resistance (see figure 11). On the other side, the coupling
capacitances of the AC principle operate as an additional large rotor earth capacitance for the LFprinciple. These capacitors can not be fully charged and therefore they limit the sensitivity.
A usual practice is the permanent application with one measuring principle. In the case of a detected failure in the protection by the self supervision a change over to the second injection principle will
be executed manually or by logic (see figure 10).
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7XR6004
Connection on
earthing brush
Figure 10:
7XT71
Rotor
Ucontrol
7XT71
Change
over
contact
Ucontrol
Umeas
Umeas
Group 1
7UM62
Group 2
7UM62
Redundancy via a change over [3]
2.2 Sensitive rotor earth fault protection with square injection method
This well proven measuring principle was successful used in the analog static relays and was
adapted for numerical applications. The basic principle with the necessary equipment is shown in
figure 11. A low frequency square wave generator with a frequency range between 1 to 3 Hz charges the rotor earth capacitances via a series resistor (RS). After these capacitances are fully charged
there can only flow a current determined by the fault resistance. With the known voltage source and
the coupling resistor the rotor earth resistance can be calculated. To avoid over functions due to DC
offsets caused by excitation voltage the protection uses only the difference voltage. The rotor earth
capacitances will always be charged in both directions. If two successive measuring results are the
same, the measuring is declared valid. This minimizes measuring errors if there is a change in the
excitation voltage.
Uout
Rotor
RS
t
Uout
Ucontrol
40 k
CE
RE
40 k
RM
Umeas
Relay
Umeas
RE = infinite
U ~ 0
t
 =RSCE
Connection on
earthing brush
Umeas
U ~ 1R
t
Figure 11:
Principle of square wave injection
Through the used low frequency the noise margin to harmonics of the excitation equipment is much
higher compared with the AC method and therefore the disturbance influence is lower. As mentioned before, a sensitivity up to 80k can be reached. The normal setting for the alarm stage is between 40 and 80k. The trip stage is set approximately at 5k.
Figure 12 gives a feeling for the measuring principle. This was recorded during a commissioning on
a 1000 MVA generator with rotating diodes. The first curve shows the control voltages. This voltage
is used for triggering the measurement and calculating the actual e.m.f.-voltage. The second curve
shows the voltage drop via the measuring shunt R M of figure 11. The change in the square waves
indicates different fault resistances which were simulated by test resistors. On the bottom there are
the binary traces with the pickup and the trip signals. The trip delay was set to 0.5 s.
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Figure 12: Record during a commissioning test
A standard question is always the connection to different excitation systems. With static exciters a
symmetrical connection is recommended. An asymmetrical connection is also possible, but the series resistors or capacitors must always be connected in parallel. Figure 13 summarizes the different ways of interfacing.
Rotor
Excitation
machine
50/60 Hz
1 - 3 Hz
a) Rotating Diodes
50/60 Hz
Rotor
Excitation transformer
1 - 3 Hz
b) Static Exciter
Figure 13: Connection to different excitation systems
Finally the testing will be briefly explained. The self supervision function in the numerical protection
detects failures in the injection equipment. But there are situations that not all broken wire conditions can be detected. May be there is an interruption on one series resistor. In this case the feeding is only via one resistor (40 k), but the assumed resistance in the firmware is 20 k instead of
actual 40 k and leads to a measuring error. Therefore the implemented test function shall be activated. With the known external test resistor a broken wire condition can reliably be detected. The
available CFC-logic allows an automatic test whose activation is possible manually or from remote
control. For more details see [3].
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3. Summary
The contribution dealt with two important topics for hydro generators, the stator and rotor earth fault
protection. These protection principles are basic functions in each large power plant. At hydro power plants there exist special operating conditions particularly in pump storage power station applications.
One major attention in the paper was the sensitivity and the protective range which can be covered.
It was clearly shown, that the 100 % stator earth fault protection with the 20-Hz-injection method
covers the full stator winding and is nearly independent from the operating conditions of the generator.
The rotor earth fault protection with a low frequency square wave injection is characterized by a
high sensitivity. High ohmic rotor earth faults can be detected. Two stages with different settings
shall be the standard, one for alarm and the other for trip.
With practical examples the performance of both protection functions was shown.
4. Literature
[1]
IEEE Guide for the application of neutral grounding in electrical utility systems.
Part II – Grounding of synchronous generator systems, published by IEEE, New York, 1989
[2]
Planning Machine Protection Systems, Siemens application guide,
No. E50400-U0089-U412-A2-7600
[3]
SIPROTEC 7UM62 V4.6, Multifunction Generator, Motor and Transformer Protection
Relay, Manual No C5300-G1176-C149-4
Author Resume
Dr. Hans-Joachim Herrmann
Born in 1952 and have 29-year’s protection experience. He graduated from Technical University of Dresden in 1977.During
1977-1991, worked as Assistant Professor on the Technical University of Zittau. Then join SIEMENS as Member of Product
Management Protection in the Energy Automation Division. Up to now, there are totally approx. 60 Contributions in Papers
and Conferences published. He is also the Co-author of a Protection Book (in Russian) and the author of a Book “Numerical
Protection” (in German).
Diego Gao
Born in 1975 and have 8-year’s protection experience. In 2004, he joined Siemens Power Automation Ltd. (SPA) serving as
a technical support engineer on generator-transformer protection.
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