FIELD OF THE INVENTION
The invention relates to a protection method of a synchronous generator to eliminate
asynchronous operation causing excessive heating including a damage of the generator.
An asynchronous generator operation is caused due to a failure of the excitation power
fed to the field winding while the generator is operating normally. Without the
excitation power the generator operates like an asynchronous generator. While it may
still deliver the real power to the system, it draws a large amount of reactive power
from the system. During such abnormal condition a large amount of current flows
through the damper winding, causing it to heat up significantly. In a synchronous
generator the function of damper winding is to assist in damping the variations in speed
as a result of any disturbance occurring on the system. But the damper winding is not
designed to carry large currents which might flow when there is an asynchronous
operation. Consequently it becomes necessary to disconnect the generator from the
system after a calculated amount of time. The amount of time for which such operation
may be tolerated will depend on the amount of load it is delivering.
BACKGROUND OF THE INVENTION
In this invention we are concerned with a synchronous generator fitted with a brushless
excitation system. Synchronous generators are of salient pole type which are usually
low speed machines used in hydro-power stations, or the non salient pole types which
are also known as turbogenerators used in thermal power stations. Both types of
generators have three-phase AC winding on stator and a field winding excited by DC
voltage on the rotor. The DC voltage fed to the field winding in the rotor needs to be
supplied from an exciter. The DC excitation source can be either fitted on the rotor of
the synchronous generator or it may be external to the rotor. In the case where the
excitation source is external a set of slip-rings and brushes are used to feed the current
to the field winding in rotor, while in the other case where the exciter is mounted on
rotor the slip-rings and brushes are not present and such excitation systems are
commonly known a brushless excitation system. The brushless excitation system
commonly adopts an AC exciter to produce the AC voltage and a set of power-electronic
devices to convert the AC voltage to DC so that it can be fed to the rotor winding of the
synchronous generator. The exciter has a field winding on its stator which is fed from
an automatic voltage regulator (AVR) to control the voltage and power factor of the
synchronous generator.
During normal operation of the synchronous generator, the winding in the stator is
connected to a power system of an essentially fixed voltage and frequency of supply,
which produces a synchronously rotating flux in the generator and the rotor assembly,
rotates at a speed which is essentially the speed of the rotating flux and is termed as
synchronous speed. The synchronous speed of the generator is a function of the
frequency of the AC supply to the poly-phase winding and the number of poles in the
stator winding. The rotor assembly also has the same number of poles. While the stator
winding creates a rotating flux in the air-gap of the synchronous generator the rotor
winding creates a matching magneto motive force (MMF) which locks with the stator
flux. Since the stator flux and the rotor, both rotate at essentially the same speed, the
generator is called a synchronous generator. The torque which is produced in the
synchronized condition is large and is supplied by a prime mover attached to the
generator shaft. In a thermal power station the prime mover can be a steam turbine
and in a hydro power station it could be a hydro-turbine. A normal operation is signified
by the state when the rotor remains synchronized while delivering the load to the
power station.
Whenever there is a change in the normal state of operation, which may be created by
sudden change in the load torque or due to the variation in AC supply, a transient
condition occurs. Any such transient condition generally creates a change in the flux
and the current established in the windings, and subsequently the generator tries to
attain another normally operating steady state condition. This newly attained steady
state then prevails till another transient condition occurs and the generator keeps
delivering the load torque while it remains synchronized to the supply. In this way a
synchronous generator operates steadily and the operation is termed as a normal
operation when a steady synchronized state of operation prevails after every transient
condition.
U.S. Patent No. 4,683,411 dated July 28, 1987 by Hamilton, Jr., et al. Assignee General
Electric Company, titled "Synchronous Generator Protection", described a digital
controller for synchronous generators which prevents excessive heating of the damper
winding. It provides for adjustment of allowable time in response to change in the
excitation voltage. It derives signals from the generator and computes the thermal limit
characteristics curves. The characteristics curves are used in a controller.
U.S. Patent Application No. U.S. 2001/0007416 Al dated July 12, 2001, by Satoshi
Koide et.al., titled "Device and method for determining step-out of synchronous
generator" is for computation of a step-out from the torque command, power
consumption and the speed. A parameter, which is essentially the ratio of the torque
command and the torque computed from power and speed, is computed which is used
as an indication of step-out. And the same is used in controller.
U.S. Patent No. 3,440,509, dated April, 22, 1969, by, Anthony Tomeo et.al., assignors
Westinghouse Electric Corporation, titled, "Resynchronizing means for brushless
synchronous generators", relates to a synchronous generator with brushless excitation
which attempts to re-synchronize a generator after it has pulled out of synchronism.
After a predetermined number of unsuccessful attempts at re-synchronization it
automatically turns the generator off.
The prior patents cited hereinabove propose hardware to do the protection in a specific
condition.
An abnormal operation is a condition when a transient state is long, or severe, which
does not permit the generator to come back to a steady operating condition soon and
consequently it causes damage to the generator. In an abnormal condition, an AC
voltage is induced in the field winding which rides on the DC voltage of the field
winding and may cause damage to the power electronic devices fitted on the exciter. As
a consequence of such failure in the excitation circuit connected to field winding - the
DC voltage fed to the field winding seizes. Consequently the generator looses
synchronism and enters the asynchronous mode of operation. As explained above, such
asynchronous mode of operation needs to be restrained.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose a protection method for a
synchronous generator against an unrestricted asynchronous operation caused by a
failure in the generator excitation supply.
Another object of the invention is to propose a protection method for a synchronous
generator against an unrestricted asynchronous operation caused by a failure in the
generator excitation supply, in which the slip-speed of the generator is determined
while it is in an asynchronous mode of operation and restrict the amount of slip such
that the amount of heating of the damper winding in rotor is restricted and does not
cause any damage to the machine.
A still another object of the invention is to propose a protection method for a
synchronous generator against an unrestricted asynchronous operation caused by a
failure in the generator excitation supply, which adapts a multi-function numerical type
generator protection relay.
A further object of the invention is to propose a protection method for a synchronous
generator against an unrestricted asynchronous operation caused by a failure in the
generator excitation supply, which is capable to compute the characteristics of load
versus allowable-time-duration, and embed the characteristics into the generator
protection relay to protect the generator from an unrestricted asynchronous operation.
A still further object of the invention is to propose a protection method for a
synchronous generator against an unrestricted asynchronous operation caused by a
failure in the generator excitation supply, in which slip versus load characteristics of a
synchronous generator is computed by using the voltage, and frequency applied to the
generator terminals along with the parameters of the generator.
Another object of the invention is to propose a protection method for a synchronous
generator against an unrestricted asynchronous operation caused by a failure in the
generator excitation supply, in which the temperature rise in the damper windings
located in the rotor, is computed.
Yet further object of the invention is to propose a protection method for a synchronous
generator against an unrestricted asynchronous operation caused by a failure in the
generator excitation supply, in which the time taken to attain the limiting value of
temperature for the given load condition, is computed.
SUMMARY OF THE INVENTION
According to the invention, the time duration which can be tolerated by the
synchronous generator given the amount of terminal voltage, frequency and the load
conditions is found and programmed into a generator protection relay. In this, method,
the generator load is computed from the running conditions and checked if it would
result in an excessive heating. A large capacity synchronous generator is known to
possess a high voltage winding on the stator and requires to be connected to a power
system grid through a transformer and a circuit breaker. The circuit breaker operates in
conjunction with a relay and a protection circuit which senses any impending abnormal
condition on the generator and takes action by sending a signal to the breaker to switch
OFF and thereby remove the electrical connection to the stator winding of the
generator, so that no harm is done to it.
According to the invention, a computation of the allowable duration of the
asynchronous mode operation on synchronous generator is made, while it is delivering
a load. The computed characteristic is then embedded in a generator protection relay.
The generator protection relay acts as a silent sentinel and protects the generator from
such severe abnormal conditions by sending a signal to the circuit breaker to trip and
thereby disconnect the generator from the mains. The generator protection relay is
generally termed as a multi-function numeric relay which possesses embedded software
programmed to perform many functions. The out-of-step operation is one among the
functions of a numeric relay. Such multi-function numeric relays are known from the
applicant's prior patent application 248/KOL/2005.
In the present invention, the protection aspect in made much simpler as it uses an
existing multi-function numeric relay. A characteristic is computed from the generator
parameters and the terminal conditions, which is stored in a multi-function numeric
relay to protect the generator from any such out-of-step condition. The characteristic
computed can be implemented in any multi-function numeric relay.
According to the invention, the characteristic parameters of a synchronous generator
are calculated from the generator design data and measurements conducted on the
generator. These parameters are namely, the winding resistances, and the reactances,
obtaining change of variable to dq model by transforming them to dq-axis reference
frame, reducing the relationship to flux linkages based on the voltages, further reducing
to two sets of unknown rotor currents in dq-reference frame, finally representing the
synchronous generator by set of coupled differential equations, which are reduced to
algebraic equations for the steady-state operations, solving the equations with the slip
and voltage of the generator, resulting in flux linkages and currents and finally
estimating the torque on calculating product of flux linkages and currents. The currents
are finally used to calculate the losses in winding and consequently causing
temperature rise.
A detailed modeling of the generator was carried out using d-q method in steady state
operating condition. The generator in question consists of a three phase AC wining in
the stator and a damper winding in rotor. The field winding is treated as open.
The objective of the invention is to compute the electromagnetic torque, given the
imposed voltages on the stator winding while it is rotating at a known speed. For this
purpose a simulation model of the synchronous generator is adopted which is the
commonly used dq-model of synchronous generator and it can be found in the
reference book [1], For the simulation purpose the voltage and currents are
transformed using the transformation method detailed in reference [1]. It contains the
matrix expression relating the dq-voltages to the flux linkages which is a set of complex
algebra equation. Knowing the voltages, the flux linkages may be computed from the
expression. Further the flux linkages are linked to the currents flowing in the windings.
The relationship between flux linkages and currents are another set of algebraic matrix
equations. From these equations the currents in different windings can be deduced and
finally the electromagnetic torque produced by the generator is given as a product of
flux linkages and currents. The next step is to compute the temperature rise in damper
windings. In this manner the characteristics of load versus the time taken to attain the
limiting temperature is computed. This characteristics is programmed into a multi
function relay which protects the generators from any unrestrained asynchronous
operation.
The parameters which are required for the computation of the characteristic are
calculated from the generator design data or the test data. These parameters are given
below:
1. rs - stator winding resistance
2. X/s - stator winding leakage reactance
3. rkq1 - q-axis damper winding resistance of rotor circuit-1 referred to stator
winding
4. rkql - q-axis damper winding resistance of rotor circuit-2 referred to stator
winding
5. rkd - d-axis damper winding resistance of rotor referred to stator winding
6. rfa - d-axis field winding resistance of rotor referred to stator winding
7. x'kq1 - q-axis damper leakage reactance of rotor circuit-1 referred to stator
winding
8. xkq2 - q-axis damper leakage reactance of rotor circuit-2 referred to stator
winding
9. Xq - q-axis synchronous reactance
10. Xmq - q-axis mutual reactance
11. Xfd - d-axis field leakage reactance of rotor winding referred to stator
winding
12. X'kd - d-axis damper leakage reactance of rotor winding referred to stator
winding
13. Xd - d-axis synchronous reactance
14. Xmd - d-axis mutual reactance
15. P - number of poles.
The input data, or the known values which are required in the calculations are, the
terminal voltages, vaS, VbS, Vcs, the rotor speed, ?b in electrical radians per second, the
base frequency, ?b in radians per second, and the base torque, Tb.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 represents the allowable time duration before disconnection of generator,
due to a given load.
Figure 2 represents schematically the internal connections of a generator
Figure 3 represents schematically the generator connection with the power system
Figure 4 represents the mechanical structure of the synchronous generator and
connection to the prime mover through a coupling.
DETAIL DESCRIPTION OF THE INVENTION
Figure 1 shows the graph of load versus the allowable time duration of the generator.
This characteristic is computed by the method detailed above. It depicts essentially the
nature of protection to be provided to the synchronous generator for an impending
asynchronous operation.
Figure 2 schematically shows the internal and external electrical connections of a
synchronous generator, including the stator and rotor windings, the brushless exciter,
diode-bridge, and the external connections to the stator and rotor windings. The
external connection to the stator winding is made to the power source and the rotor
winding external connection is made to an automatic voltage regulator. More details of
the external connection to the stator winding are shown in the Figure 3.
Figure 3 schematically shows the external connections to the synchronous generator. It
consists of a transformer and the power system grid. Since the type of generator we
are concerned with is a large capacity one and it will consist of a circuit breaker and
protection circuit. The circuit breaker operates in conjunction with a relay and
protection circuit which senses any impending abnormal condition on generator and
takes action by sending a signal to the breaker to switch OFF and thereby remove the
electrical connection to the stator winding of the generator so that no harm is done it.
The setup described in the Figure 2 and 3 consist of the items numbered 1 to 13 which
are detailed below:
DETAILED DESCRIPTION OF EACH ITEM IS GIVEN BELOW
Figures 2 and 3 shows a synchronous generator comprising:
(a) a stator winding (item no.2) on the stationary part of the synchronous
generator (item no.l) and a rotor winding (item no.3) on the rotating part of
the generator, each supported on magnetic steel structure (item nos. 16 and
17) and other supporting steel structure (item no.22), with the rotor (item
no.17) housed within the stator structure (item no.16) such that only a shaft
(item no. 18) protrudes, with rotor having no mechanical connection to the
stator except two bearings (item no.19) and thereby making it possible for the
rotor to rotate freely within the stator, and
(b) the generator further consisting of two set of electrical inputs, the electrical
power connections to the stator winding (item no.9) and the electrical
connections to the exciter winding (item no. 10). The structure and the
electrical connections delivers the electrical load at the stator winding (item
no.2) while the shaft (item no. 18) which is connected to the prime mover (item
no.20) that delivers the mechanical torque and rotation to the generator's rotor
(item no. 17).
The generator when given the specific excitation voltage at the rotor winding
(item no.3) generates at the stator winding (item no. 2) the amount of
alternating voltage at a frequency that depends on the speed and the number
of poles and will operate in synchronism with the power system grid.
When there is a significant deviation beyond the specified limits, in the
excitation voltage condition, an abnormal operation may result, for instance an
out of step operation or asynchronous operation.
When an asynchronous operation occurs the generator falls out of the
synchronized operation with the power system and this results in current in the
damper winding, which creates heating of the winding and the rotor body. If
the temperature exceeds the allowable limits this may cause severe damage to
the generator.
Whereas an embedded protection device item no. 13) is provided that senses
the impending danger of an unrestrained asynchronous operation and removes
the connection to the generator by switching off the breaker (item no.12), and
thereby protecting the synchronous generator (item no.l).
An estimation of the generator torque from the input condition is made wherein a
circuit breaker and the protection unit (item no. 13), are operated through a control and
instrumentation unit (item no. 15), the estimation comprises:
(a) The prime mover runs up the rotor and upon attaining the synchronous speed,
the automatic voltage regulator applies a DC voltage to the field of exciter and
thereby producing the voltage at the generator terminals;
(b) The control unit synchronizes the generator by closing the circuit breaker ON,
and causes the system voltage to be applied to the generator terminals and
thereby allowing the generator to get synchronized to the power system.
(c) After closing the circuit breaker and thereby synchronizing the generator a load
connected to the system may be fed by the generator with the prime mover
supplying the required torque;
(d) Having thus attaining the normal synchronous state of operation, where the
load is applied to the generator terminal, the generator field winding being
supplied by the DC excitation and the torque being supplied by the prime mover
to the generator shaft, the steady operating condition of delivering up to the
normal rated load condition is attained.
The method of estimation of torque is extended to the abnormal operating condition,
wherein, the circuit breaker remains in the ON condition and the protection unit remains
connected to the circuit, wherein the excitation current reduces beyond the limits, or
stops, leading to an asynchronous operation, which consists of a balanced nature of
currents and voltages flowing in the stator winding of generator.
A dq-transformation to the voltages is adapted which results in the transformed
voltages using the following expression:

Where the variables vas, VbS, Vcs are the voltage in the stationary frame which is applied
to the stator winding and the variable vqsr, vdsr, v0s are the dq-transformed voltages. The
angle #is given by the following expression:

Where ?(?) is the angular speed of the rotor and ? is the dummy variable of
integration. The angle ?(0) is the value of angle between the two frames, the stationary
and the dq-frame, at the instant zero, which can be fixed arbitrarily.
Upon computing the transform voltage phasorsand the flux linkages are
computed from the following expression:

After determining the flux-linkages-per-second, i.e.; ?d and ?q, the currents can be
calculated from the operational impedances as given below:
Then the electromagnetic torque developed can be calculated as:

The parameters of the generator used in the calculation are given below. These
parameters are either from the generator design or test data:
1.- stator winding resistance
2.- stator winding leakage reactance
3.- q-axis damper winding resistance of rotor circuit-1 referred to
stator winding
4.- q-axis damper winding resistance of rotor circuit-2 referred to
stator winding
5.- d-axis damper winding resistance of rotor referred to stator
winding
6.- d-axis field winding resistance of rotor referred to stator
winding
7.- q-axis damper leakage reactance of rotor circuit-1 referred to
stator winding
8.- q-axis damper leakage reactance of rotor circuit-2 referred to
stator winding
9.- q-axis synchronous reactance
10.- q-axis mutual reactance
11.- d-axis field leakage reactance of rotor winding referred to
stator winding
12.- d-axis damper leakage reactance of rotor winding referred to
stator winding
13.- d-axis synchronous reactance
14. - d-axis mutual reactance
The input data in the above calculations are the terminal voltage phasors, Vds, Vqs the
parameters, slip-s and the frequency ?e and ?b. Knowing these input data the torque
can be calculated.
The estimation of torque of synchronous generator under asynchronous operation is
computed for a given set of input voltages. When there is an asynchronous operation
measured by a change in the speed from the synchronous speed and is given by the
slip-s. When the slip-s is a large value the time duration which can be allowed is smaller
and for a smaller value of slip the duration is larger. The time duration for a given set of
voltages, frequency and slip, is computed as given below:
Id=abs(Id)
lq*abs(Iq)
Ikd =ab$(Ikd)
Ikqi=abs(lkm)
Power input = Power output + Losses
a) The power output is related to the slip and consequently the speed of the
machine.
b) During a normal operation the speed of rotor is same as synchronous speed.
During asynchronous operation the speed changes with the amount of power
delivered.
c) The variation of power generation with speed involves losses in the windings.
The losses cause the temperature of windings to increase.
d) The temperature rise of the damper winding is critical. The damper windings are
not designed to carry the currents for a continuous operation. The temperature
of the damper windings is calculated from the losses.
e) The time duration which may be allowed such that the temperature rise of
damper winding attains the limiting value is calculated.
f) The characteristics of power delivered as a function of the allowable time
duration is calculated.
g) The characteristic for a particular generator is embedded in a numerical relay
which functions to protect the generator under such abnormal operation
condition.
The torque and the allowable time duration for asynchronous operation is calculated
such that a characteristic is computed to protect the generator from an unrestricted
asynchronous operation and programmed into a multi function numerical relay which
acts to protect the generator as given below:
a) The numerical relay is an intelligent device, consisting of a microprocessor and
memory, with embedded software and functionalities therein. The relay receives
inputs from the sensors attached to it and performs the functions which are
programmed and sends the output signals to a breaker or the control and
instrumentation unit.
b) The relay acts as a silent sentinel during the normal operation of the generator.
c) When the operation of inputs from the sensors indicate an abnormal operation
the relay acts on the input data and produces an output, such as, when a
asynchronous operation occurs, the relay computes the allowable time it should
wait before sending a command to the breaker and upon exceeding this time
limit without any change in the asynchronous operation it sends a signal to the
breaker to disconnect the generator from the power system.
d) Thereby the generator and the other systems connected to it, for instance, the
damper winding, field winding and rotor forging are saved from a damaging
situation.
e) Thereby saving heavy loss and long down-time.
Reference:
[1] "Analysis of Electric machinery" by Paul C. Krause, Oleg Wasynczuk, Scott D.
Sudhoff- IEEE Power Engineering Society, IEEE No. PC 04556.
WE CLAIM
1. A method in a protection apparatus for protecting a synchronous generator (1) from
an unrestrained asynchronous operation while the generator is delivering a load to
the system, the generator comprising:
- a stator winding (2) on a stationary part of the synchronous generator, and a rotor
winding (3) on a rotating part of the generator, each supported on a magnetic steel
structure (16 and 17) including a steel structure (22), with the rotor (17) housed
within the stator structure (16) such that only a shaft (18) protrudes, with the rotor
having no mechanical connection to the stator except via two bearings (19) and
thereby making it possible for the rotor (17) to rotate freely within the stator, at
least two set of electrical inputs (9,10) one each for the electrical power connections
to the stator winding (9) and electrical connections to the exciter winding, the
electrical connections delivers the electrical load at the stator winding (2) while the
shaft (18) connected to a prime mover (20) delivers the mechanical torque and
rotation to the rotor (17), an excitation voltage when applied at the rotor winding
(3) generates at the stator winding (2) an amount of alternating voltages at a
frequency that depends on the speed and the number of poles and operate in
synchronism with the power system gird, the protection apparatus (13) comprising
an embedded device, a circuit breaker (12) for switching the generator, and a
control unit (15), the method comprising the steps of:
- applying a DC-voltage by an automatic voltage regulator upon the generator
attaining the synchronous speed to the field of exciter to produce the voltage at the
generator terminals;
- synchronizing the generator by closing the circuit breaker ON, by the control unit to
allow the generator to get synchronized to the power system;
- feeding a load connected to the system by the generator with the prime mover
supplying the required torque, after synchronizing the generator with closing of the
circuit breaker; and
- upon attaining the normal synchronous state of operation, the generator field
winding being supplied by the DC excitation and the torque being supplied by the
prime mover to the generator shaft, thereby resuming the steady operating
condition of delivering up to the normal rated load condition.
2. A method as claimed in claim 1, wherein the torque is estimated by maintaining the
circuit breaker in the ON condition and the protection unit remains connected to the
circuit, wherein the excitation current reduces beyond the limits, or stops, leading to
an asynchronous operation, which consists of:
a) a balance nature of currents and voltages are flowing in the stator winding of
generator;
wherein a dq-transformation is adapted to the voltages resulting in the
transformed voltage using the following expression:
*/` =-`1`azWhere the variables vas, Vbs, Vcs are the voltage in the stationary frame which is
applied to the stator winding and the variable vqsr, vdSr, vos are the dq-
transformed voltages. The angle ? is given by the following expression:

Where ?(?) is the angular speed of the rotor and ? is the dummy variable of
integration. The angle ?(0) is the value of angle between the two frames, the
stationary and the dq-frame, at the instant zero, which can be fixed arbitrarily,
wherein on computing the transformed voltage phasor ?q and ?d, the flux
linkages are computed from the following expression;

Wherein upon computation of the flux-linkages-per-second, i.e.; ?d and ?q, the
currents can be calculated from the operational impedances
given below:

Wherein, the currents can be calculated from the following expression:

Wherein the electromagnetic torque developed can be calculated as:

and wherein the parameters of the generator used in the calculation are either from
the generator design or test data, and constitutes as under
1. rs - stator winding resistance
2. XiS - stator winding leakage reactance
3. rkq1 - q-axis damper winding resistance of rotor circuit-1 referred to
stator winding
4. rkq2 - q-axis damper winding resistance of rotor circuit-2 referred to
stator winding
5.- d-axis damper winding resistance of rotor referred to stator
winding
6.- d-axis field winding resistance of rotor referred to stator
winding
7.- q-axis damper leakage reactance of rotor circuit-1 referred to
stator winding
8.- q-axis damper leakage reactance of rotor circuit-2 referred to
stator winding
9.- q-axis synchronous reactance
10.- q-axis mutual reactance
11.- d-axis field leakage reactance of rotor winding referred to
stator winding
12.- d-axis damper leakage reactance of rotor winding referred to
stator winding
13.- d-axis synchronous reactance
14.- d-axis mutual reactance
15.- d-axis voltage phasor
16.- q-axis voltage phasor
17.- slip
18.- Operational impedance of d-axis
19. - Operational impedance of q-axis.
the inputs data in the above calculations are the terminal voltage phasors phasors,
?ds, ?qS, the parameters, slip-s and the frequency ?eand ?b. which allow calculation
of the torque.
3. A method as claimed in claim 2, wherein the torque is computed for a given set of
input voltages, measured by a change in the speed from the synchronous speed and
is given by the slip-s, the time duration for a given set of voltages, frequency and
slip, is computed as given below:
Power input = Power output + Losses
4. The method as claimed in any of the preceding claims, wherein a characteristic is
computed to protect the generator from an unrestricted asynchronous operation is
programmed into the multi function numerical relay, wherein the numerical relay is
an intelligent device, consisting of a microprocessor and memory, with embedded
software and functionalities therein, the relay receives inputs from the sensors
attached to it and performs the functions which are programmed and sends and
output signals to a breaker or the control and instrumentation unit, the relay acts as
a sentinel during the normal operation of the generator, and wherein when the
operation of inputs from the sensors indicate an abnormal operation the relay acts
on the input data and produces an output, such as, when a asynchronous operation
occurs the relay computes the allowable time it should wait before sending a
command to the breaker and upon exceeding this time limit without any change in
the asynchronous operation it sends a signal to the breaker to disconnect the
generator from the power system.

ABSTRACT

The invention relates to a method in a protection apparatus for protecting a
synchronous generator (1) from an unrestrained asynchronous operation while the
generator is delivering a load to the system, the generator comprising a stator winding
(2) on a stationary part of the synchronous generator, and a rotor winding (3) on a
rotating part of the generator, each supported on a magnetic steel structure (16 and
17) including a steel structure (22), with the rotor (17) housed within the stator
structure (16) such that only a shaft (18) protrudes, with the rotor having no
mechanical connection to the stator except via two bearings (19) and thereby making it
possible for the rotor (17) to rotate freely within the stator, at least two set of electrical
inputs (9,10) one each for the electrical power connections to the stator winding (9)
and electrical connections to the exciter winding, the electrical connections delivers the
electrical load at the stator winding (2) while the shaft (18) connected to a prime mover
(20) delivers the mechanical torque and rotation to the rotor (17), an excitation voltage
when applied at the rotor winding (3) generates at the stator winding (2) an amount of
alternating voltages at a frequency that depends on the speed and the number of poles
and operate in synchronism with the power system gird, the protection apparatus (13)
comprising an embedded device, a circuit breaker (12) for switching the generator, and
a control unit (15), the method comprising the steps of applying a DC-voltage by an
automatic voltage regulator upon the generator attaining the synchronous speed to the
field of exciter to produce the voltage at the generator terminals; synchronizing the
generator by closing the circuit breaker ON, by the control unit to allow the generator
to get synchronized to the power system; feeding a load connected to the system by
the generator with the prime mover supplying the required torque, after synchronizing
the generator with closing of the circuit breaker; and upon attaining the normal
synchronous state of operation, the generator field winding being supplied by the DC
excitation and the torque being supplied by the prime mover to the generator shaft,
thereby resuming the steady operating condition of delivering up to the normal rated
load condition.