FIELD OF INVENTION
The invention relates to a method of protection of a synchronous motor against
a voltage dip causing an out-off-step-operation.
BACKGROUND OF INVENTION
Synchronous motor fitted with brushless excitation is known to have out-of-step
protection device so as to protect the motor and its excitation system from
damage during such an abnormal condition. Polyphase synchronous motors
usually have the AC winding on stator and a field winding excited by DC voltage
on the rotor. In large synchronous motors the polyphase winding is fed from a
fixed frequency source and the field is fed from a DC exciter. The DC excitation
source can be either fitted on the rotor of the synchronous motor 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 as
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 motor. The exciter has a field winding on its stator which is fed
from an automatic voltage regulator (AVR).
During normal operation of the synchronous motor the polyphase winding in the
stator is fed from a fixed voltage and frequency of supply, which produces a
synchronously rotating flux in the motor 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 motor is a function of the
frequency of the AC supply to the polyphase 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 motor
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 motor is called a synchronous motor. The torque which is
produced in the synchronized condition is large and is responsible for driving the
load attached to the motor. A normal operation is signified by the state when the
rotor remains synchronized while delivering the load torque.
Whenever there is a change in the normal state of operation, which may be
generated by a 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 motor tries to attain another normally operating steady state
condition. This newly attained steady state then prevails till another transient
condition occurs and the motor keeps delivering the load torque while it remains
synchronized to the supply. In this way a synchronous motor operates steadily
and the operation is termed as a normal operation when a steady synchronized
state of operation prevails after every transient condition.
An abnormal operation is a condition when a transient state is long, or severe,
which does not permit the motor to come back to a steady operating condition
soon and consequently it causes damage to the motor. One such abnormal
operating condition is the out-of-step operation, when the motor is unable to
remain synchronized and the rotor speed falls out of step with the synchronously
rotating flux created by stator. Such an abnormal condition, as the out-of-step
operation, induces AC voltage which rides on the DC voltage of the field winding
and may cause damage to the power electronic devices fitted on the exciter. The
subsequent effect may cause damage to the machine parts such as the poles of
the brushless exciter, coupling etc. The damage may even lead to a forced
outage of the motor for a long duration. Hence, the motor requires to be

protected against such out-of-step operations and this patent relates to
protection of the motor and the electronic devices from such occurrences.
One of the common reasons for such out-of-step operation is a severe
depression in the AC voltage supplied to the polyphase winding of the
synchronous motor. A prolonged dip in supply voltage is created due to abnormal
conditions which prevail on the power system. The power system is usually fitted
with breakers and other devices which are able to remove the cause of such
voltage dip within a stipulated time. However, if the disturbance persists and the
voltage-dip continues for a longer duration then an out-of-step operation may
result.
U.S.Patent No. 4,683,411 to Hamilton, Jr., et.al. Assignee General Electric
Company, entitled "Synchronous Motor Protection", describes a digital controller
for synchronous motors which prevents excessive heating of the damper
winding. It provides for adjustment of allowable time in response to changes in
excitation voltage. It derives signals from motor from there it computes the
thermal limit characteristics curves. The characteristics curves are used in a
controller.

US 2001/0007416 Al to Satoshi Koide, et.al., entitled "Device and method for
determining step-out of synchronous motor"'discloses a method for computation
of 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 a controller.
U.S.Patent No. 3,440,509, to Anthony Tomeo et.al., assignors Westinghouse
Electric Corporation, entitled, "Resynchronizing means for brushiess synchronous
motors" describes a synchronous motor with brushiess excitation which
attempts to re-synchronise a motor after it goes out of synchronism. After a
predetermined number of unsuccessful attempts at re-synchronisation, it
automatically turns the motor off.
All the patents cited above propose hardware to do the protection in a specific
condition.

OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a method of protection of a
synchronous motor against an out-of-step operation caused by a dip in the
voltage supply.
Another object of the invention is to propose a method of protection of a
synchronous motor against an out-of-step operation caused by a dip in the
voltage supply , which implements a determination of the motor torque from the
operating conditions, which drives the load to ensure generation of adequate
amount of torque by the motor.
A still another object of the invention is to propose a method of protection of a
synchronous motor against an out-of-step operation caused by a dip in the
voltage supply , which adapts a multi-function numerical type motor protection
relay.
Yet another object of the invention is to propose a method of protection of a
synchronous motor against an out-of-step operation caused by a dip in the
voltage supply, which is enabled to compute the characteristics of voltage-dip
versus allowable-time-duration, relating to the operating conditions of the motor,
the characteristic being embedded into the motor protection relay.

A further object of the invention is to propose a method of protection of a
synchronous motor against an out-of-step operation caused by a dip in the
voltage supply, which is capable of computing the torque of a synchronous
motor by using voltages and frequency applied to the stator windings including
application of the torque using the measured parameters of synchronous motor,
namely the winding resistances, and the reactances, obtaining change of
variables 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, representing the
synchronous motor by set of coupled differential and algebraic equations, solving
the equations with the dip in voltage of the motor, resulting in flux linkages and
currents, and estimate the torque on calculating product of flux linkages and
currents.
SUMMARY OF INVENTION
The present invention provides a method of computation of the duration of a
voltage-dip which may be allowed on synchronous motor terminals while it is
delivering a load. The computed characteristic is then embedded in a motor
protection relay. The motor protection relay acts as a silent sentinel and protects
the motor from such severe abnormal conditions by sending a signal to the
circuit breaker to trip and thereby disconnect the motor from the mains. The

motor protection relay is generally termed as a multi-function numeric relay
which possesses an embedded software programmed to perform a plurality of
functions. The out-of-step operation is one among the several functions of the
numeric relay.
A synchronous motor is characterized by an equivalent circuit. The parameters of
the circuit are adapted to compute the motor behavior under an out-of-step
condition. The invention allow a determination of the time duration which is
accomodatable by the synchronous motor given the amount of terminal voltage,
frequency and the load conditions.
The present invention enables the protection aspect of the synchronous motor to
be much simpler by adapting a multi-function numeric relay, with the motor
switchgear and control gear provided with a fully fledged GIS (Gas Insulated
Substation). A characteristic is computed from the motor parameters and the
terminal conditions, which is stored in a multi-function numeric relay to protect
the motor from any out-of-step condition. The characteristics computed can be
implemented in a multi-function numeric relay .

According to the invention, the time duration which can be tolerated by the
synchronous motor given the amount of terminal voltage, frequency and the load
conditions is found and programmed into a motor protection relay. In this
method the motor torque is computed from the running conditions and checked
if it would result in an out-of-step operation. The type of synchronous motor
considered in this invention is of a large capacity which possesses 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 protection circuit which senses any impending
abnormal condition on motor 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 motor, so that no harm is done to it.
According to the invention, a detailed modeling of the motor was carried out
using d-q method. The motor in question consists of a three phase AC winding in
the stator; however, the method is capable of handling a polyphase winding of
any number of phases.
The electromagnetic torque is computed given the imposed voltages on the
polyphase winding and the DC voltage in the field winding. For this purpose a
simulation model of the synchronous motor is adopted which is the commonly

used dq-model'of synchronous motor and it can be found in the reference book
[2]. For the simulation purpose the voltage and currents are transformed using
the transformation method detailed in reference [2]. It contains the matrix
expression relating the dq-voltages to the flux linkages which is a set of ordinary
differential equation, and it represents the dynamics of machine. 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 a set of algebraic matrix equations. From
these equations the currents in different windings can be deduced and finally the
electromagnetic torque produced by the motor is given as a product of flux
linkages and currents. The next step is to compute the rotor dynamics which is
given by an ordinary differential equation, involving the electromagnetic torque
produced by motor, the load torque, the inertia of rotor and the speed. The rotor
dynamic equation produces the value of speed of the motor as a function of
time.
Due to a sudden change in the system voltages the electromagnetic torque
changes and this sets up a change in the speed. When the speed deviates from
the synchronous speed the angle between the rotating frame and the position of
rotor changes. This angle is also known as the load angle since the value of this
angle depends on the load on the machine. Change in load angle will also result
into a change in the electromagnetic torque produced by the motor - which

results in a new equilibrium between the load torque and the electromagnetic
torque. However, the angle should be within a quadrature in terms of electrical
angle. A significantly larger value of the load angle will result in an unstable
situation, as the rotor will not return to a new equilibrium. This kind of situation
results in an out-of-step operation and may cause many pole-slips which may
have damaging consequences.
If the dip in voltage is restored to normal before the load angle deviates too
much the normal condition gets restored and the rotor comes back to a new
equilibrium condition. However, if the dip in voltage persists, the load angle
keeps increasing with the elapsed time. Therefore, the elapsed time becomes a
measure of the increase in load angle. The elapsed time after a dip in voltage
can be related to the safe operating characteristic of the motor. The allowable
time duration for different voltage dip conditions is simulated in the computer
and a characteristic is computed, which is similar to the characteristics shown in
the Figure 1. The characteristic computed depicts a safe zone of operation for
the motor, thus, to protect the motor from an out-of-step operation, the
characteristic is programmed into the multi function numerical relay which acts
to protect the motor when any such condition prevails.
The parameters which are required for the computation of the characteristic are
calculated from the motor design data or the test data. These parameters are
given below:
The input data, or the known values which are required in the calculations are,
the terminal voltages, the value of load torque on motor, the rotor
speed, in electrical radians per second, the base frequency, in radians per
second, and the base torque, TB
The torque is computed for a given set of input voltages. When there is a dip in
the input voltages beyond the limit, this may result in an out-of-step operation.
When the dip is for a large value the time duration which can be allowed in
smaller and for a small dip the duration is larger, for instance a flicker may not
require a disconnection. The time duration for a given set of voltages is
computed from the rotor dynamics, given the electromagnetic torque created by
the motor, the load torque and the speed of operation.
a) The electromagnetic torque and the speed are related by the following
expression

b) The above equation is an ordinary differential equation and it describes
the rotor dynamics.
c) During a normal operation the speed of rotor is same as the
synchronous speed. Due to dip in the voltages, the electromagnetic
torque changes and a mismatch between the electromagnetic torque
and the load torque occurs which creates a rotor dynamics.
Consequently the speed changes from the normal value of the
synchronous speed and the angle between the rotating field and rotor,
also known as the load angle, deviates from its normal value.
d) The deviation in the load angle beyond 90 degrees creates an out-of-
step operation.
e) If the dip in voltage is restored to normal before the load angle
deviates too much the normal condition gets restored.
f) If the dip in voltage persists the load angle keeps increasing with the
elapsed time.
g) Therefore the elapsed time becomes a cause of the increase in load
angle. The elapsed time after a dip in voltage can be related to the
safe operating zone for the motor.
h) The allowable time duration for different voltage dip conditions is
simulated in the computer and a characteristic is computed.
A characteristic is computed to protect the motor from an out-of-step operation
is programmed into the multi function numerical relay which operates as under:
a) The numerical relay is an intelligent device, consisting of a
microprocessor and a 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 signal to a breaker or the control and
instrumentation unit.
b) The relay behaves as a silent sentinel during the normal operation of
the motor.
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 voltage dip 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 voltage dip it sends a
signal to the breaker to disconnect the motor from the power system.
d) Thereby the motor and the other systems connected to it, for instance,
the coupling, load etc. are saved from a damaging situation.
e) Thereby saving heavy loss and long down-time.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1 represents the allowable time duration before disconnection of a
synchronized operating motor from the power system,due to a
dip in voltage according to the invention.
Figure 2 represents schematically the internal connections of a synchronous
motor.
Figure 3 represents schematically the motor connection with the power
system
Figure 4 represents the mechanical structure of a synchronous motor and
connection to the load through a coupling.
DETAIL DESCRIPTION OF THE INVENTION
Figure 1 shows the graph of the allowable time duration versus the amount of
dip in the terminal voltage of the motor. This characteristics is computed by the
method detailed above. It depicts essentially the nature of protection to be
provided to the synchronous motor for an impending out of step operation.

Figure 2 schematically shows the internal and external electrical connections of a
synchronous motor, 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 is
shown in the Figure 3.
Figure 3 schematically shows the external connections to the synchronous motor.
It consists of a transformer and the power system grid. Since the type of motor
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
motor 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 motor so
that no harm is done to it.
Figures 2 and 3 respectively describes an internal connectivity of a synchronous
motor, and a connectivity diagram between the synchronous motor in a power
system.

Figure 4 shows the mechanical structure of a synchronous motor which
comprises a stator winding (2) on a stationary part of the synchronous motor
(l),and a rotor winding on a rotating part (3) of the motor, each supported on a
magnetic steel structure (17&18) including additional supporting steel structures
(23), the rotor (18) housed within a stator structure (17) such that only a shaft
(19) remaining externally projected , the rotor (18) having no mechanical
connection with the stator; at least two bearings (20) allowing the rotor (18) to
rotate freely within the stator (24);
at least two set of electrical inputs, constituting an electrical power connections
to the stator winding (2) and an electrical connections to an exciter winding (5
&10); the electrical connections delivering mechanical torque causing a rotation
to a load (21) connected on the shaft (19), through a coupling (22);
- and a motor protection relay (13) enabled to sense an out-of-step operation
and remove the connection to the motor by switching off a breaker ( 12), which
allows the synchronous motor (l),including an automatic voltage regulator (8),
a coupling (22) and the load (21), the method comprising the steps of:

- starting the motor by closing the circuit breaker ON, which causes a direct
online voltage to be applied to the motor terminals;
- applying the direct online voltage to the motor terminals when the motor is in
standstill and in a free to run condition;
- allowing the started motor to attain its rated speed, while the automatic
voltage regulator inhibiting its supply and allowing the exciter field winding to
remain in a shorted condition;
- applying a DC voltage to the field of exciter when the automatic voltage
regulator attaining the related speed and thereby allowing the motor to get
synchronized to the power system; and
- delivering up to the normal rated load condition upon the motor attaining the
normal synchronous state of operation, where the load is applied to the shaft
and the motor attains the rated steady operating condition.
The estimation of torque according to the invention in an abnormal operating
condition is done wherein, the circuit breaker maintained in the ON condition
and the protection unit remains connected to the circuit, and wherein when the
input conditions changes beyond the limits, the changed input conditions being
a balanced nature of currents and voltages flowing in the polyphase stator
winding of motor, the method comprising:
- adopting a dq-transformation to the voltages resulting in the
transformed voltages using the following expression:

Where the variables vast vbst, Vcs are the voltages in the stationary
frame which is applied to the stator winding and the variable
v0s are the dq-transformed voltages, the angle ? is given by the
following expression:

?(?) 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;
- computing the transformed voltages and
computing the flux linkages from the following expression:

the voltages representing the external voltages applied to the
damper windings and the damper windings being short circuited their values are
taken as zero, the voltage exfd'r representing the voltage generated by the
exciter and depending.on the excitation given to the field winding of the exciter,
the variables can be are defined as under:

Where Dq and Dd are defined below:

- determining , from the flux-linkages the currents flowing in the stator and the
rotor circuits adapting the following expressions:

- determining the electromagnetic torque generated by the motor based on the
measured value of flux linkage and current adapting the expression given below:

- correlating the electromagnetic torque and the speed by adapting the following
expression:

The computation of the motor torque is done according to the invention based
on the parameters relating to the design or the test data of the motor which
include:

LIST OF ITEMS IN THE PATENT OUT OF STEP PROTECTION

Reference:
[1] IEC 60079-0 "Explosive atmosphere equipment - general requirements"
[2] "Analysis of Electric machinery" by Paul C. Krause, Oleg Wasynczuk, Scott
D. Sudhoff - IEEE Power Engineering Society, IEEE No PC 04556.
WE CLAIM:
l. A method of protecting a synchronous motor from an out-of-step operation
during synchronized operation with a power system source connected electrically
to the motor and delivering a mechanically driven load connected to its shaft, the
synchronous motor comprising:
- a stator winding (2) on a stationary part of the synchronous motor (1),
and a rotor winding (3) on A rotating part (25) of the motor, each
supported on a magnetic steel structure (17&18) including additional
supporting steel structures (23), the rotor (18) housed within a stator
structure (17) such that only a shaft (19) remaining externally projected,
the rotor (18) having no mechanical connection with the stator;
at least two bearings (20) allowing the rotor (18) to rotate freely within
the stator (17);
- at least two set of electrical inputs constituting an electrical power
connections to the stator winding (2) and an electrical connections to an
exciter winding (5); the
electrical connections delivering mechanical torque causing a rotation to a
load (21) connected on the shaft (19) through a coupling (22);
- and a motor protection relay (13) with the out-of-step characteristics
embedded in it and

enabled to sense an out-of-step operation and remove the connection to
the motor by switching off a breaker (12), which allows the synchronous
motor,(l) including an automatic voltage regulator (8), a coupling (22)
and the load (21), the method comprising the steps of:
- starting the motor by closing the circuit breaker ON, which causes a
direct online voltage to be applied to the motor terminals;
- applying the direct online voltage to the motor terminals when the motor
is in standstill and in a free to run condition;
- allowing the started motor to attain its rated speed, while the automatic
voltage regulator inhibiting its supply and allowing the exciter field
winding to remain in a shorted condition;
-applying a DC voltage to the field of exciter by the automatic voltage
regulator subsequent to attaining the rated speed and thereby allowing
the motor to get synchronized to the power system;and
- delivering up to the normal rated load condition upon the motor
attaining the normal synchronous state of operation, where the load is
applied to the shaft and the motor attains the rated steady operating
condition.
2. The method as claimed in claim 1, comprising the step of estimating the
torque, wherein, the circuit breaker maintained in the ON condition and the
protection unit remains connected to the circuit, and wherein when the input
conditions changes beyond the limits, the changed input conditions being
a balanced nature of currents and voltages flowing in the polyphase
stator winding of motor the method comprising:
- adopting a dq-transformation to the voltages resulting in the
transformed voltages using the following expression:

Where the variables vast Vbst vcs are the voltages in the stationary
frame which is applied to the stator winding and the variable
v0s are the dq-transformed voltages, the angle ? is given by the
following expression:

?(?) is the angular speed of the rotor and ? is the dummy variable of
integration, the angle d(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;
- computing the transformed voltages and computing the flux
linkages from the following expression:

the voltages representing the external voltages applied to the
damper windings and the damper windings being short circuited their values
are taken as zero, the voltage exfd'r representing the voltage generated by the
exciter and depending on the excitation given to the field winding of the exciter,
the variables can be defined as under:

- determining from the flux-linkages the currents flowing in the stator and the rotor
circuits adapting the following expressions:

-determining the electromagnetic torque generated by the motor based on the
measured value of flux linkage and current adapting the expression given below:

- correlating the electromagnetic torque and the speed by adapting the
following expression:

3. The method as claimed in claim 1 and 2, wherein the parameters of the motor
adapted in computation comprise one of the motor design or test data; and
wherein the parameters comprise:

3. rkq2 - q-axis damper winding resistance of rotor circuit-2
referred to stator winding
4. rkd - d-axis damper winding resistance of rotor referred to
stator winding
5. rfd - d-axis field winding resistance of rotor referred to stator
winding
6. Xkq1 - q-axis damper leakage reactance of rotor circuit-1
referred to stator winding
7. Xkq2 - q-axis damper leakage reactance of rotor circuit-2
referred to stator winding
8. Xq - q-axis synchronous reactance
9. Xmq - q-axis mutual reactance
10. Xfd - d-axis field leakage reactance of rotor winding referred to
stator winding
11. X'kd - d-axis damper leakage reactance of rotor winding
referred to stator winding
12. Xd - d-axis synchronous reactance
13. Xmd - d-axis mutual reactance
14. J- polar moment of inertia of rotor
15. H- inertia constant of rotor
16. P- number of poles, the inputs or the known values in said
computation step being the terminal voltages, Vast Vbst Vcst the value of load
torque on motor, Tlt, the rotor speed, ?rt in electrical radians per second, the
base frequency, ?br in radians per second, and the base torque, T?.
4. A method of protecting a synchronous motor from an out-of-step operation
during synchronized operation with a power system source connected electrically to
the motor and delivering a mechanically driven load connected to its shaft, the
characteristic of allowable time versus the voltage dip is also embedded into the
numerical protection relay to protect the motor when there is a voltage dip, the
synchronous motor as substantially described and illustrated herein with reference
to the accompanying drawings.
This invention relates to a simplified method of providing an out-of-step
protection to a synchronous motor against a voltage dip which may occur on the
power system feeding the voltage to the motor. Voltage dip in a power system is
a normal occurrence and happens whenever there is a disturbance like a fault or
abnormal condition in power equipment connected to the system. The dip in
voltage may be for a moment, like a flicker, or it may persist for a longer
duration. The amount of voltage dip also depends on the grid and the location of
a station. For instance, in a geographical station located in desert like place,
connected through long distance transmission lines, the grid may be weak and
may cause larger voltage dips which take longer time to restore to normal. In
this invention a simplified method of protection of the motor and other
connected equipment is claimed, when there is a voltage dip, and thereby saving
a severe abnormal condition such the out-of-step operation.
The invention uses a method outlined to compute the torque for a given set of
voltages applied to the motor terminals. As a result of the motor torque and the
load torque the rotor dynamics is computed and the time elapsed for the load
angle to deviate to a limit within the safe operating zone of stable condition is
found. The same is computed for different amount of voltage dip conditions. And
this characteristic is computed. This characteristic is subsequently is embedded
into a numerical protection relay.
The numerical protection relay is a multifunction device which provides many
protection functions. It 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 an output signal to a breaker or the control and
instrumentation unit. The characteristic of allowable time versus the voltage dip
is also embedded into the numerical protection relay. This protects the motor
when there is a voltage dip.