Reverse Power Protection of An Alternator
INTRODUCTION
Today’s complex power scenario demands uninterrupted supply for
consumer. Therefore generator is the ultimate source of today’s life and its function
is of heart of power system. Hence it needs the maximum protection from the
occurring faults.
The generator used is a synchronous machine which can either run as
generator or motor depending upon the form of input energy. These generator are
synchronized with the bus-bar that is the grid.
These are number of faults which may cause heavy damage to
generator and simultaneously to the economical situation. The faults are described in
the table. Some Abnormal condition and protection system.
Sr.
No.
Abnormal
ConditionEffects Protection
1. Thermal over
loading continuous
overloading failure
of coolant.
Over heating of stator
winding and insulation
failure.
Thermocouples of
resistor thermometer
embedded in stator slots.
2. External faults fed
by generator
Unbalanced loading stresses
on winding and shaft.
Excessive heating for
prolonged short circuit.
Negative phase sequence
protection for large
machine overload
protection for small
generator.3. Stator faults phase
to phase, phase to
earth inter turn
winding burnt out, welding
of core lamination
Biased differential
protection sensitive earth
fault protection, inter turn
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Reverse Power Protection of An Alternator
fault protection.4. Rotor Earth fault Single fault does not
harmful, second fault causes
unbalanced magnetic forces
causing damage to shaft,
bearing etc.
Rotor earth fault
protection
5. Loss of field
tripping of field
circuit breaker
Generator runs as induction
generator field failure
driving excitation current
from bus-bar speed increases
slightly.
Loss of field or
production
6. Motoring of
generator that is it
turns as such
motor.
Effect depend upon type of
prime motor and the power
drawn from the bus-bar
during motoring.
Reverse power protection
by directional power
relays direct the reversal
of power.7. Over voltage Insulation failure Lightening arrestor
connected near
connected near
generator terminal.8. Over fluxing Heating of core Over fluxing protection
by volts/
hertz relay.9. Under frequency failure of blades of Steam
turbines.
Frequency relays.
For this range of phase angle power factor is negative that is Cosφ is
negative. P=VIcosφ.
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Reverse Power Protection of An Alternator
Eventually power becomes negative which is termed as reverse
power.
CONDITION FOR REVERSE POWER
Following are the possible circumstances in which reverse power
condition may arise.
i) When number of units are operating in parallel and failure of one unit may result in
feeding power to the suffered unit from healthy units.
ii) When one of the unit from all connected parallel units is overloaded and its
consequence would be reduced speed. Due to reduction of speed, frequency
decreases and ultimately emf generated goes on decreasing and finally a critical
stage is reached where is machine (alternator) is driven as motor.
iii) When load is suddenly thrown off, terminal voltage of bus-bar increases instantly.
If this condition is not detected properly again alternator will run as synchronous
motor.
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Reverse Power Protection of An Alternator
WHAT IS REVERSE POWER ?
When the input to the turbine is stopped the generator continuous to
rotate as synchronous motor taking power from the bus-bar. During the motoring
action of the generator the power flows from the bus-bars to the machine and this
condition is called as REVERSE POWER.
GRAPHICAL REPRESENTATION
i) Normal Condition :- During normal condition angle between load current and
terminal voltage lies between 00 to 900.
For a typical inductive load the voltage current waveform is shown
below.
V I
φ 0< φ < 90°
For this range of phase angle power factor (cosφ) is positive and
therefore (P=VI cosφ) power is of forward in nature.
ii) Reverse power condition :- During abnormal condition the angle of
commencement of two quantities exceeds 900.
The waveform for this condition is shown below.
V I
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Reverse Power Protection of An Alternator
900 < φ < 2700
φ
iii) Operating times and various characters.
As the fault level increases rapidly fault clearing becomes a must.
The static relays do not have moving parts in their measuring circuits, hence relay
timer of low values can be achieved ( 1 cycle, 1 ½ cycle ) such low relay times are
impossible with conventional electro mechanical relay system which compromise
measuring relay plus several auxiliary relays.
Variety of characteristics can be obtained with static relays, therefore
selectivity, stability and adequateness can be achieved. Measurement of several
quantities such as negative phase sequence component, frequency, harmonics,
impedance etc. can be performed by static measuring unit.
iv) Representing times and overshoots
By using special circuits, the resting times and overshoot time can be
reduced, thereby the selectivity can be improved.
v) Remove Backup and Monitoring
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Reverse Power Protection of An Alternator
Static relays restricted by power line carrier can be used for remote
back up and network monitoring.
In centrally monitored systems, the backup protection is monitored by
the digital computer. The switching is carried out in such sequence that the stability
is improved.
vi) Static Relays can “think”
Complex protection scheme employ logic circuits (Logic means the
process of reasoning, induction or deduction). Suppose several conditions are
imposed on a protective system such that for certain conditions, the relay should
operate and for some other conditions, the relay should remain stable in such cases,
logic gates can be operated.
EFFECTS OF REVERSE POWER
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Reverse Power Protection of An Alternator
During reverse power condition alternator runs as synchronous motor
and the turbine acts as a load. Motoring protection is mainly for the benefit of the
prime mover and load coming on the generator bus-bar while motoring reerse power
protection measures the power flow from bus-bars to the generator running as a
motor. Normally the power taken in most cases is low of the order of 2% to 10% of
noted power, power factor and current depends on excitation level.
A single element directional power rely, sensing the direction of
power flow in any one direction, phase is sufficient. The setting depend on the type
of prime mover.
Let up see effects of motoring on different types of turbine.
1) STEAM TURBINE
Back pressure steam turbines sets should be protected by sensitive reverse power
protection. The blades of steam turbine gets overheated quickly as the steam gets
trapped, if rotated in opposite direction due to windage. In steam turbine the steam
acts like coolant of the turbine blades and maintain them at constant pressure and
temperature. If the steam flows stops the blades get overheated due to windage. In
condensing type steam turbine the heating of blades is slower hence reverse power
protection may not be necessary.
For large turbo generator with back pressure type, non condensing
steam turbines, sensitive reverse power protection with sensitivity of the order 0.5%
rated power is preferred. The relay should have directional stability for the entire
relay operating zone.
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Reverse Power Protection of An Alternator
2) RECIPROCATING PUMP :
Motoring is harmful to the engine, hence the reverse power protection
is sensitive and engine must be disconnected from generator shaft during motoring.
3) HYDRAULIC TURBINE :
The water is generally fitted with mechanical devices which detects
the low water flow because such a flow causes cavitation. However reverse power
protection may be provided to operate for motoring power less than 3% of rated
power.
4) GAS TURBINE :
The gas turbine driven generator should not be permitted to operate as
a motor because the gas turbine offers a load of 10 to 50% of full load during
motoring. The Factor to be considered are as follows.
1) Capability of prime mover to run as load.
2) Load current drawn while motoring.
The reverse power protection is generally set for 10% of rated power
in reverse condition.
Motoring of synchronous machine does not change the direction of
rotation of rotor.
STATIC RELAYS
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Reverse Power Protection of An Alternator
Electromagnetic relays have almost considered place to static relays
because of the latter’s number of advantages.
Static relays (Solid State Relay) is an electrical relay in which the
response is developed by electronic/magnetic/optical or other components without
mechanical motion of component.
1) ADVANTAGE OF STATIC RELAYS
The static relays compared to the corresponding electromagnetic relay
have many advantages.
i) Low Power Consumption
Static Relays provide less burden on current transformers and
potential transformers as compared to electromagnetic relays. In other words the
power consumption is much lower than conventional one reduced consumption has
the following merits.
Current transformers and potential transformers of less KVA rating.
Accuracy of current transformer and potential transformer is
increased.
Air gapped current transformer can be used.
Problem arising out of current transformer saturation are avoided
overall reduction in cost of current transformer and potential transformer.
ii) No Moving Contact
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Reverse Power Protection of An Alternator
Solid state devices do not have moving contacts. Therefore there are
no problems of contact bounce, arcing, contact erosion etc. in the static relay circuit.
iii) Operating times and various characters
As the fault level increases rapidly fault clearing becomes a must.
The static relays do not have moving parts in their measuring circuits, hence relay
timer of low values can be achieved ( 1 cycle, 1 ½ cycle ) such lo relay times are
impossible with conventional electro mechanical relay system which compromise
measuring relay plus several auxiliary relays.
Variety of characteristics can be obtained with static relays, therefore
selectivity, stability and adequateness can be achieved. Measurement of several
quantities such as negative phase sequence component, frequency, harmonics,
impedance etc. can be performed by static measuring unit.
iv) Representing times and overshoots
By using special circuits, the resting times and overshoot time can be
reduced, thereby the selectivity can be improved.
v) Remote Backup and Monitoring
Static relays restricted by power line carrier can be used for remote
back up and network monitoring.
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Reverse Power Protection of An Alternator
In centrally monitored systems, the backup protection is monitored by
the digital computer. The switching is carried out in such sequence that the stability
is improved.
vi) Static Relays can “think”
Complex protection scheme employ logic circuits (Logic means the
process of reasoning, induction or deduction). Suppose several conditions are
imposed on a protective system such that for certain conditions, the relay should
operate and for some other conditions, the relay should remain stable in such cases,
logic gates can be operated.
vii) Effects of Vibration and shocks
Most of the component in static relays, including the auxiliary relay in
the output stages are relatively indifferent to vibrations and shocks. The risk of
unwanted tripping is, therefore less with static relays as compared to the
electromechanical relays. This aspects makes the static relays. Unequally suitable
for earth quakes prone areas, ships, vehicles, locomotive.
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Reverse Power Protection of An Alternator
LAMINATION OF STATIC RELAY
i) Auxiliary Voltage Requirement
This disadvantage is not of any importance now as auxiliary voltage
can be obtained from station battery supply and conveniently changed to suit local
requirement.
ii) Voltage Transient
The static relays are sensitive to voltage spikes or voltage transients.
Such transients are caused by operation of breaker and isolator in the primary circuits
of current transformer and potential transformer, severe over voltage are also caused
by breaking of control circuit relay contact etc. Such voltage spikes of small
duration can damage the semiconductor components and can also cause null-
operation of relays.
Special measures are taken in static relays to overcome this difficulty.
These include, use of filter circuit in relays, screening the cable connected to relays.
iii) Temperature Dependence of Static Relays
The characteristics of semiconductors are influenced by ambient
temperature for example the amplification factor of a transistor, the forward voltage
drop of diode etc. change with temperature variation.
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Reverse Power Protection of An Alternator
Accurate measurements of relays should not be affected by
temperature variation. Relay should be accurate over wide range of temperature.
This difficulty is overcome by the following measures.
Individual component in circuit are used in such a way that change in
characteristics of components does not effect of the characteristics of complete relay.
Temperature compensation is provided by means of thermistor circuit,
digital measuring techniques etc. Thus modern static relays are designed to suit wide
limits of temperature.
iv) Price
The price of static relays is higher than the equivalent electro
mechanical types. The prices of semi-conductor components have been reduced
during past years and a further reduction is expected.
v) In electro mechanical relays the pick up of relay are rest of relays does not effect
the relay characteristics since the operation is based on the comparison between
operating torques.
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Reverse Power Protection of An Alternator
DESIGN AND APPLICATION OF
COMPONENTS
DESIGN OF TRANSFORMER
Therefore three transformer in the circuit, two of them have three
winding on each core having primary consisting of two winding current coil and
pressure coil and that of secondary consisting of only one winding.
Another remaining one transformer is for dual supply which is a
simple potential transformer.
RATTING OF TRANSFORMER
Primary Secondary
Special transformer P.c.230 v.c.c.5A 8 v 1 A
Potential transformer 230 v 16-0-16 v 200 mA
Under the design aspects of the first two transformer (Special
transformer)
As it is not possible theoretically to design such a transformer by simple design. We
have to make it possible by trail and error method.
Step wise it is such that first, we have to design simple potential
transformer of 230 v/8 v and lamp. Then turns per volt are calculated. Secondly
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Reverse Power Protection of An Alternator
design a current transformer for 5 A/1 A. Turns per volt are calculated. Then taking
their average, the specification are calculated.
The design of small transformer that is (16-0-16)v, 200 mA) rated is
made.
DESIGN DATA
A) Data presentation of special transformer
Pressure coil - 230 v Current coil turns - 84
Current coil - 5 A Secondary winding turns - 84
Secondary coil - 8 v 1 A Pressure coil conductor – 35 swg
Turns per volt (TE) - 10.34 Current coil conductor – 22 swg
Cross section area of core - 1.2 sq. inch Lamination -15 Nms, NGO
Pressure coil turns - 2400 Bobin - 15 N, Nylon 2nd
Stack - 1.25 sq. inch
Identification of PC, CC, SW
Pressure coil - Blue colour sleeve
Current coil - Green colour sleeve
Secondary winding - Red colour sleeve
B) Data specification of small transformer rated
230 v/16-0-16 v and 200 mA
Primary winding - 230 v
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Reverse Power Protection of An Alternator
Secondary winding - 16-0-16 v
Turns per volt - 21.78
Primary turns - 5250
Secondary turns - 348-0-348
Primary winding conductor - 44 swg
Secondary winding conductor- 44 swg
Laminations - 12 Nms
Stack - 0.75 sq. inch
Identification of Primary Winding, Secondary Winding, Tap
Primary winding - Black
Secondary winding - Red
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Reverse Power Protection of An Alternator
DESIGN AND OPERATION OF DUAL POWER SUPPLY
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DU
AL
PO
WE
R S
UP
PL
Y
Reverse Power Protection of An Alternator
Data Specification
Step down transformer - 230 v /32 v
(16-0-16 v)
Rectifier Bridge - G I, Wo4 G, 0528
Capacitors - 1000 µf, 50 v
Monolithic Fixed Voltage Regulator 1) IC 7812
2) IC 7912
FUNCTION OF COMPONENT
Transformer –
For stepping down the voltage, we have decided to design center
tapped 16-0-16 volts secondary because we require ± 12 v
Bridge Rectifier –
It is used to convert input AC into proportionate DC ( not pure but
unidirectional one). Its output voltage is 32 v DC
Capacitors –
These are used filtering pulsated output voltage of bridge rectifier and
for voltage division.
Therefore voltage across each capacitor = 16 v
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Reverse Power Protection of An Alternator
Monolithic Voltage Regulators –
As our circuit needs + 12 v and – 12 v for operation of op-Amp, we
used.
IC 7812 for + 12 v supply
IC 7912 for – 12 v supply
For giving constant + 12 volt
Merits of IC Voltage Regulators
i) Facilities for building positive and negative regulates voltage
ii) Battle ripple rejection
The transformer step down the incoming 230 v Ac to the required 32
v. This step down Ac voltage will be rectified in bridge rectifier. The capacitor will
remove ripples from rectified output. This DC is now an input to the combination of
three terminal regulators.
IC 7812 will give regulated + 12 v. And IC 7912 will give regulated –
12 v with respect to common point that is ground point. The ground point is
achieved by tie the common points of 7812 and 7912 to the center tap of transformer
winding that is secondary winding.
OTHER COMPONENT AND THEIR FUNCTIONS
1) Capacitor Connected Across Relay Coil
Rating – 470 µf, 25 v
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Reverse Power Protection of An Alternator
It is connected across relay coil which filters the DC voltage and
completely removes chattering of relay contacts and suppress back emf induced in
relay coil.
2) Diode connected between IC 741’s pin 6 and transistor base
Rating IN 4007
It is used to back AC and to pass only DC voltage to transistors base.
3) Transistor
Rating CL 100 (N-P-N)
It is high gain current amplifier transistor. Its base has been given the
output of IC 741 through 3.3 K Ω resistance. It drives relay actuation provided to
comparator has given voltage signal.
4) Capacitor connected across output terminal of rectifier bridges 1 and 2 are for
filtering purpose.
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Reverse Power Protection of An Alternator
VOLTAGE COMPARATOR ELEMENT
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VO
LT
AG
E C
OM
PA
RA
TO
R
Vd
Reverse Power Protection of An Alternator
IN STATIC RELAY
Voltage Comparator Element in Static Relay –
The strategic element used in out project is a voltage comparator
which gives actuating signal to relay under abnormal condition.
The function of comparator that is voltage comparator is to compare
the voltage at one of its input against the voltage at the other. Also to produce either
a high or low output voltage, depending on which input has higher level.
The comparator circuit for voltage comparator using on op-amp is as
shown in fig. By using such type of comparator we can get three types of output.
Vo = VOL for Vp < Vn
Vo = VOH for Vp < Vn
Vo = 0 for Vp < Vn
Where VOL and VOH denote low and high output levels. Thus if we
introduce the differential input voltage Vd = Vp – Vn the above equation can be
rewrite as
Vo = VOL for Vd < 0
Vo = VOH for Vd > 0
The input output characteristic for comparator is as shown in
following graph.
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VO
VOH
VOL
Reverse Power Protection of An Alternator
Thus Vp, Vn and Vd can assume all possible values while Vo is
restricted only two valve VOL and VOH
In comparator application Vd is usually in the range of mV to volts.
The voltage comparator may be inverting or non inverting type
depending upon the output from comparator that is if output voltage is high it is said
to be non inverting comparator and if it is low the comparator is said to be inverting
comparator.
RELAY AND CONTACTOR
RELAY AND CONTACTOR
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Reverse Power Protection of An Alternator
We have used a simple attracted armature type relay for tripping
circuit under abnormal condition.
Relay is a device by means of which an electric circuit is controlled
by change in other circuit.
The Relay has an energizing coil when the fault occurs input to relay
is sufficient, the coil energized and it pulls off the plate which opens the contacts and
hence the contactors are opened and protective machine is disconnected from the
supply. Attracted armature relay responds to both ac as well as dc actuating
quantities. These are essentially very fast relays.
Relay used in circuit has two set of contacts, one is Normally close
(NC) whereas other is normally open (NO)
Rating ( 12 V, 200 Ω )
We have provided an indication facility for tripping under abnormal
condition. A small buzzer will not give sound when abnormal condition is reached.
CONTACTOR
Although in our simulated model we haven’t used contactor it comes
into picture for breaking the circuit when abnormal condition is reached.
A contactor is a mechanical switching device having one position of
rest (corresponding to the position of main contacts either open or closed) operated
otherwise than by hand, capable of making, carrying and breaking overload
conditions Electromagnetic contactors are popular among all types of conductor.
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Reverse Power Protection of An Alternator
We have used contactor with 4 No. contacts and 1 NC contact.
Conductor has main contact as well as auxiliary contact the main contacts are used
for switching on and off the power circuit whereas auxiliary contacts are used in the
control circuit. The contactor coil is to be energized from a source of supply.
The difference in electromagnetic contactor and a relay is that a
contactor has three main contacts and a few auxiliary contacts whereas relay contact
has only auxiliary contacts.
WORKING
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Reverse Power Protection of An Alternator
WORKING OPERATION :
The main criterion principle of our mechanism is to make the
synchronous generator to dead state. When its prime mover faults that is when starts
running as the synchronous motor drawing power from bus-bar.
Now under these conditions we know that synchronous generator and
the bus-bar are synchronized in all aspects. All the parameter of the synchronous
generator as well as the bus-bar are in phase.
I the normal state the magnetic flux created by pressure coil and
current coil of transformer one are subtractive and that of transformer two are
additive so at the secondary side output voltage of transformer one will be less than
transformer two’s secondary. The output of both transformer is rectified by separate
bridges and their positive volt terminals are connected to comparator’s (Op-amp) pin
no.2 and 3 that is DC output of transformer one is connected to pin no.3 and that of
transformer two is connected to pin no.2 and as we have seen in normal condition
(that is alternator is supplying power to bus-bar) output of transformer two is higher
than transformer one which is connected to inverting terminal of IC 741. So at the
pin no 6 of IC 741, output voltage will be negative and as we are using transistor CL
100 (N-P-N) with output of IC 741 connected to its base. It will not conduct and coil
will not getting energized, so its Nc contacts will remain in that state only.
Suppose due to any reason Alternators terminal voltage is less than
that of bus-bar voltage, it will act as a load on bus-bar and on the verge of motoring
at that instant due to reversal of power, that is currents direction is negative with
Govt. Poly. Washim. 26
Reverse Power Protection of An Alternator
respect to voltage. Magnetic flux created by transformer one’s pressure and current
coil will be additive in nature and that of transformer two will be differentiative. In
this case positive terminal of rectifier bridge (Connected to secondary of transformer
one) will be at higher potential with respect to second bridge. So potential at pin no
three of IC 741 is high and that of pin no.2 is low, so output of Op-amp IC 741 of pin
no.6 will be positive and transistor CL 100 N-P-N will conduct because N-P-N
transistors base will get +ve potential and relay coil will get energized and will open
Nc contacts of relay. These are coupled to contacts of conductor. So contactor will
break the circuit of alternators. Thus it gets disconnected from bus-bar.
In our simulated model instead of using alternator we have utilized
basic property of single phase Ac supply. By reversing phase and neutrals of current
coil with respect to pressure coil. We obtained both normal as well as abnormal
condition and further operation takes place as stated above that is during abnormal
condition relay open its close contacts and buzzer will not gives sound.
During normal condition relay closes its contact and buzzer will gives
sound.
CONCLUSION
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Reverse Power Protection of An Alternator
Actually when the power is reversed, there is no harm to the generator
as it is synchronized machine, so when it receives the power from the grid, it starts
rotating as motor. The main and serve damage is to the turbine that is the prime
mover and generator bus-bar as load coming while motoring.
After connecting this protection circuit to generator unit we can avoid
waste of the in repair or replacement in turbine or prime mover and also economical
loss. It also avoids cascade tripping of generating units and ultimately maintaining
the system stability to higher extent.
REFERENCES
1) Switch Gear and Protection
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Reverse Power Protection of An Alternator
- S. S. Rao
2) Power System Protection
- T. S. M. Rao
3) Electrical Machine Design
- A. K. Sawhney
4) Integrated Ciecuit
- K. R. Botkar
Govt. Poly. Washim. 29