EXPERIMENT VII - eem.eskisehir.edu.tr
Transcript of EXPERIMENT VII - eem.eskisehir.edu.tr
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 1
Instructor : Assist. Prof. Dr. Sener AGALAR
TA : Res. Asst. H. Ersin EROL
ANADOLU UNIVERSITY
DEPT. OF ELECTRICAL AND ELECTRONICS
ENGINEERING
EEM 471 ELECTRICAL MACHINERY
LABORATORY
EXPERIMENT VII
3Φ Synchronous Machine
• Paralleling the Alternator with 3Φ Network
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PARALLELING THE ALTERNATOR WITH THE THREE PHASE NETWORK AND
OPERATIONS TO REGULATE THE P AND Q EXCHANGE
An operation which is frequently performed in a power station is the paralleling of
alternators with the energy distribution network. Actually, the local distribution networks are all
interconnected, to cover the whole national area; the national network is then often connected to
those of the neighbour countries for the exchange of the generated electric power, as for any
good.
After the paralleling operation, the single alternator in the power station is therefore
connected to a circuit in which many others alternators are already generating power and in
which many loads are requiring current, with a power exchange much greater than the maximum
power that the alternator may generate.
The consequences of this power disproportion are:
1) The network forces to the connected alternator its voltage and frequency values; these
values can't be changed, even when the poles excitation current or the torque on the
machine axis are varied.
2) Any operation trying to change voltage or frequency of the alternator will produce, on
the contrary, a variation of the reactive power and of the active power exchanged
between machine and network.
Given to the small magnitude of its rated power, from the paralleling with the network the DL
1026A alternator will be subjected to the same operating limits of the alternators in the power
stations and, therefore, it will be possible to experimentally verify what has been said in points 1)
and 2) above.
The paralleling of the alternator with the network may be performed, without producing a
strong short-circuit, only when:
1) The alternator's frequency is equal to the network's frequency.
2) The alternator's voltage is equal to the network's voltage.
3) The vectors of the alternator's and network's voltages are in phase.
4) The cyclic direction of the alternator's voltages coincides with that of the network's voltages.
The conditions indicated in point 4) is a direct effect of the application of condition
indicated in point 3) to the three phase voltages.
To check the equalities described in points 1) and 2), of course, only one switchable
frequency meter and voltmeter is required; the phase coincidence (point 3) may be measured
through special indicating instruments (index synchronoscopes) or through the signalling of
specially connected lamps (rotating lights synchronoscopes or flashing lights synchronoscopes).
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 3
BASIC PRINCIPLES:
1- Automatic synchronizing of a generator consists of electrically “coupling” the generator
output to another source of electrical energy and operating the generator such that its output
adds to the other source.
2- Automatic synchronizing can encompass a wide variety of conditions such as:
a) Two or more equal or similar-sized generators which, when paralleled to each other,
will operate as though they were one larger generator. This is the most common
application and reason for parallel operation.
b) Two or more unequal-sized generators which are operated in parallel as though they
were one larger generator. This is also a common condition.
c) Generator systems (which may consist of two or more individually paralleled
generators) which are operated in parallel with another electrical system which, by
comparison, is infinitely large. This is the case of operation in parallel with the normal
electrical utility source. This is commonly done for on-site peak shaving, bottom
shaving or cogeneration systems. It may be done momentarily in some special cases.
Benefits of automatically-synchronized (paralleled) systems:
1 Economy
An existing distribution system may not lend itself to being split into several sections and
handled by separate non-paralleled units. When the loads are expected to expand
substantially, the initial investment is minimized by installing one smaller generator set,
and then adding more sets in parallel as the loads increase.
2 Reliability
When a part of the emergency load is deemed very critical, it may be desirable to have
more than one generator capable of being connected to that load. When there is a normal
source outage, all generators in the system are started. The probability of having a
generator start and achieve nominal voltage and frequency is increased according to the
number of sets available. The first set ready to handle the essential load does so. As the
other generators are running and connected to the bus, the remaining loads are connected
in declining order of priority.
TYPES OF SYSTEMS:
There are two types of paralleling systems:
a) Sequential paralleling
In sequential paralleling, the engine/generator sets are connected to the bus in a
predetermined order. The lead engine is connected to the bus first. When the
engine/generator selected as number 2 is ready to be connected, a synchronizer is
connected between the output terminals of generator 2 and the bus. Then the generator is
in synchronism, its paralleling circuit breaker is closed, connecting it to the bus. Usually,
a restriction is imposed to limit the time the controls will consume in attempting to
synchronize and parallel a set to the bus before reconnecting the controls to the next set
in sequence.
b) Random paralleling
Random access permits simultaneous synchronizing of each set to the bus. The random
access method is faster than sequential paralleling but more expensive. Codes mandating
emergency loads
to be reconnected within ten seconds may require the method of operation. With diesel or
natural-gas-driven engine/generator sets, it is reasonable to expect that the emergency bus
will be established within the ten-second limit in a random access system, because any
one of the generators can be first on line.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 4
SYNCHRONIZING BASICS:
1) To successfully synchronize a generator to a bus requires some degree of instrumentation
to tell the operator what the phase relationships are between the two sources. The
simplest is two voltmeters connected to read voltage between the same phases of the
incoming generator and the bus. When the two sources are in phase and at equal voltage,
both Va and Vb will read 0 volts. (The third phase will also be the same since, if any two
are correct, the third must be correct.) When the phases are 180° out of sync, the
voltmeters will read 2 x normal system voltage. As the phases go in and out of sync the
voltmeters will drift from 0 to 2 x to 0 at a rate which depends on the slip frequency
(frequency difference). The breaker closure must occur when the voltage difference is at,
or very near, 0. Otherwise each source will be subjected to extreme currents and forces
which will damage the equipment. Out of sync voltage differences (and resultant forces)
increase rapidly with increasing phase to phase mismatch angles. In general the forces are
acceptably small if the phase angles are within about ± 15° of true synchronism.
2) Two synchronizing lights can be used in place of voltmeters. When the lights are out, the
phases are synchronized. When the phases drift out of sync, the lights will come on due
to the voltage difference. It is usual to use three lights to cater to the possibility of one
burned bulb. Bulbs must be rated for 2 x voltage.
3) A synchroscope is a pointer-type meter which incorporates the two voltmeter movements
with a single pointer. The pointer moves to a circular position dependent on the voltage
difference. At zero volts it will be located at top dead centre. The synchroscope position
is representative of voltage difference, not phase displacement angle. Any area within
about 30° to 45° of top dead centre represents a fairly small voltage difference
corresponding to a fairly small phase-to-phase displacement. A synchroscope will rotate
at the slip frequency rate.
4) All of the foregoing are instrumentation devices which will allow an operator to observe
when synchronism occurs and to initiate breaker closure accordingly. The operator must
adjust the incoming generator speed (and voltage if necessary) to obtain synchronized
conditions).
5) For automatic systems, an automatic device must be used to obtain synchronized
conditions and initiate breaker closure at the proper time. There are a wide variety of
automatic synchronizers available to interface with various types of governors. The
synchronizer can also be utilized to match voltages as well as speed.
PROTECTION DEVICES:
1) When a synchronous generator is connected to an external electrical source, it is capable
of acting as though it were an electric motor. In the case of generator sets operating in
parallel, if the engine output power fails for any reason, such as shutdown, the generator
will motor the engine at bus frequency. The required power, usually about 10 to 20% of
rated power, will be provided by other machines. To prevent this occurrence, all
paralleled generators must be fitted with a reverse-power relay. The relay is set to open
the generator breaker at about 5 to 10% reverse power.
2) The generator breaker must be rated to withstand and interrupt the available fault currents
from the load bus. This may require special breaker considerations when paralleling to an
infinite bus.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 5
3) There are many additional protective devices which can be applied to single or parallel-
operated generators. Particular application considerations will determine the
requirements.
TECHNICAL CONSIDERATONS FOR AUTOMATICALLY-SYNCHRONIZED SYSTEMS
1) The generator output must be the same as the bus; that is:
– Same number of phases
– Same phase to phase voltage
– Same phase rotation (e.g. ABC or ACB)
2) The generator and bus AC waveforms must be in identical phase relationship at the time
of breaker closure to connect them. This is called the in phase or synchronized
condition. Note that if the phase rotations are the same, then the B to B and C to C
relationship will be identical to the A to A relationship. If the phase rotations are opposite
then synchronism of all 3 phases can never be achieved. If the breaker is closed to
connect the two sources based on only one phase being in synchronism, major damage
can immediately occur.
3) Only when the two sources are inphase or synchronized (each phase voltage matched,
phase rotation matched and phase angles matched) can the two sources be connected
together.
4) Once the two sources have been connected together they will remain in synchronism no
matter what (unless the breaker(s) open and disconnect one of the sources). The two
sources are effectively “geared” together by electrical forces.
5) If the two sources are two equal generator sets, say for example 2 x 500 kW as soon as
they are in parallel, the system should now behave as though it were a single 1000 kW
generator.
6) The key to parallel operation is to make the system behave as it should. The challenge
comes from the fact that the “single” generator has two regulator exciters and two
governor systems. The characteristics of the two machines must be matched for the
“whole” system to function correctly.
7) The voltage and frequency controls of a paralleled generator not only control voltage and
frequency.
(a) Voltage control (excitation control) now controls the reactive power output of the
generator. If the generator is over excited, instead of the voltage rising the excess
excitation will result in generation and delivery of excess kVARs to the bus. If it is under
excited it will “absorb” kVAR’s from the bus. When the excitation level is exactly
correct for the actual bus voltage the generator will share the kVAR’s required by the
load.
(b) Frequency control (governor speed control) now controls the real power output of the
generator set (kWe output). If the governor frequency (speed) setting is higher than the
actual bus frequency, the governor will sense an underspeed condition and attempt to
correct the condition by increasing the fuel. This can only result in increased power
output. Likewise if the governor frequency setting is below the actual bus frequency, then
the governor will sense overspeed and react by reducing the fuel.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 6
8) In the case of two or more engine generator sets operating in parallel, it is readily
apparent that the regulators and governors must function together to achieve system
control.
9) In the case of an engine generator paralleled to an infinite bus, it is not possible to control
the infinite bus. Its regulators and governors are not accessible, and even if they were,
other considerations (such as other connected customer needs) would prevent adjusting
the bus controls to satisfy an insignificantly small paralleled generator. For paralleling
considerations a bus can start to be considered as infinite when the bus capacity is about
5 times the paralleled generator capacity. Thus if a 100 kW generator is paralleled to a
bus powered by a 1000 kW generator, it is essentially being connected to an infinite bus.
(There are exceptions to this condition but these are beyond the scope of this sales and
marketing seminar)
10) This is a classic case of “two halves do not necessarily make a whole”. However, the
control of paralleled generator(s) is in fact simple, reliable and extremely versatile.
ELECTRIC DIAGRAM
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 7
The exactly istant when the parallel operation is done is when the LI turns off and the voltage
and the frequency are equal to the network.
1 - Now, the alternator is in parallel connected with the network and, when the operations
have been correctly performed, it will perfectly balance it. The ammeter and the
wattmeters inserted between machine and network point out that there isn't any power
exchange.
2 - Try to increase the rotation speed of the driving motor. The speed remains perfectly
unchanged; the ammeter and the wattmeters show now a current and power exchange,
that increases during this adjustment (it is better to pay attention not to exceed the rated
current value of the alternator, not to reach the stability limit of the machine).
It may be verified that:
a) The real power Wa + Wb is positive. This means that it is flowing from the
alternator to the network (the position of the labelled terminals of the
ammetric and voltmetric coils of the wattmeters connected to the circuit
doesn't leave any doubt).
b) The reactive power 3 (Wa - Wb) exchanged between machine and network
is very small and doesn't significantly vary during M adjustment.
3 - Try to reduce excitation of the driving motor. Note that the tachometer reading remains
perfectly constant, on the contrary, the values of the current and of the generated real
power decrease and, at a certain point, they become zero as in the istant of the paralleling
(point 1).When this adjustment is continuated, a new exchange will be noted of current
and real power between machine and network. This time, anyway, the Wa + Wb sum is
negative. The power goes now from the network to the machine, that is therefore
operating as synchronous motor.
4 - Return to the parallel condition until the I and P exchange is zero again. Now slowly
decrease or increase the alternator's excitation current, verifying that the output voltage
doesn't change. On the contrary, a new current and power exchange rises between
machine and network (here, too, it is suggested not to exceed the rated current value).
It may be observe that:
a) The real power Wa + Wb, exchanged between machine and network, is very small and
doesn't significantly change during this adjustment.
b) The reactive power 3 (Wa - Wb), exchanged between machine and network,
significantly varies as a consequence of this adjustment. It may be easily verified that
this (inductive) reactive power shows positive values, i.e. that is flows from the
alternator to the network, when the excitation current is increased. On the contrary, it
shows negative values i.e. flows from network to alternator, when the excitation
current is decreased.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 8
CONCLUSIONS
From the test results, the following very important conclusions may be derived:
1 - The real and reactive power exchange between an alternator and the network to which
the machine is in parallel connected (where the network shows a greater power) may
be adjusted at will through simple operations on the driving motor or on the
excitation circuit.
2 - By varying the torque developed by the driving motor, almost exclusively the real
power exchange between machine and network may be modified, while the reactive
power exchange doesn't significantly vary.
3 - By varying the excitation current the reactive power exchange between machine and
network is changed, while the real power exchange doesn't significantly change.
4 - By simultaneously acting on the driving motor and on the excitation it is possible to
adjust the P and Q values and make the alternator generate at the required cos φ.
Paralleling the alternator with three-phase network.
The purpose of this test is to realise the paralleling with the three - phase network and to
verify the exchange of the power P and Q.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 9
SEQUENCE OF OPERATIONS:
When the power supply is set, perform the following operations:
1 Set the controls of the modules
VARIABLE DC OUTPUT:
FIXED THREE - PHASE OUTPUT:
VARIABLE DC OUTPUT:
(excitation)
Parallel Board:
Switch open
Knob fully turned in CCW
Switch open
Switch open
Knob fully turned in CCW
Switch open
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 10
2 Close the switch on fixed three - phase module and control the voltage value of network. Close the switch of variable dc output and regulate the knob so that the output voltage of the alternator is near to the network value. Adjust the speed so that the set rotate with a speed close to the rated one. Then control the network frequency. Adjust the alternator excitation so that the output voltage is equal to the network.
3 Observe the lamps H1-H2-H3; you will note that they turn on and off following a time progression that gives the impression of "rotating" lights. Make this rotation very slow acting on the motor excitation by the knob of the excitation rheostat
4 Close the switch (position "on") on Parallel board when HI turns off and H2 -H3 show the same luminance to do the paralleling of the alternator.
5 If the operations have been correctly performed, the alternator will perfectly balance the network and the instruments inserted between alternator and network point out that there isn't any power exchange.
6 Verify the real power exchange between alternator and network.
6.a Slowly increase the excitation of the driving motor "trying" to increase the rotation
speed of the set. Note that the speed remains perfectly unchanged while the real
power is flowing from the alternator to the network.
6.b Now decrease the excitation of the driving motor "trying" to decrease the
rotation speed of the set. Note that the speed remains perfectly unchanged while the
output power and, at a certain point, it becomes zero as in the instant of the
paralleling.
6.c Decreasing again the excitation of the driving motor, you note a new exchange of real
power from the network to the alternator, that is therefore operating as synchronous
motor.
Ptotal (W) Vout Iout Iexc
-400
-300
-200
-100
0
100
200
300
400
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 11
7 Verify the reactive power exchange between alternator and network. Again return to the
paralleling condition zero setting current and power between network and alternator by
excitation of the driving motor. Now slowly act to either decrease or increase the alternator's
excitation. Verify that the output voltage doesn't change. On the contrary a new reactive
power exchange rises between alternator and network.
Iexc Vout Iout Qtotal (VAR)
0
0.1
0.2
0.23
0.3
0.4
0.5
0.6
8 Open the switch to stop the set.