EXPERIMENT VII - Anadolu Ü · PDF fileapplication and reason for parallel operation. b)...

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT VII 1

Instructor: Assist. Prof. Dr. Sener AGALAR

TA: Res. Asst. H.Ersin EROL

Res. Asst. Mustafa DEMİRTAŞ

ANADOLU UNIVERSITY

DEPT. OF ELECTRICAL AND ELECTRONICS

ENGINEERING

EEM 471 ELECTRICAL MACHINERY

LABORATORY

EXPERIMENT VII

3Φ Synchronous Machine

Paralleling the Alternator with 3Φ Network

http://www.anadolu.edu.tr/


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).


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.


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.


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.


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


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.


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.


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


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


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.

EXPERIMENT VII - Anadolu Ü · PDF fileapplication and reason for parallel operation. b)...

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