Modified Electrical Q & Answer

Paralleling Alternators to Live Bus Bars To parallel an ac generator with another already on load, the following operations are necessary. See Figure 1and Figure 4. (a)

The terminal voltage of the incoming machine and the already running alternator on the bus bar must be equal.

(b)

The frequency of the voltage sources must be equal.

(c)

The phase sequence of the two voltages must be the same.

(d)

The emf of the incoming generator and the bus bars must be in phase.

The circuit breaker of the incoming generator must be closed when the voltages of the two generators are in phase with each other i.e. waveform of both bus bar and incoming generator are synchronised. This is done by adjusting the frequency/speed of the incoming generator and watching the synchroscope or synchronising lamps for the moment when the two voltages are in phase. If a synchroscope is used in paralleling, the two waveforms will be in phase when the synchroscope pointer remains stationery at 12 0’clock. This is not easy to achieve so in practice, when the pointer is rotating not faster than 1 revolution in 5 seconds in the fast direction, initiate closing of the circuit breaker when the pointer is at about 11 0’clock (considering circuit breaker mechanism also takes some time to close). In this way, with the incoming machine slightly fast, it will immediately assume load. If switch on when running slow, it would take motoring load which might possibly operate the reverse power relay. That is why reverse power relays are provided with time lag, to give sufficient time to correct the fault.

Bus Bars

R

Line Voltage

Y Line Voltage B

Circuit Breaker Alternator Phase Winding

Phase Voltage Figure 1

Dark Lamp Synchronising

Three lamps can be connected across the switch as shown in Figure 2 to form a simple synchroscope. When the phase angle, Ø, between bus bar and in-coming machine voltages becomes zero, VR, VY, VB, all become zero and the lamps becomes dark. The e.m.f. of the bus bar and the incoming generator are in phase. This is the required condition for closing the switch. There are two problems however. 1)

The lamps need a fair bit of voltage before their light becomes visible so they are not very accurate.

2)

When the speed is not quite correct there is not indication of whether the speed is too high or too slow.

Bus Bars

R Y B

L

L Y1

L

B1 R1 Incoming VR Generator R2

R1 VB

B2

B1 VY

Y1

Y2

V R1

VR

V

V

Y

V Y1

R2

V

Figure 2

V Y2

VB2

B1

VB

Sequence Method of Synchronising The condition for synchronising is indicated when the red lamp is out and the yellow and blue lamps are of equal brightness. This is much more accurate. Furthermore, the sequence of brightness reverses from too slow to too fast.

Bus R Y B L

L

L

V

VR

V

V Y2

R1

R2

Incoming Generator

V

B1

VR V

B2

R2

R1 VB

B2

B1 VY

Y1

Y2

Paralleling with Synchroscope

Figure 3

Bus Bars

R Y B Incoming Voltmeter

V

Incoming Generator

F

Synchroscope

Figure 4

Synchronising Power

S

V Bus Bar Voltmeter

V

Y1

If one alternator is ahead of another in phase when the synchronising switch is closed; i.e. the generator emfs are not in phase, a resultant emf, Ec ia set up in the local circuit even though the alternator e.m.f.s or terminal voltage are equal. The synchronising current, Ic set up by this e.m.f. takes power from the machine that is running faster and delivers it to the machine that is running slower. Thus the faster machine is pulled back or accelerated by motor action, until their voltages come more nearly into phase. (A time delay of say 5 seconds prevents reverse power tripping due to surges at synchronising). This action reduces Ec and therefore reduces Ic so that again, the synchronising current limits itself. The greater the phase displacement of the e.m.f.s when the synchronising switch is closed, the greater are Ec and Ic and the more violent is the action pulling the machines into phase. This sets up dangerous high values of torque on the shafts against the driving torque of the prime movers. Assume machines are synchronised when E1 of alternator 1 is Φ° in advance of its proper phase relation to E2 of alternator 2 shown as E1’.

IC

g

Ec 0 E2

E'1 •

φ

Figure 5

E 1

Resultant e.m.f. is Ec. The current Ic again lags behind Ec by the angle θ determined by the ratio of reactance to resistance in the circuit between the machines. The power P1 generated in alternator 1 is P1

=

E1Ic cos α

and power P2 generated in altrernator 2 is P2

=

E2Ic cos β

Note that P1 is positive, representing generator action and P2 is negative representing motor action, This results in pulling E1 back (clockwise) and pushing E2 ahead (counter-clockwise) thus bringing them more nearly into phase (180° to each other in the local circuit) as they should have been when synchronised. The mechanical power which is exchanged between the machines while they are out of phase and which brings them into phase is very important for the successful operation of alternators in parallel. It is called synchronising power and the circulating or synchronising current is so called because it keeps the machine in synchronism.°

PARALLEL OPERATION OF 2 ALTERNATORS

After two alternators are synchronised, it is worthwhile to consider how their operation is affected by a change of either the excitation or power input to prime-mover. Consider 2 unloaded alternators running in parallel with E1 = E2 so that the resultant voltage in the local circuit is zero. Their emf’s are in phase wrt the external circuit and in phase opposition wrt the local circuit as shown in Figure 6. Figure 6 •

Effect of change prime-mover power input on no-load E1 I

I1

I2 XS

IS Terminal volttage

R

across a load

E1

E1

E2

E2

E2 External Circuit

Local Circuit

Suppose the alternator 1 gets extra input, its rotor will accelerate and emf E1 will get ahead of E2. As a consequence there is a resultant voltage Ec = E1 - E2 in the local series circuit which will drive a current, Ic through the two armatures. The current Ic lags behind Ec by 90° (because alternator impedance is predominantly reactive). From Fig. 7 it is seen that Ic known as synchronising current is almost in phase with E1 and in phase opposition to E2. Alternator 1 is generating a power: E1 Ic cos θ1

which is positive ( θ1 < 90° )

Alternator 2 is generating a power: E2 Ic cos θ2

which is negative

( θ2 > 90° ).

In other words, alternator 1experiences a generating action tending to retard it and alternator 2 receives the power generated by Alternator 1 and thus experience motoring action tending to accelerate it. It is thus seen that there is an automatic synchronizing action, tending to retard the faster machine and accelerate the slower machine, thereby E1 maintaining the synchronism.

IC

1

Ec 2

Local Circuit

E2

Figure 7

Effect of change prime-mover power input on load Suppose the 2 alternators are now equally loaded such that I1 = I2 and internal pf cos θ1 = cos θ2. If fuel supply to alternator 1 is increased by governor setting Ic is set up as before. •

For alternator 1

New increased value of current •

=

I1’ of pf Cos θ1’

=

I2’ at pf Cos θ2’.

For alternator 2

New decreased value of current

From Figure 8 which shows the phasor diagram giving distribution of load, it is clear that

E1 I1’ Cos θ1’

E1 I1 Cos θ1

>

and

E2 I2’ Cos θ2.

Thus it can be stated that the effect of increasing fuel to prime-mover is to make it take an increase share of the load whereas the other alternator operating in parallel with it is relieved of its load by a corresponding amount. Figure 9 shows the graphical summation of vectors shown above. E1

E1

I '

I ' 1

1

' 1

' 1

I

I 1

1

IC

1

1

I C

Ec

Ec

'

I2

' I2

' 2

2

' 2

2

I2

I2

E2

E2

Figure 9 Fig-8

•

Effect of changing the excitation:

Consider again 2 alternators running in parallel at no-load with E1 = E2. If the excitation of alternator 1is increased a resultant voltage Ec = E1 - E2 is established in the local circuit. Since the impedances of the local circuit is mainly reactive, Ic lag Ec by about 90°. It is found that Ic produced armature reaction, which is demagnetising for alternator 1 and magnetising for alternator 2, tending to equalise the 2 alternator emf’s. If two alternators are equally loaded and the currents are I1 and I2 at internal pf angles θ1 and θ2 respectively. An increase in the excitation of alternator 1 gives rise to a resultant emf EC = E1 - E2 which circulates a load current Ic. This circulating current Ic must be added to I1 and I2 to get the new currents. These are I1’ for alternator I and I2’ for alternator 2. From Figure 10 which is a phasor diagram showing the effect of excitation, it is seen that though I1’ has increased, its real power component is unchanged and similarly I2’ has decreased but its real component remains the same. It can thus be said that a change in the excitation modifies the distribution of reactive power but the division of active or real power is unaltered. Thus it can be concluded that for 2 alternators working in parallel a)

a variation of prime-mover governor setting changes the distribution of active power shared by the 2 alternators.

b)

a variation of the excitation determine the terminal voltage and distribution of reactive power of the two alternators.

E1

E1

E2

E2 ' 1

I ' I

1

1

1

Ec

Ec

IC

IC

2 ' 2

I2 ' I2 E2

E2

Figure 10 Parallel Running of Generators Alternators in parallel must be driven at the same electrical speed; that is at the same frequency. The terminal voltages of all parallel connected generators must be the same, since their terminals are all respectively connected together. Hence a 4-pole alternator, driven at 1800 rpm may be operated in parallel with a 6-pole alternator driven at 1200 rpm. Each generates a 60 cycle e.m.f. and therefore is driven at the same electrical speed.