Parallel Operation of Alternators | Devices | Electrical Engineering

In this article we will discuss about:- 1. Requirements for Successful Parallel Operation 2. Synchronising of Alternators 3. Synchronising Current 4. Synchronising Power 5. Synchronising Torque 6. Effect of Reactance 7. Effect of Increasing the Excitation of One of the Alternators 8. Effect of Increasing the Driving Torque 9. Effect of Change in Speed 10. Effect of Unequal Voltages 11. Load Sharing between Two Alternators. 

Requirements for Successful Parallel Operation:

The alternators to be operated in parallel should meet some requirements so as to make their operation proper.

These requirements are as follows:

1. They must have the same output voltage rating.

2. The rated speeds of the machines should be such as to give the same frequency (f being equal to PN/120).

3. The alternators should be of the same type so as to generate voltages of the same waveform. They may differ in their kVA ratings.

4. The prime movers of the alternators should have same speed-load characteristics, which of course must be drooping ones, so as to load the generators in proportion to their output ratings.

5. The alternators should have reactance in their armatures; otherwise they will not operate in parallel successfully.

Synchronising of Alternators:

The process of connecting an alternator in parallel with another alternator or with common bus-bars (bus-bars to which a number of alternators are connected) is called synchronising.

The following conditions must be fulfilled for proper synchronising of alternators:

1. The terminal voltage of the incoming machine must be exactly equal to that of the others, or of the bus-bars connecting them.

2. The speed of the incoming machine must be such that its frequency (being equal to PN/120) equals bus-bar frequency.

3. The phase of the incoming machine voltage must be the same as that of the bus-bar voltage relative to the load i.e., the phase voltages of the incoming machine and the bus-bar should be in phase opposition. This implies that there will be no circulating current between the windings of the alternators already in operation (the bus-bars) and the incoming machine.

In case of 3-phase alternators, an additional requirement to be met is that the phase sequence of the incoming machine is the same as that of the bus-bars.

The condition of same phase sequence is checked by the phasing out at the time of commissioning of alternator and the remaining three conditions have to be checked each time the alternator is to be put in parallel with the bus-bars or other alternators already in operation.

Synchronising Current:

Once a synchronous machine is synchronised, it will tend to remain in synchronism with the other alternators. Any tendency to depart from the condition of synchronism is opposed by a synchronising torque produced due to circulating current flowing through the alternators.

When two alternators are in exact synchronism, the two alternators have equal induced emfs which are in exact phase opposition, as shown in Fig. 13.1(a). No circulating current flows round the local circuit.

When the induced emfs of the two alternators are equal in magnitude but not in exact phase opposition, as shown in Fig. 13.1 (b), their resultant emf acts round the local circuit and causes flow of current called the synchronising current, I_sy.

If any alternator (say, the second alternator) due to some disturbance tends to retard, E₂ falls back by a phase angle δ electrical degrees, as shown in Fig. 13.1 (b). Now though their induced emfs E₁ and E₂ are equal in magnitude but have a phase difference of 180° – δ. Let each of the induced emfs E₁ and E₂ be equal to E.

Resultant emf,

Where Z_s is the combined synchronous impedance per phase of the two alternators (or of one alternator only if it is connected to infinite bus-bars).

The synchronising current I_sv lags behind the resultant emf E_R by an angle θ given by θ = Tan^-1 X_s/R_e where X_s is the combined synchronous reactance and R_e is the effective resistance of the two alternators (or of one alternator only if it is connected to infinite bus-bars).

If resistance R_e is very small as compared to synchronous reactance X_s then-

Synchronising current I_sy = Eδ/X_s and lags behind E_R by 90° i.e., almost in phase with E₁.

Thus I_sy is generating current with respect to machine no. 1 and motoring current with respect to machine no. 2 (in generator action the current flows in the direction of the induced emf while in motor action current flows in a direction opposite to that of the induced emf). This current I_sy sets up a synchronising torque T_sy which tends to accelerate the motoring machine (i.e., machine no. 2) and decelerate the generating machine (i.e., machine no. 1).

Similarly if the machine no. 2 tends to speed up, induced emf E₂ will tend to advance in phase and the synchronising current I_sy will be generating current with respect to machine no. 2 and motoring current with respect to machine no. 1. The torque developed due to synchronising current I_sy will now tend to retard machine no. 2 and accelerate machine no. 1.

Thus, any departure from synchronism results in development of a synchronising torque T_sy which tends to keep the machines in synchronism.

Synchronising Power:

In above case, machine no. 1 supplies power equal to E₁I_sy cos ɸ₁, and the machine no. 2 receives power equal to E₂I_sy cos(180°- ɸ₂).

Power supplied by machine no. 1 = Power supplied to machine no. 2 + copper losses.

The power supplied by machine no. 1 is called the synchronising power and is given by the expression:

Synchronising Torque:

If T_sy be the synchronising torque in N-m, then total synchronising power:

Effect of Reactance:

Since with respect to local circuit, the emf of an alternator is in phase opposition to the emf of another alternator with which it is working in parallel, the machines run as synchronous motor with respect to another. Hence if due to certain reasons, the input to machine no. 2 is cut off, which is quite probable, it must receive wattful motor current from the other.

Consider two machines having resistance but negligible reactance. Their emfs E₁ and E₂ will be practically in phase opposition, as shown in Fig. 13.3 (a) and their resultant emf E_R will be almost in quadrature with E₁ and E₂.

The synchronising current I_sy given by the relation I_sy = ER/(R₁ + R₂) will be in phase with E_R and, therefore, in quadrature with E₁ and E₂. Hence the synchronising current I_sy will be wattless current and convey no power from machine no. 1 to machine no. 2, which needs help.

Now consider the two machines having the synchronous reactance, but no resistance. The synchronising current I will be in quadrature with resultant emf E_R, therefore, almost in phase with one of the emfs, say in phase with E₁ as shown in Fig. 13.3 (b). Thus machine no. 1 will supply power to machine no. 2 and keep the machine no. 2 running.

From the above discussions it is obvious that without reactance the machines could not develop motor and generator power, respectively to restore synchronism; parallel operation would not be possible.

Effect of Increasing the Excitation of One of the Alternators:

For simplicity, let us consider two identical alternators sharing equally a load whose power factor is cos ɸ. If both machines have exactly the same excitation it will be found that their currents I₁ and I₂ are equal in magnitude (say I each) and in phase, since the conditions are identical for both machines. The phasor diagram for the total load, for one phase is given in Fig. 13.4 (a).

Now, if the excitation of one of the alternators is increased, it will cause flow of synchronising current I_sy almost in quadrature with supply voltage V. Therefore, the load current of alternator 1, whose excitation has been increased, will be I₁, the phasor sum of I_sy and I and that of alternator 2 will be I₂, the phasor difference of I_sy and I.

Hence power factor cos ɸ, of alternator 1 decreases and that of the other improves. Because synchronising current I_sy is in quadrature with V, therefore, it does not change wattful (active) components but changes wattless (reactive) components. Hence by changing the excitation, the power factors of the alternators are changed.

If the excitation of an alternator operating in parallel with other alternators is increased above its normal value of excitation, its power factor changes in the lagging direction and its current output increases with no appreciable change in its kW load. Likewise, if the generator is under excited its power factor becomes more leading and its current output increases with no change in kW output.

This increase in current in either case is not supplied to the load but circulates between the alternators connected to the system, thereby increasing their losses and reducing their useful capacity. It is desirable in most cases, therefore, to operate each alternator at the same power factor (at or near the power factor of the load) keeping the circulating current to the minimum.

In general, the proper amount of field excitation for alternators operating in parallel is the amount of excitation each alternator would need if it were carrying its load alone at the same voltage and frequency.

Effect of Increasing the Driving Torque of One of the Alternators:

When the driving torque of one of the alternators is increased (e.g., by increasing the steam supply in case of steam turbine driven alternator), it immediately starts to accelerate. The rotor of the alternator 1 (whose driving torque has been increased) takes lead in relation of rotor of alternator no. 2, and  the condition when the rotor of alternator 1 has taken a lead of δ degrees (electrical).

This causes a phase difference between E₁ and E₂, the magnitude of induced emfs remaining the same, resulting in flow of synchronising current I_sy. Thus the power delivered by the alternator, whose driving torque has been increased, will increase by an amount equal to E₁ I_sy cos ɸ₁.

Hence by increasing the driving torque of one of the alternators, it is further loaded and other is relieved of its load. If the output of the alternator, whose driving torque has been increased, becomes more than the total load supplied, then the other alternator will run as a synchronous motor.

The kW load division between alternators is made by adjusting the governor controls. One prime-mover governor is opened while the other is closed simultaneously. Thus, the system frequency is maintained constant while the load is shifted from one machine to the other. Governor control switches are mounted on the switchboard so that the operator is able to watch the switchboard instruments while making adjustments of load division.

Effect of Change in Speed of One of the Alternators:

When the two alternators are in exact synchronism the two alternators have equal potential differences and are in exact phase opposition. No circulating current flows round the local circuit. But when the speed of machine 2 is reduced, E₂ falls back by a phase angle δ electrical degrees, as shown in Fig. 13.6, the magnitude of induced emfs E₁ and E₂ remaining the same. It will cause resultant emf E_R acting in the local circuit, which will cause a flow of synchronising current I_sy in the local circuit.

The synchronising current I_sy will lag behind E_R by θ where θ = Tan^-1 (X_s/R_e). Since R_e is negligible as compared to X_s, therefore, θ = 90°. The synchronising current I_sy will be generating current with respect to machine 1. This synchronising current sets up a synchronising torque tending to retard the generating machine 1 and accelerating the motoring machine i.e., machine 2. Thus the two machines come again in synchronism.

The frequency of the system can be raised or lowered by increasing or reducing the speed of all the machines simulta­neously.

Effect of Unequal Voltages:

Let us consider two alternators having their emfs E₁ and E₂ exactly in phase (relative to external load circuit) but of different magnitudes (E₁ > E₂). The resultant emf, E_R being equal to E₁ – E₂, acts in the local circuit and causes synchronising current I_sy around the local circuit. This synchronising current I_sy lags behind E_R or E₁ by 90° and leads E₂ by 90°, as shown in Fig. 13.7. Thus synchronising current I_sy produces demagnetising effect on machine no. 1, resulting thereby reduction in E₁ and magnetising effect on machine no. 2 resulting thereby increase in E₂. Thus difference of E₁ and E₂ is reduced and stable condition is established.

Load Sharing between Two Alternators:

Consider two machines with identical speed-load characteristics running in parallel with a common terminal voltage of V volts and load impedance Z.

Let the generated emfs of the two machines 1 and 2 operating in parallel be E₁ and E₂ respectively and synchronous impedances per phase be Z_S1 and Z_S2 respectively.

Terminal voltage of machine 1:

Similarly, terminal voltage of machine 2: