Why Interconnect Synchronous Generators?

The study of the interconnection of several synchronous generators is important because Since the demand for electricity varies during a day, also during the various seasons in a year, a modern power station employs two or more units so that one or more alternators can supply power efficiently according to the need.

Connections of several stations by a grid is economic and advantageous. This reduces the installed capacity of the stand by unit considerably and enables economic distributions of load between several stations. Also, in a country like India, where a considerable amount of power is generated by harnessing waterpower, parallel operation of steam and hydro-stations is essential to maintain continuity of supply throughout the year and also to ensure the maximum utilization of water power resources.

1.Interconnect Synchronous Generators For Load Sharing

For alternators in parallel, change in field excitation will mainly change the operating power factor of the generator and has primarily no effect on the active power delivered by the generators. The control of active power shared between alternators is affected by changing the input power to the prime mover. For example, in a thermal power station: having alternators driven by steam turbines, an increase of throttle opening and thus allowing more steam into the turbine will increase the power input.

2.Interconnect Synchronous Generators For Generator input and output

Interconnect Synchronous Generators For Generator input and output phasor diagram

For any load conditions, the output per phase is P = V I \cos { \phi }.The electrical power converted from mechanical power input is per phase

P_1=EI\cos { (\phi +\sigma ) }

Resolving E along I

P_1=EI\cos { (\phi +\delta ) } =VI\cos { \phi } +IrI=VI\cos { \phi } +I2r

The electrical input is thus the output plus the I2R loss, as might be expected. The prime mover must naturally supply also the friction, windage, and core losses.

For a given load current I at external phase-angle \phi to V, the magnitude and phase of E are determined by Zs The impedance angle \thetais arc tan(\frac { x_3 }{ r }), and using

current equation for interconnected parallel generator

These components represent I cos\phi and Isin\phi . The power converted internally is the sum of the corresponding components of the current with E cos \delta and E sin\delta , to give P_1=EI\cos { (\phi +\sigma ) }

power equation for synchronous motor connected in paralle

Thus the power developed by a synchronous machine with given values of E, V and Zs, is proportional to sin\delta or, for small angles, to\delta itself. The displacement angle \delta represents the change in relative position between the rotor and resultant pole-axes and sis proportional to the load power.

3.Synchronous Machine on Infinite Bus-bars

Assume the behavior of a single machine connected to the type of a large generating plant is not likely to disturb the voltage and frequency provided the rating of the machine is only a fraction of the total capacity of the generating plant. In the limit, we may presume that the generating plant maintains an invariable voltage and frequency at all points. In other words, a network has zero impedance and infinite rotational inertia. Synchronous machine connected to such a network is said to be operating on infinite bus-bars.

A change in the excitation changes the terminal voltage, while the power factor is determined by the load, supplied by the stand-alone synchronous generator. On the other hand, no alteration of the excitation can change the terminal voltage, when it is connected to bus bars, the power factor, however, is affected. In both cases, the power developed by a generator depends on the mechanical power supplied. Likewise, the electrical power received by a motor depends on the mechanical load applied at its shaft.

All synchronous motors and generators in normal industrial use on large power supply systems can be considered as connected to infinite bus-bars, the former because they are relatively small, the latter on account of the modern automatic voltage regulators for keeping the voltage practical, constant at all loads.

4.Basis For Drawing The General Load Diagram

Consider a synchronous machine connected to infinite bus bars (of constant-voltage, constant- frequency) of phase voltage V. Let the machine run on no-load with mechanical and core losses only supplied. If the e.m.f. E be adjusted to equality with V, no current will flow into or out of the armature on account of the exact balance between the e.m.f. and the busbar voltage. This will be the case when a synchronous generator is just parallel to the infinite bus bar.

Consider a synchronous machine connected to infinite bus bars (of constant-voltage, constant-
frequency) of phase voltage V. Let the machine run on no-load with mechanical and core losses only supplied. If the e.m.f. E be adjusted to equality with V, no current will flow into or out of the armature on account of the exact balance between the e.m.f. and the busbar voltage. This will be the case when a synchronous generator is just parallel to the infinite bus bar.

If the excitation, If is reduced, machine E will tend to be less than V, so that a leading current Ir will flow which will add to the field ampere-turns due to direct magnetizing effect of armature reaction. Under the assumption of constant synchronous impedance, this is taken into account by IrZs as the difference between E and V. The current Ir must be completely reactive because the machine is on no-load and no electrical power is being supplied to or by the machine, as it is on no-load.

If the excitation, If is reduced, machine E will tend to be less than V, so that a leading current Ir will flow which will add to the field ampere-turns due to direct magnetizing effect of armature reaction. Under the assumption of constant synchronous impedance, this is taken into account by  IrZs as the difference between E and V. The current Ir must be completely reactive because the machine is on no-load and no electrical power is being supplied to or by the machine, as it is on no-load.

If now the excitation is increased, E will tend to be greater than V. A current will, therefore, be circulated in the armature circuit, this time a lagging current which will reduce the net excitation due to the demagnetizing effect of armature reaction so that the machine will again generate a voltage equal to that of the constant bus-bar voltage. The synchronous impedance drop IrZs is, as before, the difference between E and V, and there should be only a zero-power-factor lagging current, as the machine is running on no-load.

If now the excitation is increased, E will tend to be greater than V. A current will, therefore, be circulated in the armature circuit, this time a lagging current which will reduce the net excitation due to the demagnetizing effect of armature reaction so that the machine will again generate a voltage equal to that of the constant bus-bar voltage. The synchronous impedance drop IrZs is, as before, the difference between E and V, and there should be only a zero-power-factor lagging current, as the machine is running on no-load.

5.O-Curves and V -Curves

The current loci in Fig are continued below the baseline for generator operation. The horizontal lines of constant mechanical power are now constant input and a departure from unity-power-factor working, giving increased currents, increases the I2R loss, and lowers the available electrical output. The whole system of lines depends, of course, on constant bus-bar voltage

The current loci in Fig are continued below the baseline for generator operation. The horizontal lines of constant mechanical power are now constant input and a departure from unity-power-factor working, giving increased currents, increases the I2R loss, and lowers the available electrical output. The whole system of lines depends, of course, on constant bus-bar voltage.

The circular current loci are called the O – curves for constant mechanical power. Any point P on the diagram, fixed by the percentage excitation and load, gives by the line OP the current to scale in magnitude and phase. Directly from the O-curves, Fig the V -curves, relating armature current and excitation for various constant mechanical loads can be derived.

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