Types of Electromechanical Voltage Regulators | Power Plants

The following points highlight the two main types of electromechanical voltage regulators used in power plants. The types are: 1. Tirril Automatic Voltage Regulator 2. Brown-Boveri Automatic Voltage Regulator.

Type # 1. Tirril Automatic Voltage Regulator:

It is a vibrating type voltage regulator, in which a fixed resistance is cut in and cut out of the exciter field circuit of the alternator. This is achieved by rapidly opening and closing a shunt circuit across the exciter rheostat. Its essential parts are shown in Fig. 17.5.

There are two levers at the top carrying main contacts at the facing ends. The lever on the left is controlled by the exciter magnet, which is excited by the current proportional to exciter voltage. The lever on the right is controlled by an ac magnet, known as main control magnet having both shunt and series excitations.

The ac magnet is so adjusted that with normal load and voltage, the pulls of the two coils are equal and opposite, so the lever on the right hand side is held in horizontal position. Field circuit of the exciter is provided with a rheostat, the resistance of which can be adjusted to give the required excitation.

The regulator is provided with a relay horse shoe magnet which consists of two identical windings, both of which are connected across the exciter armature. One on the left hand side permanently connected and while other on the right hand side has its circuit completed only when main contacts are closed.

As long as the main contacts are open, the relay shoe magnet remains energized, lever carrying relay contact remains pulled down and therefore, relay contacts remain opened. When the main contacts are closed the flux in the relay magnet is destroyed because of two windings of relay connected differentially, it releases the lever carrying a relay contact, so the relay contacts provided across the rheostat are closed and short circuit the rheostat.

When the load on the alternator increases, the series excitation predominates and so pulls the ac control magnet. Thus the contacts of the two levers are closed, the relay shoe magnet is de-energised and so rheostat in the exciter field is short circuited. This increases the exciter voltage and hence the excitation of the alternator. The increase in excitation of the alternator causes an increase in alternator voltage rapidly.

Now the excitation of the exciter control magnet increases due to increase in exciter voltage, pulling down the left hand lever, so opening the main contact energizing the relay and putting the rheostat again in the circuit of the exciter field before the alternator voltage has had time to increases too far.

The reverse action takes place when the load on the alternator decreases. Due to overshooting the mark principle the terminal voltage does not remain perfectly steady, but oscillates rapidly between its maximum and minimum values. The regulator is so quick acting that the variation in voltage is less than 1%. A capacitor is provided across the relay contacts to reduce the spark at the time of opening of the relay contacts.

This method can be employed to maintain the voltage at the consumer premises within permissible limits in case of short lines by designing the winding of the relay operated by the line voltage and current to allow for the regulation of a line end of the alternator.

This method is not suitable for long lines because to maintain voltage at the far end within permissible limits we will have to vary the terminal voltage of the alternator too much.

When loads are fed near the generator end and at the far end of the line, it is required to maintain the voltage at both ends of a transmission line constant. This cannot be achieved by this method. To achieve this condition other methods such as power factor control or tap changing transformers are employed.

Type # 2. Brown-Boveri Automatic Voltage Regulator:

Brown- Boveri automatic voltage regulator also operates on the “over­shooting the mark”, principle but it differs from the Tirril one. In this case the regulating resistance is gradually varied either continuously or in small steps, while in Tirril regulator the resistance is first completely inserted then completely cut out. Under steady conditions, all parts of the regulator are at rest and wear and tear is quite less as compared with that in Tirril regulator.

Brown-Boveri automatic voltage regulator is shown in Fig. 17.6 and Fig. 17.7.

Brown-Boveri automatic voltage regulator consists of three important parts:

(a) Control System:

It is essentially a split-phase induction motor, which consists of two windings A and B wound on an annular core of laminated sheet steel. The winding A is excited from two terminals of the generator (whose voltage is to be controlled) through coarse and fine resistances U and U’ and a resistance R is placed in the circuit of winding B. The ratio of resistance to reactance is made different in the two windings in order that phase angle difference may be in the currents flowing through them.

Due to phase displacement between the currents of two windings, rotating magnetic field and an electromagnetic torque acting on an aluminium drum C, which is mounted on the spindle, are produced. The electromagnetic torque acting on the aluminium drum varies with the terminal voltage of alternator.

If resistances U and U’ in series with the windings are increased or decreased, the electromagnetic torque produced will decrease or increase respectively. Hence variable resistor U is a mean by which the automatic voltage regulator can be set to operate at different voltages.

(b) Mechanical Control Torque:

Mechanical control torque is provided by the combination of two springs (a main spring and an auxiliary spring) which are so arranged that the mechanical torque provided by them is practically independent of the position of the rotor of control system. The mechanical torque under steady deflected state, is equal and in opposition to the electrical torque. The mechanical torque can be adjusted to a limited extent by a screw (which varies the torque due to an auxiliary spring).

(c) Operating System:

It consists of a regulating resistance connected in series with the field circuit of the exciter. The regulating resistance consists of a pair of resistance elements, which are connected to the contact blocks on the inside surface of which roll contact segments. The good contact between the segments and resistance sectors is made by making use of springs which press the segments outside. The two segments roll on the inside surface of the resistance sectors because of torque acting upon the aluminium drum, which itself rotates clockwise or counter-clockwise when the terminal voltage of generator changes.

(d) Damping Torque:

Damping system consisting of an aluminium disc rotating between two permanent magnets M is provided in order to prevent the oscillations of the moving system. The aluminium disc is geared to the rack P and is fastened to the aluminium drum by means of a flexible spring S acting as the recall spring. With the variations in the alternator voltage, the eddy currents induced in the disc produce the necessary damping torque, which resists the quick response of the moving system.

Operation:

Suppose that the terminal voltage of the alternator is normal at position 3, for which the voltage regulator has been set by adjusting the resistances U and U’. At this position, the electromagnetic torque is exactly balanced by the mechanical torque provided by the springs. Hence moving system is in equilibrium.

Now let us consider that the terminal voltage of the alternator rises due to decrease in load. Due to increase in terminal voltage, the electromagnetic torque will increase and therefore, being greater than the mechanical torque due to springs, will cause rotation of moving system in a clockwise direction. When this happens, the segments move out of the resistance sectors and some resistance is inserted in exciter field rheostat, thereby decreasing the field current and hence less terminal voltage on the alternator.

When the generator voltage attains its normal value, the electromagnetic torque attains its original value and system becomes stable at this new position as the mechanical torque due to springs is independent of the position of the moving system.

The reverse operations take place when the terminal voltage of the alternator decreases due to increase in load.

The generator voltage will decrease or increase slowly because of inertia of the exciter and the generator field. The regulator is made quick responding by employing “overshooting the mark principle”, i.e., if the new position for equilibrium is 4, the moving system will first overshoot upto, say, position 5 and then return to position 4, which is a true position of equilibrium for new load, under the influence of mechanical torque provided by the tightening of recall spring.

Excitation control method used to be employed in early times. This method worked well in small isolated systems where there was no local load at the sending end. Also there are limits for the excitation as well. Excitation below a certain level may cause un-stability of the system and excitation above certain level will cause overheating of the rotor. Thus the amount of regulation by this method is limited by the permissible voltage rise at the sending end and by the difficulty of designing efficient generating plant when the excitation range is too wide.