Selected Electrical Question And Answer for MEO Class 2 (2)

With reference to voltage dip in an AC insulation :

(a)                briefly explain the cause.                                                                    (3m)

(b)               state any undesirable effects.                                                              (3m)

(c)                Explain why self excited compounded generators provide an improved response.                                                                                       (4m)

Answer :-

(a)                The two stages in which voltage dip occurs are :

1)      first, the prime mover slows down, causing a lowering of frequency and the generated voltage – (emf µ rpm x flux ).

2)      Second, an increased current flows thru the impedance of the armature winding causes a further drip in output voltage.

The combination of the 2 above stages gives rise to fall sharp initial dip in voltage, followed by a slower fall to a minimum voltage.

      V

 

                                                                                                             Time (sec)

                Voltage

                 Dip

(b)               The effects of voltage dip are:

1)      causing the objectionable flickering of lights.

2)      There is a danger that due to reduced contact pressure, contact welding can occur. By British Standard specification, the contactors must operate satisfactory at 85% of normal voltage.

3)      There is a danger of motor starting at 79% of the normal voltage since torque of the motor is proportional to the square of supplied voltage.

4)      There is a danger of motor stopping due to at 2/3 of voltage, torque would low fallen to 40%. It will therefore most probably start causing an increase stator current resulting in overheating of running of motor windings.

(c)                The self regulating excited compounded  generator use the output to provide the required excitation. Because of very direct and simple circuit, the voltage recover is very much faster than in a separately excited generator which always has amore complex and indirect excitation circuits.

The current peaks of induction motor winding and cables able during starting are at a low power factor, usually of a order of 0.3 to 0.4 with a huge wattless component. The effect of the current  peaks if low power factor or of any other large load suddenly started or to a relatively small power installation will cause a voltage dip.

 

                                                                Voltage trimmer

 

            Amplifier                 Comparator                          Voltage sensing

 

                                     Silicon                 

                                     Controlled

                                     Rectifier

 

   From rotary exciter                                                                                   

   Or                                                                                                                 3 phase

   Static Exciter

   Transformer                                                                                                   440V AC

 

Sketch of AVR

The AVR corrects voltage dip in the following manner. The voltage sensing will sense a change in voltage. It produces a DC signal that is proportional to the actual generator voltage.

The signal is send to a comparator and is compared against a set DC source.

Error signal is then sent to amplifier which send control signals to a excitation current on the stator coils of the alternator by thyristor, power transistor or saturable transformer.