Single Phasing Preventer

Single Phasing Preventer:

✪The concept of single phasing is already explained in section 6.2.

✪When one of the phase is blown off due to open circuit or due to operation of fuse of that phase then three phase Induction motor still works with two phases.

✪The whole power is shared by remaining two phases, Thus, the current in these two

healthy phases increases b y √3 times. 

Causes of Single Phasing

✪increase in heating in motor windings.

✪unbalanced rotor currents produces negative phase sequence components. This component produces magnetic flux which rotates in opposite direction to main flux. There by double frequency currents are

induced in rotor causes rotor heating. 

✪This heating is not detected by replica type thermal relays used for protection of stater winding.

✪Also the phase over current relays act slowly. Hence single phasing causes major damage to rotor.

✪Single phasing preventers are generally used for small capacity motors. 

✪Fig.shows the connection scheme for single phasing preventer. 

✪Single phasing preventers are connected in secondaries of line CT.

✪These mainly contains a negative sequence filter.

✪The output of negative sequence filter is fed to a level detector. Further which sends tripping command to the starter or circuit breaker.

✪Thus it protects the motor from damage.

Single phase preventer

Basic Connection of Trip Circuit:

Basic Connection of Trip Circuit:

Fig. shows the simple arrangement of trip circuit. The connection of trip circuit is divided into three parts. 

First part is the primary winding of a current transformer (C.T.), which is connected in series with the line to be protected.

Circuit Diagram Of Trip Circuit

Second part consists of secondary winding of C.T. and the relay-operating coil

Finally the third part is the tripping circuit and it may be operated either on a.c. supply or d.c. supply. It consists of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts. 

Under normal operating condition the current flowing through the C.T.secondary and hence through relay coil is not sufficient to attract the plunger. Hence the trip circuit remains open.

As soon as fault (say short circuit) occurs at point f on the transmission line as shown in Fig. 2.3.1, the current flowing in the line increases to a high value.

This causes the flow of heavy current through the relay coil, and relay to operate by closing its contacts. 

This in turn closes the trip circuit of the breaker, making the circuit breaker open. Thus it isolates the faulty section from the rest of the healthy system.

In this way relay ensures the safety of the circuit equipment from damage and avoids the disturbance to normal working of the healthy portion of the system.

Line to Line Coupling

Line to Line Coupling :-

✥Some applications will require more dependability. When the protected line is of significant importance and the type of protection requires receipt of the signal during an internal fault, multiphase coupling improves dependability of the signal being transmitted through the fault.

✥Since the most frequent type of power system fault is a phase to ground, you can improve your chances of receiving the signal through the fault if more than one phase is used.

✥As shown in the fig. in case of line to line coupling any two line conductors can be used for transmitting a carrier signal as well as receiving the  carrier signal over a section of the transmission line.

✥In this case the carrier signal travels through the line conductors and the surrounding medium is air between the conductors. Therefore the attenuation is very less.

✥Therefore it offers much better performance during single line to ground faults.

✥This type of transmission mode is also known as aerial mode of transmission.

Diagram of Line to line Carrier Coupling

     

Bridge Transition

Bridge Transition

The main advantage of bridge transition method is that none of the motor is

disconnected from the supply during transition. 

1. All starting resistances are shorted and motors are in series.

2 & 3. Bridge link is moved, till a portion of starting resistance is connected in parallel 4. Bridge link is and motors are in parallel with individual starting with each motor.

resistances which will be gradually cut down. It is seen that during transition, the two motors remain connected to the supply. And the resistances are varied such that motor current remains almost same. Hence, torque remains the same and jerk is avoided. So uniform acceleration is obtained.

(a) Series

                               

(b) Transition

(c) Transition

                           

(d)Parallel

                              

 

POWER LOSSES AND EFFICIENCY.

POWER LOSSES AND EFFICIENCY

The power losses in a de machine consists of input power that is converted into heat. Power losses occurring in a de machine are divided into 

1. copper or electrical losses 
2. iron or magnetic losses and 
3. mechanical losses as

Efficiency 

The ratio of useful output to the total input is called the efficiency of the machine and is expressed as

Commercial or overall efficiency of a generator

Ng = Useful electrical power output

         Total mechanical power input 

Overall efficiency of a motor is given as 

Efficiency will be maximum when variable losses are equal to constant losses. The load current corresponding to maximum

efficiency is given as IL =√Pc/Ra

DC Basic and Networks True & False 1 to 30

DC Basic and Networks True & False 

1. One ampere means the flow of one coulomb each second. 

1.TRUE

2.1 coulomb charge is equal to 6.25 x 1018 electrons.

2.TRUE

3. The charge Q flow through a conductor carrying current of I amperes for 1 seconds is equal to 1/t coulombs.

3.FALSE

4. Voltage applied across a circuit, acts as a force.

4.TRUE

5. Volt is a form of potential energy.

5.TRUE

6. The ratio of voltage and electric current in a closed circuit remain constant.

6.TRUE

7.  linear resistor is one which obeys Ampere's law. 

7.FALSE

8. Specific resistance is measured in 2/m.

8.FALSE

9. The reciprocal of resistivity of a material is called its conductivity.

9.TRUE

 10. The resistance of a conductor increases at its x-sectional area decreases.

10.TRUE

11. The voltage drop across each resistor is same in case of a series circuit.

11.FALSE

12. The heating effect of electric current is always desirable.

12.FALSE

13. Two heater coils of same material are connected in parallel across the supply. Coil A has diameter and length double that of coil B Coil B will produce more heat.

13.FALSE

14. Resistance of a tungsten filament lamp decreases with the increase in supply voltage.

14.FALSE

15. If two lamps of 100 W and 40 W are connected in series across 230 V ac supply, 100 W lamp will glow brighter.

15.FALSE

16. When a resistance element of a heater gets fused, we remove a portion of it and reconnect it to the same supply. The power drawn by the heater will decrease.

16.FALSE

17. A capacitor is sort of open circuit to de.

17.TRUE

18. The current through a capacitor is zero if the voltage across it is not changing with time

18.TRUE

19. An ideal voltage source should have zero internal resistance.

19.TRUE

20. Any practical voltage source can be converted into a practical current source and vice versa.

20.TRUE

21. Constant voltage source is active and bilateral.

21.FALSE

22. Two ideal voltage sources of unequal output voltages cannot be placed in parallel

22.TRUE

23. Solution of an electric circuit will give the same result whether the source is treated as a voltage source or a current source.

23.TRUE

24. For a graph with n nodes, every Tree has (n- 1) branches. 

24.TRUE

25. The response of a circuit is time-variant if given signal x(1) ande response y(t) when the signal is x(t-T) the response is y(t- T).

25.TRUE

26. Principle of homogeneity shows linear circuit.

26.TRUE

27. 27. According to Kirchhoff's voltage law, at any junction of an electrical network, the sum of incoming currents is equal to sum of outgoing currents. 

27.FALSE

28. Superposition theorem is not applicable to a network containing time varying resistors. 

28.TRUE

29. Thevenin's theorem is quite useful when the current in one branch of a network is to

be determined or when the current in an added branch is to be determind.

29.TRUE

30. Norton's equivalent resistance is the same as Thevenin's equivalent resistance.

30.TRUE