BE8254 BASIC ELECTRICAL AND INSTRUMENTATION ENGINEERING Unit I Part A 1. Write the relations on VAR (Dec 2016) Volt-ampere reactance for inductor is V2 / XL or I2 XL Volt-ampere reactance for capacitor is V2 / XC or I2 XC 2. What are the techniques on power factor improvement? (Dec 2016) Power factor correction by static capacitance and by synchronous motor 3. What are the basic components in power transmission and distributed systems? (Dec 2016) Systems, Bulbar, load, earthling, outage, security of supply 4. What are the advantage and disadvantages of UG and OH system? (Dec 2016) Benefits of using underground cables: • • • • • • • Suitable for congested urban areas where overhead lines may be difficult or impossible to install Low maintenance Small voltage drops Fewer faults Not susceptible to shaking and shorting due to vibrations, wind, accidents, etc. Not easy to steal, make illegal connections or sabotage Poses no danger to wildlife or low flying aircraft. Disadvantages of underground cables • • • More expensive Difficult in identifying and repairing broken cables Damage to cables or electrocution may occur to people digging the ground and if they are unaware of the cable’s existence Following are the benefits or advantages of Overhead Transmission: • • • It is easy to repair and maintain. They are not rectricted by landscape i.e. they can be easily installed over river or motorway or hilly regions. Chances of electrocution are less as they run high above the ground. Cheaper to setup compare to underground transmission.
Disadvantages of Overhead Transmission • • These lines visually pollute the areas where they are installed. These lines suffer from problems such as terrorism, vandalism and lightning etc. Sometimes these lines come in the way of birds and low flying aircrafts or drones which can be dangerous. 5. Draw the current and voltage equations for star and delta connections Star Delta VL = √3 VP Line voltage = phase voltage Line current = Phase current Line current IL = √3 IP 6. What is the formula for power in three phase circuits? P = 3 Vp Ip cosФ (Or ) P = √3 VL IL cosФ 7. List out the techniques for measuring the power Three wattmeters method, Two wattmeters method, Single wattmeter method. 8. How to convert star to delta connection? 9. How to convert delta to star connection?
10. What will happen when power factor is leading in distribution of power? If there is high power factor, i.e. if the power factor is close to one: 1. Losses in form of heat will be reduced, 2. Cable becomes less bulky and easy to carry, and very cheap to afford, & 3. It also reduces over heating of transformers. 11. Why we use Vacuum Circuit Breaker in high Transmission System and not Air Circuit Breaker? Vacuum has high arc quenching property compare to air . The dielectric strength of vacuum is equal to 8 times than air. Hence vacuum circuit breaker is used in High voltage line and air circuit breaker is used in low voltage lines. 12. What is the difference between surge arrestor and lightning arrestor? Lightning Arrestor is installed outside and the effect of lightning is grounded where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge. 13. Explain the working principle of the circuit breaker Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed circuit for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) relay senses the fault and gives signal to circuit breaker. Hence the moving contact of circuit breaker separates from fixed contact and an arc is produced between them & thereby it forms the open circuit. Arc is extinguished by the Arc Quenching media like air, oil, vacuum, SF6 gas etc. 14. What is power factor? Whether it should be high or low? Why? Power factor is the ratio of true power to apparent power. It has to be ideally 1. If it is too low then cable over heating & equipment overloading will occur. if it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual power, load has to draw more current(V constant), result in more
losses if pf is good ex: 0.95 to meet actual power, load has to draw less current(V constant),result in less losses). 15. What is the difference between Isolator and Circuit Breaker? Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. it is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. Its specification is given in rated current. But circuit breaker is onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1) rated current 2) short circuit breaking capacity and 3) Instantaneous tripping current. 16. Explain types of Earthing. The two types of earthing are Equipment earthing and system earthing. In Equipment earthing: body ( non conducting part) of the equipment should be earthed to safegaurd the human beings.In System Earthing : neutral of the supply source ( Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be shifted so that unbalanced voltages will not arise. We can protect the equipment also. This type of earthing is further classified into directly earthed, Impedance earthing, resistive earthing. 17. What is the difference between MCB & MCCB, Where it can be used? MCB is miniature circuit breaker which is thermal operated for overload condition and magnetically operated for short circuit protection. It is available in small current rating. MCCB is moulded case circuit breaker which is thermal operated for overload condition and magnetically operated for short circuit protection. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100 A. 18. Where should the lighting arrestor be placed in distribution lines? Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and near power transformers in sub-stations. 19. What is the difference between over voltage relay and over current relay? The current increases not only for over voltages but also for under voltage(As most of the loads are non-linear in nature). So,the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault thereby reducing the system failure & risk of fire.So, for a better protection of the system.It should have both over voltage & over current relay. 20. When we connect the large capacitor bank in series?