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September 28, 2012

What Causes Inefficiency and Power Loss in the Transformer?

Inefficiency and the power loss in a transformer is caused by three things:
A. Hysteresis loss: which is caused by the magnetic field created in the transformer’s primary, secondary and harmonic windings. 

B. Eddy current loss: or skin effect: which is caused by the high frequency current traveling on the outer edge of the conductor. 

C. Copper loss: which is caused by the power dissipated in the windings of a transformer. Copper loss is minimized by using as large conductors as possible in the windings. 

Note: A and B are fixed losses. C varies by  I2R. When the efficiency is computed the computation is done with 100% load.

What are the types of protection of a power system?

Types of Protection:

1. Apparatus Protection
- Busbar Protection
- Motor Protection
- Generator Protection
- Transformer Protection
- Transmission Line Protection

2. System Protection
- Under-frequency Relays
- Out-of-Step Protection

- Islanding Systems
- Rate of Change of Frequency Relays

Why do we need protection of a power system?

Electrical apparatus operates at various voltage levels and may be enclosed or placed in open. Under abnormal operating conditions protection is necessary for-
  • Safety of electrical equipments.
  • Safety of human personnel.

Define measuring instrument and measurement system.

Measurement system: The procedure of measure a quantity is called the measurement system. 
Measuring instruments: The measuring instruments are used for measure the quantity. 

Write down the methods of measurement and explain them.

The methods of measurements are classified into two categories- 
a) Direct methods and 
b) Indirect methods. 

Direct methods: In these methods, the unknown quantity is directly compared against a standard. The result is expressed as a numerical number and a unit. Direct methods are quite common for the measurement of physical quantities like length, mass and time. 
Indirect methods: These methods are apply in which measurement by direct method are not possible, flexible and practicable. This method includes always error in measurement.

What is measurement? Explain the significant of measurement.

Measurements: The measurement of a given quantity is essentially an act or the result of comparison between the unknown quantity and a predefined standard. 

In fact, the measurement is the process by which one can convert physical parameters to meaningful numbers. 

In order that the result of the measurement is meaningful, there are two basic requirements: 
  1. The standard used for comparison purpose must be accurately defined and should be commonly accepted and, 
  2. The apparatus used and the method adopted must be provable. 

Significance of measurements: The importance of measurement is simply expressed in the following statement of ‘Lord Kelvin’: “I often say that when you can measure what you are speaking about and can express it in numbers, you know something about it; when you cannot express init numbers your knowledge is of a meagre and unsatisfactory kind.”

What is circuit breaker? Write the classification of circuit breakers based on the medium used for arc extinction?

Circuit breaker:
It is a piece of equipment used to break a circuit automatically under fault conditions. It breaks a circuit either manually or by remote control under normal conditions and under fault conditions.

Classification of circuit breakers based on the medium used for arc  extinction:
  • Air break circuit breaker
  • Oil circuit breaker
  • Minimum oil circuit breaker
  • Air blast circuit breaker
  • SF6 circuit breaker
  • Vacuum circuit breaker

What is the main problem of the circuit breaker?

When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem.

September 23, 2012

What are the advantages of air blast circuit breaker over oil circuit breaker?

o The risk of fire is diminished
o The arcing time is very small due to rapid buildup of dielectric strength between contacts
o The arcing products are completely removed by the blast whereas oil  deteriorates with successive operations

What are the characteristic of SF6 gas?

It has good dielectric strength and excellent arc quenching property. It is inert, non-toxic, noninflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer oil.

What is meant by electro negativity of SF6 gas?

SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro-negativity of SF6 gas.

How does the over voltage surge affect the power system?

The over voltage of the power system leads to insulation breakdown of the equipments. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line. 

Define a over current relay and an under current relay?

Over Current Relay which operates when the current in a line exceeds a predetermined value. 

Under Current Relays which operates whenever the current in a circuit drops below a predetermined value.

Differentiate between a fuse and a circuit breaker.

Fuse is a low current interrupting device. It is a copper or an aluminium wire.
Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.

What is dielectric test of a circuit breaker?

It consists of overvoltage withstand test of power frequency lightning and impulse voltages. Testa are done for both internal and external insulation with switch in both open and closed conditions.

Mention any two applications of differential relay.

  • Protection of generator & generator transformer unit; 
  • protection of large motors and busbars .

What are over and under current relays?

Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type non-directional/directional overcurrent relay, differential overcurrent relay) whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

Define operating time and resetting time of a relay.

Operating time of a relay:
It is defined as the time period extendind from the occurrence of the fault through the relay detecting the fault to the operation of the relay. 

Resetting time of a relay:
It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

Define energizing quantity.

It refers to the current or voltage which is used to activate the relay into operation.

Name the different kinds of over current relays.

  • Induction type non-directional over current relay,
  • Induction type directional over current relay & 
  • current differential relay.

What are unit system and non unit system?

A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.

Define protected zone.

Protected zone are those which are directly protected by a protective system such as relays, fuses or switchgears. If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

Give the consequences of short circuit.

Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.

What do you mean by Positive Sequence, Negative Sequence and Zero Sequence network?

When an unbalanced fault occurs, we need a way to easily analyze the network. Positive Sequence, Negative Sequence and Zero Sequence network is three method which used to determine and calculate fault in a three phase AC transmission and distribution. 

positive sequence with positive frequency, negative sequence with negative frequency, and zero sequence which are constant.
Figure: Typical Sequence Network Diagram

September 9, 2012

What is the role of the distribution substation?

Distribution Substation:
A bank of step-down transformers near the end users. The distribution substation converts voltage closer to the required usable levels. A typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

Role of the Distribution Substation:
  • A distribution substation transfers power from the transmission system to the distribution system of an area. 
  • These substations are located near the consumers localities, receive electrical power from a high-voltage transmission system and convert it to voltage levels suitable for industrial, commercial, or residential use. 
  • Distribution substations are typically the points of voltage regulation, although on long distribution circuits (of several miles/kilometers), voltage regulation equipment may also be installed along the line. 
  • In addition to transforming voltage, distribution substations also isolate faults in either the transmission or distribution systems.
Distribution substation - 20/04kV MCset metal-clad switchgear (Schneider Electric)

September 1, 2012

Write short note about Power System structure with diagram.

Power System structure with diagram: 

Electrical power system can be divided into following regions : 
  • Generating stations 
  • Transmission systems 
  • Receiving station 
  • Distribution system 
  • Load points 

Generating stations

In all these regions, there are switchgears. Bus bars are connecting bars to which a number of local feeders are connected. Bus bars operate at constant voltage. Besides the bus bars, there are other equipment in the electrical schemes such as circuit breakers, CTs, PTs, etc. These equipments can be installed according to various schemes depending upon requirements. 

Equipment in electrical scheme of Transmission system; 

The power transformers are installed between two bus bars of different voltage levels. A power transformer is the costliest, heaviest, and most important equipment in substation.

Typical station is distribution system

In 11 kV distribution system, the cost of elaborate protection may not be justified for protection transformer upto about 500 kVA. The substations are generally unattended. The H.V fuses is the only protection provided on H.V. side

Receiving system & load points for a medium size industrial work: 

The switch gear is installed in the substation of local points, such as industrial works, railway substation, cinema house, large building, foundries etc. The substation has following items: 
- Incoming line section
- Transformer section
- Secondary switching section 

What are the factors which are involved in circuit breaker rating.

Factors involved in circuit breaker rating:
The following rated quantities for HV ac circuit breeders-

  • Opening time: interval of time between energizing the shunt trip coil and instant of  separation of arcing contact in all poles
  • Breaking time: it is time between energizing of opening release and the instant of  final arc extinction. 
  • Closing time: The time interval between energizing of closing circuit and contact touch with all poles. 
  • Open close time: it is time between separation of arcing contacts all the poles and contact touch in first pole deriving open close operation. 
  • Reclosing time: Time between energizing of shunt trip release and contact toad in all poles. 
  • Close-open time: Time between contact touch in first pole and separation of arcing contact in all poles during close open operation. 
The rated characteristics of circuit breaker include rated normal current, rated voltage, rated insulation level, rated transient recovery voltage, rated short circuit breaking current, rated short circuit making current, rated operation sequence etc. 

Write short notes on Differential relays.

Differential relays:
The differential relay responds to vector difference between two or more similar quantities. 
- The differential relay has at least two actuating quantities say I1, I2
- The two or more actuating quantities should be similar i.e. current 1 & current 2.
- The relay responds to the vector difference between two i.e. I1 ~ Iwhich includes magnitude and/or phase angle difference.
Differential protection is generally a unit protection. The protected zone is exactly determined by location of CTs. The vector difference is achieved by suitable connection of CT or PT secondaries.

Write short notes on Inductive interference in a transmission line.

Inductive Interference in Transmission lines: 
Inductive interference depends upon gradient of voltage at surface of conductor or corona. The inductive reactance affected from conductor attenuation at 1000 kHz varies from 0.3 to 0.9 dB/mtr.

Describe a typical coreless type of induction furnace and its special features.

Coreless induction furnace and its special features:
The coreless furnace consists of a crucible formed from a refractory material, which contains the charge to be melted. This crucible is surrounded by a helical induction coil (also called inductor) through which the alternating current flows. The heavy currents are introduced in the charge by transformer action. The heat results from I2effect within the charge. 
The advantages of induction furnaces, in general, include faster and efficient melting, relatively low melting cost. Higher temperature can be attained, can be used for temperature holding etc. The induction furnaces are used for melting, temperature holding, and super heating purposes. 

Special features are:-
- Line frequency coreless induction furnace.
- Medium frequency coreless induction furnace. 

Describe the primary and back-up protection features that are provided for transmission lines.

A Primary and back-up protection: 
The primary protection is essential protection provided for protecting a machine. As a precautionary measure, an additional protection is generally provided and is called a “Back up protection”. The primary protection is the first to act and back-up protection is the next in the line of defence. Therefore, if primary protection fails, the back-up protection comes into action and removes the faulty part from one healthy system.  

When main protection is inoperative for purpose of maintenance, testing etc. The back-up protection acts like main protection. The back up protection can be classified as:

- Relay back-up : Same circuit breaker is used by both main and back-up protection, but protective systems are different. Separate trip coils may be provided for the same-breaker. 
- Breaker Back-up: Different breakers are provided for main and back up protection, both the breakers are being in the same station. 
- Remote back-up: Main & back up protection provided at different stations and are completely independent. 
- Centrally coordinated back-up: Here system is having central control and it can be provided with centrally controlled back-up. The central coordinating station receives information about the abnormal conditions through high frequency carrier signals. The stored programme in digital computer determines the correct switching operation as regard severity of faults, system stability etc. Main protection at various stations and back-up protection for all stations is at central control centre.

What is PLC? How does it work? Distinguish SCADA & PLC controlled system.

What is PLC? 
The PLC (Programmable Logic Controller) is a small industrial computer usually found in factories. Its main use is to replace the relay logic of a plant or process. Today, the PLC is being used in SCADA systems to do its very good programmability. Earlier PLC’s have no serial communication ports for interfacing to radio for transferring of data. Nowadays, PLC's have extensive communication features and a wide support for popular radio units being used for SCADA system. In the near future we are seeing the merging of the RTUs(Remote Terminal Unit) and the PLC’s. 

What is SCADA? 
SCADA (Supervisory Control And Data Acquisition) system refers to the combination of telemetry and data acquisition. It consists of collecting information, transferring it back to a central site, carrying out necessary analysis and control, and then displaying this data on a number of operator screens. The SCADA system is used to monitor and control a plant or equipment. Control may be automatic or can be initiated by operator commands.   

Components of SCADA System 
Components of a SCADA System a SCADA system is composed of the following: 
1. Field Instrumentation 
2. Remote Stations 
3. Communications Network 
4. Central Monitoring Station 
Traditional PLC are migrating toward to traditional DCS for instants Fisher Delta V. For application in small or medium plant many people use a PLC (Allen Bradley) and also use DCS (Delta V, PlantScappe). If you have a small plant, it is better to set PLC or DCS system which has a system starting at 5 I/O and going up to 500 I/O. 

SCADA is just that supervisory and data acquisition. It usually is not in plant but it used to supervise multiple small sites. In industry SCDA often do monitoring systems with little RTU at each well site.

Discuss about Different Motors for Industrial Drive

Different Motors for Industrial Drive:
  1. Squirrel Cage Induction Motor. This motor is quite simple but rugged and possesses high over-load capacity. It has a nearly constant speed and poor starting torque. Hence, it is used for low and medium power drives where speed control is not required as for water pumps, tube wells, lathes, drills, grinders, polishers, wood planers, fans, blowers, laundary washing machines and compressors etc. 
  2. Double Squirrel Cage Motor. It has high starting torque, large overload capacity and a nearly constant speed. Hence, it is used for driving loads which require high starting torque such as compressor pumps, reciprocating pumps, large refrigerators, crushers, boring mills, textile machinery, cranes, punches and lathes etc. 
  3. Slip-ring Induction Motor. It has high starting torque and large overload capacity. Its speed can be changed upto 50% of its normal speed. Hence, it is used for those industrial drives which require high starting torque and speed control such as lifts, pumps, winding machines, printing presses, line shafts, elevators and compressors etc. 
  4. Single-phase Synchronous Motor. Because of its constant speed, it is used in teleprinters, clocks, all kinds of timing devices, recording instruments, sound recording and reproducing systems. 
  5. Single-phase Series Motor. It possesses high starting torque and its speed can be controlled over a wide range. It is used for driving small domestic appliances like refrigerators and vacuum cleaners etc. 
  6. Repulsion Motor. It has high starting torque and is capable of wide speed control. More- over, it has high speed at high loads. Hence, it is used for drives which require large starting torque and adjustable but constant speed as in coil winding machines. 
  7. Capacitor-start Induction-run Motor. It has fairly constant speed and moderately high starting torque. Speed control is not possible. It is used for compressors, refrigerators and small portable hoists. 
  8. Capacitor-start-and-run Motor. Its operating characteristics are similar to the above motor except that it has better power factor and higher efficiency. Hence, it is used for drives requiring quiet operations.

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