Electrical Engineering Association

17/09/2018

Working principle of Power generation.

12/09/2018

09/09/2018

Primary Sources of Electrical Energy

We can produce electrical energy by converting different energies available in nature. So we should look into various natural sources of energy which we use to produce electricity. Some common sources of energy are,

1. The Sun

2. The Wind

3. The Water Head

4. The Fuel

5. The Nuclear Energy

Out of five above listed sources of energy we do not use first two in very large scale. This is because there are some limitations. But in present days water head, fuel and nuclear energy are three most primarily used natural energy sources for producing electricity. We call these three sources collectively as conventional sources of energy.

1. Energy of Sun

The Sun is the basic source of energy. The sun is the source of both heat and light. We can use both heat and light for producing electricity.

✓. Energy of Sun Heat

We focus sun rays at a small area with the help of concave mirror. The heat of the concentrated sun rays to heat up the water in boiler. The steam produced in boiler rotates a turbine. The turbine rotates an alternator to generate electricity. Although production cost of electricity is quite low here because no fuel like coal or diesel is required. However, production of electricity is not very popular. Because the area required for establishing this power plant is quite large even for smaller electrical energy generation. This is also because of unavailability of sun light in night and in cloudy weather. Also sun light varies time to time during a day. Over all the technique of producing electricity is not at all economical.

✓. Energy of Sun Light

We can use sun light directly to produce electricity. This is done by photovoltaic cell. Here, sunlight directly strikes on the surface of voltaic cell. The photovoltaic cells are basically semiconductor p n junction cells. The potential difference or voltage appears across the junction of cell due to the incident sunlight. This potential difference or voltage creates electricity in the circuit connected to the solar panel system. Solar panels for producing electricity are becoming popular now days because of limitations of other conventional resources.

2. Energy of Wind

We can use wind power to generate electricity. Where sufficient wind is available for long periods of time, we can construct efficient wind power mill to produce electricity. Here the wind mill rotates an electrical generator. As the speed of wind is not fixed we should not use the electrical energy produced by wind mill directly to the load. Instead we charge a battery connected to the system. We feed the output of the battery to load through an inverter. The main advantage of the system is that it has very low running cost, because of zero fuel cost and negligible maintenance cost.

The main disadvantages of the system are variable output, unreliable because of variable wind pressure throughout a day as well as throughout a year and production rate of electricity is also quite low compared to conventional sources of energy.

3. Energy of Water Head

When we obstruct the natural flow of water from upstream to downstream in a river by constructing a dam across the river, a head is created in this water. When we allow to flow this stored water in a controlled way through the dam the potential energy stored in this high headed water gets released in the form of kinetic energy. This kinetic energy rotates a water turbine. An alternator coupled with the shaft of the turbine generates electrical energy. The power plants which use water head to produce electricity are referred as hydroelectric power plant. Water head is the most acceptable source of electricity because it is clean, it does not cause pollution in atmosphere, it is simple in construction, it is robust and demands very less maintenance. In addition to these reasons, the dam helps irrigation in the localities and controls flood. But the construction of dam needs huge monetary investment and complex engineering. Another drawback of the system is that we can not construct a hydroelectric plant at load centre, we only can construct it in downstream river which may be far away from the load centre.

4. Energy of Fuel

Till date the fuels are the main source of electricity. We can use three types of fuel for the purpose. Solid fuel such as coal, liquid fuel such as diesel and gaseous fuel such as natural gas. Whatever may be the form of fuel that is either solid, or liquid or gaseous the basic principles is same in this system. Here heat generated due to combustion of fuel in the furnace creates steam by boiling water in a vessel called boiler. This steam is then allowed to expand through nozzles in a turbine. This creates kinetic energy on the turbine blades which turns the turbine shaft. The alternator coupled with the shaft of the turbine generates electrical energy. We refer this system of producing electricity as thermal power generating plant. Although till date fuel is the main source of electricity generation but it has a limitation of availability in nature and it is true that the availability is diminishing day by day.

5. Energy of Nuclear

Nuclear fission releases a huge quantity of energy. This energy is used to produce steam which rotates a turbine coupled with an alternator. The alternator produces electrical power. In nuclear reaction the requirement of radio active material quite small for producing a large quantity of energy. Although the cost of nuclear fuel is quite high but till it is cost effective process of generating electrical energy since the quantity of nuclear fuel used in the process is very small. It is found that one kg of uranium (radioactive material that is nuclear fuel) is equivalent to 4500 kg of coal fuel. The plants where nuclear reaction is the source of energy for boiling water to produce steam is called nuclear power plants. The nuclear power plants do have two major drawbacks.

* The establishment costs, maintenance costs and running costs of the plants are higher than that of other conventional thermal power generating plants.

* The disposal of nuclear wastes is another big issue for nuclear power plant.

08/09/2018

Working principle of an Electrical Transformer...

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07/09/2018

Capacitive Voltage Transformer (CVT)

Definition:

The capacitive voltage transformer step-down the high voltage input signals and provide the low voltage signals which can easily measure through the measuring instrument. The Capacitive voltage transformer (CVT) is also called capacitive potential transformer

Construction:

In its most basic form, the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a voltage transformer to isolate and further step down the voltage for metering devices or protective relay.

The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices. The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay.

Capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of the voltage transformer. This makes CVTs more economical than the wound voltage transformers under high voltage (over 100 kV), as the latter one requires more winding and materials.

Capacitive Voltage Transformer Working:

The capacitive potential divider is used in combination with the auxiliary transformer and the inductive element. The capacitive potential divider step-down the extra high voltage signals into a low voltage signal. The output voltage of the capacitive potential transformer is further step-down by the help of the auxiliary transformer.

Consider the circuit diagram of the capacitive potential transformer in figure 2.

The capacitor or potential divider is placed across the line whose voltage is used to be measured or controlled. Let the C1 and C2 be the capacitor placed across the transmission lines. The output of the potential divider acts as an input to the auxiliary transformer.

The capacitor places near to the ground have high capacitances as compared to that placed near the transmission line. The high value of capacitances means the impedance of that part of the potential divider becomes low. Thus, low voltages pass to the auxiliary transformer. The auxiliary transformer further step-down the voltages.

The N1 and the N2 are the numbers of turns on the primary and the secondary winding of the transformer. The meter used for measuring the low value of voltage is resistive, and the potential divider is capacitive. Thereby, the phase shift occurs, and the output will be affected. To overcome this problem, the inductance is placed in series with the auxiliary transformer.

This inductance L consists the leakage flux of the auxiliary winding of the auxiliary transformer. The value of inductances is given asThe value of inductances is adjustable. The inductance compensates the voltage drops occurs in the transformer because of the reduction of the current from the potential divider. But, in actual practice, the compensation is not possible because of the inductance losses.

L= 1/(ω^2 (C1+C2))

The voltage turn ratio of the transformer is expressed as As the value of C1 is greater than the C2. Thus the value C1/(C1+C2) is small. The low value of voltage is obtained.

V0/V1 = (C1/(C1+C2)) x (N1/N2)

The voltage transformation ratio of the capacitive potential transformer is free from the burden. The burden is the load on the secondary winding of the transformer.

Other Applications:

The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high-frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network, to communicate between substations. The CVT is installed at a point after Lightning Arrester and before Wave trap.

07/09/2018

High frequency wave trap (or) line trap

Definition:

A line trap (high-frequency stopper) is a maintenance-free parallel resonant circuit, mounted inline on high-voltage (HV) AC transmission power lines to prevent the transmission of high frequency (40 kHz to 1000 kHz) carrier signals of power line communication to unwanted destinations. Line traps are cylinder-like structures connected in series with HV transmission lines. A line trap is also called a wave trap.

The line trap acts as a barrier or filter to prevent signal losses. The inductive reactance of the line trap presents a high reactance to high-frequency signals but a low reactance to mains frequency. This prevents carrier signals from being dissipated in the substation or in a tap line or branch of the main transmission path and grounds in the case of anything happening outside of the carrier transmission path. The line trap is also used to attenuate the shunting effects of high-voltage lines.

Design:

The trap consists of three major components:

1. The main coil
2. The tuning device, and
3. The protective device (also known as a surge arrester).

The protective and tuning devices are mounted inside the main coil. A line trap may be covered with a bird barrier, in which case there are four components.

✓ The main coil is the outer part of the line trap which is made from stranded aluminum cable. The reactor coil, depending on the device, can be made up of several aluminum wires, allowing equal distribution amongst the parallel wires. The stranded aluminum coil is wound in one layer. However, when the application of more than one layer is necessary, separation between layers is required to provide a cooling duct between them to avoid overheating. The cooling duct is created with spacer bars made out of epoxy resin and fiberglass. The coil carries rated continuous power frequency currents, therefore this is the power inductor in this system. It provides a low impedance path for the electricity flow. Since the power flow is rather large at times, the coil used in a line trap must be large in terms of physical size. Hence, a line trap unit is inserted between the busbar and connection of coupling capacitor to the line. It is a parallel tuned circuit containing inductance and capacitance. It has low impedance for power frequency and high impedance to carrier frequency. This unit prevents the high frequency carrier signal from entering the neighboring line.

✓ The next major component is the tuning device. This device is securely installed inside the main coil. It adjusts blocking frequency or bandwidth, and consists of coils, capacitors, and resistors. This smaller coil is attached to both ends of the main coil. Its purpose is to create a blocking circuit which provides high impedance. There are three types of tuning devices: wideband tuning, single frequency tuning, and double frequency tuning. The tuned circuit is usually a dual-circuit broadband type. If the traps are self tuned, they do not require the use of any tuning devices. With the use of a tuning device, a line trap can be tuned to a frequency of 1000 Hz.

✓ The last main component is the protective device, which is parallel with the main coil and the tuning device. It protects the main coil and the tuning device by lowering the over-voltage levels. The bandwidth of a line trap is the frequency range over which the line trap can provide a certain specified minimum blocking impedance or resistance.

Line traps are connected in series with power line and thus their coils are rated to carry the full line current. The impedance of a line trap is very low at the power frequency and will not cause any significant voltage drop.

Application of Wave trap:

Power line carrier communication (PLCC) technology has been frequently used since 1950 by the grid stations to transmit information at high speed. Transmitting information along high-voltage lines, at high frequency, has been one of the main means of communication in electric power for over fifty years. This technology is finding wide use in building and home automation, as it avoids the need for extra wiring. The data collected from different sensors is transmitted on power lines thereby reducing the maintenance cost of the additional wiring. In some countries, this technology is also used to provide Internet connection. In order to communicate, high-frequency line traps are used as they allow substations to communicate with each other through the power lines at the same time as they transmit electrical power. In order to separate power from messages being sent, different frequencies are used. Electrical power has a frequency of 50 Hz or 60 Hz in most places, and the communication waves use frequencies such as 150 kHz and 200 kHz. Line traps consist of filter circuits that allow only power frequency waves to travel to that of electrical equipment. They also stop communication waves from traveling to equipment.

Communication is crucial for substations.

Limitation:

High frequency line traps have a temperature limit of 115 °C-180 °C depending on construction and manufacture.

04/09/2018

Electrical Substation Bus Schemes

The electrical substation is a junction point where two or more transmission lines terminate. In actuality, most EHV and HV substations can be the point where more than half a dozen of lines terminate. In many large transmission substations, the total numbers of lines terminating exceeds one or two dozen.

A substation bus scheme is the arrangement of overhead bus bar and associated switching equipment (circuit breakers and isolators) in a substation. The operational flexibility and reliability of the substation greatly depends upon the bus scheme.

The first requirement of any substation design is to avoid a total shutdown of the substation for the purpose of maintenance, or due to fault somewhere out on the line. A total shutdown of the substation means complete shutdown of all the lines connected to the substation.

Clearly, a EHV or UHV transmission substation where a large number of critical lines terminate is extremely important, and the substation should be designed to avoid total failure and interruption of minimum numbers of circuits.

The Main Criterias to be Considered During Selection of one Particular Bus - Bar Arrangement Scheme

* Simplicity of system.

* Easy maintenance of different equipments.

* Minimizing the outage during maintenance.

* Future provision of extension with growth of demand.

* Optimizing the selection of bus bar arrangement scheme so that it gives maximum return from the system.

Some very commonly used bus bar arrangement are discussed below-

1. Single Bus System

Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as show Fig 1.

Advantages of Single Bus System

* This is very simple in design.

* This is very cost effective scheme.

* This is very convenient to operate.

Disadvantages of Single Bus System

* One but major difficulty of these type of arrangement is that, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.

* The indoor 11 KV switch boards have quite often single bus bar arrangement.

2. Single Bus System with Bus Sectionalizer

Some advantages are realized if a single bus bar is sectionalized with circuit breaker. If there are more than one incoming and the incoming sources and outgoing feeders are evenly distributed on the sections as shown in the Fig 2, interruption of a system can be reduced to a reasonable extent.

Advantages of Single Bus System with Bus Sectionalizer

* If any of the sources is out of the system, still all loads can be fed by switching on the sectional circuit breaker or bus coupler breaker. If one section of the bus bar system is under maintenance, a part load of the substation can be fed by energizing the other section of the bus bar.

Disadvantages of Single Bus System with Bus Sectionalizer

* As in the case of a single bus system, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.

* The use of isolator for bus sectionalizing does not fulfill the purpose. The isolators have to be operated ‘off circuit’ and which is not possible without total interruption of bus-bar. So investment for bus-coupler breaker is required.

3. Double Bus System

In double bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus.
Actually every feeder is connected to both of the buses in parallel through individual isolator as shown in the Fig 3.

By closing any of the isolators, one can put the feeder to the associated bus. Both of the buses are energized, and total feeders are divided into two groups, one group is fed from one bus and other from other buses. But any feeder at any time can be transferred from one bus to other. There is one bus coupler breaker which should be kept close during bus transfer operation. For transfer operation, one should first close the bus coupler circuit breaker then close the isolator associated with the bus to where the feeder would be transferred and then open the isolator associated with the bus from where the feeder is transferred. Lastly, after this transfer operation, he or she should open the bus coupler breaker.

Advantages of Double Bus System

* Double Bus Bar Arrangement increases the flexibility of system.

Disadvantages of Double Bus System

* The arrangement does not permit breaker maintenance without interruption.

4. Double Breaker Bus System

In double breaker bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to double bus bar system. The only difference is that here every feeder is connected to both of the buses in parallel through individual breaker instead only isolator as shown in the figure. By closing any of the breakers and its associated isolators one can put the feeder to respective bus.

Both of the buses are energized, and total feeders are divided into two groups, one group is fed from one bus and other from other buses similar to the previous case. But any feeder at any time can be transferred from one bus to other. There is no need for bus coupler as because the operation is done by breakers instead of isolators. For transfer operation, one should first close the isolators and then the breaker associated with the bus to where the feeder would be transferred, and then he or she opens the breaker and then isolators associated with the bus from where the feeder is transferred.

5. One and A Half Breaker Bus System

This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits, only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance. For the reasons given under double breaker scheme and because of the prohibitory costs of equipment, even this scheme is not much popular. As shown in the figure that it is a simple design, two feeders are fed from two different buses through their associated breakers, and these two feeders are coupled by a third breaker which is called tiebreaker. Normally all the three breakers are closed, and power is fed to both the circuits from two buses which are operated in parallel. The tiebreaker acts as a coupler for the two feeder circuits. During the failure of any feeder breaker, the power is fed through the breaker of the second feeder and tiebreaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by the tiebreaker Fig 5.

Advantages of One and A Half Breaker Bus System

* During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from other healthy bus.

Disadvantages of One and a Half Breaker Bus System

* This scheme is much expensive due to investment for third breaker.

6. Main and Transfer Bus System

This is an alternative of a double bus system. The main conception of Main and Transfer Bus System is, here every feeder line is directly connected through an isolator to a second bus called transfer bus. The said isolator in between transfer bus and feeder line is generally called bypass isolator. The main bus is as usual connected to each feeder through a bay consists of the circuit breaker and associated isolators at both sides of the breaker. There is one bus coupler bay which couples transfer bus and main bus through a circuit breaker and associated isolators at both sides of the breaker. If necessary, the transfer bus can be energized by main bus power by closing the transfer bus coupler isolators and then breaker. Then the power in transfer bus can directly be fed to the feeder line by closing the bypass isolator. If the main circuit breaker associated with the feeder is switched off or isolated from the system, the feeder can still be fed in this way by transferring it to transfer bus.
Switching Operation for Transferring a Feeder to Transfer Bus from Main Bus without Interruption of Power
First close the isolators at both side of the bus coupler breaker.

Then close the bypass isolator of the feeder which is to be transferred to transfer bus.Now energized the transfer bus by closing the bus coupler circuit breaker from remote.

After bus coupler breaker is closed, now the power from the main bus flows to the feeder line through its main
breaker as well as bus coupler breaker via transfer bus.

Now if the main breaker of the feeder is switched off, total power flow will instantaneously shift to the bus coupler breaker, and hence this breaker will serve the purpose of protection for the feeder.
At last, the operating personnel open the isolators at both sides of the main circuit breaker to make it isolated from rest of the live system.

So, it can be concluded that in Main and Transfer Bus System the maintenance of circuit breaker is possible without any interruption of power. Because of this advantage, the scheme is very popular for 33 KV and 13 KV system.

7. Double Bus System with Bypass Isolators

This is a combination of the double bus system and main bus and transfer bus system. In Double Bus System with Bypass Isolators either bus can act as main bus and second bus as transfer bus. It permits breaker maintenance without interruption of power which is not possible in a double bus system, but it provides all the advantages of the double bus system.

It, however, requires one additional isolator (bypass isolator) for each feeder circuit and introduces slight complication in system layout. Still, this scheme is best for an optimum economy of the system, and it is the best excellent choice for 220 KV system.

8. Ring Bus System

The schematic diagram of the system is given in the figure. It provides a double feed to each feeder circuit, opening one breaker under maintenance or otherwise does not affect supply to any feeder. But this system has two major disadvantages.

One as it is a closed circuit system it is next to impossible to extend in future and hence it is unsuitable for developing systems. Secondly, during maintenance or any other reason, if any one of the circuit breaker in ring loop is switched off, the reliability of system becomes very poor as because closed loop becomes opened. Since at that moment for any tripping of any breaker in the open loop causes interruption in all the feeders between the tripped breaker and open end of the loop Fig 8.

The figure are given below respectively,

04/09/2018

Electrical Substation

Now days the electrical power demand is increasing very rapidly. For fulfilling these huge power demands the modern time requires creation of bigger and bigger power generating stations. These power generating stations may be hydro-electric, thermal or atomic. Depending upon the availability of resources these stations are constructed different places. These places may not be nearer to load centers where the actual consumption of power takes place.

So it is necessary to transmit these huge power blocks from generating station to their load centers. Long and high voltage transmission networks are needed for this purpose. Power is generated comparatively in low voltage level. It is economical to transmit power at high voltage level. Distribution of electrical power is done at lower voltage levels as specified by consumers. For maintaining these voltage levels and for providing greater stability a number of transformation and switching stations have to be created in between generating station and consumer ends. These transformation and switching stations are generally known as electrical substations. Depending upon the purposes, the substations may be classified as-

✓ Step Up Substation

Step up substations are associated with generating stations. Generation of power is limited to low voltage levels due to limitations of the rotating alternators. These generating voltages must be stepped up for economical transmission of power over long distance. So there must be a step up substation associated with generating station.

✓ Step Down Substation

The stepped up voltages must be stepped down at load centers, to different voltage levels for different purposes. Depending upon these purposes the step down substation are further categorized in different sub categories.

✓ Primary Step Down Substation

The primary step down sub stations are created nearer to load center along the primary transmission lines. Here primary transmission voltages are stepped down to different suitable voltages for secondary transmission purpose.

✓ Secondary Step Down Substation

Along the secondary transmission lines, at load center, the secondary transmission voltages are further stepped down for primary distribution purpose. The stepping down of secondary transmission voltages to primary distribution levels are done at secondary step down substation.

✓ Distribution Substation

Distribution substation are situated where the primary distribution voltages are stepped down to supply voltages for feeding the actual consumers through a distribution network.

✓ Bulk Supply or Industrial Substation

Bulk supply or industrial substation are generally a distribution substation but they are dedicated for one consumer only. An industrial consumer of large or medium supply group may be designated as bulk supply consumer. Individual step down substation is dedicated to these consumers.

✓ Mining Substation

The mining substation are very special type of substation and they need special design construction because of extra precautions for safety needed in the operation of electric supply.

✓ Mobile Substation

The mobile substations are also very special purpose substation temporarily required for construction purpose. For big construction purpose this substation fulfills the temporary power requirement during construction work.

Depending upon the constructional feature categories of substation may be divided into following manner-

✓ Outdoor Type Substation

Outdoor type substation are constructed in open air. Nearly all 132KV, 220KV, 400KV substation are outdoor type substation. Although now days special GIS (Gas insulated substation) are constructed for extra high voltage system which are generally situated under roof.

✓ Indoor Substation

The substations are constructed under roof is called indoor type substation. Generally 11 KV and sometime 33 KV substation are of this type.

✓ Underground Substation

The substation are situated at underground is called underground substation. In congested places where place for constructing distribution substation is difficult to find out, one can go for underground substation scheme.

✓ Pole Mounted Substation

Pole mounted substation are mainly distribution substation constructed on two pole, four pole and sometime six or more poles structures. In these type of substation fuse protected distribution transformer are mounted on poles along with electrical isolator switches.

✓ Converter substation

Converter substations may be associated with HVDC converter plants, traction current, or interconnected non-synchronous networks. These stations contain power electronic devices to change the frequency of current, or else convert from alternating to direct current or the reverse. Formerly rotary converters changed frequency to interconnect two systems; nowadays such substations are rare.

✓ Switching station

A switching station is a substation without transformers and operating only at a single voltage level. Switching stations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for parallelizing circuits in case of failure. An example is the switching stations for the HVDC Inga–Shaba transmission line.

A switching station may also be known as a switchyard, and these are commonly located directly adjacent to or nearby a power station. In this case the generators from the power station supply their power into the yard onto the Generator Bus on one side of the yard, and the transmission lines take their power from a Feeder Bus on the other side of the yard.

✓ Collector substation

In distributed generation projects such as a wind farm, a collector substation may be required. It resembles a distribution substation although power flow is in the opposite direction, from many wind turbines up into the transmission grid. Usually for economy of construction the collector system operates around 35 kV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation can also provide power factor correction if it is needed, metering, and control of the wind farm. In some special cases a collector substation can also contain an HVDC converter station.

Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity. Examples for such substations are Brauweiler in Germany and Hradec in the Czech Republic, where power is collected from nearby lignite-fired power plants. If no transformers are required for increasing the voltage to transmission level, the substation is a switching station.