What is Substation, Types, Selection, Equipments, Working, Method

INTRODUCTION

In a generation transmission and distribution of electrical power ac supply system is applicable and the generating stations cannot be installed in the midst of populated areas the transmission is network is unavoidable. So the power is transmit to the consumers through a network of transmission and distribution. It may be necessary at many terminals in the power system to change its characteristics such as voltage ac to dc in case of high voltage dc transmission, frequency, power factor of the supply system. Since the all substation achieve the above duties.

Let as consider that voltage is manufactured or generated at the generation station for ex 6.6 or 11 kV and the gaining suitable advantages such as smaller conductor size, less loss, more efficiency of transmission the above voltage is stepped up to either 132kV,220kV or 400kV for transmission purposes. Since step up transformer along with other required equipment performing this operation is known as “substation”. At some points ac is necessary to convert ac into dc in this case of traction and electroplating. So the further substation role this operation. The type of equipment as required by substation depends on service requirement and this can be classified as three type of substations.

Thus the substation may be explained as an assembly of apparatus which is used to change the characteristics of supply system such as voltage, frequency, ac to dc or power factors.

At the time of the distribution of electrical power also substations are required to step down the transmission voltage to appropriate value. And the substation is la connecting between generating stations and consumers of electricity. The main function of substation is to receive power transmitted at high voltage reduce it to suitable value for distribution and provide features for switching

Some substations act as switching stations where various connections between different transmission lines are made. These substations have additional functions of provide points where protective devices may be erected which can operate or disconnect the faulty circuit under abnormal conditions and provide regulation of voltage on outgoing distribution feeders. To the power factor improving devices may also be installed in a substation. Also overall monitoring of proper operation of a power system is possible with the help of substation. So the important to study of the substation.

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Fig. classification of substation

The static substations are stations from where power is transmitted to different load centers. They are either step up or primary substations or secondary substations which fed power directly to load and balance power is transmitted. The converting substations change ac power to dc power with suitable equipments for giving supply to traction electroplating or welding.

for Extra high voltage transmission substations come under the category of transformer substations which change voltage levels. They receive power at some voltage and produce of power is made at some of the voltage level between 11kV and 66kV for extra high voltage substations operate in the voltage range of up to 400kV. The voltage ranges above 400kV are handled by ultra high voltage substations.

From the distribution substations convert voltage levels to low value for the use of electrical power to consumers. The distribution is carried by 3phase , 4 wire system with line to line voltage of 400 voltage. And the line to phase voltage of 230 volts and grid substations are those from where bulk power is transmitted from one point to other in the grid. Any disturbance in these substations may causes failure to the grid.

The industrial substations supply power to the individual industrial consumers who need bulk power.

For improving the power factor of the system power factor correcting substations are erected. As the transmission line is loaded and the voltage decreases while it increases when load on the line is lowered. Thus due to inductance and capacitance of line large variations in voltage and power factor will occur. finally at the receiving ends synchronous condensers or other power factor improving devices are installed.

The substations which change the supply frequency is called as frequency change over substation. An industrial applications frequency other than supply frequency is required which can obtained from these substations

The substations are classified constructional features. There are

Indoor, Outdoor, pole mounted or underground.

For indoor substation the equipments of substation such as circuit breakers switching fuses instruments etc and the erected indoor for having better economy. The voltage handled by these substations is up to 11kV.

These substations consist of series of open and enclosed chambers or compartments to the main equipment of given installation is arranged in these compartments. These are further subdivided as substations of integrally built type where apparatus is erected on site. The cell structure are construct of concreter substations of composite built type contains assemblies which are prefabricated but installed on site. The compartments of such substation take form of metal cabinets.

Difference between Indoor and Outdoor Substations

Sl. NO. POINT INDOOR OUTDOOR SUBSTATION
1 Fault location Difficult Easy as all equipments are within view
2 Time required for erection More Less
3 Future expansion Difficult Easy
4 Amount building material required Large Small
5 Capital cost and cost of switchgear installation More Less
6 Construction work required to be done More Less
7 Operation More Less
8 Performance of all the operations maintenance and supervision In closed atmosphere In open air in all kinds of weather
9 Space required Less More

Selection of site and location for a substation

Bus bars:

Its are the common electrical component that connect electrically number of lines which are operating at the same voltage directly. These bars are of either copper or aluminum. Usually of rectangular cross section and they can be of other shapes such as round tubes, round solid bars of square tubes.

The advantage of aluminum over copper such as higher conductivity on weight basis lower cost for equal current carrying capacity and excellent corrosion resistance

Insulators

Its used to support the conductors or bus bars and confine current to the conductors and they also provide insulation to the bus bars to the most commonly used porcelain insulators of post and bushing type.

Porcelain insulators:

To the post insulator are used for bus bars consists of porcelain body cast iron cap and flanged cast iron base and the hole in the cap is threaded. So that the bus bars are either directly bolted to the cap or fixed by means of bus bar clamp.

Bushing insulators:

Its consists of porcelain shell body upper and lower locating washer used for fixing the position of bus bar or rod in shell. For current rating above 2000A bushings are designed to allow the main bus bars to be passed directly through them.

Its depending upon the service requirement several other types of insulators such as pin type, suspension type, post insulator are employed

Isolating switches:

The disconnect a part of the power system for maintenance and repair purpose isolating switches are used. These are operated after switching off the load by means of a circuit breaker and the isolators are connected on both sides of circuit breakers. Since to open isolators circuit breakers are to be opened first.

In a isolator is essentially a knife switch is designed to open a circuit under no load the lines are connected should not be carrying any current

 

 

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The above fig shows the five sections with help of insulators each section can be disconnected for repair and maintenance and required to do maintenance in section 4 then the circuit breaker in that section is to be opened first and then open the isolators 3 & 4 so the section 4 is open for maintenance. Further maintenance the isolators 3 & 4 are to be closed first and then circuit breaker is closed.

Some other cases isolators are used as circuit breaking devices. But it is limited by particular conditions such as power rating of given circuit.

Two types of isolators

There are

  • Single pole isolator
  • Three pole isolator

Circuit breakers

Its used to open or close a circuit under normal and faulty conditions and it can be manufactured in just like that way that it can be manually worked on remote control under normal conditions and automatically operated during fault. In this automatic operation the relay is preferred

Its needed as isolators can not be used to open a circuit under normal conditions as it has no provision to quench arc that is produced after opening the line.

The functions are

      • Full load current is to be carried continuously
      • Opening and closing the circuit is on no load
      • Making and breaking the normal operating current

For high voltage:

    • Air blast breaker
    • vacuum breaker
    • SF6 breakers are referred

For greater than 66kV

Low oil circuit breakers are used.

Up to 66 kV

Bulk oil circuit breakers are used.

Load interrupting switches

Its designed and used to open and close high voltage circuits under normal load conditions. Suitable arc extinguishing device is provided with the switch as its operates when line is carrying load current

Power transformers

The operation of stepping up or down the system voltage power transformers are used in the substations. At generating end the voltage is only stepped up for transmission of power although at all the continuous substations the voltage is gradually stepped down to reaches finally to working voltage level.

Now days single three phase transformer is used.

Instrument transformers

Its working voltage level of power lines and currents carried by them are normally high which can not be measured by normal devices which are designed for low values to the protective device used in the system are also designed for low values of voltages and currents. Since they can not be directly used on power lines. This problem can be overcome by use of instrument transformer which change the values of system voltage or current to a value suitable for operation of measuring instruments and protective device

The two types of instrument transformer is

  • Current Transformers
  • Potential Transformers

Current Transformers:

The CT is are step up transformer which reduces the current by a known ration. It the current coils of indicating and metering instruments and protective relays are supplied by these transformers to the primary of these transformers consist of one or more turns of thick wire connected in series with the line and the secondary consists of large number of turns of fine wire. If current to be measured is large then more number of turns are to be added in the secondary to the ratio of primary to secondary current is called as “transformation ratio” and it’s also isolate the indicating and measuring instruments from high voltages.

What is a current transformer? - Quora

Range: 100/5A for 100A current in the primary winding the secondary carries 5A. and the conversion for current is 20.

Potential transformer

The PT is step down transformer which reduces the system voltage by known ration. And the secondary of these transformers is used to supply the potential coils of measuring and indicating instruments and relays and it work an isolation for these devices against high system voltage

High Voltage Potential Transformers | Fibre & Fibre Glass Products

Range: 66kV/110V then it gives 110V for a system voltage of 66kV

Bus Bar Types:

It’s the important component of the substation. The outdoor bus bars are of two types there are

  • Rigid Type
  • Strain Type

Rigid Type:

Its made up of aluminum Pipes are used is also making connections among different components. The pedestal insulators support the bus bars and the connections and the device bus bars are spread out and it needs a large space.

Range Size
33kV 40mm
66kV 65mm
132kV 80mm
220kV 80mm
400kV 100mm

Strain type:

Its are made up of ASCR- Aluminum conductors with steel reinforcement are an overhead system of wires between two supporting structure and supported by strain type insulators and per size of the conductor the stringing tension can be limited (500-900kg)

Range Size
66kV 37/2.79mm
132kV 37/4.27mm
220kV 61/3.99mm
400kV 61/4.27mm

Single Bus Bar System:

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The single bus bar system consists to which all incoming and outgoing lines are connected and it require less maintenance. The system is used for the system voltage up to 33kV and the indoor 11kV substations always use this arrangement to the connections of the equipments are simple the system is convenient to operate. It used where relative importance of substation is less.

A bus bars was connected two incoming lines through the isolators and circuit breakers and the outgoing lines are connected to the bus bars through the transformers and the all transformer substation in the power system handle incoming and outgoing lines to depend upon the terminal substation and through substation.

In this method incoming line terminates or ends to may be placed at the end of the main line or may be situated at a point away from main line route for this case a tapping is taken from main line to supply substation

Sectionalisation Single Bus Bar system

This method is simple and cheap where the single bus bar is divided into many sections and load is equally distributed on all the sections and any two sections and the bus bars are connected by a circuit breaker and isolators. Each section performs as a separate bus bar

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The main function of this system is

  • An any one section when fault occurs of the bus that section can be isolated without affecting supply from other sections and the repairs and maintenance can be easily carried out by shutting off that section only to avoid complete shutdown of substation. The 33kV voltage process is used.
  • The bus section and the two incoming circuit breakers are so interlocked that only two breakers can be operated at a time. If any of the incoming lines between the two sections is taken out then the sectionalizes breaker can be closed and supply can be retained to that section but proper consideration should be given to other design aspects such as plinths of the transformers ratios of current transformers and power cables.

Double Bus Bar System with single Breaker:

In this system have a two bus bars there are

  • Main bus bar
  • Spare bus bar

And these two bus bars can take the entire substation load and incoming and outgoing lines can be connected to one of these bus bars with the help of bus bar coupler having of circuit breaker and isolator.

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Advantage:

  • When load can be fed from either bus so reliability and continuity of supply is maintained.
  • For this case of repair and maintenance of main bus bar the continuity of supply can be maintained through spare bus bar.

But the cost of this method is high and it can be used where loads and sequentially of supply are the main considerations than the cost. In this breaker the bus coupler section provides on load change over from one bus bar to the other to the normal bus section isolators cannot be used for breaking load currents. This method does not provide any means for the breaker maintenance without interrupting the supply.

Double Bus with Double Breaker Method:

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The single breaker method of double bus bar that the circuit breakers and isolators are not permitted for repair and maintenance without interruption of supply. So the problem can be solved by involved double breaker method which is apart from simple is very expensive and hence rarely used at substations. In this scheme is to be used then it is employed for very large generating stations where security of connections is important and this scheme is not economical and not recommendable and also when there is interconnected grid system.

Breaker and a Half method with Two Main Buses

An improvement of double breaker scheme which effects in saving of number of circuit breakers. For every two circuits one spare circuit breaker is provided and the protection scheme is more complicated.

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In this scheme not generally used as it also employed additional costs as such a method in neither desirable from economy point and not required with the interconnected grid system in which load can be easily transferred from one substation to other if any maintenance of any switchgear is to be obtained in a substation.

Main and Transfer Bus bar method

The alternative to double bus bar method. The double bus method to provide facility to change to any bus to carry maintenance of the other but it does not provide any facility for carrying out breaker and isolator maintenance. So this method exactly operate opposite to that of double bus bar method as it provides facility to carry out maintenance of breaker but no provision is made for bus maintenance.

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The maintenance on any breaker is required then there is change over from main bus to exchange bus and controlled through bus coupler breaker and this system uses an additional isolator on each circuit which increases the cost of the system to the change over from main bus to transfer bus in this system is through isolators carful interlocking is required with the bus coupler breaker.

Double Bus Bar with Bypass Isolator

In this method combination of double bus method and main and transfer bus schemes and this one of the bus bars acts as main bus while the other act as transfer bus so it provides facility for breaker maintenance.

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An additional isolator on each circuit which may introduce complexity in the layout and this method is a good compromise because of its simplicity and economy and that can be noted that the bypass switch is not continuously operated and is not provided from point the economy then temporary or shut down of line is done and direct connection is made which is known as “jumper connection”. And without using isolators maintenance of breaker is possible and this method of light load hours the whole load on one line can be transferred to other line and maintenance of that line can be obtained by making that line totally off which results in economic operations.

Mesh Scheme or Ring Bus

In this method nowadays not commonly used and its also called as ring bus

Advantages

  • Each circuit double feed opening of any one breaker for correcting and maintenance does not interrupt supply of other circuits.
  • It is lower than double bus or main and transfer bus scheme
  • The breaker maintenance is not possible without supply interruption
  • Necessary no separate bus protection is necessary as all sections of conductors in the station are protected properly

But this method affect some limitation in its operation and safety and is not well suited in developed systems.

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So it can be seen from the figure that when the mesh is closed each circuit has double feed. When any one breaker is taken out for maintenance then supply can be retained through other path. But it requires a double set of current transformers for each circuit which results in extra cost. Since it is recommended this method is not provided if circuits are more than 6. So the expansion is not possible with this method as it needed extension of mesh beyond the circuits that it supplies naturally which is extremely difficult

Ring bus bar types:

  • Simple ring
  • Rectangular ring
  • Circular ring
  • Zig – Zag ring

 

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