Table of Contents
A substation is part of electrical generation, transmission and distribution system. Substation performs several functions between the generating station and consumer.
When we plug in an electric device it is easy not to even consider where the electricity actually comes from?
The simple answer is the power generating station also known as power plant usually some place far away. Generation is the first step of production of power which will be generated in various types of power station which uses different types of energy such as hydel, wind, thermal or nuclear energy. The behaviour of electricity does not always follow our intuition which means the challenges associated with the constructing, operating, and maintaining the power grids are often complicated and sometime unexpected.
Most of the electrical power received by the substation from:
- Gas energy
- Coal energy
- Nuclear energy
- Oil energy
The voltage generated in the power station is about 12000V.
The 12000V produce in the power plant is step by the transformer which are 115Kv, 230KV and 400KV to minimize the losses on the network and increase the amount of power that can flow through it the power is then distributed over the transmission network using overhead lines and cables typical transmission voltages 115Kv, 230KV and 400KV.
We now step down the voltages using a transformer so that it can be used in industrial and residential customers. We normally transmit the power through the distributing network using cables. Normally distribution voltages are 11KV or 33Kv.
There are two main types on the transmitting network:
- Switching station
A switching station has no transformer and only operates at a single voltage level. It is used to switch electrical energy around the transmission network, connecting the power stations to the cities and local centers.
A substation takes the energy from the transmission network and steps it down to a lower voltage level using a transformer. Substation is:
- Critical for generation, transmission and distribution system
- Perform several important switching functions
- May have very voltage levels before it reaches the customer. According to the requirement of the customer whether they require high or low voltage.
- Owned and operated by electrical utility or a large industrial commercial customer. Someone has the ownership that will operate this system. For example in Pakistan most substation are operated by the NTDCL.
- Supervision and control can be done with the help of SCADA system
Three types of the potential can be find inside the substation:
- Touch Voltage
Touch voltage is the difference between the surface potential and ground potential rise at a point where the person is standing while at the same time having a hand in contact with a ground structure.
- Step voltage
When a person experience difference in surface potential bridging a distance if 1m with the feet without contacting any other ground object.
- Mesh Voltage
Mesh voltage is the difference of voltage between the potential of the soil within the grid and metallic object connect with the grid.
Each transmission substation consists of the following key elements:
Transformer is capable of receiving ac power at one voltage it can be step up or step down voltage and deliver this voltage. In this way they help to achieve better transformer efficiency while transferring the power over longer distances. The basic working principal of transformer is simple electromagnetic induction. According to this principle a varying magnetic flux associated with loop will induce an electromotive force across it. Such a fluctuating magnetic field can easily be produce by a coil and alternating emf system. Magnetic field is produce around it when current passes through it. The magnetic field produce by the coil with the fluctuating nature of the alternating current the magnetic field associated with the coil also fluctuate. This magnetic flux can be effectively linked to a secondary winding with the help of core made up of ferromagnetic material. Due to electromagnetic induction the fluctuating magnetic field will induce an emf in the secondary coils. Since the turns are arranged in a series. The net emf across the winding will be the sum of individual emf induces in each turn. Since the same magnetic flux passing through primary and secondary coil the emf per turn for both the primary and secondary coils will be same. The primary coil input voltage depends on the emf per turn. As a result the induced emf at the secondary coil is expressed as follows:
When the number of turns in the secondary winding is greater than primary winding the transformer will step up the voltage and when the number of turns in the secondary is less than primary than it will step down the voltage. But since the energy is conserved the primary and secondary currents have to obey the following relationship:
EsIs = EpIp
Three phase transformer use three single phase transformer but with slightly different coil configuration. Here the primary and secondary coils sits concentrically and similarly two more such windings will be used in the configuration of the three phase transformer. Transformer with high power rating generally employs a special type of winding known as disc type winding. Where a separate disk winding are connected in series through outer and inner crossover. The low voltage winding are connected in delta configuration and high voltage are connected in a star configuration. Thus the line voltage further rises to belt 3.5 times at high voltage side. This also means a three phase step up transformer we can draw four output wires three phase power wire and one neutral.
High voltage bushing are required to bring out the electrical energy. The core of the transformer is made up of thin insulated steel laminations. Such steel laminations are stacked together to form three phase limbs. The energy losses due to eddy current can be reduced by thin laminations. The low voltage windage usually sits near the core. For the dissipation of heat the transformer is immersed in cooling oil. By using natural convection process the oil dissipates the heat. The heat in the transformer will be absorbed by the oil.
Circuit breaker generally 145KV and above rating circuit breakers are SF6 circuit breaker. Usually single pole circuit breaker is used in the substation the conductors are connected at the top and bottom of the palm. The SF6 gauge in the circuit breaker is used to monitor the gas pressure. The circuit breaker has two main components one interrupter where closed and open operation of contacts takes place to drive mechanism first lets see what is inside this interrupter. This is air tight chamber filled with SF6 inside the porcelain housing we can see two main contacts one is male contact which is moving and the other one is female contact which is fixed this is how these two contacts look like if we look closely two more contacts relatively smaller these contacts are called as our contacts this one is male our contact which is fixed and this other one our female our contact which is moving this is nozzle which is made of non-metallic insulating material function of the nozzle is to direct SF6 flows towards arcing region to accelerate quenching. Pistons in the circuit breakers are used to push SF6 to blow it on arc for effective our quenching piston rod is coupled with insulating rod which runs through the stack and drive piston rod and subsequently moving contacts during operation. The opening and closing of the contacts occurs:
When the interrupter is in open condition and close command is given to the circuit moving contact move towards the fixed contacts before the closing arc is formed because the distance between our contacts less than between main contacts. Our contacts more like during this operation SF6 gas get accumulated in the region between pistons and main contact. Finally arc and main contacts get close and arc is quenched this is how closed operation takes place inside the interrupter. Now we will check the open operation this closed condition one strip or an open command is given to the circuit breakers moving contacts to move away from fixed contact. Firstly main contact open and then our contact open hence arc is produced between our contacts during this time SF6 accumulated between main contacts and pistons get compressed due to contact movement is pushed into the arc region through nozzle hence arc is successfully quenched this is how open operation takes place inside the interrupter.
The isolator is mechanical switch which isolates a part of circuit from the system according to the requirements. Isolator is manually operated mechanical switch which separately a part of electric power. Isolator can also be operated by motorized mechanism. Isolator is a switch which is always operated at off load either we open or close the isolator.
On load: when current is flowing in the circuit
Off load: when current is not flowing in the circuit
Types of isolator:
- Double break isolator
- Single break isolator
- Pantograph type isolator
Depending upon the position in the power system the isolator can be categorized as:
Bus side Isolator:
This isolator is directly connected with the bus.
Line side Isolator:
This isolator is situated at line side of any feeder.
Transfer side Isolator:
This isolator is directly connected with transfer bus.
Busbar (also Bus bar) is common connection point or a node for multiple incoming and outgoing circuits. Such as power lines or feeders as we know it is impractical to connect multiple conductors at one point hence we use busbar where these connection can be done spaciously conveniently so let’s start with different bus bar schemes or system in electrical substation one single bus system this is the most basic and simple busbar system as we can in the diagram in this type all incoming and outgoing base such as lines transformer feeders are directly connected to single bus.
It is most easy and convenient for operation also it is very economical because of its least capital cost among all busbar systems.
Single bus Sectionalized system:
This is single bus sectionalized system this is single bus system with additional circuit breaker and isolator making two different sections of bus hence called as single bus sectionalized system. Since there are two sections separated by circuit breaker fault on one section does not affect the other section.
Current transformer are used to transform standardized primary current into standardized secondary current. Current transformer is a current measuring device used to safely reproduce a low level current that accurately represents a higher current level for the purpose of metering and protection. The ac currents converted in this way are much smaller than the primary flowing currents and can be directly proceed by the connected protection control and measuring system. The basic principle of the of the current transformer is simple electromagnetic induction. By applying Maxwell equations specifically amperes law we can say that if a magnetic field is integrated around a close loop of wire the value of that integral is equal to the net current enclosed by the loop. Current transformer consisting of magnetic core it is closed loop instrument and the core of the transformer consists of secondary winding .The primary winding of the CT the main loop has the wire with the current we wish to measure pass through the center of the core. The primary winding that carries the main current is said to have single loop for winding the wire produces the magnetic field that drives the current on the secondary winding which is used as the output of the
- the current on the secondary winding is proportional to the current flowing through the center of the core. Typically the secondary rating is 5A to 1A. for example with a 1000V rating or a turns ratio 200 to 1. When 1000A on the primary circuit would produce 5A on the secondary winding. Current transformers are primarily used for metering and protection applications. Current transformer is also used to monitor power and power factor so that real and reactive power can be optimized.
This transformer is used for measurement and protection. Potential transformer transforms the voltage from high voltage to low voltage. The potential transformer is step down transformer so that primary winding of the transformer will contain more turns than the secondary winding. As we have low voltage it the secondary side we can easily measure the voltage through voltmeter.
The potential transformer is used for high voltage measuring. This used for the protection for over voltage and under voltage to protect the lines. Potential transformer is also used for the synchronization of the generator with the line.
The surge arrestor is used for the prevention of the lightning and voltage spikes. The surge arrestors are used to divert abnormal high voltage to ground caused due to lightning phenomenon or switching operation without affecting the continuity of the supply. Thus it saves the electrical equipment from any possible damage due to high voltage surges. The difference between the lightning arrestors and surge is that it lightning arrestor divert only light surges while the surge arrestors divert switching surges and as well as lightning surges. Surge arrestor is connected between the phase and ground. The upper part of the surge arrestor is connected with the line while the lower part is connected with the ground. Surge arrestors are used at the input so that the high frequency voltage that came from the lightning cannot come in the equipment. Voltage spikes are created when we on or off the system. When the voltage is suddenly increase than it is called voltage spike. The surge arrestor consists of Zinc oxide (ZnO). The zinc oxide has the property when maximum voltage came in it then it minimizes the resistance and in the normal operation the resistance is maximum. The upper ring of the surge arrestor is known as gradient ring. This ring distributes the lightning voltage.
Power lines transmit energy from one location to another and are the life blood of the electrical system. There are certain situations which can cause the power line to fail such as:
- Equipment failure by human error like using wrong connection
- When Lightning strikes due to which spikes in voltage will occur
- When animals spinning two lines
Over current protection work well when the power flowed in only one direction. Because the fault can be located current magnitude. The relay will closest to the fault will trip first in a properly coordinated system because of the large current flowing into it and its smaller picks up setting. Current can flow an any direction on an electrical grid so it is almost impossible to set all of the relay to operate correctly for every possible scenario.
Earthing of the substation:
The materials which are require for earthing are:
Typically three types of conductors are used for earthing the substation copper clad steel, steel and aluminum.
The ground metal fencing is used to enclose substation with energized electrical equipment or conductor.
Working of Substation:
These feed power into the substation
These connect the different circuits together.
These allows the different bus bars to connect together.
These take power out of the substation.
There are many possible ways to connect the incomers, bus bars and feeders together here are the most common:
This is very simple circuit where the high voltage incomer to the transformer. where transformer step down the voltage to the LV feeder. These are widely used in network to step down the voltage to a lower level.
The high voltage and low level voltage connection to the transformer can either be via a cable or open terminal substation. Let’s see how this circuit works in practice, we can see that high voltage incomer are live we close the isolator on the incomer before closing the incomer circuit breaker. This will energize the transformer and now close the low voltage circuit breaker again we close the isolator before closing the circuit breaker. Power is now flowing from high voltage incomer to the transformer through low voltage feeder.
Single busbar substation:
This is simple configuration to connect the incomer to several feeders. Lets see how this configuration works. Firstly we close the isolator on the incomer before closing the incomer circuit breaker. The bus bar is now energized now we will energize one of the feeder. Again we close the feeder before closing the feeder circuit breaker. Power is now flowing from the high incomer to the low voltage feeder.
The main issue with this type of configuration is if the main supply lost or the fault occurs on the busbar. All the feeders will lose the supply. Solution for this problem is that we will use split busbar substation to give it more security we will use separate incomers which will be alternative supply. We will also add bus section circuit breaker. So when one busbar is out of service. It can still supply several feeders. Let now close the circuit breaker. For normal configuration when both incomers are available if each incomer has its own bus bar. When the bus section breaker open. Now let what will happen when we lose one of the incomer. Firstly we close the isolator before the bus section circuit breaker. In normal practice we leave the busbar isolator permanently close if one of the incomer fails we can quickly close the bus section circuit breaker and energize that bus bar. This arrangement is also called 2 out of 3 systems. For the normal operation if 2 of the circuit breaker is closed at one time.
Double bus bar configuration:
Another popular configuration is double bus bar configuration. This configuration provides multiple options to connect the incomers to the feeders together. We have four bus bar in this arrangement. Each of the incomer is connected to one of the two bus bar using the bus bar isolators. We can also add bus coupler circuits to connect the upper and lower bus bars together. This now close the circuit breaker the normal configuration is feeding up to feed to bus bar as we can see income of one of feeding bus bar one and two and two is feeding bus bar three before. All the feature of the arrangement is that we can change which bus bar a feeder is connected to without disconnect the load this is done what called no load change of sequence. This can be achieving by firstly we will close one of the bus coupler to connect the two bus bar together the bus coupler has inbuilt synchronizing relay to make sure that the parameters on each bus bar the same once the bus coupler is closed we can close second bus isolators on each of the feeders once this is done we can open the original bus bar isolator and we have now change over which feeders the bus bar is connected to we then open the bus coupler circuit breaker again splitting the bus bar. Additional bus bar is loaded to give the network operator more options to connect the incomer of the feeders together having one bus bar put incomer is a common configuration to provide even more flexibility the bus bars are connected together using both section and both couplers. Ideally it should be possible to connect any feeder to incomer or balance the loads on the network and allow flexibility when a fault occur all the circuits need to be taken out of the service for maintenance.
In this arrangement we have two common bus bars connected with base which have three circuit breaker each. The base can have incomer on feeder alternatively two feeders this configuration is common for transmission level switching station where power is simply flowing in out of the station i.e no transformers we can connect any incomers 20 feeder via busbar so the arrangement is very flexible and close the circuit breakers. In this arrangement power is flowing from incomer one and to the feeders connected to that busbar incomer is connected to the busbar. When we release energy to the incomer 1 the arrangement is automatically configured with the incomers now feeding both busbars on all of the loads are connected with them. The breaker and half arrangement is popular around the world as it gives a cost efficient way providing a very flexible design which is easy to expand should the system demands increase number of the days that can be tired together is virtually limited and I have worked on high voltage transmission substation .
Why stones are used in the substation:
It is usually asked in the job interview that why we use ballast or stones in the substation?
The answer to the question is that the stones maintain resistivity on the soil and it act as insulator. Normally in substation we have two types of faults one is permanent fault and other is resistive fault. In the resistive fault when the wire break and fall down on the earth due to which the current will try to ground but due to the stones the resistance is very high and will act as insulator and will not cause the current to ground. It will maintain the resistivity on the surface layer. If the stones were not present than the current will be ground due to which high amount of current will flow in the wire and the transmission wires may be damage.