Substation design planning and layout
Substations are facilities in a power station, where voltage transformation or switching is being carried out, and this process is done using equipment which includes; surge arresters, voltage transformers, isolators with and without earth blade, circuit breakers, current transformers, transformer, generators, relays, including others. A substation can either be for transmission or distribution. However, there are different types of substation design. These types are based on their functionality.
Type of substation design
- Transmission substation
- Distribution substation
- Switching substation
For transmission, a substation can either be a primary transmission substation, which has a voltage ranging from 330/132KV. While for the secondary transmission substation, the voltage ranges from 132/33KV. The voltage level for transmission substation, help to indicate if it’s primary or secondary. However, there are factors that can help one differentiate between these two.
The injection distribution substation is classified under the distribution substation because of its voltage level which ranges 33/11KV. Moreover, it’s necessary to understand that all process is stepping down of these voltages. Nevertheless, the voltage at 11KV can still be stepped down to 0.415KV. This stepping down system is classified as a mini distribution substation because there’s still voltage transformation.
However, in switching substation, no voltage transformation is done, and the input voltage is the same as output voltage. Moreover, you can have more than one incoming and also more outgoing. During grid collapse, and to maintain continuity, switching from one incomer to another can be done. Most importantly, during switching, synchronization takes places and this has an effect on the frequency, phases, and voltages.
Substations can be of different classes, and as such, they can be classified as either, indoor or outdoor substations. The design planning of a substation is factored on this classification. However, for outdoor substation, all power equipment is placed in an open-air, while for indoor substation, apparatus is laid inside the building of the substation.
Substation design planning and layout
- Type of substation: this consideration could either be an indoor or outdoor substation.
- The land area should be considered because this will affect the layout of the substation.
- Cost: this includes the overall cost of running the substation.
- Reliability of supply, which involves the availability of supply under stated conditions, over a given period of time.
- Extendability of supply. Extension of load capacity should be considered also, due to the population growth of consumers. This consideration helps for continuity of supply when loads increases.
- Maintainability, this is the ability to carry out maintenance on the substation, at any given point in time.
- Operational flexibility and protection arrangements. Substations are designed to be flexible in carrying out switching operations.
Substations transmit power through transmission lines, which could be overhead or through an underground cable. However, the overhead transmission line is the most commonly used system of power transmission or distribution. Among other major considerations on transmission lines, are short circuit rating of the line.
Line short circuit could be as a result of the line to line fault or line to earth fault. These faults can result in a high amount of current flowing through the fault path. However, knowing that electrical heating is a function of the square of the current and the resistance.
During short circuit fault, the resistance at that point drive to zero and current have much tendency to flow path of least resistance than higher resistance, as such more current will flow through that path. The increase in the amount of current flowing through the fault path will lead to electrical heating.
In the transmission line, the major way of preventing short circuits is to consider the minimum(standard) clearance between each phase and also with the earth. However, in cables, water can’t enter the cable. Nevertheless, there’s what is know as Ingress of water which is something to consider also.
The protection unit in a substation uses breakers. These breakers are rated based on fault MVA. However, this is the potential increase in the nominal MVA.
Switchyard layout configuration-substation design
The switchyard layout is an outdoor system. This segment is inside the substation layout. However, a lot of people miss up the switchyard layout with that of the substation layout. In a substation, it’s easy to identify where the switchyard is. The switchyard is most at times on gravel, that’s how you know your switchyard.
Basically, there are three circuits present in substation layout and this includes;
- Line circuit, this circuit helps for line protection.
- The transformer circuit helps to protect the transformers present in a substation.
- Coupling circuit.
The substation layout consists of both the switchyard configuration and the indoor configuration. The switchyard layout is an outdoor system, and the compasses both the line and transformer circuit. Gravels are placed only on the switchyard layout. However, indoor control relay panels are placed indoor, to help control the outdoor switchyard circuits.
The Line circuit
The Line circuit includes;
- Surge Arrester
- Voltage transformer
- Isolators with earth blade
- Circuit breaker
- Current transformer
A surge arrester is an electrical device that protects an electrical system from a sudden increase of voltage(voltage transit) caused by an external or internal source. Meanwhile, surge arresters are used as part of the protective equipment in a power transmission and distribution substation.
A voltage transformer also called a potential transformer, is an electrical device that allows high voltages to be measured without causing damage to the measuring device. Meanwhile, the winding of a voltage transformer is connected in parallel to the line to be protected and it uses a step-down transformer to help step down the voltage at the secondary winding before the voltage is being measured. Therefore, with the help of a voltage transformer, a 120V voltmeter can be used to measure a voltage up to 11kV without causing damage to the voltmeter.
Isolators are a mechanical switching device with an electrical configuration to either help to create an electrical path for continuity of power or to disconnect an electrical path. Meanwhile, isolators serve as a protective device in a line circuit of substations.
A circuit breaker is an automatic electrical protecting device with a tripping mechanism to help cut supply on the line when faults caused by short-circuiting or overload on the line occurs. Typically, circuit breakers protect the line from faults that results in a high amount of current being drawn in the circuit, knowing that excessive flow of current above the rate value will result in overheating of the line insulator thereby resulting to fire.
Another line protective device is the current transformer. The current transformer is an electrical protective device used to measure the high level of alternating current. The current transformer uses a step-up transformer to reduce the voltage at the secondary winding before being measured. However, the current at the secondary is proportionate with the primary. Therefore, with the help of a current transformer, a 5A ammeter can be used to measure current up to 200A.
Substations configurations are of different types. However, each of these types is factored into the design planning of a substation.
Type of substation configuration-substation design
The following are the types of substation configuration;
- Type A1-Type AA1
- Series of Type B
- Type A2- TypeAA2
- The Type A3-Type AA3
- Type A4-Type AA4
- Modified Type A4
- Type A5-Type AA5
The A-Series and AA series are differentiated by the buses. All A’s have a double bus, and AAs are of a single bus. Extensibility and maintainability are among the important considerations when designing substations. The modified type A4 has its line protection unit trade-off.
For a full injection distribution substation, the type B configuration is not used. The type B configuration uses a Ring main unit system and this type of configuration only have application for transformers ranging from 2.5MVA and below. So no need for a control room here.
In a substation or transmission lines, there is a minimum(standard) clearance between humans and lines. So when an operator wants to work on a double line, apparently the line has to be knocked out because that’s not the clearance set aside for humans.
However, substations can operate over a long period of time before subjected to maintenance. An isolator plays a vital role during the maintenance of substations. Although an isolator is not to be operated on load. For this reason, the circuit breaker should be operated before the isolator.
Moreover, isolators can either be operated manually or motorized. The type of isolator used in this function is factored on the voltage rating of the substation. The manual operated isolator, find application for voltage rating up to 145KV. While motorized isolators are used for voltage rating ranging from 245-420KV.
Substation design layout
The design of a substation is subjected to several considerations. These consideration helps to attain a perfect layout for substations. Most importantly, the geographical nature of the land is to be considered first. The nature of the land helps to structure the design of a substation.
Substations consist of the control building which is isolated from the switchyard. Here, the equipment placed on the switchyard can be controlled, even without reaching out to the switchyard. Most standard of a substation control building has an area of 18m by 10m. With two wide access doors, for easy access.
However, the control building entails offices for substation operators. The control building is placed at the land edge of the substation, usually, 2m away from the fence. The position of the control building is factored on where the substation transformer is placed.
The road entrance of the substation is usually 4-4.5m wide. With a gatehouse, which is the resident of the gate personnel. However, going to the other side of the switchyard, which is placed 1m to 1.5m away from the roadside inside the substation.
Substations have feeders, supplying a different area of consumers. The number of feeders depends on the rating of the substation transformer. This is kept at the appropriate number to avoid the overloading of the transformer.
Safety in substations
During a fault, which will essentially lead to an increase in voltage. Ideally, this excessive increase will flow to Mother Earth. This voltage decreases exponentially with time and the rate at which it fades away is dependent on the resistance level of the earth, which implies that the lower the earth resistance the faster the fading. This resistance can either be low or high.
However, for a good earthing system, the earth resistance has to be low, to excessively drag more current to itself. In other words, low earth resistance will take a shorter time for the fault to fade off. When the resistance of the earth is high, it will take a longer time for the fault to fade away.
Although the rate of absorption of this fault through the earth is dependent on a split factor. This factor determines the amount of current that goes radially and goes down the earth. Moreover, there is two important safety to avoid electrical faults.
- Step potential
- Touch potential
During an electrical fault, it is advised for everyone to avoid these two potential contact. For step potential, during a fault that drives down to the earth, but with split factor, the fault current spread radially about 60m away from that point. The human body presents a path flow for electrical current. So during a fault, when the human leg is apart, it creates different potential.
Since current flows through a potential level to another, as such current will flow through the human body, from one of the legs to the other, and this will lead to death. So it’s expected for the human legs to be placed very close to each other and walk closely with the legs still close with each other or moving with just one of the legs if possible, to avoid creating a different potential level.
For Touch potential, the human body tends to allow current flow through. Any of the body part making contact with the faulty electrical point will create a pathway for current to flow to the earth. This, on the other hand, will result in death. To avoid touch potential, it’s advisable to avoid any form of body contact with any material that has the tendency for the flow of electrons.
However, personal protective equipment (PPE) is required for anyone present in a substation, to help prevent and avoid health or safety risks.
FAQs on Substations
Reason why we don’t open an isolator on load?
There’s no quenching technique on isolators. During isolation of high voltage lines, an arc is produced and without an arc quenching medium, it will result in a fire outbreak. However, the isolator is open after the circuit breaker opens and they must be closed before the circuit breaker.
Why is the switchyard layout placed on gravel?
Gravels offers high resistance, making it a good insulator. However, the current passes through a path of low resistance. So during a fault, that will subject the operator at risk, but the operator standing on the gravel will help to prevent electrocution.
Graves are not combustible, so it doesn’t burn.
It helps to avoid transfer potential in a substation.
Why is oil circuit breakers not preferable in a substation?
During braking, an arc is produced. Using an oil circuit breaker as a medium of arc quenching, residuals are produced over time and this will reduce the insulation properties of the breaker. However, a slow breaking will lead to a fire outbreak.