It is said that the substation is a very important part of the power system. Let’s discuss it in detail.

1. What is a substation?

Substations are groups of equipment installed to protect and control the transmission of electric power. The substation switchgear is an integral part of the substation, which plays the role of carrying, connecting, and breaking the normal current load. Furthermore, manufacturers of compact substations ensure the conversion of energy from medium voltage to low voltage or from low voltage to medium voltage in secondary distribution.

2. Different types of substations

Now, talking about the types of substations, packaged substations can be classified based on the following parameters.

  • Based on construction and design
  • Based on the job role
  • Service-Based Hiring
  • Based on the electricity network
  • Based on the operating voltage
  • Based on insulation


1) Based on design and construction methods

Here we will discuss substation types that are mainly distinguished by their physical characteristics, i.e. how they are designed and built. These include:

#1. Indoor substation

An indoor substation is a substation in which equipment is installed inside a building. The voltage range is around 11,000 V but can go up to 33,000 V. Although this kind of substation has a complex structure, it can be easily detected once a fault occurs. The space required is very small. Electrical switchgear helps to achieve this.

#2. Outdoor substation

Such substation equipment is located outside the building. In this case, the voltage range is 66 kV to 75 kV. The structure is not complicated, but failures are not easy to detect in this case. More space is required.

#3. Substation on pole

A pole-mounted substation is a substation mounted on a pole. Substations of this type are said to be the cheapest.

#4. Ground-based substation

A ground-based substation is fenced and all equipment is assembled.

#5. Underground substation

This type of substation is mainly used in restricted areas. According to the main purpose, it is used to transmit electricity directly to the city center.


2) Based on job role

In this section, we will discuss substations that differ in terms of performance. Each step in power transmission and distribution requires a specific type of substation.

#1. Step-up or primary substation

A step-up substation receives power from a facility and increases the voltage with a power transformer. Step-up substations are used in wind farms, hydropower projects, etc. The voltage is increased from 11 kV to 200 kV.

#2. Primary grid substation

Primary grid substations come into operation to reduce the voltage for primary or boost transmission. The output of the primary grid substation is used as the input of the secondary substation, and the voltage of the secondary substation is lowered again for further transmission. At a suitable load center, it is created.

#3. The second substation

This comes into play after the main grid substation. The output of the main grid substation serves as its input. It works along the second transmission line. It reduces the voltage from 66 kV to 11 kV.

#4. Step-down or distribution substation

Step-down substations are located in the grid at the switching point. It changes the transfer voltage to a secondary transfer voltage. The voltage range is about 69 kV. It is close to the user end and is a pole-mounted substation.

#5. Bulk Substation

This type of substation is located on high-voltage transmission lines that provide large amounts of electricity for industrial needs.

#6. Mobile substation

Mobile substations are actually substations on wheels. Transformers, circuit breakers, and busbars are mounted on a self-contained semitrailer.


3) Based on service needs

Next, we classify substations according to the services they provide. We will examine the use of different types of substations in the transmission of power in detail.

#1. Substation

A substation is a substation that distributes electricity and also steps up and down the voltage of an AC power system. The substation consists of one or two power transformers, power distribution equipment, control, protection devices, and auxiliary equipment. It is a key element of a substation. All in all, it’s where big power is converted into usable power and vice versa.

Substations are divided into four categories – step-up substations, primary grid substations, secondary substations, and distribution substations.

#2. Frequency conversion substation

Frequency conversion is a top priority, and frequency conversion substations emerge as the times require. It is a motor generator that changes the power of alternating current from one frequency to another. It is mainly used in industry.

#3. Switch substation

Switching substations or switchyards are substations without transformers. It operates on a single voltage level. It performs switching and does not change the input and output voltage levels.

#4. Conversion substation

A substation is a substation that converts alternating current in power transmission to direct current. Alternating current is used domestically, while direct current is sent abroad via submarine cables.

#5. Power factor correction substation

Power transmission is concerned with improving power factors. A power factor correction substation works in this respect, i.e. the transmission’s power factor is improved.

4) Based on the power network

This one here is related to the power of the substation. That is, we are talking about the capacity of substations here, and we will also know that each area has different types of substations according to their capacity.

#1. Grid substation

Grid substations enter the system where voltage must be stepped up or down. This is where the voltage is changed as needed and the next process is started. Its main purpose is to transmit electricity over great distances.

#2. Town substation

Town substations are distributed in the town. The voltage range is 33/11 kV.

#3. Local substations

Substations that step down transmission voltage from 11 kV to 415 kV are called location substations. They are positioned in a way that is geared toward domestic and local purposes.

#4. City substation

Urban substations are substations similar to town and local substations. It makes the necessary changes in voltage to convert it from transmission lines to distribution lines.


5) Based on the operating voltage

As the voltage must be changed as needed, we have those substations that are differentiated by voltage.

#1. High voltage substation

In this type of substation, electricity is taken from one point, converted, and distributed to another. The high-voltage substation consists of high-voltage switchgear, medium-voltage switchgear, high-voltage equipment and transformers, auxiliary equipment, control systems, protection equipment, etc., with a voltage range of 11 kV to 66 kV.

#2. UHV substation

The voltage range in EHV substations is 132 kV to 400 kV. In addition to stepping down and stepping up the voltage of the distribution network, they also spread across the country. The types of equipment in UHV substations include bus bars, insulators, power transformers, circuit breakers, isolators, transformers, lightning arresters, neutral grounding reactors, current limiting reactors, shunt reactors, and capacitor banks.

#3. UHV substation

When the voltage range is above 400kV, this substation is called a UHV substation.

6) Based on insulation

Here, we discuss insulation-based substations.

#1. Gas-insulated substation

Substations with gas insulation are high-voltage substations with sealed main structures. It is also a kind of withdrawable switchgear. A dielectric gas such as SF6 or sulfur hexafluoride gas is used as the insulating medium. These components are electrical busbars, galvanic isolators or disconnectors, circuit breakers, circuit transformers, voltage transformers, earthing switches, surge arresters, or lightning arresters.

As mentioned earlier, electrical switching devices are responsible for making, breaking, and carrying normal current loads. Hence, vital importance must be given while engaging a gis switchgear manufacturer.

Types of gas-insulated substations:

Phase-isolated gas-insulated substation – In this type, all phases of the interval are assembled individually. For each phase, one pole of the circuit breaker, one pole of the galvanic isolator, and the phase assembly of the current transformer are connected together.

Integrated three-phase gas insulated substation – There are three phases of current transformers, three phases of disconnectors, and three phases of circuit breakers in this type.

Hybrid gas insulated substation system – it is a combination of isolated phase and three phases gas insulated substation. The simplest connection of a three-phase system and the isolation of phases prevents phase-to-phase faults, this design utilizes the best of both worlds to provide the best quality. Hence the name “hybrid”.

Compact gas-insulated substation – A metal enclosure encloses the functional elements of this type of substation, just like the RMU substation.

Highly integrated gas-insulated substation – a very efficient development in the field of substations. In this type of substation, all substation equipment is enclosed in one enclosure.

#2. Air-insulated substation

The air-insulated substation is a substation in which the main circuit is insulated from the ground by an insulator such as porcelain. Air-insulated substations are the most common type of substation. The substation works well for low voltages and is very efficient when the area is large. It also takes less time to erect.

3. Substations indoors and outdoors

Following are the key points of difference between indoor and outdoor substations.


1) Location

Speaking of differences, first, we have a location. When the equipment is installed inside the unit, it is called an indoor substation. If it is installed outdoors, it is called an outdoor substation.

2) Build

In terms of structure, indoor substations consist of a series of open and closed chambers or compartments. The equipment of the device is arranged in these compartments. The compartment is divided into the control room, indicating instrument and protection device room, main busbar room, current transformer room, and cable sealing box room. All in all, its design is complex.

For outdoor switchgear, the construction is not as complicated as for indoor substations, but considerable space is required. The cost of installing electrical switchgear is cheaper in this regard. Pole-mounted substations and foundation-mounted substations are two types of outdoor substations. In pole-mounted substations, the equipment is of the outdoor type. High-voltage transformer fuses are used for the protection of high-voltage transmission lines.

For low-voltage lines, it is also equipped with a low-voltage switch. In ground-based substations, the equipment is relatively heavy and complex. Therefore, the location must be suitable.

3) Voltage

For indoor substations, it is typically as high as 11,000 V. It is scalable from 33,000 V to 66,000 V.  For outdoor substations, the voltage is between 55 kV and 765 kV.

4) Equipment type

Due to the complex construction, the equipment is not easy to see. In outdoor substations, however, it is clearly visible.

5) Handling failure conditions

In indoor substations, faults are easy to spot due to the attention paid to every detail during setup, but in outdoor substations the situation is different. The fault is not obvious, so it can be very complicated to deal with.

6) Maintenance

Indoor substation equipment is complicated, and naturally, the maintenance cost in this area is also high. The situation with outdoor substations is less complicated, so low maintenance will do the trick.

7) Repair and replacement

Even though the fault detection process is easier due to the system installed, any kind of repair or replacement of the equipment in the indoor substation is not easily done. But for outdoor substations, repair, and replacement is a difficult process.

8) Security

In terms of safety, indoor substations are much safer. Because the outdoor substation is not complicated, safety is not enough.

9) Clear

Since all components are individually insulated in the indoor substation, there are fewer gaps between them. Components such as bus bars, OH wires, etc. are not insulated, so the gap is larger.


10) Space

There is less space between devices in indoor substations. It is more in outdoor substations.

11) Upgrade

Indoor substations have fewer upgrades. It is more in outdoor substations.

4. AIS and GIS substation

AIS and GIS have many significant differences besides the dielectric used. They are fully described below.

1) Advantages

There are advantages of AIS substation:

1. It is the best choice for building substations in places with large spaces.

2. AIS switchgear does not require a lot of costs. They can be built at a reasonable cost compared to GIS switchgear.

3. Compared with the high-voltage GIS substation, the construction speed of the AIS substation is faster.

4. Since the equipment is transparent, once a failure occurs, there is no need for frequent or expensive maintenance. Detection will be easier and faster.

There are advantages of GIS substation:

1. The high-voltage GIS substation adopts a metal shell to ensure a safe working environment.

2. Traditional AIS systems require minimal space because the distance between active and passive switchgear parts is minimized.

3. Components are separated into compartments for accurate connection and assembly.

4. Like AIS, GIS substations do not require extensive maintenance as they are built to provide maximum environmental protection.

5. SF6 gas is known not to deplete or age for about 40 years. Therefore, it is not necessary to replace it regularly.

6. GIS can be assembled quickly.


2) Disadvantages

There are advantages of GIS substation:

1. Air-insulated switchgear requires more floor space than gas-insulated substations because they are designed to be erected in large spaces.

2. It has a set of maintenance requirements that must be met on a regular basis, which can be expensive.

3. Compared with SF6 gas-insulated switchgear, the air is a poor dielectric medium. Moisture and secondary factors such as humidity and pollution require additional space to increase efficiency.

There are disadvantages of GIS substations:

1. The installation of GIS substations requires high capital.

2. It would be more economical to transport the SF6 gas supply over rough terrain.

3. GIS gas-insulated substations also require a lot of maintenance. Maintenance should also be performed by trained personnel.

4. Internal faults should be rectified as soon as possible, which can be costly in GIS switchgear.

5. GIS substation failures often lead to long power outages.

6. Although the gas is mostly harmless, it may decompose into hazardous by-products such as metal fluoride powder due to difficulties in flashing. This can endanger the person’s health by causing physical suffocation and other breathing difficulties.

3) Main hardware

For primary appliances, you need primary hardware. On the other hand, the cost of ancillary equipment such as supports, cables, land, buildings, controls, security, and monitoring may result in a small cost difference between the two systems.

4) Maintenance

SF6 gas-insulated switchgear requires considerably less maintenance. As a general rule, GIS systems only require visual inspections every four years or more, depending on the manufacturer’s recommendations. Lubrication can be done in GIS systems for about 20 years. When lubricating, always follow the instructions of the switchgear company.

AIS systems should be inspected annually to biennially, according to the specific manufacturer’s recommendations. Unlike GIS gas-insulated substations, where individual compartments and elements are thoroughly insulated and monitored, all compartments must be checked during inspections. Every two years, AIS circuit breakers require about four hours of maintenance.

Due to the complexity of inspections, AIS systems require more effort in maintenance. As part of the inspection, technicians will tighten, pull, clean, lubricate and vacuum the equipment. In medium voltage gas-insulated switchgear, which is hermetically sealed, copper oxidation is not visible. These components are also protected from environmental damage by being sealed.

Since circuit breakers and disconnectors in MV GIS have a lower probability of failure than AIS throughout their lifetime, mv GIS has lower maintenance costs than AIS switchgear.


5) Operation

In terms of their operation, GIS systems are easier to manage day-to-day because they can be accessed from the front rather than the rear. They also have their own built-in test equipment. An arc flash is a flashover of electrical current that deviates from its intended path and travels through the air from one conductor to another or to the ground.

It is possible for arc flashes to occur in electrical systems, but they are uncommon in GIS switchgear since all internal components are shielded and only the cable compartment is accessible. No cables or links can come into contact with live conductors as they are fully insulated. Training in MV GIS switchgear is more expensive than training in AIS switchgear.

6) Design features

Since air-insulated substations must be installed in high-grade areas of the space, they are often located outdoors. The internal components of AIS air-insulated switchgear should also be air insulated. Some other features of AIS switchgear are

  • less construction time
  • Most suitable for substations with lower rated voltage.
  • vast space
  • Safe to place outdoors.

Switchgear designed for high-voltage gas insulation has one or more of the following characteristics:

  • limited space
  • extreme weather conditions
  • low environmental impact required
  • less maintenance

7) Cost

The initial capital expenditure is not sufficient to evaluate the entire substation project economically. Overall costs should be assessed by considering major hardware, maintenance, operation, outage, and disposal costs. Here is a cost comparison of AIS switchgear and GIS equipment:

  • Since both substations are the main machinery, the main hardware is used. Costs for support, conductors, land, and equipment such as installation, management, protection, and monitoring are often high. This can lead to very little cost difference between the two systems.
  • Due to the lower probability of failure of circuit breakers and disconnectors in GIS, maintenance costs of high voltage GIS are lower compared to AIS.
  • The cost of GIS training is much higher than that of AIS training.

Cost depends on various factors. To calculate the total cost, a budget should also be prepared for post-use decommissioning and disposal costs.

8) Environmental impact

While both AIS air-insulated switchgear and GIS equipment are inherently non-hazardous, specific environmental impacts require attention as substations use electricity and natural gas, both of which should be protected at all costs. The following are the environmental impacts of mv GIS and AIS:

#1. Noise

Air-insulated circuit breakers and load break switches generate a lot of momentary noise when they are triggered. Other noise sources of substation switchgear include corona discharge, arcing during the switching operation, etc. Noise levels are low as the machinery in the HV GIS installation is fully enclosed.

SF6 gas in the enclosure is a convenient and efficient sound absorber, as new arc interruption techniques in SF6 gas require lower energy levels, smaller spring-operated devices with lower noise levels have been developed.

#2. Gas leak

SF6 gas and other lubricating oils may leak into the environment under certain conditions. Since SF6 is classified as a greenhouse gas, manufacturers and consumers should take all necessary precautions to prevent construction and operational releases. In order to prevent gas leakage, various countermeasures are usually taken.

However, when comparing the two substations, the sealing face lengths in AIS and GIS are different. In GIS, the SF6 seal has the longest overall length. However, in an AIS it is likely that oil-filled equipment will be placed. Due to the fact that GIS equipment is usually installed indoors, it reduces environmental exposure and extends the life of the containment system at the same time.

Compared to AIS, HV GIS systems are safer in terms of gas leakage. In addition to the above, the manufacturer provides extensive instructions on installation, service, maintenance, repair, and proper disposal to minimize disposal losses.


#3. Weather conditions

All technologies must be designed to withstand the weather parameters of the location where they are installed, such as temperature, humidity, ice, rain or snow, wind, solar radiation, etc. In this case, there were no significant differences between the systems. MV GIS systems, however, are usually installed indoors, so weather issues are not an issue.

#4. Pollution

Contamination on switchgear is proportional to the number of insulators and bushings. As a result, medium voltage gas-insulated switchgear systems have some significant advantages. Naturally, indoor switchgear has many advantages over outdoor AIS.

#5. Corrosion

Any equipment installed outdoors is subject to corrosion due to environmental influences, including even exposure to extreme conditions such as acid rain, marine environments, or man-made pollutants. However, since GIS is often deployed indoors, it has an advantage over AIS in this case.

#6. Earthquake conditions

If a substation is located in a seismically active area, regardless of the technology, proper engineering design and seismic testing in compliance with the relevant requirements must be employed. On the other hand, AIS is more likely to be damaged after an earthquake due to the separately installed structure and foundation, but GIS may respond more robustly due to its design. Due to their compactness and low center of gravity, GIS units function optimally in seismic environments.

5. Conclusion

Simply put, a substation is a path through which electricity transmission is transformed into distribution lines suitable for use. As we have discussed various substations and their subcategories, we have seen that various factors and scenarios influence the type of substation that is appropriate for a particular situation. Contact a well-known manufacturer of complete substations if you need high-voltage switchgear installed at your high-voltage substation.

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