Electric Substations — How do they work?

The challenges associated with construction, operation, and maintenance of the power grid are often complicated. Many of those challenges are overcome at the facility end which, at first glance, often look like a chaotic and dangerous mess of wires and equipment, but actually serves a number of essential roles in electrical grid, is the SUBSTATION.

Electrical substations have an utmost importance in electric distribution facility (see video explanation here), converts AC voltages from one level to another level or change nature of supply voltage i.e., from AC to DC or vice versa. The general layout of a substation consists of conductors that run along the entire substation. To avoid shutting down the entire substation, we need switches that can isolate equipment, transfer load, and control the flow of electricity along the bus. Switching in a substation is a carefully controlled procedure with specially designed equipment to handle high voltages for the protection of equipment.

Types of Substation

The substations can be classified on the basis the following

• Voltage levels
• Operation
• Applications
• Construction features.

1. On the basis of voltage levels

On the basis of voltage levels, substations can be categorized as:

• Low Voltage (LV) Substations: 0.24KV - 0.6 KV
• Medium Voltage (MV) Substations: 2.4KV - 69KV
• High (HV) Substations: 115KV - 765 kV
• Extra - High (EHV) & Ultra-High Voltages (UHV) Substations: 800KV - 1,100KV

These voltage level values may vary regionally. Above mentioned are specified for North American region.

2. On the Basis of Operation:

A substation can be used to transform voltage levels or for rectification or to improve power factor. Based on service requirement, substations can be classified as:

Switching substations

Switching substations allow switching between power lines without altering the transmitted voltages. They also isolate the faulted zones in the power systems and de-energize the faulted equipment to maintain grid stability.

Collecting substations

Power from distributed generation plants like solar plants, wind farms, hydroelectric projects etc., can be collected from multiple sources and synchronized with the grid power in Collector-type Substations.

Transformer Substations

Substations used to step up or step down the voltage level of an AC power system for power distribution are commonly termed as transformer substations. Power Substations are usually located near generating stations to increase the generated voltage level for transmission of electric power over the long distances. Distribution substations stepdown the voltage to a lower value according to consumer requirement. They are located near the load centers to feed power to consumers..

Converting Substations

These substations are used in DC transmission system to convert three-phase AC Voltage to DC and vice versa by deploying converters, harmonic filters, and synchronous condensers at sending and receiving ends of transmission system.

Power Factor Substations

To compensate the power losses during the transmission of electric power, synchronous condenser is used as a power factor correction device. Substations which deploy the use of capacitor banks or synchronous condensers are known as power factor correction substations.

3. On the basis of Applications

The following is the classification of substations based on the application aspect.

Grid substations:

• Primary substations are present on the primary side of transmission lines and are connected to bulk load centers. Voltages are stepped down here for secondary transmission.
• Secondary substations are lined at the secondary transmission line ends for the purpose of distribution.

Distribution substations: The distribution substations are located at the place where voltages of primary distribution are being stepped down and power is supplied to residential consumers.

Depending on the type of equipment used and configuration, these substations could be classified as

• Conventional – Outdoor type with air-insulated equipment
• Indoor type with air-insulated equipment
• SF6 Gas Insulated Substation
• Outdoor type with gas-insulated equipment
• Indoor type with gas-insulated equipment
• Composite Substation or Hybrid Substation combination of above two.

Mobile substations:

The Mobile substations are specifically and temporarily employed for dedicated purposes for instance large scale constructions etc. These substations are a source of temporary electrical supply and allow easy maintenance, and efficient protection from blackouts, fires, weather disturbances etc.

Industrial Substations:

TThe industrial substations, also known as bulk substations, are for dedicated consumers only e.g., industries requiring bulk power to be supplied.

Mining Substations:

This substation is dedicated for the control of electrical power supply from the surface to mine power station lying underground. They are required to be carefully designed for increased safety precautionary measures.

4. On the Basis of Construction Features:

Indoor Substation

These substations are divided into different compartments for controlling and metering of devices. Equipment are installed within the building of substation to avoid the exposure of transmission line with hazardous chemicals.

Outdoor Substation

As the name implies, these substations have equipment located out. Outdoor substation require  large area clearance between the live conductors.  These substations are further classified as:

Pole Mounted Substations (PMS)

These substations are erected for mounting power distribution transformer in the localities. The pole mounted substation shall be located in non- hazardous overhead obstructions free environment.

Foundation Mounted Substation

Surface Mounted Substation: This substation mount the transformers on ground surface having capacity of 33,000 volts or above.

Comparison Table of Indoor and Outdoor Substation:

Layout of a Substation:

Substation Components

Transformer

How to select type of transformer?

The selection for the type of transformer depends on the location of substation. As, non-flammable fluid-insulated transformers and sealed transformers are suitable for both indoor and outdoor use. Whereas Liquid-immersed transformers insulated with flammable liquids can be installed only indoor when located in fireproof vaults, thus they are normally suitable only for outdoor substation according to NFPA 70 and NFPA 70B. In addition to liquid type transformer, ventilated dry-type transformers are suitable only for indoor locations, and can be used for outdoor application only when mounted in a specific designed enclosure.

What should be the size of transformer?

The size of transformer in terms of kVA should be capable of holding sudden overload condition. Along with this, transformer size should be selected considering additional load in future. For 12.47kV/480V, dry type transformers are in range of 750-2500 kVA and for liquid filled transformer usually 750-3750 kVA size is preferred.

Why should you choose a transformer with minimum no load losses?

While selecting the transformer, it is economical to select a transformer with reduced losses regardless of the fact if its initial cost is more than the transformer with higher losses.

The formula below is used to evaluate the cost of transformer based on no load and full load losses is:

C_c = A P_o +B P_k

Where:

C_c = Capital Cost

A = per unit cost due to no-load losses

P_o = no-load losses

B = per unit cost due to load losses

P_k = losses due to the load

Assume two transformers: XFMR1 & XFMR2

Cost of losses for a period of 10 years will be:

XFMR1: A x P_o = 13578 x 0.098 x 10 = $13306

B x P_k = 12380 x 0.13 x 10 = $16094

Total cost of losses equal to approximately $29400.

XFMR2: A x P_o = 24528 x 0.098 x 10 = $24037

B x P_k = 17882 x 0.13 x 10 = $23247

Total cost of losses equal to approximately $47285. The saving calculated is therefore $17884 in 10 years. For detailed video explanation, click here.

Switchgear

Switchgear refers to integrated collection of devices to control and isolate electrical equipment through monitoring and protective equipment for power system protection. Click here to learn more about Equipment Maintenance Plan.

Monitoring Equipment

Substation needs extensive consideration while designing and selecting protective device. One of these devices includes relay for protection of substation equipment. In order to feed incoming line current to relay, instrument transformers are used practically. These transformers are used to stepdown high values of current into low current range which small size relay can easily bear to operate effectively, thus eliminating the need of large relays in the protection scheme for substations. Standard current ratings on secondary side of CT are 5 amperes or 1 ampere.

Power systems have evolved according to the needs of consumers, and operators over the past decades. SCADA systems were developed which allow remote monitoring and control of the system's key parameters.

Protection Devices

Circuit Breaker

A Circuit Breaker is characterized as "a mechanical isolating device equipped for making and breaking of current flow under normal and fault condition, for example, a short circuit" (IEEE Standard C.37.100-1992). For lower voltages, the circuit breakers can be located in a sealed container under vacuum to avoid electricity conducting in the air between the contacts. For higher voltage, breakers are often submerged in tanks filled with non-conductive oil or dense dielectric gas.

Circuit breakers are commonly grouped by the capability of medium such as air, oil, vacuum and SF6 gas to suppress the electrical arc. The widely used breakers are air, vacuum, oil and SF6 gas circuit breaker.

Air circuit breaker had been in practice for high voltage systems above 110 kV. Air blast breakers are no longer manufactured and have been replaced by breakers using SF6 technology.

Vacuum circuit breakers are extensively employed for switchgear applications up to 38kV.  

Two designs exist for oil circuit breaker: bulk oil and minimum oil circuit breaker. Ecological concerns and guidelines constrained the need of oil regulation and maintenance coupled with high cost, prompted the selection of the SF6 gas electrical switch in lieu of the oil circuit.

SF6 is 100 times more effective than an air circuit breaker as it employs SF6 gas mixed with another gas to avoid the liquification of SF6 gas. It is used for both medium and high voltage electrical power systems from 33KV to 800KV.

Circuit breakers shall have the following ratings:

• The continuous current rating of a circuit breaker must be higher than the maximum continuous current drawn by the load.
• The interrupting rating of a circuit breaker shall not be less than the maximum fault current for which circuit breaker is designed to interrupt this fault current.
• The momentary rating of a circuit breaker shall not be less than the maximum asymmetrical fault current at the point of installation.
• The rated maximum voltage of a circuit breaker shall not be less than the maximum circuit voltage.

High-Voltage Fuses

Fuses are generally applied for protecting power transformers in substations to provide interruption of permanent faults. Switchgear and substations that utilize high-voltage fuses should be selected according to following standard ratings:

• Continuous Current Rating: The continuous current rating of interrupter switches must not be less than the maximum continuous current at the point of installation.
• Voltage Rating: The maximum voltage rating should be equal or higher than the maximum circuit voltage.
• Supply Terminals: The supply terminals of fused interrupter switches shall be installed at the top of the switch enclosure, so as to prevent persons from accidentally contacting energized parts or dropping tools into energized parts.

Load Break Switch

Load break switches isolates the substation equipment while maintenance operation. Typically, a disconnect switch is installed on both side of a substation equipment. They are designed to carry load currents continuously and momentarily carry short circuit currents for defined duration without any overheating.

As the name suggest, a load break switch is a disconnect switch which break and make a circuit.. Arcing horns are equipped with disconnect switches to interrupt minor amount of charging or magnetizing current easily. This capability of current interruption is dependent on the arcing horn material used (typically copper or stainless steel), switch break type (vertical break, double end break, double end break Vee, center break, center break Vee, or single side break), phase spacing, and switch mounting position (horizontal upright, vertical, or under hung).

High-speed arcing horns also known as whip horns, or quick break whips. They are appropriate for voltage rating under 161 kV.

Relay

The automatic protective switchgear mainly consists of the relay and circuit breaker. These devices protect the generators, transmission lines, motors and other heavy electrical equipment.

The types of relay used in substations are:

• Differential relay
• Over Current relay(OCR):
• Instantaneous OCR,
• Definite time OCR,
• Inverse time relay with definite minimum time(IDMT)
• Auto reclosure relay
• OCR relay with earth fault relay

Surge Arrester

The protective equipment used to protect substation equipment primarily from high voltage produced due to lightning strike or switching surges are referred as surge arrester. They deploy non-linear resistors which produces a short circuit condition for high voltage, leading to a low resistance path for spark to flow. Thus, protecting connected equipment from surges of high voltage.

The most common surge arrester is metal oxide arresters. MOSA’s are used for above 1000 volts circuit under 48-62 Hz power systems to limit the voltage surges. The standard methods for the selection and location of arresters in the system are summarized in section 5 of IEEE Std C62.22™-2009.

The parameters which define the performance of surge arresters are:

Rated voltage:

The following table determines a minimum value of surge arresters rated voltage (Ur) based on system voltage (Um):

Protective Characteristics:

The surge arrester should be capable of withstanding high voltage transient without damaging of insulation provided. Lightning impulse and switching impulse protection level are calculated for estimating the insulation withstand of the equipment for lightning and switching impulses.

These levels depend on the distance between equipment to be protected and the arrester. The lower value of lightning protection margin states the lack of protection for the equipment which is not with in the protection or in close vicinity of the arresters. Click here to learn more about Surge Protection Devices.

Calculation for Feeder Size and Overcurrent Protection

The allowable ampacity of feeder conductor size should not be less than 125% of the continuous load along with the non-continuous load. Article 310 of NFPA 70 NEC 2017 explains the ampacity ratings, mechanical strength, uses and general requirements for the conductor. Table 310.15(B) shows size of conductor based on ambient temperature up to 2000 Volts. For THHW insulated copper conductor at 750 C, required size is 4/0 kcmil for load amperage of 230A.

Table 310.15 (B) Conductor size based on Ampacities of Insulated Conductors

Article 240 of NFPA 70 covers the basic requirements for overcurrent protection devices operated at system rated voltage below and above 1000 volts. The overcurrent protective device should be capable to allow 125% of the continuous and non-continuous load as discussed in Article 215.3.

The steps to calculate feeder size and rating of OCR device can be summarized as:

With the advancement in automation system for electrical substation, an extensive consideration should be given while selecting the equipment to provide high reliability to power systems.

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  About The Author

  Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.