Types and objects of the SubStations

Substations

A collection of electrical appliances (e.g., bus bars, switch carriers, power transformers, etc.) that converts some of the characteristics of electrical energy (e.g., conversion of AC voltage to DC voltage, changing the level of voltage, frequency, and power factor, etc.) from one form to another, is called electrical substation. Through the incoming transmission lines to a sub-station, a high voltage (132 KV, 220 KV, or 500 KV) AC electrical power is obtained from the generating machine or power station. And converts these high voltages to low voltage (11 KV, or 33 KV) according to needs through step-down transformers and provides it to outgoing distribution lines. A place, where the aforementioned task is performed, is known as a sub-station or a grid station. A sub-station is an important section of any power system, which produces an important linkage between the generating station, transmission system, distribution system, and load point. In other words, works as a medium or source of coordination between a consumer and the powerhouse. Substations actually work as sources for the supply of electrical energy in the local electrical distribution areas, where they are located.

Basically, an electrical sub-station consists of several incoming circuits and outgoing circuits connected through a common bus bar system. The bus bars are such conducting bars, to which a number of incoming or outgoing circuits are connected. Every circuit consists of some specific electrical appliances e.g., circuit, breakers, isolators, earthing, switches, current, transformers, voltage transformers, etc., which are coordinated with each other in a sequence in such a way that a circuit can be turned OFF by hand in a normal condition (i.e., when there is no fault) and this circuit turns OFF automatically during abnormal conditions e.g., short circuit, etc.

Functions or Objects of Substations

A substation performs the following tasks;

(1). A continuous supply of required electrical power to the consumer

(2). A maximum security of the supply

(3). To minimize the possibility of a fault duration

(4). To normalize the performance of the network and plants

(5). Supply of electrical power within specific voltage limits

(6). Supplying energy to the consumer at the minimum possible rates

(7). To act as switching for connecting the incoming lines from different power stations

(8). To receive high voltage energy from the generating stations and reduce these high voltages to a reasonable value for the local distribution

(9). To convert AC to DC or DC to AC

(10). To supply electrical power within the required frequency limits (49.5 Hz and 50.5 Hz) by converting a high frequency to a low frequency or low frequency to a high frequency

(11). To disconnect the security appliances installed in substations in the event of a fault under an automatic system

(12). To regulate the voltages of the outgoing feeders from the substation (i.e., to control unwanted fluctuations taking place in voltages)

(13). To improve the power factor with the help of synchronous condensers installed in the substations

(14). To control the street lighting equipment or appliances

(15). To establish communication among different substations

(16). To distribute load economically and develop various pertinent assignments

(17). To inspect a fault, specify its reason and bring improvement in order to avoid faults

(18). To ensure electric supply on different load points through the network

(19). To provide a facility for changing the transmission route

(20). To do tape changing

Types of Substations

Substations can be divided into the following two types;

(A). According to Service

(B). According to Design

Type of Substations According to Service

The classification of the substations according to service or purpose, is done as follows;

(1). Step Up Transformer Substation

A substation, which changes the voltage level of the electric supply, is known as a step-up transformer substation. In other words, substations that convert electrical power from one voltage to another voltage, are known as step-up transformer substations. These substations are connected to the generating stations. These substations can further be divided into the following forms;

(i). Transmission or Primary Substation

These substations get electric power from a generating station or a powerhouse. As the generation voltages are low (normally 11 KV or 11.5 KV or 12 KV), therefore this station steps up these voltages up to the primary transmission voltage (220 KV or 500 KV), so that they could be transmitted over long distances. These substations are built along primary transmission lines at appropriate load centers. In figure 3.1, a transmission substation has been depicted.

(ii). Sub-transmission or Secondary Substation

These are the substations that get high voltages from the primary substation and provide it on the secondary transmission by stepping it down (11 KV, 33 KV, 66 KV, 132 KV). After stepping down primary transmission voltage into suitable secondary voltage, secondary transmission lines are then spread from secondary substations to the load points. The secondary substations are constructed along these secondary transmission lines up to the load points, where primary and secondary distribution is done by means of stepping down the voltage further.

(iii). Distribution Substation

These are the substations, which get power from a secondary substation and lower voltage further to provide it for the secondary distribution. In other words, these are the substations, where voltages are stepped down further and brought up to the supply voltage. Electric supply is provided to the consumers through a network of distribution and service lines with the help of these substations.

(2). Industrial Substation

Large industrial consumers, who require more power, are instructed to set up a separate substation. The substations which are specially established for industrial consumers are known as industrial substations. These types of substations are normally distribution substations as well.

(3). Switching Substation

A substation that just functions like switching, without changing the voltage level of the transmission lines, is known as a switching substation. In other words, these substations do not change voltages of the incoming as well as outgoing transmission lines (that’s voltages of the incoming and outgoing transmission lines remain equal), rather they are used only for the switching of the power lines.

(4). Power Factor Correction Substation

The substations, which are used to improve or increase the system’s power factor, are known as power factor correction substations. We know that the power factor of long transmission lines decreases from unity owing to the inductance, as a result, the line current increases. As a result of an increase in the line current, copper losses also increase. Thus, in order to minimize transmission losses and to improve the overall system performance, it is inevitable to improve the power factor (that’s bringing it to the proximity of unity). Therefore, power factor improvement substations are built at the receiving end of the transmission lines to improve power factor, which comprises power factor improvement appliances i.e., synchronous motors, synchronous condensers, etc. Remember that such substations are also known as synchronous substations.

(5). Frequency Changer Substation

The substation which changes the normal supply frequency is called the frequency changer substation. The need for changes in frequency depends on industrial uses.

(6). Converting Substation

A substation that converts AC to DC, is known as converting substation. We know that power generation, transmission, and distribution are done in AC. However, a need for DC power is felt to fulfill certain objectives. For example, DC power is used for electric traction, electroplating, battery charging, DC motors, etc. The substation which converts AC power to DC power through appropriate appliances (e.g., rectifiers or ignitron), are known as converting substations.

Types of Substations According to Design

According to the design, there are two types of substations;

(1). Indoor Substation

(2). Outdoor Substation

(1). Indoor Substation 

A substation, that has been constructed inside a building, is known as an indoor substation. In other words, substations, the entire appliances that are installed within the substation premises, are called indoor substations. The objective behind installing all appliances or components within the building is to safeguard them against environmental contamination (e.g., dirt and dust, smoke, etc.), moisture, chemical evaporations, gases, etc., so that iron-made items do not suffer any damage. Such substations are usually built up to 11 KV and they are preferred in the densely populated areas. However, these substations can also be set up from 33 KV to 66 KV. The indoor type substations consist of a series of numerous open and enclosed chambers or compartments. The required instruments or appliances for these substations are set within these compartments in an orderly manner. The chamber or an empty place, wherein instruments or gadgets of a bus bar, or connection of a bus bar are set up, is called a cell or a compartment.

There are further two types of indoor substations according to construction;

(i). Substations of the Integrally Built Type

The substations which are constructed from the scratch by physically visiting the site is called substation of the integrally built type. In this type of substation, cells or compartments are constructed from cement or concrete, upon which an iron frame is placed. All electrical items or appliances are installed on this iron frame. In other words, in this type of station, iron frames are placed above the concrete or bricks constructed cells or chambers, above which all appliances are fitted on site.  

(ii). Substations of the Composite Built-Up Type

In this type of substation, pre-fabricated parts or assemblies manufactured in a workshop or factory are assembled on-site inside a switch room of the substation. The compartments of these types of substations are generally in the shape of metal compartments or enclosed cells, wherein each compartment consists of instruments of the main connection cell. An oil circuit, a load interrupter switch, and one or more than one voltage transformers are installed inside the iron cupboard.

(iii). Unit Type Factory Fabricated Substations & Metal Clad Switch Boards

As the name suggests, these are such types of substations, which are fully manufactured in electrical workshops of the factories and then transported to their desired installation spots. After the installation of the substation’s switchboard, only a connection for the incoming and outgoing power circuits has to be required. The cells or compartments of unit-type switchboards tend to be fully metal-clad compartment-shaped. These metal-clad compartments are usually divided into different compartments or parts on the substation, which include the control compartment, indicating and metering instruments compartment and protective devices compartment, circuit breaker and operating mechanism compartment, current transformer, and cable ceiling box compartment. This has been elucidated vide figure 3.3.

The diagram of a unit-type metal-clad switchboard, which consists of a number of compartments, has been illustrated in figure 3.4.

Figure 3.4 – General view of a Unit-type metal-clad switchboard

Advantages of Indoor Substations

(1). Remain protected from dirt and dust, smoke, etc.

(2). External dangers and weather conditions have no influence on them

(3). As faults occur rarely, therefore seldom a need for its repair arises

(4). They can be installed in densely populated areas

(5). They have a long-life span and are reliable

(6). They seem quite pretty in appearance

(7). They require a small space as compared to the outdoor substations

(8). They are simple from the construction point of view

(9). As all appliances or instruments are inside a building, therefore switching issues do not arise during a rainy season

(10). They are relatively more protected against external threats e.g., warfare, lightning, bridging, leakage, etc.

Disadvantages

(1). They have a high initial cost

(2). Finding some fault tends to be difficult

(3). Further extension is not possible whenever required

(4). Their erection requires more time

(5). They are not normally constructed with a rating above 66 KV

(6). The installation of a switchgear involves a huge expense

(7). Requires more construction material

(8). It is less popular compared to the outdoor substation

Outdoor Substation

These are a type of substations, in which all the fitted instruments e.g., bus bars, circuit breakers, isolators, current transformers, voltage transformers, power transformers, etc. are erected in an open field or an open switchyard instead of installing them within any building. As all electrical gadgets or instruments and switch gears etc. are clearly visible due to being erected under an open sky, therefore such type of substations is called open type or outdoor type substations. For security purposes, a fence is constructed around it. Bus bars are provided support through a post insulator or strain insulator. For supporting other electrical instruments, insulators, and incoming and outgoing lines, a galvanized steel structure is constructed above the substation.

As the outdoor substations have a rating of above 66 KV, therefore a relatively large mutual distance or clearance between the conductors is retained due to such high voltages. Appropriate ground clearance is achieved through erecting electrical appliances above the supporting structure. In our stations, mostly these types of substations are used.

There are basically two types of an outdoor substation;

(i). Pole-mounted Substations

(ii). Foundation-mounted Substation

(i). Pole–Mounted Substation

These types of substations are constructed for the distribution of electric power within the localities through the installation of 250 KV capacity distribution transformers. A beautiful platform is constructed through a thick and sound single pole or H pole or with the help of four poles, above which a transformer is fixed. Such types of substations are cheap, simple, and small in size. All the instruments of this substation are of an outdoor type, which is erected above the supporting structure of the H.T distribution lines (i.e., on poles, etc.). For turning ON or OFF an H.T (high tension) transmission line, a triple pole mechanically operated (T.P.M.O) switch is used. For the protection of the H.T side, an H.T fuse unit is installed. For the control of the L.T (low tension) side, an iron-clad L.T switch having proper capacity fuses on it is installed. To protect transformers against the peril of lightning, lightning arresters are installed above the H.T line. Substations are earthed on one or two spots.

The repairing costs on the substation are low. If a large number of such substations are installed in some towns, then its distribution at a low price is possible. However, as a result of an increase in the number of transformers, KVA also increases, no-load losses increase, and thus the price per KVA increases as well. This is the cheapest type of any substation below 11 KV voltage because there is no need to construct any type of building for it. These are distribution substations are as well which are erected above poles. Normally, transformers up to 100 KVA capacity are installed above a double pole structure (or between two poles) and transformers from 100 KVA to 250 KVA capacity are erected above a proper platform on a four-pole structure (i.e., between four poles). These types of pole-mounted substations are installed in densely populated areas. As all instruments are bare and visible to the eyes, it is easy to find out any sort of a fault and it’s repairing also involves a very low cost.

(ii). Foundation Mounted Substation

As a foundation is constructed to install transformers and other heavy instruments on these types of substations in open ground or open switchyard, therefore such substations are called foundation mounted substations. All components along with the transformer are connected together as a unit and then erected on the foundation. For security purposes, a fence, wall, or barbed wire is constructed around the unit. For primary and secondary distribution (more than 250 KVA) foundation-mounted outdoor type substations are always constructed. As appliances or components installed on such substations are extremely bulky and heavy, therefore such a place is selected for these substations where easy access to a heavy transport is possible. At the time of selection of someplace, points like the availability of land, cost, and the availability of an additional place for expansion purposes in the future, are kept in mind. However, the locality, where power is going to be distributed, must be closer to the substation as far as possible, so that less cost incurs on the supporting structures and distribution lines. On both sides of these substations (i.e., H.T and L.T) switchgear is installed consisting of circuit breakers. However, due to the reliability of transformers nowadays, circuit breakers are installed only on the H.T side from an economic point of view. Moreover, as bus bars and other related components tend to be under an open sky, the mutual distance or clearance between the conductors is retained large according to the voltage. The supporting structure is erected at a suitable height and an appropriate ground clearance is also obtained. These types of substations are mostly used for primary and secondary transmission. In figure 3.6, a foundation-mounted outdoor substation has been illustrated.

Figure 3.6 – foundation mounted outdoor substation

Advantages of Outdoor Substations

(1). As all equipment is visible to the eyes, therefore fault finding is relatively easy

(2). An extension in the substation is possible as and when required

(3). The reaction time of an outdoor substation is low

(4). Requires fewer building materials

(5). The construction work comparatively becomes less

(6). The cost of switchgear installation is low

(7). As relatively a large distance is retained between two nearby equipment, therefore chances of an internal fault are fewer

(8). It is easy to look after equipment or instruments etc.

(9). Its repair is easy and cheap

(10). They have a simple design and serve longer

(11). They have a low initial cost and are constructed with a rating of 66KV

(12). They are more popular compared to indoor substations.

Disadvantages

(1). A large space is required for outdoor substations

(2). Whatever the kind of weather, the equipment has to be repaired under an open sky in case of a fault

(3). Dust etc. mounts on the components of a substation, and temperature fluctuations always have an impact on these substations

(4). They remain vulnerable in a situation of external dangers (e.g., war, lightning, bridging, leakage, etc.)

(5). It requires repairs frequently due to more external faults

(6). Such substations are not erected in densely populated areas

(7). They have a less beautiful outward appearance

(8). They are not normally manufactured with low ratings

In spite of the aforementioned drawbacks, outdoor substations are manufactured and used in the power system on a large scale.

Comparison Between Indoor & Outdoor Substations

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