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IPD and Illumination Unit: 4 : Distribution Substation of Industrial Plant 4.1 Introduction The distribution substation of an industrial plant contains the higher voltage portion of the system, starting with the purchased-power service or generators and, then protective devices, switching equipment, measuring instruments, circuits, and all transformers with secondary voltages of 400/220 V.

Distribution substation is generally comprised of the following major components:

1.Supply Line 2.Transformers 3.Busbars 4.Switchgear 5.Outcoming feeders 6.Switching apparatus 7.Switches 8.Fuses 9.Circuit breakers 10.Surge voltage protection 11.Grounding 1. Supply Line Distribution substation is connected to a sub-transmission system via at least one supply line, which is often called a primary feeder. However, it is typical for a distribution substation to be supplied by two or more supply lines to increase reliability of the power supply in case one supply line is disconnected. 2. Transformers Transformers “step down” supply line voltage to distribution level voltage. Distribution substation usually employs three-phase transformers. However, banks of single-phase transformers can also be used. For reliability and maintenance purposes, two transformers are typically employed at the substation, but the number can vary depending on the importance of the consumers fed from the substation and the distribution system design in general. 3. Busbars Busbars are used to carry large current and to distribute current to multiple circuits within switchgear or equipment. Plug-in devices with circuit breakers or fusible switches may be installed and wired without de-energizing the busbars if so specified by the manufacturer. 4. Switchgear Switchgear is a general term covering primary switching and interrupting devices together with its control and regulating equipment. Power switchgear includes breakers, disconnect switches, main bus conductors, interconnecting wiring, support structures with insulators, enclosures, and secondary devices for monitoring and control. Switchgear can be of outdoor or indoor types, or a combination of both. Outdoor switchgear is typically used for voltages above 26 kV, whereas indoor switchgear is commonly for voltages below 26 kV.

6. Switching Apparatus Switching apparatus is needed to connect or disconnect elements of the power system to or from other elements of the system. Switching apparatus includes switches, fuses, circuit breakers, and service protectors. a) Switches Switches are used for isolation, load interruption, and transferring service between different sources of supply. Isolating switches are used to provide visible disconnect to enable safe access to the isolated equipment. b) Fuses Fuses are used as an overcurrent protective device with a circuit-opening fusible link that is heated and severed as overcurrent passes through it. Fuses are available in a wide range of voltage, current, and interrupting ratings, current-limiting types, and for indoor and outdoor applications. c) Circuit breakers Circuit breakers are devices designed to open and close a circuit either automatically or manually. When applied within its rating, an automatic circuit breaker must be capable of opening a circuit automatically on a predetermined overload of current without damaging itself or adjacent elements. Circuit breakers are available for the entire system voltage range, and may be as furnished single-pole, double-pole, triple-pole, or four-pole, and arranged for indoor or outside use. Sulfur hexafluoride (SF6) gas-insulated circuit breakers are available for medium and high voltages, such as gas-insulated substations. 7. Surge Voltage Protection Transient overvoltages are due to natural and inherent characteristics of power systems. Overvoltages may be caused by lightning or by a sudden change of system conditions (such as switching operations, faults, load rejection, etc.), or both. Generally, the overvoltage types can be classified as lightning generated and as switching generated. 8. Grounding Grounding is divided into two categories: power system grounding and equipment grounding. Power system grounding means that at some location in the system there are intentional electric connections between the electric system phase conductors and ground (earth).

4.2 classification of Substation The substations can be classified in several ways including the following:

1 Classification based on voltage levels e.g. : A.C. Substation : EHV, HV, MV, LV; HVDC Substation. 2 Classification based on Outdoor or Indoor a. Outdoor substation is under open sky. b. Indoor substation is inside a building. 3 Classification based on configuration a. Conventional Air insulated outdoor substation or b. SF6 Gas Insulated Substation (GIS) c. Composite substations having combination of the above two 4 Classification based on application Step Up Substation – Associated with generating station as the generating voltage is low. a. Primary Grid Substation – Created at suitable load centre along primary transmission lines. b. Secondary Substation – Along secondary transmission line. c. Distribution Substation – Created where the transmission line voltage is step down to supply voltage. d. Bulk supply and industrial substation – Similar to distribution sub-station but created separately for each consumer. e. Mining Substation – Needs special design consideration because of extra precaution for safety needed in the operation of electric supply. f Mobile Substation – Temporary requirement. For Distribution to Industrial Plant Substations are arranged in following way (i) Indoor sub-station (ii) Outdoor sub-station (iii) Underground sub-station (iv) Pole-mounted sub-station (i) Indoor sub-stations- For voltages upto 11 kV, the equipment of the sub-station is installed indoor because of economic considerations. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltages upto 66 kV.

(ii) Outdoor sub-stations. For voltages beyond 66 kV, equipment is invariably installed outdoor. It is because for such voltages, the clearances between conductors and the space

required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor.

(iii) Underground sub-stations. In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the sub-station is created to underground .

(iv) Pole-mounted sub-stations. This is an outdoor sub-station with equipment installed overhead on H-pole or 4-pole structure. It is the cheapest form of sub-station for voltages not exceeding 11kV (or 33 kV in some cases). Electric power is almost distributed in localities through such substations.

4.5 Selection and location of a site for an Electrical Substation

The following factor is considered while making site selection for a substation. A) Type of substation – The category of the substation is important for its location. For example, a step-up transformer is a point where power from various sources is pooled and step-up for long distance transmission should be located as cool as possible to minimise the losses. Similarly, the step-down transformer should be located nearer to the load centre to reduce transmission losses, the cost of the distribution system and better reliability of supply. B) Availability of suitable and sufficient land – The land selected for a substation should be level and open from all sides. It should not be waterlogged particularly in the rainy season. The site selected for substation should be such that approach of transmission lines and their take off can be easily possible without any obstruction.The places nearer to airdrome, shooting practice ground, etc. should be avoided.

C) Communication facility – Suitable communication facility is desirable at a proposed station, both during and after its construction. It is better, therefore, to select the site alongside the existing road to facilitate an easier and cheaper transportation. D) Atmospheric Pollution – The atmosphere around the ground factories produces metal corroding gas, air fumes, conductive dust, etc. And the area near the sea coast may be more humid and is harmful to the proper running of the power system. Thus, the substation should not be located near the factories or sea coast. E) Availability of Essential Facilities to the Staff – The site should be such where staff can be provided essential facilities like school, hospital, drinking water, housing, etc. F) Drainage Facility – The site selected for the proposed substations should have proper drainage arrangement or the possibility of making effective drainage, avoid pollution of air and growth of micro-organism and

4.6 Schematic Diagram of Substation The main connection scheme is drawn keeping in view the following factors: (i) General bus-bar arrangement, (ii) Operating voltage, (iii) Number of incoming and outgoing lines,

(iv) Number of transformers, (v) Safety to equipment, (vi) Safety to operating personnel, and (vii) Future extension requirement. The main connection diagram drawn for a substation shows the arrangements of all the circuits with its main bus-bars.  For simplicity and to facilitate reading, all the electrical connections of a substation can be represented by a single line diagram. It is understood that all the phases are connected identically.

The substation has an additional function like they provide points where safety devices may be installed to disconnect equipment or circuit in the event of the fault. The synchronous condenser is placed at the end of the transmission line for improving the power factor and for measuring the operation at the various part of the power system. Street lighting, as well as the switching control for street lighting, can be installed in a substation.

4.7 Equipment and Measuring Accessories for Substations and Switch- gear Installation The working of the electrical equipment used in the substation is explained below in detail. A) Isolator – The isolator connects or disconnects the incoming circuit when the supply is already interrupted. It is also used for breaking the charging current of the transmission line. The isolator is placed on the supply side of the circuit breaker so that the circuit breaker is isolated from the live parts of the maintenance. B) Lightning Arrester – The lightning arrester is a protective device which protects the system

from lightning effects. It has two terminals one is high voltage and the other is the ground voltage. The high voltage terminal is connected to the transmission line and the ground terminal passes the high voltage surges to earth. C) CT Metering – The metering CT measure and records the current when their secondary terminal is connected to the metering equipment panel. D) PT Metering – A voltage transformer may be defined as an instrument transformer for the transformation of voltage from a higher value to the lower value. E) Step-down Transformer – The step-down transformer converts the high voltage current into the low voltage current. F) Capacitor Bank – The capacitor bank consists series or parallel connection of the capacitor. The main function of the capacitor bank is to improve the power factor of the line. It draws the leading current to the line by reducing the reactive component of the circuit. G) Circuit Breaker – The circuit breaker interrupts the abnormal or faults current to flow through the line. It is the type of electrical switch which open or closes the contacts when the fault occurs in the system. The outgoing feeder supplies the input power to the consumer end. H) Batteries- In distribution substations, for automatic control circuits and protective relay system, as well as emergency lighting circuits, batteries are required to supply power. Station battery is assembled of a certain number accumulator cell depending on the operating voltage of the respective DC circuit. Storage batteries are two types; lead acid and acid-alkaline batteries. Lead acid battery is most commonly used in power stations and substations because their voltage is high and very cheap. I) Switchyard- The switch-yard houses transformers, circuit breakers and switches for connecting and disconnecting the transformers and circuit breakers. It also has lightning arrestors for the protection of the power station against lightning strokes. K) Indicating and metering Instruments:- Ammeters, Voltmeters, Wattmeters, kWh meters, KVARH meters, power factor meters reactive volt-amperes meters are installed in substations to control and maintain a watch over the current flowing through the circuits and over the power loads. L) Carrier-current Equipment Such equipment is installed in the substations for communication, relaying, telemetering or for supervisory control. The equipment is suitably mounted in a room known as carrier room and connected to the high voltage power circuit.

M) Insulator:- It is used in generating stations and substations to fix and insulate the bus bars systems. They may be subdivided into post and bushing type. A post insulator consists of a porcelain body and their cap is made up of cast iron. It is directly bolted to the busbars by means of bus-bar clamps. A bushing or through insulator consists of porcelain-shell body, upper and lower locating washes used for fixing the position of bus-bar or rod in a shell....