CHAPTER 1 INTRODUCTION  1.1   INTRODUCTION : The present-day electrical power system is A.C. i.e. electric power is generated, transmitted and distributed in the form of alternating current. The electric  power is produced at the power stations which are located at favourable places, generally quite away from the consumers. It is delivered to the consumers through a large network of transmission and distribution. At many places in the line of the  power system, it may be desirable and necessary to change some characteristic (e.g. voltage, A.C. to D.C., frequency, Power factor etc.) of electric supply. This is accomplished by suitable apparatus called sub-station. For example, generation voltage (11KV or 6.6KV) at the power station is stepped up to high voltage (say 220KV or 132KV) for transmission of electric power. The assembly of apparatus (e.g. transformer etc.) used for this purpose is the sub-station. Similarly, near the consumer’s localities, the voltage may have to be stepped down to utilization level. This job is again accomplished by a suitable apparatus called ‘ substation. 1.2 CONSTRUCTION OF A SUBSTATION At the time of constructing a substation, we have to consider some factors which affect the substation efficiency like selection of site.   2 1.2.1 SELECTION OF SITE: Main points to be considered while selecting the site for EHV Sub-Station are as follows: i) The site chosen should be as near to the load centre as possible. ii) It should be easily approachable by road or rail for transportation of equipments. iii) Land should be fairly levelled to minimize development cost. iv) The source of water should be as near to the site as possible. This is because water is required for various construction activities; (Especially civil works,), earthing and for drinking purposes etc. v) The sub-station site should be as near to the town / city but should be clear of  public places, aerodromes, and Military / police installations. vi) The land should be have sufficient ground area to accommodate substation equipments, buildings, staff quarters, space for storage of material, such as store yards and store sheds etc. with roads and space for future expansion. vii) Set back distances from various roads such as National Highways, State Highways should be observed as per the regulations in force. viii) While selecting the land for the substation preference to be given to the Govt. land over Private land. ix) The land should not have water logging problem. x) The site should permit easy and safe approach to outlets for EHV lines.   CHAPTER 2 CLASSIFICATION OF SUBSTATIONS There are several ways of classifying sub-stations. However, the two most important ways of classifying them are according to (1) service requirement and (2) constructional features. . 1 ACCORDING TO THE REQUIREMENT: A sub-station may be called upon to change voltage level or improve power factor or convert A.C. power into D.C. power etc. According to the service requirement, sub-stations may be classified into: (i)Transformer sub-stations:  Those sub-stations which change the voltage level of electric supply are called transformer sub-stations. These sub-stations receive  power at some voltage and deliver it at some other voltage. Obviously, transformer will be the main component in such sub-stations. Most of the sub-stations in the  power system are of this type. (ii)Switching sub-stations:  These sub-stations do not change the voltage level i.e. incoming and outgoing lines have the same voltage. However, they simply  perform the switching operations of power lines. (iii)Power factor correction sub-stations : Those sub-stations which improve the power factor of the system are called power factor correction sub-stations. Such sub-stations are generally located at the receiving end of transmission lines. These sub-stations generally use synchronous condensers as the power factor improvement equipment. (iv)Frequency changer sub-stations:  Those sub-stations which change the supply frequency are known as frequency changer sub-stations. Such a frequency change may be required for industrial utilization. (v)Converting sub-stations:  Those sub-stations which change A.C. power into D.C. power are called converting sub-stations. These sub-stations receive A.C. power and convert it into D.C. power with suitable apparatus (e.g. ignitron) to supply for such purposes as traction, electroplating, electric welding etc.   (vi)Industrial sub-stations:-  Those sub-stations which supply power to individual industrial concerns are known as industrial sub-stations. 2.2 ACCORDING TO THE CONSTRUCTIONAL FEATURES:  A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments etc.) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-stations are classified as: Indoor sub-station Outdoor sub-station Underground sub-station Pole-mounted sub-station Indoor sub-stations:-  For voltages up to 11KV, 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 up to 66 KV Outdoor sub-stations:-  For voltages beyond 66KV, equipment is invariably installed out-door. 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. 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 underground. Pole-Mounted sub-stations:-  This is an outdoor sub-station with equipment installed over-head 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 sub-station. Fig: 2.1 CONSTRUCTION OF THE SUBSTATION. CHAPTER 3   SINGLE LINE DIAGRAM (SLD) A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the concerned Electrical System which includes all the required electrical equipment connection sequence wise from the point of entrance of Power up to the end of the scope of the mentioned Work. As in the case of 132KV Substation, the SLD shall show Lightening Arrestor, C.T/P.T Unit, Isolators, Protection and Metering P.T & C.T. Circuit Breakers, again Isolators and circuit Breakers, Main Power Transformer, all protective devices/relays and other special equipment like CVT, GUARD RINGS, etc as per design criteria. And the symbols are shown below. There are several feeders enter into the substation and carrying out the power. As these feeders enter the station they are to pass through various instruments. 3.1 FEEDER CERCUIT 1. Lightening arrestors; 2. CVT; 3. Wave trap; 4. Isolators with earth switch 5. Current transformer; 6. Circuit breaker; 7. Feeder Bus isolator 8. BUS; 9. Potential transformer in the bus with a bus isolator 3.2 TRANSFORMER CIRCUIT:   i)HV side: 1. Transformer bus Isolator 2. Circuit breaker 3. Current transformer 4. Lightning Arrestors 5. Auto Transformer 100MVA (220/132KV) ii)LV side:  1. Lightening arrestors 2. Current transformer 3. Circuit breaker 4. Bus Isolator. 5. Bus 6. Potential transformer with a bus isolator 7. A capacitor bank attached to the bus 3.3 AUXILIARY SUPPLY: 220V.Battery system: To control and protect the substation equipment the 220 volts DC battery system is necessary. It is provided in the main control room. It will be discussed below.   Fig: 3.1 SINGLE LINE DIAGRAM OF A 220/132KV SUBSTATION WARANGAL. Lightening arrestors are the instruments that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used. The lightening arrestors do not let the lightening to fall on the station. If some lightening occurs the arrestors pull the lightening and ground it to the earth. In any substation the main important is of protection which is firstly done by these lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the lightening to the ground. These are located at the entrance of the transmission line in to the substation and as near as possible to the transformer terminals. LA will be provided on the support insulators to facilitate leakage current measurement and to count the no of surges discharged through the LA. LA bottom flange will be earthed via leakage ammeter and surge counter. Leakage current is to be recorded periodically. If the leakage current enters into the red range from the green range, the LA is prone for failure. Hence, it is to be replaced. There should be independent earth pit for LA in each phase so as to facilitate fast discharging and to raise the earth potential. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground. Fig 4(i) shows the basic form of a surge diverter. It consists of a spark gap in series with a non- linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-linear resistance is that its resistance increases as the voltage (or current) increases and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in Fig 4 (ii). Fig.4.1 (i) Surge diverter (ii)Characteristics of the non linear resister    4.1.2.The action of the Lightning Arrester or surge diverter is as under: (i) Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting.   (ii) On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line. (iii) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to over voltage, the arc would be a short circuit on the power system and may cause power-follow current in the arrester. Since the characteristic of the resistor is to offer low resistance to high voltage (or current), it gives the effect of short circuit. After the surge is over, the resistor offers high resistance to make the gap non conducting. 4.1.3. Guide for selection of LA: (i)Before selecting the LA it should be ascertained whether the system is effectively earthed, non-effectively earthed or having isolated neutral. (ii)The system neutrals are considered to be effectively earthed when the co-efficient of earthing does not exceed 80%. In this case, the reactance ratio X0/ X1 (zero sequence reactance/positive sequence reactance) is positive and less than 3 and at the same time the resistance ratio RO/X1 (zero sequence resistance/positive sequence reactance) is less than 1 at any point on the system. For this system the arrestor rating will be 80% of the highest  phase to phase system voltage.   11 (iii) The LA voltage rating corresponding to the system voltages normal are indicated below : Rated system Voltage (KV) Highest system Voltage (KV) Arrester rating in KV Effectively earthed systems 11 12 9 33 36 30 66 72.5 60 132 145 120/132 (latex) 220 245 198/216 (latex) 400 420 336 Table: 4.1.3 LA voltage rating 4.1.4 LOCATION OF LIGHTING ARRESTORS : The LAs employed for protecting transformers should be installed as close as  possible to the transformer. The electrical circuit length between LA and the transformer bushing terminal should not exceed the limits given below: Rated system Voltage KV BIL KV Peak Max. distance between L.A and Transformer bushing terminal (inclusive of lead length) (in metres) Effectively earthed 11 75 12.0 33 200 18.0 66 325 24.0 132 550 35.0 650 43.0 220 900 Closes to Transformer 1050 400 1425 1550 Table: 4.1.4 The limits of LA and Transformers