Building Science - Power Supply System - Theory PDF

ID-S205_04 BUILDING SCIENCE Electrical System  ELECTRICAL SERVICES  POWER SUPPLY SYSTEM L  ELECRICAL DRAWING AND SINGLE LINE DIAGRAM E  WIRING SYSTEM AND ELECTRICAL INSTALLATIONS C T U R E  SINGAPORE POWER LTD & SUBSIDIARIES  SP Services Ltd (SPSL), SP Powe...

ID-S205_04 BUILDING SCIENCE Electrical System  ELECTRICAL SERVICES  POWER SUPPLY SYSTEM L  ELECRICAL DRAWING AND SINGLE LINE DIAGRAM E  WIRING SYSTEM AND ELECTRICAL INSTALLATIONS C T U R E  SINGAPORE POWER LTD & SUBSIDIARIES  SP Services Ltd (SPSL), SP PowerAssets Ltd (SPPA) and SP PowerGrid Ltd (SPPG) are subsidiaries of Singapore Power Ltd. L  SPSL provides support services: E  offers terms and conditions of service connection; arranges for service connection C  for the electricity market, It receives requests for electricity service connection; T  Energisation/turn-on; and collects transmission charges, security deposits and U  charges for other services on behalf of SPPA. SPSL also bills customers R for consumption  SPPA is the Transmission Licensee and owner of the transmission and E distribution network.  SPPG, the managing agent for and on behalf of SPPA, develops, operates and maintains the transmission and distribution facilities. L E C Power Supply System T U R E  Power Supply System MODE OF POWER SUPPLY  The mode of power supply depends on:  Capacity of building load  Location of building relative to supply network L  Based on the above consideration, the voltage level at which the power is supplied to various buildings is presented below E SUPPLY VOLTAGES AND CONNECTED LOADS C  230V, 50 Hz, single-phase, up to a maximum of 23kVA, 100A  400V, 50 Hz, 3-phase, 4-wire up to 2000 kVA T  22kV, 50 Hz, 3-phase, 3-wire for a Contracted Capacity of: U  between 1,700 kW and 12,750kW for 2 HT 22kV services  between 12,751kW and 25,500kW for 4 HT 22kV services. R  66kV, 50 Hz, 3-phase, 3-wire for connection with minimum Contracted Capacity E of 25,500 kW.  230kV, 50 Hz, 3-phase, 3-wire for connection with minimum Contracted Capacity of 85,000 kW  Voltage variation or fluctuation at the supply point is maintained as far as possible within ± 6% of the nominal voltage.  Power Supply System INTRODUCTION TO POWER SYSTEM  High Voltage : > 1000 V alternated current (AC) and >1500 V direct current  Low Voltage: 50-1000 V alternated current (AC) and 120-1500 V direct current  Extra Low Voltage: < 50 V alternated current and < 120 V direct current L  SELV/PELV (Separated ELV) : 0 – 12 V alternated current and 0 – 30 V direct current E  SELV (For dry area) : 0 – 25 V alternated current and 0 – 60 V direct current C Low-Voltage System T A low-voltage (LV) system refer to distribution voltages below 1000V. Typical nominal voltages in this range are 240,380,400,415,480,550 and 600 V. In Singapore, a LV U system refers to the three-phase four-wire system of 400 V between line-to-line, and 230 R V between each line to neutral. LV is not only supply voltage, it is also the utilisation E voltage if most of the electrical appliances. Consumer whose incoming supply is 22KV or 6.6KV will have to design and install their own HT and LV systems. For consumer taking LV supply from the utility, the LV system prior to incoming supply will be managed by the utility and these consumers have to design and install only their own inter LV network.  Power Supply System CONSUMERS TAKING HIGH VOLTAGE SUPPLY L E C T U R All the equipment such as high voltage switchgears, transformers, low E voltage switchgears and associated cabling must be supplied installed and maintained by the consumer  Power Supply System Consumers Taking Low Voltage Supply Consumer load < 23 kVA at 230 V or 400 V L E C Consumer load > 23 kVA T and < 280 kVA at 400 V U R E Consumer load > 280 kVA at 400 V  Power Supply System Power Supply System Electricity is generated in power stations, usually located far from the consumers. After generation, the electricity is being transmitted to sub-stations near the load centres and then distributed to consumers. L E C T U R E  Power Supply System Generation One form of electricity generation is by thermal power station. These power stations employ steam turbines to run the alternators. L The steam is obtained from high-pressure boilers. The fuel burnt in the boilers can be solid, liquid or gaseous. The liquid fuels used are fuel oil, crude oil, E petrol or paraffin oil. Generally fuel oil is used for firing the boilers. The other C types of fuels are better quality and are used in internal combustion engine. T The scheme of thermal generation can be divided into two phases: U  Formation of steam in the boiler house  Generation of electrical power in the generation (Turbine) room R E  Power Supply System Generation Figure shows below a power flow diagram for a thermal power station. In the boiler house the fuel is burnt and the water is converted into high-pressure steam [converts chemical energy to thermal energy], which is further heated in a superheater. The superheated steam passes the turbine rotating the turbine L blades. Thermal energy is converted into mechanical energy. E The pressure of the steam decreases and its volume increases, after imparting energy to the turbine rotor it passes out of the turbine blades into the condenser C (vacuum). In the condenser the cold water is circulated with the help of pump that condenses the low-pressure wet steam. This condensed water is then heated T again and converted into high-pressure steam to rotate the turbine blades. U The turbine in the generation room acts as a prime mover [converts mechanical R energy to electrical energy] of the alternator, which generates electrical energy generally at a voltage of 11–16kV. This energy is then fed through the generator E switchboards and circuit breakers to transformers. The transformer step-up the voltage of the generator to a higher values at 400kV or 230kV for transmission. The overall efficiency of the Thermal Power Station varies from 20% to 26% depending on the plant capacity.  Power Supply System Thermal Power Station L E C T U R E  Power Supply System Transmission The ideal arrangement for supply of electricity is to have a power station located right at the load centre and generate power at the utilisation voltage. Then transmission system can be eliminated. However, it is obviously not feasible to L have a power station right in the city centre and also it is not technically feasible to generate power in a large scale at the utilisation voltage. An electrical system E operated at 400V can only supply up to a maximum demand of 3 to 4MW. At a higher voltage of 22kV, the maximum demand can be increased to 200MW. C Transmission of electrical energy by high voltage circuit is required in order to T bring bulk energy from a remote source to a load centre and at the same time to interconnect power stations. The interconnection would increase the reliability of U supply and provide the spooling of generating plants so that the standby capacity can be reduced. R The reason for the transformation from a lower voltage to one considerably high E for transmission is that it is much more economical to transmit bulk supplies of electrical energy by using the highest voltage possible. In this way, the overhead lines or underground cables need have only comparatively small conductors, with the minimum electrical losses (I2R).  Power Supply System Transmission  To illustrate this aspect of electricity transmission, a conductor of 18mm diameter is sufficient to transmit 50000kW at 132000V. To transmit the same L amount of power at 250V, the conductor diameter would have to be something like 400mm. E C  In Singapore the transmission network consists of three levels, 400kV, T 230kV and 66kV. The transmission system in Singapore is totally using the UNDERGROUND CABLE NETWORK. U R E  Power Supply System Distribution L E C T U R E Figure: Electricity Transmission and Distribution System  Power Supply System Distribution The main function of a distribution system is to receive electric power from large, bulk power sources and to distribute electric power to consumers at various voltage levels with acceptable degrees of reliability. The most commonly used nominal voltages are 3.3kV, 6.6kV, 11kV, 22kV and 33kV. L Depending on the load density and the annual growth rate in a service area, the E tendency is toward higher distribution voltage especially for urban areas that have an increasing consumption of electrical energy. C By selecting a higher distribution voltage, appreciable savings in overall cost can T be achieved if the load density within the service area is high. U A simple transmission and distribution system is shown in Figure. In Singapore, R the primary distribution voltages adopted are 22kV and 6.6kV, and the secondary distribution voltage at utilisation level is 400V. E Connecting to one phase and neutral of a three-phase 400V supply gives 230V single-phase supply suitable for domestic consumers.  Power Supply System Distribution In the city centre or industrial estate, where the load density is high, it is distributed at 22kV and stepped down directly to the utilisation voltage through 22kV/400V transformer. In areas where the load density is low, it is distributed at 6.6kV and stepped down through 6.6kV/400V transformers. L E The voltage variation or fluctuation at the supply point is maintained, as far as it is practical, to within ±6% of the normal voltage. The supply frequency is 50Hz and C the variation is maintained at ±1%. T All distribution networks in Singapore are by underground cables from substations placed near to the load centre and supplied at 22kV or 6.6kV. U Transformers in these local substations reduce the voltage to 400V, three-phase and neutral distributor cables connect this supply to consumers. R E  Power Supply System Electricity Transmission and Distribution System L E C T U R E A typical 400V/230V distribution system  Power Supply System Three-Phase Four-Wire AC System The voltage between any phase conductor and the neutral will be 230V, known as single-phase. Colours Red, Yellow and Blue identify the three phases. The neutral is always black. Supplies to premises are always connected to different phases to balance the loads. L A three-phase, four wire AC system is illustrated in Figure. There are three ‘live’ conductors called ‘phases’ or ‘lines’. The voltage between any of these three E phases is usually 400V. The star point is earthed to an earth electrode sunk into the ground below the substation, and from this point is taken the fourth conductor, C the ‘neutral’. If the consumer is a small one, a house for instance, the supply cable contains T two cables, a live and neutral. The colour of the live will depend on the phase from which it has been taken. U A three-phase 400V supply is used for supplying small industrial and commercial loads such as factory, schools, blocks of flat, commercial building, etc. The higher R voltage (400V) is used generally for motors. The lighting loads are connected across the other phases and the neutral such that when the whole installation is E operating, the loads across the three phases are reasonably balanced so that each phase conductor carries approximately the same current. A single-phase 230V supply is usually provided for individual domestic consumers like our home.  Power Supply System Three-Phase Four-Wire AC System L E C T U R E Three-phase four-wire distribution system  Power Supply System 6.6 kV Substation Plan L E C T U R E  Power Supply System 6.6 kV Substation Section L E C T U R E  Power Supply System 6.6 kV Substation Elevation L E C T U R E  Power Supply System TYPE OF TRANSFORMERS  Two types of liquid filled transformers  Mineral oil filled transformers  Silicon oil filled transformers L  Cast resin dry type transformers E C T U R E  Power Supply System CABLES  A cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protective covering.  Although several types of cables are available, the type of cable to be used L depends on working voltage and service requirements. E C T U R E  Figure above shows the general constructional features of a paper insulated underground cable  The basic components are conductor, insulation and serving. Other components such as lead sheath, bedding and armouring varies from cable to cable  Power Supply System CABLES L E C SINGLE-CORE PVC 3 OR 4 CORE PVC/SWA/PVC VOLTAGES UP TO 1000 V T  Available in standard sizes from 1.5  Used as main incoming cable U mm2 to 300 mm2 to MSB or in urban low voltage  Rated current from 13 A to 540 A power distribution R (Depends on method of installation)  Used for wiring to socket outlets, E wiring to equipment & appliance and for submain cabling  Used in conduits, trunking or on cable trays  Power Supply System CABLES  3 & 4 core – 16 mm2 to 400 mm2 (100 A to 740 A) L E XLPE CABLES – 600/1000V (ARMOURED) C T  Single core – 50 mm2 to U 1000 mm2 (275 A to 1775 A) R  3 & 4 core – 16 mm2 to 400 mm2 (100 A to 740 A) E XLPE CABLES – 600/1000V (NON-ARMOURED)