The Distribution Board Provided - Report Example

DISTRIBUTION BOARD CIRCUITRY EXPLAINED INTRODUCTION This report gives a detailed of the distribution board provided. Three major aspects have been considered and meticulously explained. They form the three objectives of this task. A comprehensive description of all equipment used including their functions in the system. A careful stepwise description of the power flow from the HV supply to the LV load side plus calculation of the total load. Finally, a clear outline of the cable types used, their sizes, advantages and disadvantages. A distribution board is defined as “a component of an electricity supply system which divides an electrical power feed into subsidiary circuits, while providing a protective fuse or circuit breaker for each circuit, in a common enclosure.” DEFINITION OF EQUIPMENT AND PARTS. Feeder: conductors that connect the power supply source and a final branch circuit over-current device. N/C and N/O: these are the normally closed and normally open contacts respectively. Normally closed (N/C) contact is one which is in conducting state when it or the device operating it is in a de-energized state. Normally open (N/O) contact is always in a non-conducting state when it or the device operating it is in a de-energized state. These two connect or disconnect the line from the load. Metering circuit breaker: The American National Standards Institute (ANSI) defines a circuit breaker as: “A mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions. Also capable of making and carrying for a specified time and breaking currents under specified abnormal circuit conditions, such as those of a short circuit.” The metering circuit breaker here protects the meter circuit from faults that could cause adverse effects on them. It operates by switching itself off in case of a fault. When the fault is rectified it is switched on. Remote emergency trip push button: It is a switch usually located at a safer location and in this case is used to shut down the power to the metering on emergency. 22kV/110V VTS – Metering Voltage Transformer: It is a transformer used to monitor power line voltage. This steps down line power from 22kV to 110V that can be applied to the measuring instruments without damaging them. 400/5A, 100/5A CTS - Metering Current Transformer: It is a transformer used to monitor power line current. It isolates the meters from the large current of the line and provides a reduced current that can be consumed by the measuring instrument. The 400/5A reduces current from 400 amperes to 5 amperes suitable for metering instruments. Likewise, 100A/5A steps down current form 100A to 5A to be used in metering. TX2 1000kVA 10,000/400V Transformer: A step-down Delta/Wye transformer that converts 10kV to 400V for low voltage consumption. It has a rating of 1000kVA. The delta/wye configuration is the most commonly used in 3-phase transformers because the wye on the secondary side gives a fourth conductor, the neutral, which is useful in LV distribution. It facilitates the use of single phase power from a 3-phase LV line. Main Incomer: Found on the main distribution board and it acts like a switch which is responsible for switching the phases and neutrals for the whole power supply. 6 Stage 200kVAr PFC Unit with Auto Control: This is a power factor correction (PFC) unit with a 200kVAr rating. The control parameter is VAR, and not PF (power factor). Target PF value is just used to calculate the capacitive VAR required to be added/removed to achieve the desired. Future Generator: A provision for the connection of a generator that will supplement the power supplied by the main incomer to the main board. A.C. generators or alternators which operate on the same fundamental principles of electromagnetic induction as DC generators. Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. The generator could be single phase but preferably 3-phase due to the nature of the system (main board has a 3-phase supply). MCCBs (Molded Case Circuit Breakers) – rated current up to 2500 A. They have thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings. Is closed mechanically and can be opened electrically as well as mechanically. This category includes: 400A MERLIN GERIN NS400N with STR23SE@MAX 800A MERLIN GERIN C800N with STR205D@MAX attached on the feeds to SUB A1, A2, B and C, 63A MERLIN GERIN Dehnventil NS100N with LSTR22SE@MAX, 250A MERLIN GERIN NS250N with STR22SE@MAX 60A MERLIN GERIN NS160N with STR22SE@MAX attached to the boards A1, A2, B1, B2, B3, C1, and C. ACBs – Air Circuit Breakers It is a circuit breaker which operates in air at atmospheric pressure. They are usually of two types namely: Plain air circuit breaker and Air blast Circuit Breaker In this category are the three 1000A MERLIN GERIN MASTERPACT ACB M08-M63H1 with STR28D@MAX attached to the feeds to Main Board in the Main Switch room. Both ACB and MCCB serve the same function, which is to isolate a circuit either because you want the power off or to isolate because there is a fault. They both use trip units which monitor the current flowing through the break and trip when it gets too high. Isolating Switches: They are used to completely de-energize an electrical circuit to allow for maintenance or service. It can be manually or automatically operated. High-voltage isolation switches are used to allow isolation of apparatus such as circuit breakers, transformers, and transmission lines, for maintenance On the board they are the 250A, 400A and 800A Isolating Switches. Digital meter: The digital meter on the future generator circuit is used to measure the power the generator contributes to the main board. Surge Protector: Also called surge suppressor is an appliance designed to protect electrical devices from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold. Lightning Arrestors – (surge protectors are smaller versions of lightning arresters) are devices connected between each electrical conductor in power and the Earth. These prevent the flow of the normal power to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a power line is struck by lightning or is near to a lightning strike. If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances. The lightning arrestors in the board have been used to surge-protect the system. POWER FLOW The incoming high voltage power (10kV) supply feeders from the ESB enters the New ESB Ring-main through the N/C-N/O contacts that keep the power connected or disconnected from the rest of the distribution board. Power is then passed through the metering circuit breaker which opens on fault thus protecting the meter instruments. Attached to this breaker is a trip mechanism that determines the conditions under which the power is cut off. An emergency trip button is also attached and can manually operate the circuit breaker from a safer location in case of an emergency. The Contractor and Supply Authority meters are then attached to take various measurements succeeded by current transformer (CTS) and voltage transformer (VTS). These two monitor the current and voltage in the line respectively. The electric power is then driven through another circuit breaker that controls the power to the TX1 Feeder. It has a current transformer and a trip that together initiate a trip in the breaker in case the set current is exceeded as a result protecting the TX1. In the Main Switch Room, a large step down Delta/Wye transformer with a rating of 1000kVA, 10,000/400V is used to step the power from 10kV down to 400V for low voltage consumption. The output of the TX2 forms the Main Incomer. A 1000A ACB circuit breaker is attached to it to protect the Main Board from faults in the incoming power. A 100A TPN isolator precedes a 415V surge arrestor that cushions the main board from surges in the main incoming power. The power factor correction (PFC) unit and the future generator are each connected to the main board through a 1000A ACB circuit breaker for protection purposes. Feeds to Subs A1, A2, B and C are then tapped from the main board through MCCB circuit breakers which protect the SUBs from over-current and short-circuit. BLOCK A It contains two SUBs, A1 and A2 which are both fed from the main board in the Main switch room. SUB A1 Power from the main board is passed through a 400A MCCB circuit breaker which acts as an overall protection for the entire Sub A1; it trips when current exceeds 400A. Power then goes through a 400A isolating switch which is used to de-energize the Sub A1 in case of maintenance. From the main board of Sub A1, the various loads are supplied and each supply is safeguarded by a surge protector, lightning arrestor and an MCCB circuit breaker. Loads for SUB A1/1 up to A1/8 except A1/5 use 63A MCCB circuit breaker. SUB A1/5 uses a higher MCCB with rating of 160A indicating that it serves a larger load compared to the other seven. This is also evident from the larger diameter of the cable used. SUB A2 It has the largest load of the four Subs. Its supply from the main passes through an 800A MCCB circuit breaker for fault protection and then through an 800A Isolating switch that provides isolation during maintenance. Sub A2 main board then serves as the supply to the 8 feeds. Surge protectors are used to counter surges in the feeds and MCCB breakers to bar over-current. 63A MCCB and 250A MCCB breakers are used where the latter protects higher load feeds. BLOCK B SUB B It is supplied through a 400A MCCB breaker and 400A Isolating switch then the supply is mounted onto SUB B main board. Power is then distributed to the three SUBs via 250A MCCB breakers and 250A isolating switches. B1 A 250A MCCB breaker and a 250A isolating switch precede the supply to the main board and they serve the same protection purposes as in the previous SUBs. From the board, the power leaves to the respective 11 feeds through surge protectors and MCCB circuit breakers. Different cables are used to deliver it to the load. Cable sizes are dependent on the current through them. B2 Just like B1, B2 is fed from the main board through a 250A MCCB breaker and a 250A isolating switch. The layout remains inherently constant where the power upon entering the main board of B2 leaves through surge protectors, different rating MCCBs then to the load elements. It has 9 feeds. B3 The supply undergoes virtually the same treatment; through a 250A circuit breaker, isolating switch, main board, surge protectors, 63A MCCB breakers then to the load. It has 8 feeds. BLOCK C SUB C The whole block is wired through a 400A MCCB circuit breaker succeeded by a 400A isolating switch after which the power is mounted onto the SUB C main board. This then forms the supply to both C1 and C2. C1 C1 is protected through MCCB breaker and an isolating switch both with a rating of 160A. This part has the fewest elements – three feeds each passing through a surge protector and a 63A MCCB breaker. Power is then delivered to the connected load elements. C2 Connection to the main board of C2 is through a 160A MCCB circuit breaker and a 160A isolating switch. It is then surge protected and proofed against over-current using a surge protector and and 63A MCCB circuit breaker respectively before being delivered to the appliance consuming it. TOTAL LOAD DETERMINATION Theory To establish the total load of the system, we consider the transformer TX2 on full load conditions. We then express the total (maximum) load in terms of full load current of the transformer. This is because when all the appliances are connected, they will draw a cumulative current I from the supply, in this case the transformer TX2. Therefore it is conceivably correct why we express the total load in terms of full load current on the transformer. TX2 is taken to be a 3-phase transformer because of its Delta/Wye connection. Generally, the power P in VA for a system with voltage V and current I is given by: P = V* I For single phase power the formula remains the same. However for 3-phase power the formula is as given below: kVA rating for a 3-phase system = (square root of 3) * Secondary Voltage * Current * (1/1000) Let P = kVA rating of transformer V = secondary voltage of transformer in volts I = full load current of the transformer in amperes P = 1.732 * V * I * 1/1000 (in kVA) Therefore, I = (P*1000) / (1.732 * V) = (1000 * 1000) / (1.732 * 400) =1443.418 A Full load current = 1443.418 A The circuit breaker after the TX2 transformer has a rating of 1000A meaning that the maximum allowable load of this system is 1000A. CABLES USED The cables used are of two categories: the XLPE/SWA/PVC and LSF CPC cables XLPE/SWA/PVC cables are used for the phases while LSF CPC cables are used for earthing. XLPE/SWA/PVC SWA – Steel Wire Armored cable is a hard-wearing power cable designed for the supply of mains electricity. It can be broken down into its constituent parts as follows: Conductor – consists of plain stranded copper Thermosetting insulation – XLPE (Cross-linked polyethylene). It has better electrical properties than PVC and is therefore used for medium- and high-voltage applications. It has more resistance to deformation at higher temperatures than PVC. It also has good water resistance. Insulation in cables ensures that conductors and other metal substances do not come into contact with each other. Bedding – Polyvinyl chloride (PVC) bedding is used to provide a protective boundary between inner and outer layers of the cable. PVC is a low voltage cable insulation. It is clean to handle and is reasonably resistant to oils and other chemicals. However, when burnt, it emits dense smoke and corrosive hydrogen chloride gas and its physical characteristics change with temperature: when cold it becomes hard and difficult to strip. At high temperatures the material becomes soft so that conductors which are pressing on the insulation (e.g. at bends) migrate through it, sometimes moving to the edge of the insulation risking short circuiting. Armor – Steel wire armor provides mechanical protection making the cable withstand higher stresses, be buried directly and used in external or underground projects. The armoring is normally connected to earth and can sometimes be used as the circuit protective conductor (earth wire) for the equipment supplied by cable. Sheath – formed by a black PVC sheath which holds all components of the cable together and provides additional protection from external stresses. LSF CPC Low Smoke and Fume (LSF) Circuit Protective Conductor (CPC). They have reduced smoke and corrosive gas emissions in cases of fire compared to PVC and XLPE. Circuit Protective Conductor is the name used to refer to the protective earth wiring of all metal parts of a building. Earthing is the process by which all metal parts in a buildings are electrically connected together and then linked to earth. This is done to prevent any metal component within a building becoming dangerous should it become live due to an electrical fault or damage. The link conductors are referred to as Circuit Protective Conductors (CPC). Any fault should cause the circuit protection device to operate and isolate the incoming mains. The cables used can be classified into low voltage cables and low voltage service cable. Low Voltage Cables 300mm.sq.1c.Cu XLPE/SWA/PVC + 2x50mm.sq CPC This is a 3-phase underground cable used to transmit low voltage power. It has an advantage of offering superb mechanical and electrical protection though it is expensive to install and less flexible. Given that it is an underground installation, it is suitable for crowded places like towns thus sparing us the dangers associated with overhead. In this board, it has been used to transmit 400V from the transformer to the main board and has a high current carrying capacity of 1000A. 300mm.sq.4c.Cu XLPE/SWA/PVC + 150mm.sq.LSF CPC 240mm.sq.4c.Cu XLPE/SWA/PVC + 240mm.sq.LSF CPC 185mm.sq.4c.Cu. XLPE/SWA/PVC + 185mm.sq. LSF CPC 150mm.sq.4c.Cu. XLPE/SWA/PVC + 95mm.sq. LSF CPC 120mm.sq.4c.Cu. XLPE/SWA/PVC + 70mm.sq. LSF CPC Low Voltage Service Cables They are smaller in diameter as compared to the above low voltage cables. The cross-section is small because they carry less current to the loads connected to them. Their small size makes them relatively cheap to acquire, install and consequently maintain. They occupy less space and can virtually be piped easily as they traverse to their destinations. This category has the following cables: 95mm.sq.4c.Cu XLPE/SWA/PVC + 35mm.sq.LSF CPC 70mm.sq.4c.Cu XLPE/SWA/PVC + 35mm.sq.LSF CPC 70mm.sq.4c.Cu XLPE/SWA/PVC + 25mm.sq.LSF CPC 50mm.sq.4c.Cu XLPE/SWA/PVC + 10mm.sq.LSF CPC 35mm.sq.4c.Cu XLPE/SWA/PVC + 10mm.sq.LSF CPC 25mm.sq.4c.Cu XLPE/SWA/PVC + 10mm.sq.LSF CPC 16mm.sq.4c.Cu XLPE/SWA/PVC + 10mm.sq.LSF CPC Read More