Power Distribution Systems and Transmission Line Supplies - Assignment Example

ENGINEER QUESTIONS Client Insert Name Client Insert Institution Client Insert Date Due Engineer Questions PART 1 ANSWERS Q1. Explain why different voltages are used in power distribution systems This is used to ensure efficiency and effectiveness of power transmission. There has to be sections within the distribution systems that carry high voltage to ensure that the system carries more power for a given current load and other sections with low voltage to ensure increased reliability (Ayanda 2002). Q2. What is the available line current (Amps, 3 phase) from a 2000 kVA, 3 phase transformer at 400 volts? Q3. If a 22kV phase to phase, 3 phase transmission line supplies 4200 kW at 0.85 power factor, what is the 3 phase line current (LL) in the 22kV Transmission Lines? Q4. If the phase to phase voltage of a 3 phase star connected electrical system is 33kV, what is the phase to neutral voltage? Q5. Explain with the aid of sketch: i. Discrimination Discrimination in electrical terms refers to the protection of a distribution network using such appliances as fuses, and circuit breakers. It is the whole system involved in the protection process including cables to ensure that electrical faults do not cause damage to distribution networks (Ayanda 2002). It enables the protection through tripping in a way that minimizing the disruption to the whole distribution network as it is done on a co-ordination study (Moore 2010). There are different ways that discrimination can be achieved and previously, it was done by “tracing the protection curve of one device and overlying it on the upstream device to see there was no crossover (condition where they both would trip)” (Moore 2010). Nowadays, there are special software that are now used. Figure 1 below shows a typical Discrimination Circuit. Fig 1: Discrimination Circuit Source: (Moore 2010, p. 26) ii. Cascading Cascading helps circuit breakers to have enhanced breaking capacity by helping to limit high short-circuit currents (Almarshoud 2004). The following animation shows how cascading occurs when a single failure causes failure in the whole circuit. Normally working Network Single Failure Occurs Whole Circuit Fails Source: (Almarshoud 2004, p. 128). PART 2 ANSWERS A building has the following areas Car parking 1500m2 Office 15000m2 Warehouse 3000m2 Plant 1500m2 Based on the following typical loads Car park 10W/m2 Office 90W/m2 Warehouse 50VA/m2 Plant 15VA/m2 Calculate the following assuming a power factor of 0.92 i. Estimated maximum demand for the building in kVA ii. Transformer capacity in kVA, allowing a 20% spare allowance for further capacity? iii. Current in the secondary (400v) terminals of the transformer PART 3 ANSWERS Q1. Which insulation can withstand hotter long term operating temperatures? PVC or Cross Linked Polyethylene (XLIPE)? XLIPE insulation withstands hotter long term operating temperatures being able to withstand strengths of up to 120 – 1500C having a maximum rated conductor temperature of 900C which can stretch to 1400C based on the quality of the materials used in making the XLIPE (Grigsby et al. 2001). In addition to this, these materials have a short-circuit rating of up to 2500C having superior dielectric properties and this makes it ideal for a wide range of voltage, from -10kV to 50 kV AC and several hundred for kV DC (Almarshoud 2004). Q2. Which of the three primary cable sizing criteria will determine the cross sectional area of a very short length of cable attached to a transformer via a very slow acting protection device? Explain your reasons The cable sizing criteria that would be used for this case is the Starting and Running Voltage Drops in Cable approach. This would be the most appropriate criteria since it ensures that the cross-sectional area of the cable selected is good enough to keep the voltage drop across the cable conductor with a good limit to ensure that the equipment supplied with the power through the cable optimal during starting and running condition of the appliance (Matthews 2010). In other words, this helps the voltage drop between the load and the supply load does not exceed the current that will be safe and efficient for the starting and running of the appliances attached to the source. Since the cable to be chosen is very small and the system has a very slow acting protection device, it means the cable to be chosen will have to determined using the voltage carrying capacity of such a conductor being the most appropriate criterion for selection (Flurscheim 2009). Q3. What is the short circuit capacity of a cable and why is it important? The short circuit capacity of a cable is the amount of load that a cable can carry without being damaged. It shows the loading bearing of such a cable which is important in understanding the size of a such a cable that is required to withstand the worst short-circuit currents flowing through it (Matthews 2010). Q4. Calculate the required disconnection time for a 63-Amp circuit breaker protecting a 16mm2 Cu PVC/PVC 4C+E cable with a prospective fault level of 15kA (refer to attached tables. State your assumptions The Minimum cable size based on the above information and due to short circuit temperature rise is obtained from the following formula: From the above equation, the following is the key; A – Stands for the minimum cross-sectional area of the cable in mm2 i – Stands for the prospective short circuit current in Amperes, A t – Stands for the duration of the short circuit in seconds, s k – Stands for the short circuit temperature rise constant, k. For Copper cable, k is calculated using the following formula: , where  is the initial conductor temperature in degrees Celsius and is the final conductor temperature in degrees Celsius Therefore, k for this situation is,  From the first equation, t can be determined by  The following are the assumptions made for this question: The conductor temperature is 600C and the ambient temperature is 250C The disconnection time has under normal conditions without any excessive power loss that damages any of the devices attached to the network PART 4 ANSWERS Assume conductor temperature of 600C and ambient temperature of 250C. Provide a cable sizing solution for the following problem: Operating voltage – 400V three phase Conductor – Copper Current carrying Capacity – 325 Amps per phase Maximum acceptable Volt Drop – 2.5% Prospective Fault Levels – 15kA Route length – 95m Insulation type – XLPE/PVC Installation type – Single core cables installed in trefoil, spaced from surface and on one tray, with no other circuits From the formula of maximum cable length, the prospective short circuit current can be obtained as shown below: , from this formula; From the above formula, I can be obtained from; The Minimum cable size based on the above information and due to short circuit temperature rise is obtained from the following formula: From the above equation, the following is the key; A – Stands for the minimum cross-sectional area of the cable in mm2 i – Stands for the prospective short circuit current in Amperes, A t – Stands for the duration of the short circuit in seconds, s k – Stands for the short circuit temperature rise constant, k. For Copper cable, k is calculated using the following formula: , where  is the initial conductor temperature in degrees Celsius and is the final conductor temperature in degrees Celsius Therefore, k for this situation is,  Therefore, Bibliography Almarshoud, A. 2004. "Performance of Grid-Connected Induction Generator under Naturally Commutated AC Voltage Controller." Electric Power Components and Systems, vol.32, pp.7, pp. 122 – 132. Ayanda, V. 2002. Electrical Systems Design. London: Prentice Hall. Flurscheim, C. 2009. Power Circuit Breaker Theory and Design. 2nd edn. New York: IET. Grigsby, L., et al. 2001. The Electric Power Engineering Handbook. USA: CRC Press. Matthews, J. 2010. Introduction to the Design and Analysis of Building Electrical Systems. New York: Springer. Moore, G. 2010. “Electric Cables Handbook.” 3rd edn. An excellent reference book for cables, vol. 3, no. 5, pp. 23 – 27. Read More