CHAPTER 8 Effect of HT Distribution Feeder Voltage on Distribution Transformer Losses

Transcription

1 CHAPTER 8 Effect of HT Distribution Feeder Voltage on Distribution Transformer Losses 8.1 Introduction The present level of Transmission and Distribution (T & D) losses in Indian power system is estimated to be 23%. The T& D losses in India is very high compared to the losses in the power systems of the developed countries like USA, Canada, USSR, Korea and Japan. The T & D losses in developed countries are as shown in the table 8.1. Table 8.1. T & D Losses in Developed Countries Sl.No Name of Country T & D Losses in % 1 USA 8.8% 2 Canada 8.5% 3 USSR 8.2% 4 Korea 11.0% 5 Japan 6.0% It has been reported that the losses in power system in Japan and Korea were also of the order of 20%. Adopting modern techniques of Distribution system planning and design, they have brought down the losses to the international level of 6 to 8%. There is a need to take up similar excises in respect of Indian power systems to reduce T & D losses, when the country is facing severe power shortages need to emphasis. The typical existing T & D losses break-up is shown in the Table 8.2. Table 8.2. T & D losses Break-up for Indian Power System Sl. No System Break-up of losses In % % of Contribution of each voltage level to total losses 1 EHV System 4.5% 19.6% 2 33 KV System 4.0% 17.4% 3 11 KV System 7.0% 30.4% 4 LV System 7.5% 32.6% Total Losses 23.0% 100.0% Source: 95

2 The 11KV HT Distribution System contributes about 30% of total T & D losses of Indian Power System. The main contributing factor for the losses in the area is the wrong HT Distribution system practice chosen by India coupled with non-adherence to prescribed norms of voltage drops. The 11KV HT Distribution system losses include the losses in 11KV/415 Volts Transformers and 11KV feeders Line Losses. Transformer is the most energy efficient equipment used in industry. The nominal efficiency of the power Transformer is in the order of 99 to 99.5% and for Distribution Transformer is in the order of 98 to 99% though the efficiency of Transformer is very high, it offers a good potential to save energy. An attempt was made to increase the operating efficiency of the Transformer by maintain the voltage profile, proper loading and power factor correction. In Tamilnadu Electricity Board (TNEB) power supply, 11KV HT Distribution voltage varies from Peak to off-peak periods. The voltage variation is due to Demand variation and reactive power variation. To establish the voltage variations, an 11KV HT Distribution feeder named Airport Feeder is taken as sample feeder for analysis purpose and Feeder Voltage are measured in Four Locations. The Airport feeder receives power from an 110KV/11KV 20MVA Sub-station located at a place called Pykara. The approximate length of the Feeder is 5.4 Kilometres. The Airport feeder is a radial feeder and meets a demand of approximately 5MVA during peak hours and 1MVA demand during off-peak hours. The feeder consists of various types of loads like domestic, Institution, industrial and commercial loads. The 11KV HT Distribution feeder Voltage was recorded for 24Hours in four different locations of equal distance at 11KV/433 Volts Distribution Transformer end and the effect of voltage variation on the Distribution Transformer losses and efficiency was studied and concluded. 8.2 Transformer Losses The Transformer losses are mainly governed by the input supply voltage, load factor and power factor of load. Transformer losses are classified as core and copper losses Core Loss Core loss is voltage dependent loss and increase exponentially with respect to voltage. It is associated with power consumed to sustain the magnetic field in the Transformer s steel core, whenever the Transformer energized. Core losses are caused by two factors; Hysteresis and eddy 96

3 current losses. Hysteresis loss is energy loss, last due to reversing the magnetic field in the core as the magnetizing alternating current rises and falls in reverse direction. Eddy current loss is result of induced currents circulating in the core. Total core loss is given by, P c = P e + P h Core loss remains constant and is independent of the load. Figure 8.1. Shows Per unit variation of Core loss with respect to per unit HT input voltage for Distribution Transformer. HT voltage Vs core loss Core loss (pu) HT Voltage (pu) Figure 8.1. Per Unit Variation of Core Loss with respect to Per Unit of HT input voltage Courtesy : Indo-tech Transformers Limited, Chennai, India Copper Loss Copper loss is also called load loss and is associated with full load current flow in the Transformer windings. Copper loss is power lost in primary and secondary windings of a Transformer due to ohmic resistance of the windings. The typical loss values of 11KV / 433 V Distribution Transformers are shown in the Table

4 Table 8.3. Typical Loss values of 11KV/433V Distribution Transformers Transformer Capacity in KVA Iron Loss in Full Load Copper Loss in Courtesy : Indo-tech Transformers Limited, Chennai,India 8.3 Voltage Fluctuation Control in Transformer It is necessary to keep the power supply voltage close to the rated voltage, by using voltage control techniques. Whenever the supply voltage is less than the Rated value there is a chance of nuisance tripping of voltage sensitivity devices. The voltage control is achieved by performing tap changing in Transformer. There are two types of tap-changers. One is called as onload tap changer (OLTC) and another is called as Off-load tap changers Off-load tap changer It is a device fitted in the Distribution Transformer, which is used to vary the voltage transformation ratio. In this type the output voltage can be varied only after isolating the Transformer from power supply. This type of tap changer is commonly used in Distribution Transformer used by TNEB On-load tap changer The voltage levels can vary online with the help of OLTC facility without isolating the connected load to the Transformer to minimize the magnetization losses and to reduce the nuisance tripping. In general, Power Transformers in major sub-stations are provided with OLTC facility. The down stream Distribution Transformers can be provided with off circuit tap changer. 98

5 The on load tap changer can be put in auto mode or manual mode depending on the requirement. The industries availing power supply at 11KV level, preferred to select OLTC for their Distribution Transformers to maintain the LT output voltage constant irrespective of their utility grid voltage variations. The use of OLTC to maintain the LT voltage at constant level helps majority of the industries to minimize energy consumption, by avoiding over voltage during off-peak hours. The OLTC provision in Distribution Transformer is helpful to optimize the LT voltage based on load factor of Induction motor. In State Electricity Board, installing Distribution Transformer with OLTC provision become expensive and requires a little bit increase in maintenance requirements. However the necessity of OLTC below 1000 KVA can be considered after calculating the cost economics. The line drop compensation technique is used to adjust the tap position automatically to maintain the voltage in feeder end in power Transformer with OLTC facility. 8.4 Voltage Variation in 11KV HT Distribution Feeder The 11KV HT Distribution feeder Voltage of State Electricity Board varies from Peak hours to Off-peak hours in a day. Before implementation of ABT, the peak hours HT Distribution voltage goes below 11KV and typical value is 10.2 to 10.8KV at feeder ends. After implementation of ABT, the peak hour s voltage is close to 11KV. But during off-peak hours, the voltage rise to the level of 11.4 to 11.6 KV even at feeder end. The Voltage profile of 11KV HT Distribution system was recorded with the help of Hioki Make Power Quality Analyser in four different locations of 11KV/433 Volts Distribution Transformers and recorded voltage profile data are used to analyse the energy saving purpose. The HT Distribution feeder voltage was first sampled (Sample 1) at an 11KV/433 Volts Distribution Transformer located close to the main 110KV / 11KV 20MVA substation (Pykara Substation) feeding point. The HT Distribution Voltage was second sampled at 1.8 Kilometres away (Pasumalai) from the feeding point. The HT Distribution feeder voltage was third sampled at 3.6 Kilometres away (Thiagarajar College of Engineering) from feeding point. The HT Distribution feeder voltage was fourth sampled at 4.8 Kilometres away (Feeder End point) from feeding point. Figure 8.2 shows all the four HT Distribution feeder voltage samples recorded for 24 Hours in a day at four different locations of Distribution Transformer end. 99

6 Combination of all voltage samples pm HT voltage (kv) time Figure KV HT Distribution Feeder Voltage Samples Profiles Figure 8.2. Shows that majority of the time the 11KV Distribution feeder voltage is above 11.2KV. The feeder voltage varies form 10.8 KV to 11.8 KV. In all samples it s observed that the voltage varies in the band between 11.6 KV and 11.4 KV occurs 65% of time in a day (During Offpeak Hours and Normal Hours). During Peak Hours and part of Normal Hours the voltage varies between 10.8 and 11.4 KV and occurs for 35% of time in a day. After the implementation of Available based Tariff in Indian Power System, the 11KV HT Distribution voltage varies from 11.2 to 11.8 KV even during peak hours (6.00 PM to 9.00 P.M). Before ABT implementation, the 110KV/11KV 20MVA Sub-station tries to maintain 11.4KV at Transformer on 11KV side in order to maintain the Feeder end voltage at 11KV. After the implementation of ABT, due to Reactive VAR compensation, the feeder drop has come down. There is a need to revise the sending end voltage of 11KV Feeder to 11.2 KV to minimize over voltage during off-peak hours. The occurrences of different voltage samples in % of time are shown in the Table

7 Table 8.3. Comparison of the Occurrences of Different Voltage Sample in % of Time Sample1 in % HT time Sample2 in % time Sample3 In % time Sample4 in % time voltage in Voltage Voltage Occurrence Voltage Occurrence Voltage Occurrence KV Occurrence Reasons for Wide Voltage Variation in 11KV HT Distribution System The major reasons for the wide voltage variation in 11KV HT Distribution System were investigated with the help of State Electricity Board Officials and observed the following reasons for the variations. Absence of OLTC Facility in 110 KV / 11KV Power Transformers / Sub-stations Non- Familiar about the reliability of On-load Tap Changing Gear Effect of Installation of Reactive VAR Compensator Effect of Power Factor Incentives to HT and LT Industries Absence of OLTC Facility Majority of the 110KV/11KV power Transformer installed after the year 1980 by state electricity boards are provided with On-Load Tap Changing (OLTC) gear Facility. But the 110KV/11KV power Transformer installed before the year 1980, are provided with off-load tap changers. In Power Transformers provided with off-load tap changing gears, to change the tap from one position to another position requires switching off the power supply and will disturb the operation of power system. Hence maintaining the HT Distribution voltage profile becomes a tough 101

8 task in Power Transformers with off-load Tap changing facility. The HT Distribution voltage can be maintained within the prescribed limit with the help of OLTC facility in power Transformers Reliability of OLTC The tap changers are designed for operations (to change one tap position to another tap position) during its meaningful life. After that the contacts are replaced to extend it life further. The State Electricity Board operating person feels that the frequent use of OLTC will increase the maintenance in state electricity board. But the cost of savings by maintaining the voltage profile to minimize the Distribution Transformer losses is huge compare to the maintenance cost of the power Transformers. This thesis work recommends utilizing the Automatic mode of operation of the OLTC to maintain feeder voltage during Off-peak hours Effect of Installation of Reactive VAR Capacitor After the implementation of Available based tariff in Indian power system, the state electricity board has taken measures to improve the power factor of the 11KV Distribution System by installing HT capacitors at every Distribution Transformer feeding end. This improves the power factor and minimizes the feeder drop Effect of Power Factor Incentives for HT and LT Industries After the implementation of Available based tariff, the HT industries are penalized when their power factor is below 0.9 Lagging and encouraged by incentives for maintaining power factor above 0.95 Lagging. Similarly the LT industrial units are penalized, when their power factor is below 0.85 Lagging and encouraged by incentives for maintaining power factor above 0.90 lagging. Initial period of power factor penalty and incentive schemes, the HT and LT industries tray to avoid the penalty, after an year majority of the HT and LT industries maintaining the power factor close to unity to avail full incentives. This improves the 11KV Distribution Feeder voltage and minimizes the voltage drop. 8.6 Effect of Automatic OLTC Performance A trail was conducted to record the voltage profile of an 11KV Distribution feeder voltage by putting the OLTC of the 110KV/11KV 20MVA Power Transformer (feeding Sub-station at Pykara) in automatic mode to maintain the voltage at 11.2 KV at the Transformer 11KV Side. The voltage 102

9 profile of the 11KV Distribution feeder is recorded at Sample point 4 (11KV HT Feeder End) for 24Hours in a day. The 11KV Distribution Feeder voltage varies from peak hours to off-peak hours between 10.9 and 11.2 KV at the tail end of the feeder. The HT Distribution voltage recorded by putting the OLTC in Auto mode to maintain feeder voltage between 10.8 and 11.2 KV is shown in Figure 8.3. OLTC performance curve voltage (kv) :26:45 16:31:45 18:36:45 20:41:45 22:46:45 0:51:45 2:56:45 5:01:45 7:06:45 9:11:45 11:16:45 13:21:45 15:26:45 time Figure 8.3. Effect of OLTC performance on 11KV Distribution Feeder Voltage Profile 8.7 Effect of Over Voltage in Transformer Loss When HT Voltage is increased, the core loss of the Transformer will increase but in contrast the HT copper loss will decrease. In Distribution Transformers, the increase in core loss is more than decrease in HT copper loss and leads to increase in total Transformer losses in majority of the cases. A great potential of energy can be saved, if this effect can be nullified or minimized by adjusting the Transformer taps optimally with the help of on-load tap changing facilities in the main power Transformer (110KV/11KV) feeding to the 11KV Distribution feeder lines. Based on the observation made, this thesis work proposes to maintain the HT Distribution feeder Voltage between 10.8 and 11.2 KV. If the Power Transformer step down voltage and maintained at 11.2 KV, then the Distribution Transformer close to main 110KV/11KV Sub-station 103

10 will receive 11.2 KV and the Distribution Transformer at the far end of HT Distribution will receive 10.8KV at the worst cases. The off-load Tap position of Distribution Transformers can be maintained in such a way to receive rated 415 Volts. The Distribution Transformer close to main sub-station can be operate at Tap position No.2 and at middle of the HT feeder can operate at Tap position No.3 and tail end of HT feeder can be operate at Tap position No.4. For a well-designed 11KV system the HT Distribution drop is maximum of 0.2 KV (200Volts) and some of the 11KV Distribution system having voltage drop of less than 0.2KV are still operating with the state electricity board (Especially priority feeders). This clearly indicates there is a possibility to maintaining the 11KV Distribution voltage between 10.8 KV and 11.2KV to minimize the core loss in Distribution Transformers. But it is not feasible in case of main sub-station Transformer feeding to feeder without OLTC provision. The commonly used sizes of Distribution Transformers by the State Electricity Boards are 11KV/ 433V 100KVA, 250KVA and 500KVA. An attempt was made to calculate the savings potential theoretically due to High input 11KV input voltage of Distribution Transformer of the commonly used Size at various loading conditions. The losses at various voltage conditions are shown in the Table 8.5, 8.6 and 8.7. Table 8.5. Losses at various Voltage conditions for 100KVA Distribution Transformer HT HT Copper Voltage Core loss in in KV in

11 Table 8.6. Losses at various Voltage conditions for 250KVA Distribution Transformer HT voltage In KV Core loss in Table 8.7. Losses at various Voltage conditions for 500KVA Distribution Transformer HT voltage in KV Core Loss in

12 8.8 Loading of Transformer Loading plays a major role on Transformer operating efficiency. In any Electrical equipment as the load factor increases towards the capacity the efficiency will be high. In case of Transformer the maximum efficiency occurs well below the rated load. As we are aware by the theory, in case of Transformer the maximum efficiency occurs when the copper loss is equal to the iron loss. In case of a Transformer, maximum efficiency occurs between 40 and 60 % of the load factor. The reason for this is the iron loss is only 10 to 15 % of full load copper loss in case of Distribution Transformer and 15 to 20 % of full load copper loss in case of Power Transformer. At the same time, it is not recommended to go for additional capacity of Transformer to increase operating efficiency and reduce load factor to 40 to 60% because of the very high initial investment for the additional Transformer Effect of Load Factor The load factor of the Distribution Transformer varies from off-peak hours to Peak hours and varies from 25% to 100%. During Peak hours the rise in 11KV Distribution feeder voltage is helpful to minimize the over all Distribution Transformer losses. When the load factor of the Distribution Transformers between 80 and 100%, the increase in core loss due to rise in voltage is less than the decrease in copper loss due to reduction in HT side current. The 80 to 100% Load Factor occurs in Distribution Transformer only during Peak Hours, but the 11KV feeder voltage is more or less equal to the rated voltage. During off-peak hours the voltage raises more than the rated and Load factor of Distribution Transformer is between 25 and 50%. In this case the increase in Core Loss due to rise in voltage is more than the decrease in copper loss due to reduction in HT Side current. The voltage measurements done on 11KV Feeders show that the 11KV voltage varies from 11.4 KV to 11.8KV during off-peak hours. The Load factor of TNEB Distribution Transformer feeding to commercial and domestic load is around 25 to 50% during off-peak hours. Majority of the cases the Distribution Transformer feeding to commercial and domestic load has only 25% load factor during off-peak hours. The effects of 11KV HT Distribution feeder voltage on the Distribution Transformer at various load factors are analysed. The Distribution Transformer losses are high, when the load is less than the rated load. An analysis was carryout to calculate the losses at 25% load and 50% load. Based on the analysis made on this thesis work, the Distribution Transformer losses are less, 106

13 when input voltage is between 10.8 KV and 11.2 KV. An attempt was made to quantify the savings potential in TNEB Distribution Transformer feeding to commercial and domestic load. The saving potential in TNEB Distribution Transformer by maintaining the voltage profile of 11KV supply between 10.8 KV and 11.2KV was calculated assuming 50% Total No. of Transformers are loaded to 25% load factor during off-peak and remaining 50%Transformer loaded to 50% load factor. 8.9 Benefits of Maintaining Voltage Maintaining the 11KV HT Distribution Feeder voltage between 10.8 KV and 11.2 KV will save energy in Distribution Transformers by minimizing the voltage related losses during off-peak hours and normal hours. This also saves the equipment connected to the Distribution Transformer in case of over voltages. In case of a 250 KVA Transformer by keeping the 11KV rated supply voltage between 10.8 and 11.2 KV during off-peak hours (approx. 8 hrs) will yield saving in energy of 0.85 KWH a day. The annual saving estimated by maintaining the voltage in 11KV power supply will save around Indian Rupees.15 Millions in 100 KVA, 250 KVA and 500 KVA Transformers alone. Energy Conservation not only save money, it can also reduce the emission of greenhouse gases such as carbon dioxide (CO 2), which is released when electricity is generated and environment Cost Benefits The Number of Distribution Transformer in operation for supplying power to Commercial, Small Scale Industries and Domestic Load in TNEB, Chennai is as follows: Total number of 11KV/ 433 Volts 100 KVA Transformer = 49,127 Total number of 11KV/433 Volts 250 KVA Transformer = 16,816 Total number of 11KV/ 433 Volts 500 KVA Transformer = 4,780 For estimating the saving potential, the cost of electrical energy is taken as the average generation cost for TNEB. The average generation cost for TNEB is Indian Rupees 1.86/KWH. This was used to calculate the savings potential in TNEB Distribution Transformers to determine the annual cost savings of these three size Transformers. The annual cost savings potential in TNEB due to maintaining the 11KV HT Distribution Voltage within the proposed voltage band of 10.8 KV to 11.2 KV is shown in the Table

14 Table 8.8. Annual Cost Savings due to maintaining the 11KV HT Distribution Voltage Savings in case of Savings in case of Annual Cost of 25% Load Factor in Indian 50% Load Factor in Indian Savings Rupees/year Rupees/year 100 KVA 4,479,137 2,801, KVA 3,287,918 2,009, KVA 1,713,434 1,129, Summary In this chapter, the need of maintaining 11KV HT Distribution System Feeder Voltage between 10.8KV and 11.2KV is discussed to minimize the Distribution Transformers losses during off-peak hours by State Electricity Board with the help of OLTC facility in Transformer. The targeted value of feeder voltage from 10.8 KV to 11.2 KV can be regulated with the help of OLTC provided in the Power Transformers. There is a need to revise the 11KV Distribution feeder sending end voltage due to implementation of ABT in the Regional grid. A trail was conducted to record the voltage profile of an 11KV power supply by putting the OLTC in automatic mode in the Power Transformer to maintain the voltage at 11.2 at the Transformer 11KV Side. The 11KV Distribution Feeder voltage varies between 10.9 and 11.2 KV at the tail end of the feeder. This voltage is ideal for the 11KV HT Distribution system. This also reduces the core saturation and increases the life span of the Distribution Transformers. The annual savings potential by maintaining the 11KV power supply voltage profile between 10.8KV and 11.2 KV for the 100 KVA, 250 KVA and 500KVA Transformers is Indian Rupees.15.7 Millions. The On-load tap-changing gear plays a major role in maintaining the voltage profile of the 11KV HT Distribution Feeder voltage. But the usage of OLTC was limited due to lag of awareness in maintaining the voltage during off-peak hours. This thesis work recommends operating the OLTC in automatic voltage correction mode especially during off-peak hours. The trail taken in a 110KV sub-station operating the Power Transformer with OLTC in Automatic mode has maintain the voltage of 11KV power supply between 10.8 KV and 11.2 KV. This can helpful to minimize the T & D losses of India. Hence it is strongly recommended to maintain the 11KV Distribution feeder voltage preferably ± 2 to 3 %, to minimize the Distribution losses in the power system in India and developing countries. 108