CHAPTER 3 DEVELOPMENT OF DISTRIBUTION SIMULATION PACKAGE FOR LOAD ANALYSIS OF LV NETWORK

Transcription

1 78 CHAPTER 3 DEVELOPMENT OF DISTRIBUTION SIMULATION PACKAGE FOR LOAD ANALYSIS OF LV NETWORK 3.1 INTRODUCTION Distribution loads vary in response to temperature, time of the day, day of the week and other factors such as humidity precipitation and season. The effects of daily load patterns of a typical low voltage network (secondary distribution network) need to be studied in depth. This requires a detailed collection of distribution transformer load recordings of all electrical parameters such as voltage, current, power and power factor for all three phases. Load analysis is the detailed systematic study of all load recordings to derive significant conclusions. 3.2 LITERATURE SURVEY Valentina Cecchi and et al (2007) have designed instrumentation and measurement configuration for network configuration and meter placement. In meter placement studies, the goal is optimal monitoring of the network to observe events, sensing three-phase, neutral voltage and current signals at each measurement bus and operate controllable network devices such as automatic switches. The work is in primary distribution network and it is not extended up to secondary distribution network.

2 79 Aderiano Galindo Leal and et al (2009) describes artificial neural network approach for loss evaluation of distribution transformer. Load profiles of distribution transformer and consumers of different category namely residential, industrial, commercial are presented in their work. The authors have recorded seven-day load profiles of distribution transformers of Brazilian distribution utility which proves uniformity in stochastic nature of the loads. That is, load profiles of different categories of consumers can be grouped under clusters as residential, commercial and industrial. But it requires daily load profiles of consumers whereas daily load profiles of distribution transformer are only available in developed country s scenario. In this thesis, Distribution simulation package (DSP) has been developed for detailed load analysis of LV network. It uses LabVIEW as off-line distribution monitoring system. The developed package is capable of displaying the distribution parameters namely, voltage, current and power factor in any hour of day. Also power and energy measurement is done for every half-an-hour and displayed. Low voltage distribution network power quality parameters and issues can be studied. The distribution simulation package developed is used in this thesis for load analysis and further optimization of load balancing and loss reduction. 3.3 DEVELOPMENT OF DISTRIBUTION SIMULATION PACKAGE In the existing system of distribution network, energy meters are provided for energy accounting. There is no means of sensing unbalance currents, voltage unbalance and power factor correction requirement for continuous 24 hours in three phases of LT Feeder. In other words, load curves, voltage curves, energy curves and power factor curves for individual three phases for full day are not available for monitoring, analyzing and controlling the LV network. The individual phase of distribution transformer

3 80 can be monitored by taking the reading whenever required. Hence there arises a need for the development of distribution simulation package for LV network for systematic analysis. The modules developed in the DSP are listed below along with their associated function. Load Survey Module: Collection of 30 minute readings on the daily load pattern of distribution transformer. Power measurement Module: The measurement of power (Real, Reactive and Apparent) and display of voltage current (vi-profile), power factor and power in the front panel for each phase R, Y and B. Display Module: The display of voltage graph showing all three phases, current graph with all three phases, power graph showing all three phases and total power for any selected day for the low voltage distribution network. These graphs are effectively utilized for load analysis and to study the power quality performance of low voltage distribution network. Unbalance Prediction Module: Prediction of unbalance in the network for the day selected and display them with LED indicators Design of Simulation Package In the design of simulation package, LabVIEW is used as software simulation tool. In the existing system of distribution network, the distribution transformers are fixed with energy meters in the secondary of the distribution transformer and energy meter readings are downloaded with common meter reading instrument (CMRI). The energy meter reading includes voltage current profile (vi profile) and power factor. It can be used

4 81 for power measurement. With additional functionalities developed in this work like plotting of all electrical parameters on graph, prediction of unbalance current, LabVIEW based system can be termed as effective Monitoring Module (MM) for low voltage distribution networks. LabVIEW version 7.0 graphical environments virtual instrumentation (VI) based on simulation is used as the software platform. This is due to its superiority and simplicity in design, software intuitive graphical programming capability, effective data flow programming and block diagram approach Load Survey Module The phase voltages, line currents and power factor of all three phases are monitored every half an hour in the meter fixed in the secondary of the distribution transformer. The voltage curve and load curve are obtained from these values. The active, the reactive and the apparent power are computed from these quantities after determining the phase angle. VI-sub modules are developed and the parameters listed below are plotted. Individual phase voltage. Individual phase current. Individual phase active power. Individual phase reactive power. Individual phase apparent power. Individual phase power factor. With the above concepts, the front panel and block diagram are developed for three phase loads by downloading the vi-profile (voltage current profile) from energy meter installed in the distribution transformer and simulating the setup using practical values. From the actual values obtained load unbalance is predicted.

5 Power Measurement Module Design The energy meter reading which includes vi-profile with power factor serves as one of the inputs to LabVIEW system. It consists of 30 minute readings and hence 48 samples per day. The other input for power calculation and display of voltage graph, current graph, power graph and energy graph is the day input. For particular transformer and particular day of the month selected, all graphs of electrical parameters and power measurement has been designed in the block diagram of LabVIEW and displayed in the Front panel. Figure 3.1 shows the subvi for displaying the viprofile on a particular date. Figure 3.1 SubVI for displaying vi-profile on a specific Date The subvi for the measurement of power is shown in Figure 3.2. Figure 3.2 SubVI for Measurement of Power

6 83 The file path is the data downloaded from the CMRI, i.e., distribution transformer secondary reading. Then the sub-vi calculates the individual phase real power, reactive power and apparent power. The front panel design for measurement of power is displayed in Figure 3.3. Figure 3.4 shows the design of block diagram. Figure 3.3 Front Panel for Measurement of Power

7 Figure 3.4 Block Diagram for Measurement of Power 84

8 85 The LabVIEW front panel displays the vi-profile on a particular date with power and energy measurement. It reads the vi-profile and computes the real power, reactive power, apparent power and energy, kwh. Graphical representation of voltage, current and power is plotted. Figure 3.5 shows the subvi for the plotting of individual phase currents on the particular phases. Figure 3.6 shows the subvi for the plotting of individual phase voltages on particular phase. Figure 3.5 SubVI for Load Graph Display Figure 3.6 SubVI for Plotting of Individual Phase Voltages The load analysis is done for any particular day for transformer. The performance of low voltage distribution network with reference to power

9 86 quality issues is studied in depth and unbalance of load can be predicted. The power factor has to be maintained constant for study purpose and practically it has to be done by including capacitor banks dynamically with the load and insisting the consumers to maintain the power factor of Unbalance Prediction Module In the distribution network, current consumption depends on the temperature of the day, time of the day and day of the week and other factors such as humidity precipitation and season. Load analysis of transformer has to be done for full month to check for the consistency and stochastic nature of the loads. To balance the current in the secondary distribution network, only the peak load period is considered since the loads are predominant at peak load. The unbalance effect is more during peak loads. By attempting load balance during peak hours gives better performance throughout the day. SubVI for finding the maximum current in a particular day and particular phase is shown in Figure 3.7. Figure 3.7 SubVI for Maximum Current in a Particular Phase

10 87 The subvi asks for the file path and date as inputs. The file path contains the secondary voltage reading of the distribution transformer, hence 30 minute readings of current in that phase of low voltage distribution network and it asks to enter a date. Then subvi displays the maximum current in a particular phase and it also displays current consumption in the peak load hours. In similar way, the LabVIEW is designed for all three phases. The maximum current consumption in each phase is I Rmax, I Ymax, and I Bmax. The optimum current (I opt ) is given by equation (2.2) The difference between I opt and I Rmax is then determined. Similarly the difference between I opt and I Ymax, I opt and I Bmax is computed. If the difference is positive then that phase is considered as overloaded and if the difference is negative then that phase is considered to be under loaded. If the difference is within the tolerance value, then that load is perfectly balanced. SubVI for prediction of unbalance is shown in Figure 3.8. Figure 3.8 SubVI for Prediction of Current Unbalance

11 88 If there is unbalance, then it is displayed by the LED indicators as in Figure 3.9. Unbalance in particular phase is indicated by RED indication and balanced condition of particular phase is indicated by GREEN indication. R Y B R Y B Balanced Condition in all phases Unbalanced Condition in all phases R Y B R Y B Balanced Condition in two phases Unbalanced Condition two phases Figure 3.9 Display of Balanced/Unbalanced condition in LV Network For the peak period, four hours is selected. There are 9 sets of reading for every half-an-hour. For example, if the peak period is 18 hrs to 22 hrs there will be nine set of half an-hour reading starting with 18hrs reading

12 89 and ending with 22hrs reading. LED display indicates the time of unbalance during peak hours and extent of unbalance studied from load graph display. LED display is only indicative of unbalanced or balanced condition. The extent of unbalance and load balancing techniques is further analyzed. Shifting of consumers is suggested and LV distribution network is optimized with further study in this thesis. 3.4 LOAD ANALYSIS Load analysis of distribution transformers with different loading patterns deduces significant inferences for this thesis. For performing load analysis, distribution simulation package developed as discussed in section 3.2 is effectively utilized. The following transformers are considered as case study. Sample distribution transformer with medium loads (Urban DT- ML) Sample distribution transformer with heavy loads (Urban DT- HL) Sample distribution transformer with low loads (Urban DT- LL) Sample distribution transformer with distributed loads (Rural DT-DL) Load graphs of 5 numbers distribution transformers selected at random across the country, India. Typical loads on low voltage networks are stochastic by nature. However it has to be ensured that there is similarity in stochastic nature throughout the day. For arriving at practical and effective conclusion for formulating this thesis the above transformers have to be extensively studied

13 90 with reference to loading patterns and their impact on performance of low voltage distribution network. Also study has to be performed for the entire day for transformers with different load patterns and different categories of consumers namely, domestic, commercial and industrial. The study includes the following for each distribution transformer and recorded as mentioned. Input to the package Low voltage distribution album of distribution transformer. Meter readings of distribution transformer (30minute readings). Output from the package Power measurement Voltage graph Load (Current Graph) Power graph and Total power The selected distribution transformer ratings are 500 kva, 250 kva and 100 kva with different loading patterns like heavy load (current more than 500 A), medium load (current more than 250 A) low load (current more than 100 A) and distributed load (current less than 100 A). The capacity of a distribution transformer is determined by the amount of current it can carry continuously at rated voltage without exceeding the design temperature. The transformers are rated in kilovolt-amperes (kva) since the capacity is limited by the load current which is proportional to the kva regardless of the power factor. The standard ratings are 10, 16, 25, 63, 100, 160, 200, 250, 315, 400, 500, 630, 1000, 1250, 1600, 2000 and 2500 kva for 11 kv distribution

14 91 transformers ( Central electricity authority, Government of India website) and 100, 160, 200, 315, 400, 500, 630, 1000, 1250, 1600, 2000, 2500 kva for 33 kv distribution transformers. The allowable losses for standard kva ratings are itemized in Table 3.1 reproduced from Daily allowable peak loads for normal life expectancy is also a critical factor in allowing peak load. It can be seen that losses at 100% loading is more than double the losses at 50% loading. Table 3.1 Allowable losses for standard kva ratings S.No Losses at Rated Voltage, Frequency at 75 C Voltage Ratio Rating (kva) Maximum Losses at 50% Loading (Watts) Maximum Losses at 100% Loading (Watts) / / / / / / / / / / / / / / / / / / / / / / /

15 Sample Distribution Transformer with Medium loads (Urban DT- ML) The schematic diagram with low voltage distribution album is shown in Figure Energy Measurement of Urban DT- ML (Distribution Simulation Package Output) for one day is shown in Figure 3.11 and load graphs for 4 different days taken at random are shown in Figure 3.12, Figure 3.13, Figure 3.14 and Figure Sample Distribution Transformer with Heavy loads (Urban DT- HL) The schematic diagram with low voltage distribution album is shown in Figure Energy Measurement of Urban DT- HL for one day is shown in Figure 3.17 and load graphs for 4 different days taken at random are shown in Figure 3.18, Figure 3.19, Figure 3.20 and Figure Sample Distribution Transformer with Light loads (Urban DT- LL) The schematic diagram with low voltage distribution album is shown in Figure Energy Measurement of Urban DT- LL for one day is shown in Figure 3.23 and load graphs for 4 different days taken at random are shown in Figure 3.24, Figure 3.25, Figure 3.26 and Figure Sample Distribution Transformer with Distributed loads (Rural DT-DL) The schematic diagram with low voltage distribution album is as shown in Figure Energy Measurement of Urban DT- DL for one day is shown in Figure 3.29 and load graphs for 4 different days taken at random are shown in Figure 3.30, Figure 3.31, Figure 3.32 and Figure 3.33.

16 93 108, , , Figure 3.10 LV Album of Urban DT- ML Measurement of power and energy in low voltage distribution network Voltage Current Active Power Power Factor Total Time R Y B R Y B R Y B R Y B Total Power KWH 0:00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :19: :30: :00: :30: :00: :30: :03: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: Distribution Transformer Cumulative kilo watt hour (kwh) Figure 3.11 Energy Measurement of Urban DT- ML

17 Date Date Figure Load Graphs of Urban DT- ML Figure Load Graphs of Urban DT- ML 94

18 Date Date Figure Load Graphs of Urban DT- ML Figure Load Graphs of Urban DT- ML 95

19 96 Figure 3.16 LV Album of Urban DT- HL Figure 3.17 Energy Measurement of Urban DT- HL

20 Date Date Figure Load Graphs of Urban DT- HL Figure Load Graphs of Urban DT- HL 97

21 Date Date Figure Load Graphs of Urban DT- HL Figure Load Graphs of Urban DT- HL 98

22 99 Figure 3.22 LV Album of Urban DT- LL Figure 3.23 Energy Measurement of Urban DT- LL

23 Figure Load Graphs of Urban DT- LL Figure Load Graphs of Urban DT- LL 100

24 Figure Load Graphs of Urban DT- LL Figure Load Graphs of Urban DT- LL 101

25 102 Figure 3.28 LV Album of Urban DT-DL Measurement of power and energy in low voltage distribution network Date Time Voltage Current Active Power Power Factor Total R Y B R Y B R Y B R Y B Power kwh :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :03: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: :00: :30: Distribution Transformer Cumulative kilo watt hour (kwh) Figure 3.29 Energy Measurement of Urban DT- DL

26 Date Date Figure 3.30 Load Graphs of Urban DT- DL Figure 3.31 Load Graphs of Urban DT- DL 103

27 Date Date Figure 3.32 Load Graphs of Urban DT- DL Figure 3.33 Load Graphs of Urban DT- DL 104

28 Load Graphs of Distribution Transformers selected at random Five numbers distribution transformers with different load patterns have been selected from different parts of the country, India. The set of energy meter readings of these transformers are obtained from Central Power Research Institute, Bangalore, India for study purpose. The set of load graphs is shown in Figure 3.34, Figure 3.35, Figure 3.36, Figure 3.37 and Figure The graphs are drawn from energy meter readings of selected distribution transformers to study the stochastic nature of load and to substantiate the deductions derived from distribution transformers already discussed. In the distribution transformers selected, 4 numbers are in unbalanced condition and one number in balanced condition. In unbalanced distribution transformers unbalance exists throughout the day for any selected day (Figure 3.34, Figure 3.35, Figure 3.36, and Figure 3.37). Similarly once the loads are balanced all three phases remain balanced throughout the day as seen from the graph (Figure 3.38). Load shedding is the period of time when distribution transformer is not energized. The feeder feeding that distribution transformer is switched off to match demand and supply in the case of demand exceeding supply. It can be verified from the load graph whenever there is load shedding after that the current in the phases of transformer increases as contrast to normal period.

29 Figure 3.34 Load Graphs of DT-Random1 106

30 Figure 3.34 (continued) 107

31 Figure (continued) 108

32 Figure 3.35 Load Graphs of DT-Random2 109

33 Figure 3.35 (continued) 110

34 Figure 3.35 (continued) 111

35 Figure 3.36 Load Graphs of DT-Random3 112

36 Figure 3.37 Load Graphs of DT-Random4 113

37 Figure 3.37 Load Graphs of DT-Random4 114

38 Figure 3.38 Load Graphs of DT-Random5 115

39 Figure 3.38 (continued) 116

40 117 Figure 3.38 (continued) 3.5 LOAD ANALYSIS INFERENCES The detailed analysis of distribution transformers with different types of loading patterns leads to very interesting findings. Such data are not available in scientific way for analysis across the country, India. Therefore, utmost care has been taken to collect the data from different places like metro, urban and rural populated regions. There are at least data from 100 transformers analyzed before presenting the inferences. Out of 100 transformers, 9 transformers have been presented in this work. The graphs displayed from random samples indicate few exceptions which are also analyzed Significant Inferences Though the load utilization of individual consumers is a variable factor, there is uniformity found in stochastic nature. The per-capita consumption of electricity is high in urban compared to rural areas.

41 118 The peaks and valleys in load graphs tend to follow similarity though not identical in all types of distribution network. The high peak occurs in approximately same band of one hour every day, the major variation may be two hours which is also very rare. The peak load of the transformer occurs when majority of the consumers connected to distribution network utilizes most of the loads. It occurs in the same time band of peak load of the distribution transformer due to prevailing culture and habit of the people. The percentage of unbalance between phases is observed to be proportionate and hence value of unbalance will be maximum during peak loads. If the transformer is in balanced condition it remains balanced throughout the day as seen from load graph of DT-Random5 shown in Figure Exceptions Analysis By analyzing load graph of DT-Random1, after load shedding, a peak more than the regular graph is noticed. It indicates if LV network is unbalanced, the effect of unbalance is more after load shedding. By analyzing load graph of DT-Random2, for few days, it is slightly different from other load graphs of the same distribution transformer. On analysis it is found that for the same distribution transformer the load pattern was little different during Sundays and local holidays compared to week days. It has been observed that the distribution transformer caters the commercial loads in

42 119 all three phases, mostly shops and hence load reduced on Sunday during day time. By analyzing the load graphs of DT-Random3 it was found that there was no exception and it was uniform in all days. By analyzing the load graph of DT-Random3 which is typically catering the consumers of rural area further, it has been observed that phase-y is used only for approximately 7 hours split into two bands of 4hrs 30mts and 2hrs 30mts. On analyzing the consumer loads of that area, it is composed largely of cottage units, making of appalams, a snack deep fried or roasted, appalam is made from rice or black gram flour. The entire process of making of appalams is manual. Once made by the women at home, it has now grown into an industry. Appalam makers consider a sunny and windless day most suitable for making the product. They make appalam during morning hours, put them in the sun during afternoon and pack them in the evening. Hence there appears two slots of energy usage in such industries. By analyzing the load graph of DT-Random4 it is seen that it has morning peak and occurrence of morning peak is almost in the same slot everyday. The unbalance is the maximum during peaks. Typical loads on low voltage networks are stochastic by nature. However it has been ensured that there is similarity in stochastic nature throughout the day as seen from load graphs of distribution transformers of different load patterns. It has been verified that if R phase is overloaded followed by Y phase and then the B phase, the same load pattern continues throughout the day. Also peak load band approximately occurs within the

43 120 same time band in different days of the same transformer. All the inferences made out of low voltage distribution network load analysis prove that optimization of low voltage distribution network can be achieved by successful reconfiguration of consumers by off-line methodologies in costeffective manner. This distribution package serves as a backbone for the research study.