SCADA for Power Factor Correction in Distribution Systems

Transcription

1 ISSN No: SCADA for Power Factor Correction in Distribution Systems K.Somasundaram, M.Thiyagarajan, S.Sriram* Department of EEE, SASURIE College of Engineering, Vijayamangalam, Tiruppur, Tamilnadu, India. *Corresponding Author: S.Sriram Received: 22/10/2015, Revised: 18/12/2015 and Accepted: 15/04/2016 Abstract There are two kinds of energy losses, resistive and reactive, in transmission lines and transformers. The resistive component of the load can not avoided but the reactive component can be minimised. When a heavy inductive load is connected to the power grid, a large positive reactive power component is added. Increase in phase lagging causes high energy loss in the distribution network. Capacitors are widely used to minimize the power loss and to minimize the voltage flicker in the distribution networks. SCADA with fuzzy logic is used for acquiring online data, controlling processes, and monitoring from remote locations. LABVIEW software is used in this system which has three major functions. First, it allows the user to acquire RTU measured parameters namely, line voltage, load current and reactive power. Second, it calculates the required capacitors to be added to the line for achieving unity power factor using fuzzy control logic and send commands to the RTU to activate the capacitor bank Ishitv Technologies LLP All rights reserved. Keywords: PFC,SCADA 1. Introduction Energy losses in transmission lines and transformers are of two kinds: resistive and reactive. The former is caused by resistive component of the load and cannot be avoided. The latter, coming from reactive component of the load, can be avoided. Reactive losses come from circuit capacitance (negative), and circuit inductance (positive). When a heavy inductive load is connected to the power grid, a large positive reactive power component is added, thereby increasing observed power load. Increase in phase lagging will cause more energy loss in the distribution network. Capacitors are widely used in distribution system to reduce power loss to maintain the voltage profile within permissible limits and to minimize the voltage flicker in power distribution networks. The extent of these benefits depends on the location, size, type and number of capacitors and the fast method of capacitor insertion. 2.SCADA 225

2 The fast method of total power loss calculation can be manipulated using Supervisory Control and Data Acquisition System (SCADA) with personal computer. SCADA, is a system for acquiring online data, controlling processes, and monitoring from remote locations. Now-a-days many companies are implementing SCADA architecture through communication network to monitor important infrastructure components such as power plants, oil and gas pipelines, chemical refineries, flood control dams, waste water systems and vital systems.tennesse Valley Authority has implemented SCADA in various hydro electric generation facilities to reduce operating costs by consolidating the control of multiple plants at one location. TVA said that this type of control still required the attention of an operator and the response time is high. Advanced Plant Management System developed by a company is a supervisory real-time control system widely used in the electrical power generation industry discussed plant start-up management, load and gas predictor, alarm analysis, remote vibration monitoring, emission monitoring and reporting. Minorinishi, et al discussed SCADA for Okutataragi Pumped Storage Power Station which is used to find a fault tree based operation guidance function, a bearing temperature function and a vibrating monitoring function. The above mentioned papers do not discuss fuzzy based SCADA for reducing energy loss in power distribution systems. In this paper fuzzy logic is used to control power loss in distribution systems. Data acquisition operation includes measurements of voltages, load currents, power factors, reactive power and estimation of power loss. Electronic circuit based on operational amplifier and PIC microcontroller have been designed and built to do all the measurements and to insert the required shunt capacitors at the right time and allocation. To find the optimal solution of the complicated combinational problem, the fuzzy logic is used which imitates the process of natural evolution. This tool proved to be very successful for capacitor evaluation and insertion. A special feature of the fuzzy set approach is the capability of handling the uncertain and linguistic terms such as heavy load to describe the system loading condition. It acts as a decision maker to reduce the power loss. 2.1 Inductive Circuits Most industrial loads are inductive in nature, e.g. motors and transformers. Due to the inductive reactance of the load, the current drawn by the load lags behind the voltage waveform electrically by angle. The magnitude of ф is proportional to the inductive reactance. Since the current lags behind the voltage, inductive loads are said to have a lagging power factor. Some examples of inductive motors are air conditioning units, refrigerators, freezers, washers, dryers, dishwashers, pool pumps, vacuum cleaners, furnace blower motors, fans, etc. 2.2 Power Factor The power factor depends on the electrical loads type; it is usually that the current lagging voltage and the angle of lagging are varied as the load varied and the daily time elapse. Increasing the lagging will cause more energy loss in the distribution network. This problem has been overcome by inserting shunt capacitors in the distribution network which is essentially reduces the current drawn from the network. 2.3 Capacitors Corrected Power Factor The fundamental function of shunt capacitors is to regulate the voltage and reactive power flows at the point where they are inserted, in other words they are improving the power factor of the load. In typical distribution 226

3 loads, the current lags the voltage; the cosine of the angle between current and voltage is known as the power factor of the circuit. Capacitors draw leading reactive power from the source; i.e. they supply lagging reactive power to the load. Capacitors are characterized by leading kvar in the phase diagram or power triangle. This is opposite to the inductive kvar which is explained in Figure 1. Fig.1: Phase diagram for power factor correction The angle ф is the phase angle between the voltage and current waveforms. The reactive power is defined by Q= (S 2 -P 2 ) A capacitor of Q kvar will compensate for the inductive kvar and produce cos(ф) =1. It is not common practice to produce cos(ф) = 1 with capacitors because this may result in overcompensation due to load changes and the response time of the controller. Generally public utilities specify a value (cos (ф 2 )) to which the existing power factor (cos (ф 1 )) should be corrected. The reactive power to be compensated is determined as follows. Q N = P ( tanø 1 tanø 2 ) Connection and rating of capacitors A general expression for the kvar rating of a capacitor (single-phase connection) is: Q N =U N I N Q N =U N (U N /X N ) X N =1/(w C N ) = 1/(2π f N C N ) Q N = U 2 N. ω C N = U 2 N 2π f N C N Capacitor in single-phase application The capacitor is connected across the phase and neutral and is subjected to the phase voltage. The above equation, without any change, is applicable to such capacitors. Capacitor in three-phase application Star connection The partial capacitor is subjected to a voltage of U N / 3. Thus total kvar compensation of all three partial capacitors [Figure 2]: 227

4 Fig.2: Star Connection Delta connection The capacitor is subjected to line voltage U N, phase to phase [Figure 3]. Thus total kvar compensation: Fig. 3: Delta Connection From the above equations it follows that for the desired Q kvar: C DELTA = C STAR / 3 Thus for the same amount of kvar compensation a star connection requires the triple capacitance of a delta connection. On the other hand, for the same nominal voltage U N in delta connection a 3 thicker dielectric film is required to get similar values of electric field strength The present SCADA system is based on microcontroller (PIC 16F877A) which comprises three modules: 1.Control Unit or Remote Terminal Unit (RTU), 2. Monitoring Unit and 3.Communication Network. (1) CONTROL UNIT The main functions of the control unit are measurement of voltage, current and phase lag between load current and line voltage. It is a closed loop control system, which is designed using the microcontroller. 228

5 The system control unit [Figure 4] comprises of the following: (1) Central processing unit and (ii) Input/output unit. MEMOR Y INPUTPORT ADC CLOC K CPU PPI OUTPUTPORT SSR Fig. 4: BLOCK DIAGRAM OF CONTROL UNIT (i) Central Processing Unit The CPU comprises a microcontroller and additional components like memory, input/output modules and clock drivers. The main work of the CPU is to execute the program stored in the memory and generate suitable signals to their input and output ports. At the beginning, shunt capacitors are applied to the substation bus for correcting to an initial power factor, for example, 90 percent. Then, a load flow run is performed to determine the total system Kilovoltamperes and kilowatt losses at this level and capacitors kilovars are noted.these signals invoke the measurement of load current, line voltage and the angle between current and voltage waveform. Depending upon the phase difference in the measured data which cause the power losses, the required capacitors added to the line is calculated. By applying suitable signal to the output port additional capacitors are applied to substation bus under test to increase the power factor and another load flow run is made. This process of iteration is applied until the power factor becomes unity. (ii) Input/output Section In order to measure line Voltage and load current in each phase, a current and voltage transformer with rectifier units are used to convert the actual signal into DC voltage [Figure 5]. These voltages are measured with ADC. The phase difference is measured with simple operational amplifier in comparator mode. The output of the comparator is in the form of pulse train. The width of the pulse is equal to phase difference which is measured with one of the I/O line. Similarly for other two phases, line voltage, load current and phase differences are measured continuously. Fig. 5A Circuit Diagram for Input Section I/O CPU ADC Voltage Fig 5B Block Diagram for Input Section The output circuits which consist of set of power transistors (NO 055) are controlled by the output signal of the MUX Phase Angle Current 229

6 control unit to drive the solid state relays which insert or disconnect the shunt capacitors to make the power factor unity[figure 6]. The solid state relay provides complete isolation between the line voltage and the control circuit Fuzzy Model Description A fuzzy-based decision maker (FDM) has been used to compute the required shunt capacitor to improve the power factor according to the measured power factor (A) and measured reactive power (B), as shown in Fig. 7. The fuzzy controller consists of 35 rules based on 7 fuzzy regions. The measured variables are inverted into suitable linguistic variables. The following linguistic variables are used for measured power factor (X); Very Small (VS), Small (Sb), Medium (M), Large (L) and Very Large (VL). The fuzzy sets definition for measured reactive power (Y) and calculated output variable (Z) are; Very Small (VS), Small (S), Rather Small (RS), Medium (M), Rather Large (RL), Large (L) and Very Large (VL). Figures 8, 9, and 10 shows the membership functions of the input variable measured power factor, the measured reactive power and output variable of the capacitor respectively, under their initial conditions before applying the Simple Tuning algorithm. Figure 8 shows the universe of discourse and fuzzy sets of these variables. Fuzzy sets can have a variety of shapes; however, a triangular or a trapezoid can often provide an adequate representation of the knowledge. Fuzzy systems allow to cope with uncertainties due measurement in realistic cases, using techniques inspired in human reasoning and they allow introducing explicit human knowledge, but their construction need expert knowledge to be transformed into rules. System Software The system software of the automatic power factor correction system is used to monitor the status of the power loss in the distribution system in the control room. This software developed in LABVIEW has three major functions[figure 11]. First, it allows the user to acquire RTU measured parameters namely, line voltage, load current and reactive power. Second, it calculates the required capacitors to be added to the line for achieving unity power factor using fuzzy control logic and send commands to the RTU to activate the capacitor bank. Finally, it helps to diagnose the problem of the SCADA system. 3.Conclusion Now-a-days there are many industrial sectors where SCADA has been deployed to enable better utilization of human assets, as well as better management of capital assets. Monitoring power factor in sub-station is just one of those areas but has presented some significant technical challenges. The convergence of low power consumption microcontroller based electronics with affordable telecommunications has enabled these challenges to be overcome. This is improving the management of Power distribution systems worldwide. References: [1] Jianguo, Zhou, et al. "Load balancing and reactive power compensation based on capacitor banks shunt compensation in low voltage distribution networks."control Conference (CCC), st Chinese. IEEE, [2] Mujawar, Irfan I., et al. "An Innovative TCR Compensator for Closed Loop Reactive Power Compensation of Dynamic Loads." Changes 2.1 (2014). [3] Tinggren, R., et al. "Power factor controller-an integrated power quality device."transmission and Distribution Conference, 1999 IEEE. Vol. 2. IEEE, [4] Lippincott, Colin. "Secure wireless data communications for distribution automation in the Smart Grid."Innovative Smart Grid Technologies (ISGT), 2012 IEEE PES. IEEE,

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