Optimal Placement Approach of Phasor Measuring Unit by GPS

Transcription

1 Optimal Placement Approach of Phasor Measuring Unit by GPS B. Phani Ranga Raja 1, K. Naresh 2, A. Balaji 3, M. Rambabu 4 Assistant Professor in Usha Rama College of Engineering and Technology Department of Electrical and Electrics Engineering 2 naresh5kelothu@gmail.com, 3 baluurce@gmail.com, 4 ramhey05@gmail.com Abstract The PMU is a power system device, capable of measuring the synchrized voltage and current phasor in a power system this project gives an idea about synchrized phasor measurement (SPM) based Wide Area Mitoring System (WAMS) using Phasor Measurement Unit (PMU) placed at various locatis in electrical power network. They are synchrized by the Global Positiing System (GPS) satellites. A Mat lab based Simulink model of the phasor measurement unit and phasor data ccentrator for data storage and a comm reference time data is also developed in mat lab. Optimal PMU placement in power system network is an important task.synchrized measurements make it possible to directly measure phase angles between correspding phasors in different locatis within the power system A PMU placement strategy is developed and analyzed IEEE- 3 bus test system. Improved informati allows fast and reliable emergency actis which reduces need for relatively transmissi margins required by potential power system disturbances. Index Terms Phasor measurement unit (PMU s),wide area mitoring, integer programming method,state estimati MATLAB/Simulink I. INTRODUCTION Electrical energy is an essential ingredient for the industrial and all-round development of any country. It is a coveted form of energy, because it can be generated centrally in bulk and transmitted ecomically over lg distances. Further it can be adapted easily and efficiently to domestic and industrial applicatis, particularly for lighting purposes and mechanical work. Cventially, electric energy is obtained by cversi from fossil fuels (Coal, Oil, Natural gas), nuclear and hydro sources. Heat energy released by burning fossil fuels or by fissi of nuclear material is cverted to electricity by first cverting heat energy to the mechanical form through a thermo cycle and then cverting mechanical energy through generators to the electrical form. With the ever increasing per capita energy csumpti and expentially rising populati, technologists already see the end of the earth s n-replenish able fuel sources. A coordinated worldwide acti plan is necessary to ensure that energy supply to humanity is assured for a lg time and at low ecomic cost. For this, the power system operator should be able to mitor the power system at every instant of time which helps to properly perform the Energy Management System(EMS)functis like security analysis, voltage stability analysis, optimal load dispatch etc. Phasors are the basic tools of ac circuit analysis usually introduced as a means of representing steady state sinusoidal waveforms of fundamental power frequency. Synchrized Phasor Measurement Units are the instruments that use Global Positiing System (GPS) transmissis to synchrize measurements of positive sequence voltage phasors at network buses and positive sequence current phasors in the lines cnected to those buses. In order to ensure the secure and ecomical operati of the power system, the operators must be able to mitor the practical states of the power system. State Estimati is e of the most important functis in power system mitoring. It provides the platform for advanced security mitoring applicatis such as ctingency analysis and optimal power flow. Traditially State Estimati is accomplished by the State Estimator in the ctrol center based measurements received from Supervisory Ctrol and Data Acquisiti System (SCADA). These measurements are commly provided by Remote Terminal units (RTU) at the Substatis and include real/reactive power flows, power injectis and magnitudes of bus voltages and branch currents. Due to nlinear relati between measurements and state variables, this state estimati becomes n-linear and iterative calculati is required for finding the cvergent point. This process has high computatial burden and sometimes it fails to cverge [4]. From the time the first measurement is taken to the time the state estimati is available, several secds or minutes may have elapsed. Hence the state estimators available in the present day ctrol centers are restricted to steady state applicatis ly. At present many state estimati methods in power system are the static state estimati. Currently with the development of GPS and its applicatis in the power system, the time transferring functi of GPS is used widely in the power system. One of the applicatis, which will be significantly affected by the introducti of, is the State Estimator. The measurement data from the PMU is carried to ctrol centre 2223

2 more quickly than the measurement data from SCADA. So, with the introducti of PMU and its capability for metering bus voltage and branch current phasors with high accuracy, the State Estimati becomes linear and therefore its speed and accuracy will be improved and allows mitoring of dynamic phenomena in the power system. This improvement for power system mitoring opens the way to Wide Area Mitoring System (WAMS). The first step for state estimati is gathering measurement data from substatis. These measurements must be sufficient so as to make system observable and the state estimati could be performed. If all buses of a system are placed with the system is completely observable and do not need any more calculati. Aiming at the factor of Price, Technology and Communicati ability the PMU cannot be equipped at all buses in the system. Moreover as a csequence of Ohm s law, When a PMU is placed at a bus, neighboring bus also becomes observable with a lesser number of than the number of buses. The accuracy of State Estimati solutis is dependent the quality of data as well as PMU placement cfigurati and redundancy. II. HISTORIC OF PMU S The phase voltage phasors of the power network buses are always of particular importance to the power system engineers. It is known that active power flow in a power supply line is almost proportial to the sine of the angle between voltages at the two terminals of the line. Because many of the planning and operatial csideratis in a power network are directly involved with the flow of active power, the measurement of the angle differences between the transmissis is of importance for many years. These systems LORAN-C, GO satellite cnectis and HBG radio cnectis (in Europe) to find the time synchrizati of the reference at different places in a power system. The next rising zero crossing of a phase voltage was used to make the local phase angle with respect to the reference time. With the difference of the measured angles a comm reference point in two places, set the phase angle between voltages at the two buses. Measurement accuracies achieved in these systems were the order of 40μs. Single-phase voltage angles were measured, and no attempt was made to measure the prevailing voltage phasor magnitude. Also, it was a taken into account the harmics in the waveform. These methods for measuring phase angle differences are not suitable for generalizati for wide-area phasor measurement systems and e-of-a-kind systems that are no lger used. The modern period of phasor measurement technology has its start in research cducted computer relaying of transmissi lines. Early work transmissi line relaying with micro-processor based relays showed that the available computer power in 1970 s was barely sufficient to manage the calculatis needed to perform all the transmissi line relaying functis. A significant porti of the computatis was dedicated to solving six fault loop equatis at each sample time in order to determine if any e of the ten types of faults possible a three phase transmissi line are present. The search for methods which would eliminate the need to solve the six equatis finally yielded a new relaying technique which was based symmetrical compent analysis of line voltages and currents. Using symmetrical compents and certain quantities derived from them are to perform all fault calculatis with a single equati. A new symmetrical compent-based algorithm for protecting a transmissi line was described. As a part of this theory, efficient algorithms for computing symmetrical compents of three-phase voltages and currents were described, and the calculati of positive-sequence voltages and currents using the algorithms gave an motivati for the development of modern phasor measurement systems. It was so recognized that the positive sequence measurement (a part of the symmetrical compent calculati) is of great value in its own right. Positive-sequence voltages of a network cstitute the state vector of a power system, and it is of fundamental importance in all of power system analysis. The Global Positiing System (GPS) was beginning to be fully set up around that time. It became clear that, this system offered the most effective way of synchrizing power system measurements over great distances. The first prototypes of the modern phasor measurement units () using GPS were built at Virginia Tech in early 1980s. The prototype PMU units built at Virginia Tech were deployed at a few substatis of the Bneville Power Administrati, the American Electric Power Service Corporati, and the New York Power Authority. The first commercial manufacture of with Virginia Tech collaborati was started by Macro dyne in At present, a number of manufacturers offer as a commercial product, and deployment of power systems is being carried out in sincere in many countries around the world. Alg with the development of as measurement tools, research was going applicatis of the measurements provided by the. The technology of synchrized phasor measurements has come of period, and most modern power systems around the world are in the process of installing wide-area measurement systemscsisting of the phasor measurement units. III. BLOCK DIAGRAM OF PMU S The PMU manufactured by different manufacturers differ from each other in many important aspects. Therefore, it is difficult to discuss the hardware cfigurati PMU in a manner which is universally applicable. However, it is possible to discuss a generic PMU, which capture the essence of the principal compents. 2224

3 Figure 3 shows the complete block diagram of modern PMU. All elements of the PMU with the excepti of the GPS receiver are to be found in computer relays as well. Ana log inp Anti-alias ing filters GPSre ceiver Phase-locked oscillator A/D Figure 3 Block diagram of Modern PMU. Phasor rmicro-proce ssor MODEM The analog inputs of PMU are currents and voltages obtained from the secdary windings of the current and voltage transformers. All three phase currents and voltages are used to produce the positive-sequence measurement. The current and voltage signals are cverted to voltages with appropriate shunts or instrument transformers (typically within the range of ±10 volts). so that they are matched with the requirements of the analog-to digital cverters. The anti-aliasing filter is used to filter out from the input waveform frequencies above the Nyquist rate. GPS is capable of providing timing signal of the order of 1 micro secd at any locati around the world. The phase locked oscillator cverts the GPS from 1 pulse per secd into a sequence of high-speed timing pulses used in the waveform sampling. Sampling rates have been going up steadily over the years starting with a rate of 12 samples per cycle of the nominal power frequency in the first to as high as 96 or 128 samples per cycle in more modern devices. The microprocessor executes the DFT phasor calculatis. Finally the principle output of the PMU is the time stamped measurement to be transferred over the communicati links through suitable modems. There are some unique properties of PMU s as follows: It provides time synchrized sub-secd data which is used for wide area mitoring. It is directly providing the phase angles at high sub-secd rate. It improves post disturbances assessment capability using high resoluti time synchrized data. IV. OPTIMAL PLACEMENT OF PMU S A PMU is able to measure the voltage phasor of the installed bus and the current phasors of some or all the lines cnected to that bus. The following generalized rules can be used for PMU placement. Rule 1: Assign e voltage measurement to a bus where a PMU is placed, including e current measurement to each branch cnected to the bus itself. Rule 2: Assign e voltage pseudo-measurement to each node reached by an- other equipped with a PMU. Rule 3: Assign e current pseudo-measurement to each branch cnecting two buses where voltages are known. This allows intercnecting observed zes. Rule 4:Assign e current pseudo-measurement to each branch where current can be indirectly calculated by the Kirchhoff current law (KCL).The observability cditis that have to be met for selecting the placement of PMU sets are Cditi 1: For PMU installed at a bus, the bus voltage phasor and the current phasors of all incident branches are known. Cditi 2: If e end voltage phasor and the current phasor of a branch are known, then the voltage phasor at the other end of the branch can be calculated. Cditi 3: If voltage phasors of both ends of a branch are known, then the current phasor of this branch can be directly obtained. Cditi 4: If there is a zero-injecti bus without PMU and the current phasors of the incident branches are all known but e, then the current phasor of the unknown branch can be calculated using KCL. Cditi 5: If the voltage phasor of a zero-injecti bus is unknown and the voltage phasors of all adjacent buses are known, then the voltage phasor of the zero-injecti bus can be obtained through node voltage equatis. Cditi 6: If the voltage phasors of a set of adjacent zero injecti buses are unknown, but the voltage phasors of all the adjacent buses to that set are known, then the voltage phasors of zero injecti buses can be computed by node voltage equatis. The two different procedures that are followed to solve OPP Problem are: 1. Integer Programming based Procedure 2. Graph Theoretic Procedure followed by Topological Observability Analysis An example of an optimally placed set of in a 14-bus system is shown in Figure 4.1 In this system, there are three placed at buses 2, 6 and 9 respectively. Bus 7 is the ly zero injecti bus. The PMU at bus 2 can not ly measure the voltage phasor of bus 2, but also the current phasors of branches 2-1, 2-3, 2-4 and 2-5. Using Ohm s law, the voltage phasors at buses 1, 3, 4 and 5 can be obtained from the branch currents and the voltage at bus 2. Having determined voltage phasors at buses 1, 2, 3, 4, and 5, the current phasors of branches 1-5, 3-4 and 4-5 can be calculated. Following the same logic, PMU at bus 6 can measure the voltage phasor at bus 6 and the current phasors of branches 6-5, 6-11, 6-12 and 6-13, thus allowing the calculati of the voltage phasors at buses 5, 11, 12, 13 and the current phasor of branch PMU at bus 9 can 2225

4 measure the voltage phasor at bus 9 and the current phasors of branches 9-4, 9-7, 9-10, 9-14 and allow the calculati of the voltage phasors at buses 4, 7, 10, 14, and the current phasors of branches 4-7. As voltage phasors of buses 10, 11, 13, 14 are known, current phasors of branches and can now also be calculated. Using the known current phasors of branches 4-7 and 9-7, and the zero injecti at bus 7, the current phasor of branch 7-8 can be derived using the Kirchhoff s Current Law. The ly remaining unknown voltage phasor at bus 8 can now be calculated by using the voltage phasor at bus 7 and the current phasor of branch 7-8. Thus the entire system becomes observable by placing ly three at buses 2, 6, 9 and by csidering the zero injecti at bus 7. IV. SIMULATION MODEL OF PMU S The procedures explained in the last chapter are implemented in MATLAB Software and tested IEEE 14-bus test system. V.OPTIMAL PLACEMENT OF PMU S Integer Programming based Procedure Case 1: A system with no cvential measurements and/or zero injectis. Case 2: A system with some flow measurements and no zero-injectis. Case 3: A system with both flow measurements & zero injectis (a) Forming N-Linear Cstraints (b) Topology Transformati The OPP Problem for the above 3 cases are solved for IEEE 14-bus test system, and the Optimal placement of is given in Table 5.1: Table 5.1 Optimal Placement of for IEEE 14-bus system by Integer Programming Based Procedure. Case 1 Case 2 (a) Case 3 (b) 4 2,6,8,9 4 2,6,8,9 3 2,6,9 Fig 4. Simulink block diagram of PMU s Figure 4 shows a Simulink diagram explaining the procedure of measuring the voltage or current analog signal. The external time source is an absolute time reference from a global positiing system (GPS) receiver, which delivers a phase-locked sampling clock pulse to the Analog-to-Digital cverter system. The sampled data are cverted to a complex number which represents the phasor of the sampled waveform. Phasors of the three phases are combined to produce the positive sequence measurement. The figure includes a hardware low-pass filter (Hardware LPF) for anti-aliasing and ananalog-to-digital (A/D) cverter for analog-to-digital cversi. The system of supervisi permits capturing records of the same event at different points in the power system with a unique time reference, the phasor measurement units at present are located strategically, with the purpose of capturing informati the impact of ctingencies at the local or system level. VII. CONCLUSIONS We can cclude that a Prototype Simulink modelling of PMU is carried out in MATLAB. From this work we are able to achieve the Voltage and Currents measurements in Phasor form with accurate Time stamping. So Limitati of SCADA measurement of accurate time stamped measurements is rectified. By developing optimal PMU Placement strategy with the aim of achieving complete observability of the power system in steady state cditis by using integer programming method. 2226

5 REFERENCES 1. I.J.Nagrath, D.P.Kothari, A Text book Modern Power System Analysis, Tata McGraw-Hill Publishing Company Limited, New Delhi. 2. A.G.Phadke, Synchrized Phasor Measurements in Power Systems, IEEE comput. Applicat. Power vol.6, no.2 Apr 1993 pp Ali Abur, Bei Xu, Optimal Placement of Phasor Measurement Units for State Estimati, Texas A & M University, PSERC Publicati, Oct Allen J. Wood, Bruce F. Wollenberg, Power Generati Operati and Ctrol, Secd Editi, A Wiley-Interscience Publicati. 5. A.G.Phadke, J.S.Thorp, A Text book Synchrized Phasor Measurements and their Applicatis, Springer Publicatis. 6. B.PhaniRanga Raja, B.Ramesh, Synchrized Phasor Measurements Based Power System Dynamic State Estimati Internatial Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.8, pp