Keywords PMU；; WAMS wide-area phasor measurement: applied technology
1, preface
The synchronous phasor measuring device PMU was first proposed in 1980. With the application of GPS in civil field, PMU came into being, and several major power grid accidents around the world promoted the application of PMU and WAMS (Wide Area Measurement System) synchronous phasor measurement technology based on PMU in the system. According to the accurate phasor data provided by PMU, engineers and technicians can determine the sequence of a series of events of system failure, and determine the cause and point of system failure. The field test and research results show that WAMW technology has broad application prospects in power system stability prediction and control, state estimation and dynamic monitoring, relay protection, model verification, fault location and so on.
2. Main technical problems of 2.PMU
PMU requires that the synchronization error should not exceed 1? S, the phasor amplitude error is less than 0.2%, the angle error is only 0.2 degrees, the frequency measurement is 45-55hz, the error is less than 0.005Hz, and the data can be continuously recorded for 14 days, and the fastest is 100Hz. When the GPS signal is lost, it can automatically keep the time. If the GPS lock is lost for 60 minutes, the error will not exceed 55? S. transmitting dynamic data according to standard protocol. The main technical problems of PMU include synchronous acquisition and phasor calculation.
2. 1 synchronous acquisition
The structure of a typical PMU is shown in Figure 2- 1. The basic principle is that the phase-locked oscillator divides the 1pps signal given by the GPS receiver into a series of pulses for sampling, and the AC signal is quantized by the A/D analog-to-digital converter after filtering, and then the discrete Fourier transform is carried out by the microprocessor to calculate the phasor. Microprocessor can also use symmetrical component method to calculate positive sequence phasor. PUM device assembly timestamp, positive sequence phasor, etc. Convert them into messages according to specific standards and transmit them to a remote data concentrator. Collect PMU information from each data concentrator to provide data for the protection, control and monitoring of the whole system.
2.2 phasor calculation
Phasor measurement algorithms mainly include discrete Fourier transform (DFT) and zero-crossing detection.
2.2. 1 zero crossing detection method
The zero-crossing detection method only needs to compare the zero-crossing time of the measured power frequency signal with a certain standard time, and the phase angle difference can be obtained, which is a relatively intuitive synchronous phasor measurement method. For 50Hz power frequency signal, the voltage phase angle difference between substation and reference station is that if the phase of each cycle can be compared and the real-time measurement of phase angle can be improved, it is necessary to establish a standard 50Hz signal based on the accurate crystal oscillator clock in WAMS CPU, and the CPU will stamp it at the time when the voltage crosses zero, and then calculate the phase angle difference of each node voltage relative to the standard 50Hz signal. The zero-crossing detection method is simple in principle and easy to realize, but it is easily influenced by harmonics, noise and aperiodic components, with low accuracy and poor real-time performance, and needs to be combined with other technical means.
2.2.2 Discrete Fourier Transform Method
DFT is one of the most widely used algorithms in phasor calculation of power system. DFT has filtering function, which can accurately calculate the DC component, fundamental component and harmonic component in the signal, and the calculation accuracy is not affected by DC component and harmonic component.
N is the number of sampling points in each period, and X is the effective value of phasor, that is, the sampling value. This phasor calculation method can eliminate the influence of the whole harmonic component, but the input signal needs to be low-pass filtered before phasor calculation to prevent frequency domain aliasing.
DFT phasor calculation requires the sampling frequency to be an integer multiple of the fundamental signal period. When the signal frequency is not synchronized with the sampling frequency, the phase of the periodic sampling signal is discontinuous at the beginning and end, which will lead to frequency leakage and calculation error. Fixed interval sampling method and equiangular sampling method can reduce this adverse effect.
2.2.3 Digital differentiation method
The digital differential method uses the characteristics of sine, and after the difference, the signal frequency can be converted into coefficients, and the phasor calculation formula can be obtained after the quotient of time domain variables is reduced. The digital differential method is essentially based on Lagrange interpolation curve fitting method and digital differential method. Digital differentiation method has the advantages of small calculation, high precision and short time consumption, but the algorithm does not have anti-interference ability, which limits its application scope. Digital differential method can suppress the interference of harmonics to a certain extent by choosing appropriate data interval, but it is difficult to obtain good results for random interference and aperiodic components, and filtering is still needed.
2.3 Factors affecting measurement accuracy
The frequency of the system is not fixed. When the signal is a non-power frequency signal, the fixed sampling window is inconsistent with the signal period, so it is necessary to compensate the error of the non-power frequency signal, otherwise the accuracy of the phasor measurement unit will be affected to some extent. Harmonics in the system will also affect the accuracy of PMU measurement. When the phasor calculation method adopts DFT, it can eliminate integer harmonics and play a certain filtering role. In addition, transient distortion will also affect the accuracy of PMU phasor measurement.
3. Application of 3.PMU in power system
1993, American engineers and technicians developed the first PMU device, which marked the practical application of synchronous phasor measurement technology in power system and promoted the popularization and application of synchronous phasor measurement technology to a new stage. With the rapid development of PMU application research and engineering implementation by major power companies and scientific research institutions, synchronous phasor measurement technology based on PUM will have broad application prospects in power system protection, power system control and power system monitoring.
3. 1 dynamic process monitoring and recording
3. 1. 1 power system fault recording
The transmission capacity of early communication channels was low and expensive. At first, almost the only application of PMU was fault recording. At present, fault recording is still the most basic and very important application of PMU. It includes the fault record of conventional protection and the behavior record of the system under disturbance.
3.2 Monitoring, identification and suppression of low-frequency oscillation of the system
The low-frequency oscillation of power system has become one of the most important factors that restrict the transmission capacity of power grid and endanger the safe and stable operation of power grid. WAMS based on PMU can synchronously measure the dynamic process of power system online, and can quickly measure the generator power angle, angular velocity, internal potential, bus voltage and other measurements closely related to the generator electromechanical transient, and transmit the information to the dispatching center in time, which provides an information platform for realizing online analysis of low-frequency oscillation in the whole network. WAMS configures PSS parameters according to the identification results, which effectively suppresses low-frequency oscillation. System operators can know the damping characteristics, oscillation frequency and related units that often occur in the power grid, know the main oscillation problems in the current power grid in advance, adjust the parameters of the control system, arrange the operation mode reasonably, and formulate the correction control scheme in advance.
4. Conclusion
The wide-area phasor synchronous measurement technology based on PMU is still a new thing, which will have broad development prospects in power system, and PMU/WAMS will have greater development space. Intelligent dispatch is the core of smart grid, and wide-area phasor measurement technology is the basis of realizing smart grid and an important means to ensure grid security. Intelligent control is one of the important links of smart grid, and WAMS system based on PMU is the key to ensure intelligent control. In order to realize the grand goal of building a strong smart grid in China, wide area measurement system will be an important part.
refer to
Peng Hai. Research on adaptive protection of power system based on WAN [D]. Master thesis of Southwest Jiaotong University, 2006.
Juck Zhang. Research on synchronous phasor measurement unit PMU. Master thesis of Guangxi University, 2007.
You Yan, Zhang Ye, Sui Huibin, Lu Jinchuan. Wide area protection system based on PMU [J]. Shandong Electric Power Science and Technology, 2005(3).