Electrical power system is not a simple thing, but a complex man made system. It should supply electrical energy to the consumers. In the view of increasing probability for outages due to the system overloads, which are caused by ever increasing demand for electrical power, utilities are examining what modern information technology can contribute to improve the situation. Now-a-days, Wide Area Measurement Systems (WAMS) are being built around the world and will be an important necessary tool for the reliable operation of power systems in future. In order to have continuous monitoring Synchrophasors have become a vital part of WAMS. With the adaptation of recent developments of information technology, Phasor Measurement Units (PMU) are widely used in the day to day operation of contemporary power systems. PMU gives phasor values of voltage and current which are GPS time stamped. Grid monitoring in real-time is essential for ensuring stable operation of the grid. The grid must be completely observable, i.e., the voltages, currents and power flow at each and every bus must be available for complete analysis of the power system. Number of PMUs were installed across the world for different applications such as post fault analysis, state estimation and protection of power system. Since the cost of PMUs are very high, they are placed optimally in the power system such that complete power system is observable.
In power system, different type of harmonics rides on the fundamental sinusoidal waveform, which results in the non-sinusoidal waveform.So, in order to eliminate higher order harmonics, analog signal is passed through the Anti-aliasingfilter (AAF).
With the advancement of technology in both computation and communication, the conventional power grids undertook a major revolution to become smarter power grids.
Unlike conventional power grids, smart power grids are incorporated with synchrophasor measurement units like PMUs, distributed generation system and SPC. SPC is included with Intelligent Energy Management System (IEMS), Main Control Centre (MCC), data servers and Phasor Data Concentrators (PDC). SPC monitors the grid in real time and undertakes necessary control and protection actions through wide area control systems. There are two DFT algorithms to estimate phasor: Recursive DFT algorithm and Non recursive DFT algorithm. Non Recursive DFT algorithm is most frequently used algorithm since it is more stable than the Recursive DFT algorithm.
Consider a sinusoidal signal as given below
where ƒ0 is fundamental frequency, and is sampled at a sampling frequency of Nƒ0. The sampling angle θ = (2π/N), where N is the number of samples per cycle.
The phasor estimate for the consecutive window of samples is given as
Fig. 1. Architecture of Phasor Measurement Unit…
Fig. 2. Connection of PMU to the Grid…
Fig. 3. Revolution in conventional power grid to smart power grid…
Implementation of PMU in Laboratory
PMU can be implemented by different software packages. The following design of PMU is developed using NI LabVIEW software package. The steps involved in the development of PMU using LabVIEW are demonstrated in Figure 5.
The data is stored in .csv (Comma separated value) format. The sample of data being stored is shown in Table I
Fig. 4. Implementation of PMU by using NI LabVIEW…
Fig 5. Development of PMU using LabView…
Synchrophasor Technology vs SCADA
Before the revolution of PMU, SCADA became vital part in the power systems. But SCADA has primarily two major drawbacks namely asynchronous scanning and phase angle calculation. In SCADA, RTUs (Remote Terminal Units) transfer data to master. In other words, master collects data from all the RTUs sequentially, this is called data pooling. Let us consider a case that there are 100 RTUs, master is polling data from 72nd RTU. In this time suppose there is a disturbance or fault near 25th RTU, but master will not poll data. After completion of data polling from all 100 RTUs, master will come back again from RTU 1 and so on. So, the data we are getting from the SCADA is not live data. Hence, in state estimator, operation is not accurate. This drawback is called Asynchronous scanning.In SCADA the phase angle will not be obtained. So, angle has to be calculated separately by using different algorithms proposed in the literature. Therefore, it adds additional delay.
The above two drawbacks can be easily overcome by the PMU because all the PMUs are synchronised to GPS, hence data obtained at PDC is live data. PMUs give phasor values of voltages and currents, so the phase angle calculation delay is also overcome.
Nowadays, most of the power grids are equipped with the digital relays for increasing the reliable and fast protection. The PMU also has to be installed for improving the situational awareness conveyed to the power system operators. Instead of installing PMU and digital relays separately the digital relay can be implemented using PMU and that will lead to decreasing the number of devices in power systems and economics of designing. While doing improvement in monitoring by installing PMU we can enhance the operation of the digital relay also.
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