Monitoring of Angular Separation in Power system: Part 7 State Estimation (SE) tool in EMS
So far, we have discussed three methods for angular separation monitoring and their limitations are discussed below:
?The next method that grid operators utilize for angular separation monitoring is State estimator (SE) tool available in Energy Management System(EMS).?This method is one of the most widely used methods by system operators across the world for monitoring angular separation during real-time operation.
Real-time operation is carried out based on two kinds of information which are Real-time data of power system from various components (generators, transmission, distribution and others) and analytics of past operational data (from last few minutes to a few years). These data are available to control center based on implemented supervisory control and data acquisition (SCADA) system. SCADA system access power system data of observable/monitored network using Remote terminal unit (RTU) placed at substation/generating plant and other required places. These data are not completely time synchronized (except event data) and are reported to the control center in 1-20 second intervals. The details range from analog values (V, I, P, Q, f), digital values (breaker/isolator status), meteorological data (Temperature, wind speed) etc. Grid operator uses the measured data for monitoring and operating power system using Energy Management System (EMS). EMS is basically an interface between data from SCADA and its interpretation for usage by grid operators.?It uses graphics to make it easier for operators to monitor the power system. However, these data are noise prone and can also be non available, have latency issues. to counter these issues State estimator is used.?Further SCADA data do not provide direct angle measurement due to latency, common reference availability etc.
For further details on SCADA/EMS do check out : https://nptel.ac.in/courses/108106022 (Course on EMS and SCADA at NPTEL)
Now coming back to the discussion on how EMS system can use real-time data for angular separation monitoring. This is possible through the State estimation tool.?State Estimation was introduced by Gauss and Legendre (around 1800). The basic idea was to “fine tune” State Variables (Voltage and Angle at each bus) by minimizing the sum of the residual squares. This is the well-known Least Squares (LS) method. ?State Estimation was applied to Power Systems by Schweppe and Wildes in the late 1960s in a real-time environment (Check vintage papers provided at the end of this article).
To simply understand the fact, we have voltage, active and reactive power measured data from the SCADA system and also network topology and its model parameters (like impedance of transmission lines). If a transmission line impedance is known and both end voltage as well line power flow is known, then it can easily calculate the power angle. Further, by taking one of the nodes in the system can be taken as a slack bus node where the angle will be taken as zero. Based on this node the entire power system angular separation can be calculated.?However, the catch is that many of these measured quantities will have some inherent noise, all data at a time may not be available or some of the data can be bad data, inaccurate network parameters etc. This information is not known to the tool which is performing this task.?Here comes the role of the weighted least square method (popularly used SE algo) through which such issues are handled. The algorithm basically tries to reduce the noise in known parameters and detect any bad quality data and estimate voltages and angles for all nodes.
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Let us not go too deep into how the overall state estimation work as it will result in taking out all the fun. The output of SE is estimated voltages and angles in the system and based on which other parameters can be easily calculated. ?Now with this, system operators will have a real-time angle calculated for the entire system with respect to one reference node. The overall accuracy of SE estimated values is very good if the measured quantities have low errors and observability of the network is highest. It also provides input that which of the real-time data is suspected of having high errors which help operators in getting these validated and corrected. Thus overall, it also helps in the tuning of the SE.
At control center, SE runs at each 1-5 minutes basis for monitoring of power system. Further, the estimated system parameter details can be used as input to load flow analysis, contingency analysis, dynamic security assessment (DSA) tools and economic dispatch etc. due to its high accuracy over the measured system data.?
SE is a vast exploratory area to learn and some of the vintage papers, major reports are shared below in reference. In addition, there has been further development in state estimation techniques with synchrophasor data which also provides angular measurement. This has resulted in linear as well as hybrid SE.
Vintage Papers on State Estimation:
Report on state estimator by POSOCO : "Report on Improvements on Usage of State Estimation in Load Dispatch Centers in India", POSOCO, June 2015. Online: https://posoco.in/download/report-on-state-estimator-public/?wpdmdl=531