Synchrophasor Based Ocillation Detection: A Case Study for Indian Power Grid

The Indian electrical grid is one of the largest & complex networks in the world. Such complex system is subjected to stress or disturbances which manifest in the form of low frequency oscillations. Monitoring of these oscillations is necessary as they can disrupt the system if they are sustained for a longer period of time with significant magnitude. This paper presents the analysis of low frequency oscillation modes using the data from Phasor measurement unit located in Western and northern region of the country. It reports case studies of disturbances that occurred in the Indian power grid. The modes with low frequency oscillations were observed in both Western& northern region of the Indian grid.The Matrix Pencil technique was used to identify the oscillatory modes in system during the occurrence of the event. The importance of identifying critical oscillatory modes to improve the power system operation, has been brought out in this paper. Keywords—Phasor measurement unit, oscillations, power system stability, inter-area oscillations


INTRODUCTION
The Indian electrical grid is one of the largest and complex power grids in the world with an installed capacity of 227 GW [1]. It consists of five regional grids i.e. NR (northern Region), ER (Eastern Region), NER (North-Eastern Region), WR (Western Region) & SR (Southern Region) operating synchronously since December 2013. The operation & control of such complex network is carried out by the hierarchical network of load control centres. National load dispatch center (NLDC), five regional load despatch centres (RLDCs) thirty three state load despatch centres (SLDCs).
The system operation has now become complex due to integration of high capacity transmission lines, renewables etc. leading to several unforeseen stability problems into the system. The decision making time by the operator of such a large grid has to be reduced with the complexity & stabilityproblems especially small signal stability problems due tostress in the system caused by higher loading levels. Thus facilitating the need for advanced monitoring and visualization tools. The technological advent of phasor measurement unit (PMU), has come out to be as tool which provides the grid operator with the real time view of the system. Indian grid has installed a number of PMUs at various locations [2]. PMU provides the time synchronised measurements of voltage and current phasors along with frequency & rateof change of frequency (ROCOF)synchronised with GlobalPositioning System (GPS) satellite [3]. These measurementsare utilised for power system operation & for analysis of eventsin post-despatch scenario [4].Various applications with benefits of PMU are explained in [5]. These PMU measurements canalso be used for Low frequency oscillation (LFO) detection [6].
This paper presents a case study that demonstrates the application of PMU to detect LFOs in the system & actions tobe taken to damp these oscillations.The paper is organised asfollows:Section II reviews the theoretical background on smallsignal stability, LFOs and proposed technique for LFO modedetection Matrix pencil method. Section IIIand IV presents brief description of the events that occurred in the WR and discusses the results of modal analysis. Section V, concludes the paper.

II. THEORETICAL BACKGROUND
A. Small signal stability Small signal disturbances during power system operationmay occur due to several reasons thereby affecting the powersystem. The ability of power system to be in steady state dueto such disturbances is called small signal stability. Transientstability is associated with the ability of power system tomaintain synchronism when subjected to large disturbanceslike line faults, bus fault, generator trip etc. During thesedisturbances the electromagnetic & mechanical torques ofeach synchronous machine need to be maintained. Disturbances can occur in the interconnected power system due to faults, load changesand when these disturbances occur oscillations usually arise in the system. These oscillations are acceptable as long as they decay [7,8]. It is very important to monitor theseoscillations to ensure that no lightly damped oscillatory modes exist in the system asthey threaten the reliable operation of the power system. The small signal instabilityissues need to be addressed since most of the blackouts have been associated with it [9]. The oscillations can be regarded as the characteristic of the system i.e. oscillatoryparameters are dependent on the physical infrastructure. Every power grid is unique inits physical connections thus presence of oscillations vary with the networks. The powersystem oscillations are complex and difficult to analyse.

B. Matrix Pencil method
Matrix pencil method is an efficient approach to fit measured date set with sum of exponentials. This method is just a one step process of finding signal poles directly from the Eigen values of the matrix developed. It directly estimates the parameters for the exponential terms in 1 to an observed measurement [1, 2].
Next SVD of matrix [Y] is calculated which gives: Here Thus The poles of the signal are given by non-zero Eigen values of Once n & poles ( ) are known residues are solved using least square sense.  The measurements at Badrawati indicate sharp fall in the frequency and ROCOF. Three spikes were observed in the measurements with, and the third spike was the severe one indicating tripping of all elements. The ROCOF reached approximately 0.17 Hz/sec when the event occurred. The frequency also started dropping and reached 49.8 Hz within 1 sec.

Fig. 2. Line voltages recorded at Badrawati
The line voltages recorded by PMU placed at Bhadrawati are shown in Fig. 2. The disturbance at Parli is indicated by sharp pulses.It can observed that after the event occurred at Parli, line voltages reduced.   The results of modal analysis using Prony method have been tabulated in Table-1. The voltage measurements at recorded by PMU at Raipur in WR and Agra in NR have been used for analysis. It can be observed from the results that the modes with frequencies of 0.6 Hz, 0.8 Hz, 1.5 Hz and 2.7 Hz, were identified with negative and close to zero damping.
The 0.6 Hz oscillatory mode was estimated to have negative damping both in Raipur as well as Agra. The 0.8 Hz, 1.5Hz and 2.7 Hz modes were observed to have close to zero damping but comparatively lower amplitudes as compared to the 0.6 Hz mode. Since the 0.6 Hz inter-area oscillatory mode has negative damping and higher amplitude is quantified as the critical mode in the system. The amplitude of all the oscillatory modes at Raipur are more than those observed at Agra. This is because Agra is located far from the place where the event occurred (i.e. Parli).   The line currents measurements for Raipur-Bhadrawati line are shown in Fig.8. This recorded data also indicates the event occurrence with the sharp pulses. It can also be observed that with the event occurrence power flow on this line increased which lead to decreased line voltages at Raipur thus leading to increased line currents. This can be clearly observed in Fig. 8.
Modal analysis of the power flow measurement on Raipur-Bhadrawati line was carried out using matrix pencil technique in-order to identify the oscillatory modes excited in the system due to the disturbance. The analysis was carried out for two instances Duration 1 (before) and Duration 2 (after) the disturbance in-order study the behavior of the modes.

V. CONCLUSION
Low frequency oscillations (LFOs) are inherent to interconnected power systems. These oscillations need to be stable in order to have secure power system operations. The Indian grid has recently installed a number of PMUs in the system which increase the situational awareness amongst the operators. This paper tries to demonstrate one possible application of PMU measurements in the power system i.e. identification of low frequency oscillations. This paper uses Matrix pencil for identification of LFOs. This paper also indicates that as the location of PMU increases from the event located the amplitude of modal parameters is observed to have reduced. Thus it can be concluded that the PMU placed far away from the event location will indicate lower amplitude for a critical mode observed in the system. Some of the devices in the power system to counteract negative damping include PSS inexcitation system of generator & controls of FACTS devices.The real time actions by system operator include generationre-despatch, load shedding, circuit switching etc. to relievethe stress in the system. This case study has indicated theimportance of PMUs & their location towards identificationof LFOs and its source identification. Extensive research needs to be carried out on theoptimum placement of PMUs to record the events & foranalysis of LFOs in the system.
Ongoing work deals with developing tools for advanced monitoring of the power grid operations. With increase in the number of PMUs placed in the system will lead to increased observability.