Gujarat Energy Transmission Company Limited (GETCO) is one of the most progressive state transmission utilities in the country in terms of technology deployment. Over the years, GETCO has added significant line length and increased its number of substations, which stand at 61,056 ckt. km and 1,869 respectively, as of March 2018. However, it faces challenges in system operations owing to the increasing integration of renewable energy in the state grid. Further, the rapid addition of thermal generation capacity, long distance power flows, multiple players and increasing customer expectations increase the complexity of system operations. In order to mitigate these challenges, GETCO is implementing wide area monitoring systems (WAMS) at its state load despatch centre (SLDC). In recent times, WAMS has emerged as an efficient solution for addressing reliability and operational concerns in power supply and generation. It enhances real-time power transfer capabilities, enables automatic corrective actions like adaptive islanding, allows better visualisation through state measurements and provides decision support tools, etc.
The basic infrastructure of WAMS technology comprises phasor measurement units (PMUs), wideband communication network and phasor data concentrator (PDC) units. As part of WAMS implementation, PMUs are dispersed throughout the electricity grid and placed at strategic locations in order to cover the diverse footprint of the grid. The PDC unit is installed at the central location. It collects the information from PMUs and sends out alerts and alarms in emergency situations. It facilitates the development of different types of analytics for smooth grid operation. The PMU data is also transmitted to the supervisory control and data acquisition (SCADA) system.
WAMS technology requires a high bandwidth communication network for rapid data transfer, which should match the frequency of sampling of the PMU data. The communication infrastructure is a critical backbone in the WAMS architecture. The PMU devices are then connected to one or many control centres over the communication network.
WAMS in Gujarat
Under WAMS Phase I, 113 PMUs have been installed in the state at 25 strategic locations. Each PMU has five analoge channels for measurement. The PMUs have been installed at 10 400 kV substations, 13 220 kV substations, two generating substations at Adani Power’s Mundra thermal power projects, and at the Wankabori Thermal Power Station (WTPS). In total, there are 452 lines and transformers being monitored through PMUs.
With regard to technical specifications, PMUs in Gujarat belong to P (Performance) class and are compatible with the IEEE C37.118 – 2005 and 2011 standard. The reporting rate of PMUs could be 25 or 50 samples per second. At present, it is selected at 25 samples per second and timestamped using GPS. Currently, the phasor capacity is of 1,500, which is further extendable up to 3,000. Its data storage capability is triggered every 30 days and the data is stored in the historian server for one year so that the utility can analyse the data up till one year. The PMUs measure bus voltage, line current at an angular position, frequency, and breaker and isolator status.
Management of the huge quantum of data collected by PMUs requires sophisticated analytics at the SLDC such as online oscillation mode (OscM) identification, hybrid state estimation (HSE) and dynamic security assessment (DSA). At the Gujarat SLDC, OscM has already been commissioned, HSE is installed but is currently at the development stage and DSA is still undergoing prototype development. In OscM, the interarea mode range is 0.2-0.5 Hz, the local mode range is 0.8-1.8 Hz and the interplant mode range is 1.5-3 Hz.
The HSE rate is determined by SCADA’s sampling rate. There are alarms and logs on limit violations and the results can be visualised through software. If the data is not received from a particular PMU, state estimation proceeds with the available measurements.
WAMS applications
At the Gujarat SLDC, the key functions of WAMS include enhancing situational awareness, forensic analysis of faults or grid incidents, and analysis of oscillations in the power system.
Enhancing situational awareness involves visualising the magnitude and angle of all three voltages/current phases, as well as the frequency, rate of change of frequency, and the angular separation between a pair of nodes. Forensic analysis involves the detection of grid events within a region, their sequence and the type and time of fault; fault clearance; voltage recovery post fault clearance; possible protection operation; and single phase auto reclosing in extra high voltage lines; etc. The analysis of oscillations in the power system includes the detection of time, duration, amplitude, the frequency and type of oscillations (inter area or local), the nature of oscillations (damped or undamped), and the amplitude and damping factor. Further, the PMU data is used for condition monitoring of equipment, protection of equipment, quick decision-making with regard to system restoration, etc.
Conclusion
WAMS implementation has immensely benefited GETCO as it helps compute the available transmission capacity on the basis of accurate real-time measurements instead of the coarser measurements or simulation methods used earlier. This increases the effective capacity of congested corridors, and increases transmission asset utilisation. WAMS has not only helped in detecting the issues, but also in identifying the root cause of the problems. It assists in taking preventive and corrective actions. With PMUs, it has become possible to accurately predict grid conditions through developed analytics.
GETCO is also implementing the second phase of the WAMS project. It is planning to install another 182 PMUs at 62 locations. It is also planning to cover all generating stations, 440 kV substations, interface points, strategic 220 kV substations, major load pockets and renewable energy stations, under the second phase of the project.
(Based on inputs from a presentation by D.P. Parmar, JE, SCADA, GETCO)