Application of phase angle measurement for real time security monitoring of Indian Electric Power System – An experience

The Indian electric power system has expanded manifold and is on a high growth phase. The five synchronous separate geopolitical grids, which developed independently, have been integrated gradually. In August 2006, the 25 GW Northern grid was synchronised to the “North East-East-West” grid to form the North East—East-West grid now designated as the “N-E-W grid” having installed capacity of around 90 GW (peak 60 GW). The 25 GW South India grid (installed capacity of over 36 GW), is presently connected asynchronously to the N-E-W-grid through 4 GW of HVDC links Fig. 1.

The power system in India is operated on a regional basis through its regional control centres that are equipped with conventional SCADA/EMS system. With the above developments, there has been a paradigm shift in power system operation in India. Knowledge of the “neighbouring system” became as important as the knowledge of the host region for real-time security assessment and monitoring of the integrated system. However as the National control centre is yet to be commissioned, the real-time data exchange between the regional control centres is extremely limited. This limitation was overcome through a novel use of phase angle measurement at strategic locations.

In the Indian grids, phase shifters have not been deployed. However, hybrid system of AC and DC interregional ties provides an opportunity to modulate power flow on the AC links. The two HVDC back to-back stations within the NEW grid enables us to measure phase angle between the two AC buses on either side of the HVDC blocks. The AC buses on either side of the HVDC back-to-back blocks are geographically at the same substation but were separated thousands of kilometres electrically after formation of N-E-W grid. Measurements of phase angle difference between these two adjacent (buses) points provide a signature of the system state. The phase angle measured in the manner described above is telemetered to the regional control centres. World over Synchrophasor technology is being considered for such wide area monitoring. This has been exhaustively documented in international literature^[1–8]. Monitoring and assessment of system state using phase angle measurement with the help of SCADA system would not give system behaviour for low frequency oscillations (inter-area oscillations) but it is still a good visualization tool as it captures several quantities to indicate the ‘pulse’ of the system.

The phase angle differences across different nodes are a measure of static stress across the grid and its proximity to instability and therefore can be monitored with respect to predetermined stability threshold limits. These threshold limits are being put based on the past experiences of system separation and off line simulations. The novel approach has improved system visualization and situational awareness of the system operators in the “NEW-grid”. The paper describes the deployment of direct phase angle measurement for monitoring the security of a large grid and would be a precursor to the deployment of Synchrophasor technology in the Indian electric power systems.