The integration of 175 GW of renewable energy by 2022 requires grid balancing support. One of the key options to address renewable variability is grid-connected battery energy storage systems (BESS). To examine the efficacy of BESS in India, Power Grid Corporation of India Limited (Powergrid) launched a 1 MW pilot project with 500 kWh battery technologies and an associated battery management system, in Puducherry. The storage system is being used for frequency regulation and energy time-shift applications. A summary of the pilot project and its outcomes…
Objective
Powergrid implemented the BESS project with the following objectives:
- Prepare proof of concept for BESS frequency regulation and energy time-shift applications
- Obtain first-hand experience of different battery technologies and battery management systems before large-scale deployment
- Seek policy/regulatory advocacy for deployment of such technologies
Results
Powergrid commissioned the BESS project based on both advanced lead acid and lithium ion in April 2017. The BESS system has since been operating in grid-connected mode. The application mode that is, frequency regulation/ energy time shift can be changed as and when required. Technical analysis is carried out on several aspects of the storage system and the experience is regularly shared by Powergrid at technical conferences and meetings. Some of the key results include 10 per cent of peak load shaving through the energy time shift application of BESS. The AC-to-AC efficiency of lithium iron phosphate and advanced lead acid BESS is 80-85 per cent and 70-75 per cent respectively. The normalised root mean square error between PCS command and power output of BESS is 14 per cent and 18 per cent for lithium iron phosphate and advanced lead acid batteries respectively.
Lessons learnt
Every battery technology has a different rated depth of discharge for specified cycles. To bring all the technologies on a single platform during tendering, the useful capacity was defined. During operation, each cycle depth is different. Hence, evaluating the cycle life of batteries was difficult.
In the frequency regulation application of BESS, it was observed that the BESS fluctuates around 50 Hz frequency between charging and discharging modes. To resolve the problem, a dead band was introduced in the control algorithm of the BESS. This reduced the frequent charge-discharge of the battery considerably. Also, the distribution of frequency of the Indian grid is not symmetrical on either side of 50 Hz. It was observed that the BESS remained idle for a considerable amount of time because of this. Thus, the control algorithm of BESS was changed appropriately to reduce the idle time of BESS. Further, it was noted that the lithium iron phosphate battery is more energy dense than the advanced lead acid battery. A lead acid has 1.5 times greater footprint than a lithium iron phosphate BESS of the same size. Further, BESS provides a very quick response rate to changing grid frequency signals.
There is no limitation in terms of ramp rate for providing grid support in both advanced lead acid and lithium iron phosphate BESS. Also, being a tropical country, air-conditioning requirements are high. Overall, the efficiency is affected due to auxiliary power consumption (mainly for air-conditioning) by about 5 per cent.
Conclusion
Increasingly, digital technologies are being employed to better track and dynamically manage electricity production, transmission, distribution and use – a small part of the broader societal trend towards digitalisation. Energy storage systems are an emerging source of power system flexibility, and are likely to play a pivotal role in next-generation electric grids, acting as a flexible bridge between the needs of utilities (and other energy service providers) and their customers.
Based on a casebook prepared by the International Smart Grid Action Network on energy storage systems