Report Shows How Best To Use Solar, Batteries To Offset EV Impact On Transformers

A new study funded by a Demonstration of Energy & Efficiency Developments (DEED) scholarship from the American Public Power Association analyzes optimal strategies that can mitigate transformer loss of life resulting from a high penetration of electric vehicles (EVs).

The premise of the study, PV Generation, EV & Stationary Battery Optimal Control & Management in Distribution Grids, was that the accelerating adoption of electric vehicles could cause the overloading of some transformers, leading to premature ageing and potentially failure and added expense for utilities and their customers.

The study found the optimized use of a residential battery energy storage system (BESS) with or without photovoltaic (PV) solar panels in areas with a high penetration of electric vehicles can mitigate transformer loss of life. In the scenario that included PV solar, however, the high price of a solar installation made the economic impact of a solar installation uneconomic without subsidies.

The study was accepted for presentation at the 53rd North American Power Symposium next month.

The study, by Milad Soleimani, a research assistant and Ph.D. candidate with the department of electrical and computer engineering at Texas A&M University in College Station, looked at seven use-case scenarios.

To evaluate each scenario, a one-year study was conducted using the data available for 2018 in Bryan/College Station, Texas. Bryan Texas Utilities was the sponsor of the DEED grant.

The study looked at 12 electric vehicles and 10 charging slots. The nominal power of the transformer connected to the buildings was 63 kilovolts (kVA) and the total PV generation capacity of each building was 10 kilowatts (kW). The rated power of the BESS inverter was 5 kW. It was assumed that the electric vehicles charger was unidirectional, and the electric vehicle could only be charged. Nissan Leafs and Chevy Bolts were used in the study.

• The first scenario, a, was a base case in which there were no electric vehicles, no solar power and no BESS;
• The second case, b, had electric vehicles but no solar power and no BESS;
• The third case, c, had electric vehicles, solar PV, but no BESS;
• The fourth case, d, had electric vehicles, BESS but no solar power, and the batteries were charging schedule was optimized using only real time prices of wholesale power;
• The fifth case, e, had electric vehicles, no solar power and BESS whose charging schedule was optimized using both real time prices and calculations to account for stresses that can shorten a transformer’s life;
• The sixth case, f, had electric vehicles, solar power and BESS whose charging schedule was optimized using price only; and
• The seventh case, g, had electric vehicles, solar power and BESS whose charging schedule was optimized using price and transformer loss of life calculations.

In the analysis, the results show that the impact of PV solar installations on mitigating transformer loss of life are negligible (scenario three or c). The fourth and sixth (d and f) scenarios show that operating a BESS on price only not only does not mitigate transformer loss of life it can, in some cases, have a negative impact on transformer life.

The result showed, however, that the risk of transformer loss of life can be significantly reduced by using BESS optimized for price and transformer loss of life mitigation, as in scenarios five (e) and seven (g). The study noted, however, that because of the high cost of PV solar, combining solar power with BESS, even when optimized, may not be economic without subsidies.

In the cases where the technologies provide a good return, utilities benefit, therefore, utilities “should be investors,” Soleimani wrote. That can be done, he said, by “developing and providing rebates and other incentive programs to motivate consumers to deploy these resources.”

The study concludes that optimizing BESS for both price and transformer loss of life mitigation is “essential for making the investment in BESS profitable.”

Other key findings of the study were that the proper sizing of a BESS is essential to making the system more profitable and the impact of PV solar generation without BESS support is relatively low.