Voltage Regulation with Rooftop Solar PV in Hawai’i – What are the Impacts to the Utility and PV Customers?

By Julieta Giraldez, Andy Hoke, Earle Ifuku, Reid Ueda and Marc Asano

The combination of renewable portfolio standards (RPS), which require utilities to generate a specified amount of electricity from renewable sources, falling costs of renewable generation, and the financial incentives for residential distributed generation have dramatically increased the penetration of renewable energy across the U.S. Distributed electricity generation in the residential sector, including solar photovoltaic (PV) and wind technologies, increased tenfold between 2010–2015, and this rise is expected to continue in the near future. This has changed the way both transmission and distribution departments in utilities across the country plan for the future needs of the power grid. In an era of increasing distributed energy resources (DER) penetration, a new framework is emerging among utilities seeking to proactively plan for DER integration.

The Hawaiian Electric Companies continue to lead U.S. in the integration of privately owned rooftop solar PV systems, with more than 15% of the total customers having installed such a unit—including an estimated 26% of single-family homes. In areas where distributed PV is concentrated, all the power fed back into the grid can cause voltages to rise (among other challenges). The Hawaiian Electric Companies, as well as other utilities nationwide, have prepared new grid modernization plans to address these issues. A key element of the Companies’ grid modernization strategy is to utilize new technologies—including storage and PV systems with grid-supportive advanced inverters. Such technologies will help to more than triple the amount of additional private rooftop solar PV systems penetration.

The Hawaiian Electric Companies collaborated with the inverter manufacturers, system integrators, and solar industry members of the Smart Inverter Technical Working Group Hawai‘i (SITWG) to partner with the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) to research the implementation of advanced inverter grid support functions (GSF). Using technical guidance and detailed circuit data from the Companies’ engineers, and stakeholder input from the SITWG members, NREL led a study that explored different modes of voltage-regulation GSF. The Voltage Regulation Operating Strategies (VROS) study helped to better understand the tradeoffs of the grid-support benefits and solar energy curtailment impacts from the activation of selected voltage-regulation advanced inverter functions.

The activation of voltage-regulation GSF can provide solar customers with a “non-wire alternative” option to potentially more costly distribution circuit upgrades. The traditional utility interconnection requirements such as IEEE 1547-2003 required inverters to disconnect when the grid is operating outside the prescribed boundaries for voltage and frequency. The recent publication of UL 1741 Supplement SA (September 2016), however, permits the newer “grid supportive” advanced inverters to be certified with the capability to stay online and provide support to the grid.

Whether it is a reactive power function such as a volt-VAR droop, a fixed non-unity power factor, and/or a real power curtailment function such as volt-watt droop, the economic impacts to the customer and the reliability and operational impacts to the utility needed to be better understood. To the customer’s end, the economic impact may be reduced PV system energy production or solar energy curtailment. To the utility side, the reliability and operational impacts can be the increased reactive power absorption and the excessive operation of existing traditional voltage regulation equipment (causing accelerated equipment replacement). However, both the utility and its customers, thanks to GSF, may benefit from improved voltage regulation and power quality, as well as from the ability to interconnect more distributed PV at lower interconnection costs.

In Hawai‘i, reactive power or VAR priority mode is proposed to be implemented for reactive power GSF to ensure that voltage support is available when it is most needed. In this mode, if the inverter capacity is not larger than the DC panel size rating, real power curtailment can occur during peak production times. Typically, PV system designs try to maximize the economic investment by using a higher ratio of DC panel size to AC inverter rating. Whether it is fixed power factor or volt-VAR reactive power support, the inverter size can limit the real power generation during peak producing hours. Additionally, if volt-watt is implemented in combination with fixed power factor or volt-VAR, increased solar energy curtailment could be realized if the customer experiences very high voltages.

The HECO and NREL VROS study found that for the two high penetration feeders selected in the study, volt-VAR is always as effective or more effective than fixed power factor 0.95 absorbing at regulating voltages during PV system production hours. Note that the volt-VAR curve settings in the study can absorb/produce up to 0.44 pu reactive power (corresponding to 0.9 power factor at full output), whereas the default fixed power factor in Hawai‘i is absorbed 0.95 power factor. The study also concluded that because volt-VAR is a voltage-based control, the advanced inverter provides proportional reactive power support only when voltages are within the sloping region of the volt-VAR curve (droop), as opposed to fixed power factor which continuously absorbs reactive power even when VAR support was not needed. Consequently volt-VAR in the study always resulted in:

• Less energy curtailment to the customers with volt-VAR activated relative to fixed power factor, and
• Less reactive power demand at the feeder-head.

With regard to the impact of solar PV with GSF on utility operations, activating any GSF in new PV systems had no adverse impact to the utility’s voltage regulation equipment (substation load tap changer) in terms of increasing total number of operations. However, even though the use of volt-VAR resulted in less increase of reactive power demand at the feeder level, as compared to fixed power factor 0.95 absorbing, the increase in reactive power demand in the aggregate of an entire distribution system with very high penetrations of volt-VAR could have an adverse impact on the bulk power system. The potential impact of GSF in the transmission system should be further explored.

As far as the overall solar energy curtailment impact due to combining reactive power GSF with volt-watt (real power curtailment function), the activation of volt-watt relied on the effectiveness of either fixed power factor or volt-VAR first to regulate voltage. As such, Hawaiian Electric’s designed the activation of volt-watt to operate as a system protection function used only when voltages exceed 1.06 pu. The combined activation of volt-VAR and volt-watt results in less customer curtailment due to volt-watt alone or when used in combination with 0.95 fixed power factor setting.

Finally, the annual amount of solar energy curtailment to customers with GSF including the combination of reactive power GSF (volt-VAR or fixed power factor) with real power curtailment (volt-watt) resulted in relatively low values compared to what industry stakeholders expected at the beginning of the study. The recommended volt-VAR combined with volt-watt setting, in the “near-term” PV-penetration cases modeled in the study, resulted in annual solar energy curtailment of less than 0.5% per customer for 95% of the customers, and less than 5% for the remaining 5% of the customers. For customers experiencing solar energy curtailment, a possible mitigation strategy would be to use demand response to lower the voltage at the customer meter and reduce the activation of inverter GSF. A second phase of the study is investigating the impact of smart loads and customer sited battery storage on feeder voltages.

Hawaiian Electric and NREL are currently conducting a small field trial to verify the VROS study results. Simultaneously, Hawaiian Electric is offering the activation of GSF as a low-cost option for interconnecting PV systems on high penetration circuits, and have proposed the activation of volt-VAR and volt-watt in its interconnection Rule 14H.

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