Smart Inverter Control of PV Solar Farms and Energy Storage Systems

I. Project Concept

The high penetration of renewable generation in distribution systems has introduced more uncertainties and technical challenges in the operation of the utility distribution system. Due to the intermittent nature of renewable generation, it is essential to maintain system voltage profile and facilitate smoother integration of inverter based distributed generation. Optimum power distribution also entails the reduction of distribution losses and provision of adequate power quality and availability.

Elexicon Energy Inc., in partnership with the University of Western Ontario (UWO), has received a project grant from the LDC Tomorrow Fund managed by the Mearie Group. The project involves the application of a novel smart inverter control of PV solar farms (during night and day) and Energy Storage Systems by utilizing their unused inverter capacities to provide dynamic reactive power exchange with the distribution grids for achieving:

  1. Line Loss Minimization, and
  2. Conservation Voltage Reduction (CVR) for reduction of peak loads and overall system loads

This project is based on a novel patent-protected technology of using PV solar farms as a dynamic reactive power compensator STATCOM, termed PV-STATCOM, with their unused inverter capacity.

  • The benefits of the proposed PV-STATCOM technology are demonstrated through simulation studies on the electrical distribution feeders of Elexicon Energy to which solar farms and inverter based Battery Energy Storage Systems (BESS) are connected.
  • The annual energy savings and the corresponding financial benefits for Elexicon are quantified.
  • The proposed technology can be applied on PV solar systems and BESS installed in any distribution utility in Canada, bringing significant benefits to Canadian utilities.

II. PV-STATCOM technology

Solar farms are idle during nights with their entire inverter capacity unutilized. During early mornings and later afternoons, a substantial portion of the inverter capacity remains unused as well. The innovative PV-STATCOM technology offers the following advantages:

  • The PV-STATCOM technology can provide a 24/7 functionality of a STATCOM at more than 10 times lower cost compared to a conventional STATCOM or Static Var Compensator (SVC) for the same grid support functions. This is because only some additional PV-STATCOM controls with their associated measurement circuits need to be installed on the existing inverters and existing electrical/civil infrastructure of a PV solar farm (substation, transformers, bus-work, breakers, protection systems, etc.) to transform a solar farm into a STATCOM.
  • This technology is also applicable to inverter based wind farms, inverter based energy storage systems and electric vehicle charging stations, during periods when their inverter capacities are not being fully used. The proposed PV-STATCOM control does not interfere with the power production of solar PV systems or active power exchange of BESS. It only utilizes the unused/idle inverter capacity of solar farms and BESS.
  • PV-STATCOM technology responds within 1-2 cycles as compared to 1-2 seconds for the volt-var or volt-watt smart inverter functions.
  •  The PV-STATCOM technology can provide several novel benefits on 24/7 basis in both transmission and distribution systems as below.

Distribution System Applications of PV-STATCOM:

  •  Dynamic voltage control as a STATCOM
  • Increasing the connectivity of wind farms during night (when wind is high)
  • Eliminating the need for STATCOMs in enhancing connectivity of PV solar systems
  • Preventing instability of remotely located critical induction motors
  • Simultaneous Conservation Voltage Reduction (CVR) and Line Losses Reduction by controlling solar PV systems and BESS as STATCOM
  • Mitigation of short circuit currents from PV inverters

Transmission System Applications of PV-STATCOM:

  • Enhancing power transmission capacity through power oscillation damping and increased system stability
  • Simultaneous Fast Frequency Regulation and Power Oscillation damping
  • Prevention of subsynchronous resonance (SSR)
  • Alleviation of Fault Induced Delayed Voltage Recovery (FIDVR)

III. First in Canada Field Demonstration of PV-STATCOM

The nighttime and daytime “PV-STATCOM” technology was demonstrated for the first time in Canada, (and perhaps in the world), on 13th December 2016, in the utility network of Bluewater Power, Sarnia, Ontario. It was demonstrated that a 10 kW PV solar farm autonomously transformed into a PV-STATCOM to stabilize a critical induction motor during a severe system disturbance.

IV. Patents

Thirteen patents have already been granted in US, Canada, Europe and China on different aspects of PV-STATCOM technology. Thirteen patents are presently pending in Europe, India, US, Canada and China.

V. Application Potential of PV-STATCOM

If a utility or customer has determined that it must invest in a FACTS Controller (STATCOM or SVC) to solve any of its T&D system problem, and, if a solar farm is nearby, the solar farm with PV-STATCOM technology can provide the same service to the utility/customer at about 10 times lower cost.

  • This brings huge cost saving for the utility/customer.

VI. Benefits to Utilities

  1. Better network performance for utilities
  2. Significant savings in using this technology compared to equivalent sized STATCOMs, SVCs, DVARs.

VII. Benefits to Solar Farms

New revenue making opportunities for PV solar farms for providing the above grid support functions both during night and day, in addition to sale of real power.

VIII. Project Benefits:

Line Loss Minimization: Line losses are typically 4-5% in North American transmission and distribution systems. Considering an average Ontario demand of 18,000 MW during daytime, the line losses are 720 MW. This line loss, if saved, can easily supply the power needs of almost half a million people. Hence, even a fractional reduction in line losses can have substantial benefits.

Conservative Voltage Reduction:
Conservation voltage reduction (CVR) is an established technique used worldwide to reduce both peak demand and overall energy consumption in power distribution systems. CVR involves intentionally lowering the voltages on distribution circuits in the lower portion of the utility acceptable voltage range. It has been shown through several utility studies and implementations that a 1% reduction in bus voltages can reduce the energy consumption by 0.7 – 1.5% depending on the type of loads.

Volt/Var control strategies involving voltage regulators, transformer taps, and switched capacitors are conventionally used in distribution systems for line loss reduction and CVR. Smart inverter controls (Volt-var, etc) on solar farms have been considered for providing line loss reduction and CVR, but only during daytime.

However, a combination of solar PV systems and BESS over night and day has not been used so far for providing CVR and line loss reduction.

This project uses PV solar farms both during nighttime and daytime, and Battery Energy Storage Systems over 24-hour period as STATCOMs with a response time of 1-2 cycles. This allows very rapid, almost continuous implementation of optimal voltages in the distribution networks to achieve both line loss reduction and load reduction through CVR. Solar farms have entire inverter capacity available during nighttime. Also, there is a substantial unused inverter capacity during early mornings and late afternoons. This unused PV inverter capacity complemented by the unused inverter capacity of BESS over the 24 hour period will be utilized for reactive power exchange as STATCOM for CVR and line loss reduction.

This project aims to demonstrate the benefits of the novel PV-STATCOM technology in the distribution network of Elexicon. Both optimal load flow studies and electromagnetic transient simulation studies are being conducted on the distribution networks of Elexicon which have solar farms and Energy Storage Systems connected. The energy savings and peak load reduction that can be accomplished on Elexicon systems using PV-STATCOM technology based on actual unused inverter capacities of solar farms and ESS are determined over daily and monthly periods. These savings are subsequently quantified in dollars for Elexicon.

IX. Initial Project Results:

  • The benefits of the implementing the night and day PV-STATCOM technology on the 120 kW PV solar system connected to Elexicon office building for achieving line loss reduction and CVR are evaluated.
  • Industrial grade PSS/E software is used for the simulation studies.
  • The study system is modelled in PSS/E and studies are carried out using the Optimal Power Flow (OPF) functionality of the software.
  • The studies are done utilizing PV solar, and load data provided by Elexicon.
  • The PV-STATCOM technology for various objectives are tested by conducting OPF studies for every 5 minutes for whole day (24 hours) over the month of June 2020
  • The studies are conducted for a weekday and weekend, and the results are extrapolated to estimate the savings for a year.
  •  The Elexicon office building had an average power factor of 0.87 in the month of June, as depicted in Fig.1. The PV-STATCOM technology improves the power factor from 0.87 to 0.99, as illustrated in Fig. 2.
  • Power factor correction provided by the PV-STATCOM reduces the line current as shown in Fig. 3.
  • The percent reduction in line losses in the distribution feeder supplying power to the Elexicon office building are portrayed in Fig. 4. The line losses are reduced by an average amount of 21.5% by using PV-STATCOM.
  • The next set of studies will evaluate the benefit of PV-STATCOM technology on PV solar systems and BESS in providing CVR and line losses in the Elexicon microgrid.

Fig. 1. Power factor of Elexicon office building without PV-STATCOM control of the solar farm

Fig. 2. Power factor of Elexicon office building with PV-STATCOM control of the solar farm

Fig. 3. Normalized current flow into the Elexicon office building during weekday without and with PV-STATCOM control of the solar farm

Fig. 4. Percent reduction in line losses in the feeder supplying the Elexicon office building during weekday with the application of PV-STATCOM control of the solar farm

The PV-STATCOM technology can provide several novel benefits on 24/7 basis in both transmission and distribution systems.