As the solar industry continues to grow and the grid sees a higher penetration of photovoltaic generation, grid operators are forced to think differently about how they manage the increasing number of distributed generation solar projects.
As a result, solar inverters are becoming significant contributors to next-generation power management on the 21st-century grid. Historically, grid-connected inverters have been treated as negative loads, and the focus was entirely on energy harvest and active power production of the solar PV system. While obtaining maximum energy harvest is still a priority, utilities are seeing the value of the ancillary services that inverters can provide to support grid stability.
For example, the electric system suffers when there is significant VAR content in a transmission network segment, resulting in lowered capacity and voltage. Inverters can provide power factor and VAR support during this common occurrence to help maintain grid voltage and offset the need for installation of expensive voltage management devices. Similar voltage and load dynamics exist within the distribution network, as well, but there may not be a system operator managing the distributed PV resources.
In such a case, the grid operator would benefit from coordinated and autonomous control by the inverters to provide ancillary services where inverters can provide nearly instantaneous set point changes based on VAR requests, again offsetting the need for capacitor banks or excessive cycling of voltage regulating devices. This requires the inverter to be quite flexible in its grid integration controls capabilities. To help meet this industry need, Advanced Energy is contributing to many of the policy and standards development activities in North America that are bringing consistency to grid interactive controls.
Key grid interaction controls and capabilities that inverters must provide for the solar-powered grid include controls for active power with associated ramp rates during transitions, reactive power (kVAR), power factor, and configurable voltage and frequency trip points. To access these control functions, operators should have the choice of configuring the inverter directly, or integrating the inverter’s grid support functions on existing SCADA and distribution management systems. Additional grid support controls utility-scale inverters should provide:
Active Power Curtailment: Specifies an upper limit for inverter active power output. When the output of the PV array at its maximum power point exceeds the power set point, the Active Power Curtailment control increases the PV voltage to reduce the power output of the array.
Active Power Ramp Rate: Defines the rate at which the active power production by the inverter ramps up or down, limited by the available DC power output of the array.
Figure 1 – Shown here is the transition between maximum power set points and the associated ramp rate
Reactive Power Control: Sets the level of reactive power (kVAR or Q) generation or consumption, and operates within the constraints of the inverter’s power envelope and current irradiance conditions. For example, Advanced Energy’s AE 500NX-1kV has a total capacity of ±165kVAR (subject to array and grid conditions).
Reactive Power Ramp Rate: Defines the rate at which the reactive power production by the inverter transitions between set points.
Power Factor (PF) Control: Sets the ratio of real power to apparent power, allowing for sourcing or sinking of VARs to maintain voltage and increase efficiency in the power system. For example, Advanced Energy’s AE 500NX-1kV is capable of ±0.90 PF (subject to array and grid conditions).
Figure 2 – The power triangle above represents the relationship between apparent power, real power, and the resulting reactive power.
Power Factor Ramp Rate: Defines the rate at which the inverter transitions between different power factor set points.
Voltage Ride-Through: This control enables configuration of the desired window of operation under zero-, low-, and high-voltage ride-through conditions. The graph below shows the range of the AE 500NX-1kV (red line), as well as a few of the prominent regulatory trip curves mandated throughout the United States.
Figure 3: The VRT window for the AE 500NX-1kV inverter supports multiple regional profiles
Frequency Ride-Through: The capability to ride through both high and low variations in frequency. For example, Advanced Energy’s 500NX-1kV supports a range from 63 Hz to 57 Hz with configurable trip limits to meet various regulatory requirements.
PV integration challenges are inherent neither to distributed generation solar projects, nor to PV in general. Rather, the challenge is being exacerbated by regulations that prevent inverters from providing support functions that could help stabilize the grid. Organizations across the industry are addressing the issue and bringing consistency to grid-interactive controls. The California Public Utility Commission summarizes the issue well in its Advanced Inverter Technologies report and discusses the promising future of distributed generation:
Advanced inverter functionalities may lend significant improvement to the stability, reliability, and efficiency, of the electric power distribution system in the U.S. Distribution automation systems implemented by utilities will be central to the integration of these functionalities, which require protection, control, and communication to reach full efficacy. Implementation of reactive power support functions can permit distributed energy resources (DER) to respond to loading conditions to minimize losses and improve the quality of supplied power. By the same token, ride-through of adverse voltage and frequency conditions may enable inverter response to mitigate the impact of unexpected conditions, maintain interconnection, and thereby lend resiliency to these resources. At present, U.S. compliance-based standards for interoperability and performance tend to inhibit the implementation of these functionalities, but they are being revised to consider safe and reliable augmentation of inverter functionality to support increased penetration of DER.
To help meet the grid support needs of transmission and distribution utilities, Advanced Energy provides a comprehensive suite of utility-interactive inverter controls and recommends an optional Inverter Master Controller to simplify integration in large-scale PV plants. To ensure that these advanced control capabilities meet the evolving needs of operators, Advanced Energy will continue to participate in the industry collaboration activities that are shaping the integration of grid support functionality into inverters.
For more information on Advanced Energy’s grid interactive controls and capabilities, see the company’s case study detailing how its 500 kW/1kV inverters successfully overcame low voltage events at a PG&E substation here.