The inverter performs the essential job of turning the DC power generated by a photovoltaic array into the AC power utilized by the electric grid. Much like the grid itself, we’re hearing that inverters are becoming smart, and that smart inverters can enhance the value of solar, for the utility and the customer.
Inverters have come a long way. They began as electromechanical devices, were improved with vacuum tubes and glass-filled tubes, then became silicon-based devices and now are equipped with digital technology. They have evolved from square wave to modified sine wave to pure sine wave as their consumer and grid needs have changed over time:
1. Off-Grid (1980s – today): Square and modified sine wave inverters were driven by consumer’s basic electricity needs, which then transitioned to pure sine wave inverters as technology improved and equipment needed cleaner signals, particularly for electronics
2. Basic On-Grid (1990s – today): Pure sine wave inverters were upgraded as distributed solar and wind became grid-citizens and needed to meet safety and power quality standards.
Image Source: Projectfreepower.com
What is the need for a smart inverter?
With the rapid increase in the number of distributed solar systems installed each year, and as more utility customers also become generators , there is a corresponding rise in the functionality expected of inverters. This shift involves moving from just maintaining minimum safety and power quality standards to measuring and reacting to grid conditions in a proactive, dynamic environment.
Image Source: NREL
(dotted lines are allowable voltage range, red line is voltage in top circuit without solar, blue line is voltage in bottom circuit with solar)
As the graphic above illustrates, voltage on the distribution system decreases as the distance from the utility substation increases - no surprise there. However as PV penetration gets high enough it will increase the voltage on the system. If the amount of solar generation on that circuit is high enough, the voltage will exceed the normal allowable range even at the same distance from the substation. The intermittency of solar due to environmental conditions (cloud movement, etc.) can also cause problems.
How can inverters support the grid?
Smart inverter technology enables more intelligent, grid-focused capabilities such as:
• Maintaining connection to the grid instead of tripping from minor disturbances due to voltage or frequency range fluctuations outside of normal ranges
• Producing not only real power but also generating or consuming reactive power, helping with voltage swings inherent in solar PV systems; and,
• Facilitating real-time communications, allowing for remote control to enable/disable functions or change set points as needed by the grid
All of these contribute to the stability of the electric grid. Learning from experience in Europe with grid issues hopefully will allow the US to stay at the front of this issue. In fact current codes are in the process of being updated. Unfortunately, changes to standards and past practices can be tedious and slow.
What is needed to move forward?
Smart inverter standards with the appropriate functions and settings can keep the electric grid up and running even with high penetrations of intermittent generator resources. The Western Electric Industry Leaders (WEIL), a group of electric utility executives across the Western Interconnect, issued a paper on August 7, 2013 addressed to governors, commissioners, and legislators regarding the need to update inverter standards.
SEPA sees smart inverters as being a key part of solar expansion in the future, especially in high penetration areas. Utilities and the industry will have to work together to define standards, market mechanisms, and how information can be exchanged in the real time. We will be releasing a paper on how smart inverters can be leveraged in the first quarter of 2014. Join the conversation at SEPA Connect.