The Secret to Maintaining the Stability of a Highly Renewable Island Grid – pv magazine USA

Maintaining grid stability is the number one priority for grid operators, and over the last century, various technologies and strategies have emerged and been implemented to help with load management, frequency regulation, and black start capabilities, among other things. Most of these solutions are designed to work with the high-inertia grid provided by rotating generators. However, as the number of solar PV and other inverter-based power sources on the grid increases, they often replace rotating generators, a source of high inertia, leaving grid operators with small and islanded systems to manage low-inertia grids with tools designed for high-inertia grids. It doesn’t work.

One of the major problems with low-inertia island systems is that the rate of change of frequency (RoCoF) is faster on a low-inertia network than on a high-inertia network. This means that the speed of response to correct a frequency deviation must occur within milliseconds on a low-inertia network, whereas a high-inertia network can rely on this inertia to get it through the first five to ten seconds before having to rebalance. Traditional frequency control methods, such as generator response and load shedding, are simply not fast enough for low-inertia networks.

The solid lines in this graph represent the falling frequency in low-, medium-, and high-inertia systems. As the steep drop in the yellow line (low inertia) shows, the frequency falls much faster in the low-inertia system than in the high-inertia system (red line).

To combat this problem, low-inertia grid operators turn to traditional solutions, such as increasing the number of fossil-fueled spinning generators to compensate for the decrease in system inertia. Then, because they have to keep an additional generator running to respond to such an event, and that generator is generating electricity, operators resort to curtailing the renewable energy generated by inverter-based resources because they now have excess power. In addition to wasting the renewable energy generated, this approach creates a vicious cycle that adds unnecessary redundancy, costs, and is at odds with environmental and sustainability initiatives.

Solving the inertia deficit

This may seem counterintuitive to operators familiar with traditional grid management methods, but the key to stabilizing the destabilizing effects of more renewables on the grid is — more renewables. And the key to managing more renewables is — software in the form of a fast, precise controller. Renewables can make up for lost inertia by offering synthetic inertia in the form of fast or fast frequency response, and the controller is the brains behind detecting grid disturbances and ensuring that inverter-based resources are dispatched within milliseconds to rebalance any deviations.

A critical part of this approach is the integration of a battery energy storage system (BESS). The BESS acts as a shock absorber, capable of absorbing or releasing power from/to the grid to compensate for changes in production, load, or frequency. When the BESS is paired with an advanced, high-speed controller, the BESS can be called upon to perform additional grid management functions, increasing its own return on investment. These additional BESS functions include:

• Shifting Energy: Absorbing excess solar PV energy during periods of high production and sending it during periods of low production. This reduces the need for curtailment, captures generated energy that would otherwise be lost, and increases the ability to respond to spikes in demand.
• Ramp Control: Solar PV production is intermittent and can be highly variable during weather events, where cloud cover can cause rapid peaks and valleys in power output. BESS can absorb these peaks and lift the valleys to smooth and stabilize power output.
• Frequency adjustment: Providing fast frequency response to solve the high RoCoF problem in low-inertia networks is extremely simple because the power of the BESS can be immediately used to solve the frequency deviation problem.

A multi-level, high-speed controller is needed to manage all of these use cases in a single battery. The controller must be able to generate a plan in advance that takes into account the anticipated load requirements of the network and be able to adjust that plan in response to current events. Without parallel processing capabilities that can learn, schedule, sort, and issue commands, the BESS may be full when it needs to absorb and empty when it needs to send. Of course, it is possible to have dedicated BESS units for each use case, but given the amount of downtime that the BESS is idle between use cases, it makes more sense to package all the use cases into one. This saves capital costs and helps in cases where there may be physical constraints that prevent multiple BESS units from being installed.

So far, we’ve revealed that the “secret” to maintaining the stability of a highly renewable grid is to integrate a multi-level, high-speed BESS+ controller into the grid. But what about the inverters, where do they come in?

Will grid-tie inverters help?

When it comes to tools designed for the 21st century grid, grid-forming inverters hold great promise. Unlike grid followers, grid-forming inverters do not require a fully functional grid to “follow” them and determine their own set points. This makes them ideal for managing inverter-based assets in low-inertia grids.

When combined with renewable energy sources such as solar PV or BESS, grid forming inverters can help with grid support services such as black start and frequency management. However, there are some services they cannot help with, and worse, when multiple grid forming inverters are configured in a network, they can compete with each other to try to restabilize the network after a disturbance, resulting in more destabilization. Therefore, they cannot offer a complete solution to low inertia grid problems.

The inverters need something to control them all. This is where the multilevel controller comes in. A multilevel, high-speed controller establishes and enforces a hierarchy of control over all the energy resources on the grid, allowing each resource to contribute when and how it needs to, according to the controller’s commands. It can work with both grid-forming and grid-following inverters, and integrate with existing grid resources. Furthermore, if it is both grid-aware and hardware-aware, the controller will ensure that operations remain within the system’s constraints.

With visibility across the entire network and its assets, the multi-level controller can take a holistic approach and make real-time decisions that take into account network constraints and operator priorities. This leads to fewer outages and faster restorations in the event of unavoidable outages.

Islands looking to reduce their dependence on fossil fuel-based power generation must abandon traditional grid management methods and adopt 21st century grid tools. Photovoltaics, wind generation, fast inverters and BESSs are all part of the new technology mix, and when combined with multi-level, fast controllers, they have proven their worth in real island environments.

Tim Allen, CEO of PXiSE Energy Solutions, brings more than 22 years of experience in utility-scale solar, wind and energy storage projects, software controls, investor-owned utilities, independent power producers and clean development spaces. His unique skill set, beginning with an electrical engineering degree from CalPoly, offers experienced perspectives and relationships that position him to lead PXiSE into the future.