If you’re like most people who’ve gone to a conference lately – or read this blog from its inception – you’ve already heard warnings about what could happen to grid voltage and stability when stray clouds waft over neighborhood solar arrays and block PV generation. The sudden drop of renewable power is what many people point to as the key challenge of variable generation resources.
After all, that’s why utilities are looking for ways to “firm” renewable generation, which is the process of backing variable resources up with some combination of fast-ramping power or demand-side management to jump in when power production subsides. But, while loss of power gets most of the attention, over-production is an equally daunting challenge for grid operators.
Too much of a good thing
To understand how over-production can impact distribution systems, look at Germany, where solar installations were contributing as much as 40 percent of peak power demand by the end of 2012. Researchers from that country’s Fraunhofer Institute for Wind Energy and Energy System Technology wrote a great article for IEEE’s Power and Energy magazine.
In it, they explain that high PV penetration creates several issues, and one is reverse power flows that result when PV generation exceeds the local demand. The Fraunhofer research team used data from a University of Stuttgart study that showed one low-voltage distribution system’s load and PV generation. During a 24-hour period, PV capacity in that LV system topped load by some 900 percent.
When that happens, the resulting reverse power flows can create voltage rise and grid instability. Worse, it can leave distribution system operators with the chore of reinforcing lines, replacing transformers and performing other maintenance necessitated by all that PV.
DERMS to the Rescue – the California Story
Regardless of the issue – power loss or over-production – utilities can shore up their distribution systems with a distributed energy resources management system (DERMS). As a case in point, we recently worked with a large utility in California to optimize DERs, including customer loads, to address voltage sags and spikes that accompany high PV penetrations. To do this, the DERMS system uses data from its own network devices as well as from a host of other sources, including advanced metering infrastructure and distribution-system sensors. Using these data plus optimization algorithms, the DERMS tracks and computes the most beneficial control points for each grid asset every two seconds based on a number of parameters.
So, as an example of how a DERMS monitors just one of hundreds or thousands of connected assets, the monitoring and control points of a battery inverter include AC voltage, current, frequency, active power, reactive power, as well as DC voltage and current, its state of charge, its state of health, cell voltages, cell temperatures and control modes. The system takes these data points and also layers in other factors. Is the battery inverter online and available for control? Is it already on control? What are its upper and lower control limits? What are its storage level and ramp rate?
After considering those issues for just this one device, the DERMS then decides if it should charge the battery to help boost load or feed the grid to support high demand. Although response from this one battery may be small, the DERMS is doing these same kinds of monitoring and control operations with thousands of devices to help the utility keep is low-voltage distribution system in balance.
The utility using this solution is relying on the DERMS to support both optimal power flow and volt/VAR control. To this end, the utility supplies its desired nominal voltage/VAR flow along a feeder. Then the DERMS, which interfaces with the utility distribution management system, supports control of all the devices along the feeder, including connected customer loads as well as utility equipment, such as capacitor banks.
Likewise, the utility supplies its kW delta between the substation or feeder peak thresholds and the total uncontrolled load on the substation or feeder. As part of its optimal power flow calculations, the DERMS manages network-connected assets to minimize the operating cost of the feeder. That type of calculating involves a large number of inputs, including line losses for the feeder, conservation voltage reduction savings, capacitor switching costs and constraints, VAR and power factor penalties and constraints, as well historical demand, solar forecast, temperature and more.
Yes, there is a huge number of elements calculated by the DERMS system but, ultimately, this system really has one overriding and remarkable benefit. Using a DERMS, a utility can use grid-edge assets and behind-the-meter resources to meet its local capacity needs and accommodate an influx of PV generation. And in the end, it doesn’t really matter whether solar PV pushes too much or too little power onto the system. Given the right software, a utility can meet either challenge.