Despite energy-agenda uncertainty coming off a major election cycle, increasing renewables and distributed generation are on a potential short list of trends not likely to change. What is changing is how we accommodate a growing number and variety of generating sources on a network originally designed for one-way power flows. After becoming important control variables way back in the 1970s due to the energy crisis, voltage and Volt Ampere Reactive (VAR) optimization are taking on increased importance anew with our increasingly complex power grid.
Volt/VAR optimization (VVO), stated simply, is managing voltage levels and reactive power to achieve more efficient grid operation by reducing energy consumption, system losses and even peak demand for short periods. It also serves in situations of varying voltage, demand, frequency, and VAR conditions caused by generators and load. Moreover, since roughly 40% of total power system losses occur at the distribution level, voltage optimization programs often are focused on utility distribution systems and may be referred to as conservation voltage reduction (CVR) programs. Reducing supply voltage to the maximum extent allowed by code reduces energy consumption (inefficient usage) and losses.
In a study conducted by EPRI and SMUD, researchers found voltage reduction employed at 14 substations for peak reduction and CVR resulted in an average energy consumption reduction of 0.4% to 0.7% for each percent of voltage reduction. As one might expect, the results varied by the type of circuit loads: residential, commercial or industrial. Studies like this one and many others sponsored through EPRI’s Smart Grid Demonstration initiative have confirmed that huge energy savings would be possible nationally with widespread VVO.
Conversely, without a focus on VVO, utilities are finding it increasingly difficult to keep voltage along feeders within acceptable ranges with the increasing penetration of DERs (see "Actual Power Quality"/T&D World November 2020). Under/over voltage conditions and frequently related power quality issues may exacerbate energy losses, cause utility equipment malfunctions and damage customer electrical appliances. In addition, poor voltage/VAR management can reverse expected benefits of renewables and distributed generation (DG) by increasing losses, potentially resulting in more emissions.
Traditional VVO conducted on primarily unidirectional power systems, was achieved via adjustment of substation power transformer load-tap changers, line regulator auto transformers, and switched shunt capacitors. While these tools may be individually automated, traditional controls were not necessarily integrated to seamlessly function in unison as necessitated by a complex grid with multiple, sometimes intermittent power sources.
Modern VVO systems may be part of a distribution management system and employ near/real time information and modeling to provide system-wide, coordinated control for unbalanced distribution networks. Newer tools include electronic control components such as thyristers, smart inverters, static VAR compensators and static synchronous compensators. Smart inverters which are increasingly being required for asynchronous generators allow automatic synchronization to the grid, helping the generator match voltage, frequency, amplitude, and phase angle. Smart inverters also can generate and absorb VARS when required. At the transmission level and elsewhere, Flexible AC Transmission systems (FACTS) utilize static VAR compensators and static synchronous compensators. Advanced digital controls offer greater functionality, speed, and adjustment range for VVO in modern systems.
VVO on distribution systems with a growing variety of DER and load sources is facilitated by sensors on our smart grids as well as another now common monitoring tool-automated meters. Automated meter infrastructure (AMI) can provide system operators with near real-time information about abnormal distribution system conditions, such as voltage swings or outages. Such feedback is particularly important as utilities strive to balance loss reduction using CVR programs while maintaining minimum voltage requirements for customers at the grid edge.
The growing availability of data is only part of a comprehensive VVO solution on today’s grid. As effectively described in an article by Xiaoming Feng and William Peterson from ABB, VVO systems must simultaneously balance power flow equations, voltage constraints, current constraints, tap change constraints, shunt capacitor constraints, etc., for an entire feeder system or network. Several vendors including etap, Eaton, Oracle, Veritone, Trilliant, ABB and Schneider Electric offer specialized analytic and control systems designed for these functions.
Utilities optimizing their distribution systems today often are addressing challenges in addition to VVO. A growing number of companies are turning to advanced distribution management systems (ADMS) to better prepare for the future. CenterPoint Energy is using an ADMS to supercharge its storm preparedness/response program with a DSCADA system that communicate with 2,000 intelligent grid switching devices. APS is using its ADMS system to deal with a huge influx of solar resources, tying advanced analytics, inputs from AMI and an advanced communications network into a comprehensive management tool. Whether addressing voltage and reactive power conditions on the grid the old-fashioned way or with newer comprehensive management systems, utilities focused on these parameters are improving safety, equipment longevity and conservation for their customers.
