Storage in the Power Grid

The electric grid is in a rapid state of change. The arrival of intermittent renewable resources is creating significant new operating challenges for the utilities. Batteries and other storage devices are suggested as a means of providing local firming and backup for intermittent resources. This has led to a common focus on storing and returning energy to the local power system, be that a residence, commercial or industrial establishment. This storage/return can be used to reduce peak demand or provide backup, or to provide reserve capacity or flexibility for the grid operator.

But there is another area where storage devices may provide useful support, and the provision of two or more services at one time may well be the difference that makes the installation viable.

Batteries and other storage devices are generally equipped with an inverter, a device that converts between DC power, required for the battery, and AC power required for the grid. The user can manage the magnitude and direction of power flow by managing the inverter.

The inverter often has another capability that may add value, and that is the ability to provide or absorb reactive power from the AC source. Reactive power is actually real power that flows in one direction for half of each electrical cycle, and in the other direction for the other half cycle. There is real flow of power, but because the power flows equally in both directions over a full cycle (60 times each second), no energy is delivered.

Reactive power, measured in VARs, is used by utilities to manage voltage. The addition of high penetrations of intermittent generation is often being restricted because the intermittent capacity is causing voltage changes outside the allowable range, something that is seen by all customers that are nearby. Local inverters have the capability to support voltage, but there are some caveats. IEEE 1547 and California Rule 21 do not permit local inverters to manage voltage, but if the set point for reactive power or voltage is controlled by the utility, this restriction should be ineffective. The utility is responsible for system voltage, and any distributed control needs to be coordinated with other utility devices that control voltage.

So a site with storage and smart inverters may be capable of providing a number of key services that will support both the customer and the utility. Some of these are:

  • Backup power and peak demand management for the customer
  • Peak demand reduction for the utility
  • Local distribution voltage management for the utility
  • Provision of reserve and flexibility capacity for the utility

The actual location at an industrial, commercial or residential site may not be so important as the characteristics of the load and intermittent generation that are connected nearby. In the case of voltage management, utilities find themselves using old substation equipment (LTCs) to manage reactive power, and this setup is not optimal. Reactive power delivered to a customer causes real power losses in distribution, so the best place to locate devices that can supply or absorb reactive power to manage local voltages is as close as possible to any source that causes this need. It could be an industrial site, or at the other end, it could be a residential site with large vehicle chargers.

This technology has potential real value for the future. Getting the best economic value from the storage will require that it be used to meet as many needs as possible, and with new technology in communications, optimization, and control, it should be fully capable of delivering an outstanding service that will improve power quality for everyone, while reducing costs at the same time.

This topic is discussed as part of The Big Question in the September/October 2016 issue of Renewable Energy World magazine, which comes out in early September. You can subscribe here.

Lead image credit: U.S. Department of Energy

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