Replacing Bulk Generation Grid Services with Smart Grid Regulation

By Jose Medina Palomo and Sasan Mokhtari

It has been long argued that distributed generation units based on renewable sources are not providing their share of grid services. This current state of affairs, however, is rapidly changing.

The improvements in communications and controls, coupled with the new generation of Smart Inverters are allowing Distributed Energy Resources (DER) including renewables and controllable loads to participate in providing Grid Services such as Frequency Response, Automated Generation Control (AGC) Regulation and Volt/VAR control. This creates a paradigm shift, where DER enhance the reliability of the system rather than force bulk generation to provide a disproportionate amount of grid services.

DER include any resource (generation or controllable load) connected to the Distribution system behind or in front of the meter. DER are managed by a Distributed Energy Resources Management System (DERMS). The main functions of a DERMS are modeling and registration of DER, aggregating devices into manageable resources, computing the availability per resource, offering this available capacity, sending control signals to devices, and preparing performance reports after events.

Throughout the USA, several pilot projects have been testing the ability of DER to provide grid services. These pilot projects rely on real time measurement, the calculation of available capacity per grid service, high speed communication to receive and send control signals, and intelligent disaggregation of the signal to the different end devices.

Following is a description of how the Smart Grid is enabling the provision of AGC Regulation, Frequency Response, and Volt/VAR support by DER.

AGC Regulation

AGC regulation is the ability of a resource to respond to a changing control signal every 2 to 4 seconds by modulating its active power output. This AGC signal fluctuates up and down with the upper and lower limits established by the resource regulation capability. This grid service is traditionally provided by fast responding generators such as Hydro Generators (HG) and Combustion Gas Turbines (CT).

With the advances in Smart Grid, AGC can now be provided by controllable loads such as water heaters, heat pumps, Building Management Systems (BMS), batteries [including Electric Vehicles (EVs)], and Distributed Renewable Generation through their smart inverters.

DERMS receives an AGC signal, processes it, and sends proper set-points to the available end devices. Performance Indices computed in a field demonstration of AGC indicated that the demand-side resources scored as high as utility scale batteries, above the traditional bulk generation (HG and CTs), and well within the limits established by the energy industry to provide regulation.

Frequency Response

Frequency response is a fast response from a resource to increase or decrease generation in response to a frequency fluctuation from nominal. The response takes places in hundreds of milliseconds, to several seconds. Traditionally, this service is provided by bulk generation through rotating inertia, and generator governor automatic response to measured frequency. Thanks to advances in Smart Grid technology, it can now also be provided by DER which locally decrease load (increase generation), or increase load (decrease generation) in response to the sign and value of the measured frequency deviation.

DERMS computes the available capacity and determines the increase or decrease in the DER under control. DERMS sends these settings to the end devices for autonomous local operation, and monitors their performance.

Volt/VAR Support

DERMS Volt/VAR control uses the capability of DER Smart Inverters to regulate not only active, but also reactive power. Smart Inverters are rated in apparent power, meaning that even at maximum active power generation they still can change the reactive power generation or consumption without decreasing active power. Such inverters can be used for VAR support in case of normal flow direction (from the substation), or to provide voltage regulation in the areas of reverse flow.

DER Inverter systems are usually operated autonomously, responding to the local measurements by changing both active and reactive power output. The inverter response is implemented through local controller settings (active power, reactive power or power factor) or pre-loaded curves of different types. Typical curves are reactive power and active power as a function of voltage, and power factor as a function of active power.

DERMS manages DER Inverter systems (directly or through aggregators) providing both voltage regulation and VAR support. In case when DER generation does not change the direction of the flow in the distribution grid, DERMS can respond to the Utility Volt/VAR requirement to support reactive power at a desired location (for example, per feeder). This functionality allows Utility Volt/VAR to use additional sources of reactive power. There is some analogy with AGC regulation, but the signals have periodicity in minutes, and end devices are aggregated per location.

In the presence of reverse flow in any distribution grid area, the Utility Volt/VAR cannot regulate voltage in this area. In this case, DERMS provides voltage regulation by coordinating responses of the Smart Inverters in this area.

Concluding Remarks

Enabled by Smart Grid technological advances in monitoring and controlling, DERs can now be operated to supply Grid Services. Currently, the provision of Grid Services by DERs serves a supporting role to the traditional Power Grid. As DER penetration increases technology must continue to advance to deliver Grid Services in an environment where system inertia is being replaced by emulated inertia.

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