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Power Conditioning Systems for Renewables, Storage, and Microgrids

Summary:

This project develops the measurement science necessary to support the widespread use of advanced power electronics to provided new functionality in the smart grid, as needed to support integration of new variable resources. The introduction of variable renewables, storage and microgrids into today's electrical grid requires conversion of electric power from one form to another (AC to/from DC and/or conversion between different voltage levels), and requires conditioning the power quality to what is needed by the subsystems being integrated. These functions are performed by Power Conditioning Systems (PCSs) that are a key enabler to utilizing renewables, storage and microgrids on a large scale.  This project develops measurement methods for PCSs needed for these applications, and supports PCS performance standards development to provide smart grid­interactive interfaces for these devices.  The PCS grid applications supported include smart grid interfaces for individual renewable/clean energy and storage systems including plug­in vehicles used as storage, as well as microgrids, and DC circuits.  A key goal of this project is to demonstrate interoperability of these devices in laboratory-emulated microgrid scenarios as a precursor to deployment in selected building and campus scale microgrid demonstrations at NIST.

Description:

Objective - To establish standards and measurement methods for smart grid and microgrid Power Conditioning Systems needed to transition from today's low penetration of non­dispatchable intermittent renewable energy sources to the future high penetrations of dispatchable smart grid-­interactive distributed generators, storage, and microgrids.

What is the new technical idea? The term Power Conditioning System (PCS) refers to the general class of devices that use power electronics technologies to convert electric power from one form to another; for example, converting between direct current (DC) and alternating current (AC), and/or converting between different voltage levels, and/or providing specific power qualities required by the subsystems being interfaced by the PCS.

Many "loads" on the power grid today are already interfaced through PCSs that provide the type  of electricity needed by the load and also provide valuable grid interface characteristics such as unity power factor (phase of AC current draw is aligned with AC voltage) and reduced waveform harmonics (reduced sinusoidal distortion of     load current). The transition to PCS­based loads occurred over the last three decades, starting with low power loads and evolving toward high power loads such as today's large variable speed electric motor drives (up to 100 MW).  The grid "power delivery system" itself has also begun to use PCSs such as Flexible AC Transmission System (FACTS) devices that inject corrective power waveforms into the grid, and High Voltage DC Transmission (HVDC) stations that convert between AC and DC for long distance transmission (at 1000 kV, 1000 MW levels).

On the other hand, only a fraction of power generators on the grid today are PCS­based (<<1% overall), but we are on the verge of a transformation to much higher penetration levels of PCS­based generators (>10%) that will occur over only a few years. The transformation is partially due to the addition of renewable/clean energy sources that produce DC (photovoltaic and fuel cell) or variable AC (wind turbines) and thus require a PCS to convert to regulated AC meeting grid interconnection requirements.  The distributed nature of solar energy also poses unique challenges in simultaneously meeting the requirements to provide grid stability by remaining connected during abnormal grid conditions, while also ensuring safety by de­energizing or separating into a microgrid island when the distribution grid goes down. Microgrids also provide resiliency and power quality advantages to consumers and can contribute to overall stability of the grid. Advanced smart grid­interactive PCS­ based generator and microgrid functions developed as a result of this project enable solutions to these and many other issues and will enable distributed generators to provide grid interactive functions that increase their value proposition.

Future grid architectures involving fleets of stationary microgrids plus tactical mobile microgrids can play a critical role during disaster response involving wide­area electricity outages by enabling individual microgrids to continue to operate or to be brought back up before transmission lines and substations are restored. (An example of the benefit of this technology during a natural disaster is the microgrid in Sendai, Japan, after the Great East Japan Earthquake.  Connecticut, for example, is actively pursuing microgrid technology and has established a microgrid grant and loan pilot program.) In the future, tactical mobile microgrids consisting of compact, lightweight PCS units on trucks might be used to rapidly integrate diverse types of generators, storage, loads and feeders during wide area disaster recovery efforts. Disaster­ recovery capability might also be integrated intrinsically within power conditioning units of critical infrastructure equipment such as nuclear power plant cooling systems or municipal flood pump stations so that they can rapidly interface to alternate electricity sources during disaster recovery.  The coordination of PCS, Distributed Energy Resources (DER) and microgrid technology and standards development performed by this project will aid in more rapid adoption of advanced disaster recovery strategies.

What is the research plan? This NIST project addresses the critical standards and metrology gaps needed to support the transformation to high penetration levels of PCS­based distributed generators, storage and microgrids.  The project is enabling DER to be used as multi­functional operational assets to manage local and regional grid operations including the ability to island portions of the grid into resilient self­sustainable microgrids. Microgrids manage their own local resources and operations in both grid connected and islanded mode, and appear as a single controllable entity to the larger grid. Microgrids are an architecural construct that enable multi-level distributed control of the rapidly increasing numbers of DERs, controllable loads and other intelegent electric devices that are being connected to the grid.

The project plan has two tasks that address: 1) standards for advanced interface functionalities of PCS­based generators, storage, and microgrids, and 2) application integration through conformity and interoperability testing and transition to demonstrations. The first Task is accomplished by leveraging the Smart Grid Interoperability Panel (SGIP) Distributed Renewables, Generators and Storage (DRGS) Domain Expert Working Group (DEWG) led by this NIST project. To accomplish the second task, the project is designing and constructing and operating the Microgrid/PCS Interoperability Testbed within NIST Smart Grid Testbed (located in the basement of building 220).

The Microgrid/PCS Interoperability Testbed enables testing interoperability of PCS­-based devices and controllers in microgrid scenarios. The lab also includes electrical interconnection and information exchange with devices and systems from other Smart Grid Projects located in adjacent labs withing the overall Smart Grid Testbed.  The Microgrid/PCS Interoperability Testbed is being configured to support four different scales of microgrids:

  • Thrust A (residential customer microgrid)
  • Thrust B (light commercial facility microgrid)
  • Thrust C (DC microgrids)
  • Thrust D (heavy-commercial and industrial microgrids)

Each Thrust will consist of the following four phases:

Phase 1: power equipment configuration and safety

Phase 2: microgrid equipment configuration and testing P

hase 3: adjacent lab equipment interconnection, safety and testing

Phase 4: transition microgrid/PCS equipment to demonstrations, eg.,microgrid equipment and smart inverters in netzero house