Hardware in-the-loop testing for microgrids

Author: Kati Sidwall, RTDS Technologies, Canada

Inverter Testing - the Challenge and Benefit of Power Hardware in the Loop

It is possible to connect physical power electronic energy conversion hardware to a simulated grid through a power interface - often referred to as "power hardware in the loop" (or PHIL) testing. PHIL requires a four-quadrant amplifier, which can both source and sink real and reactive power, to serve as the interface between the simulated system and the physical hardware. Beyond that, several interface methods and test setups are possible. (Figure 1).
In one case, the DC side of a commercially-available microinverter was connected to a 250W solar panel. The inverter's AC side was connected to the output terminal of an amplifier, which provides values from the simulated grid. The inverter current, measured by a sensor on the amplifier, was sent back to the real time simulator in order to close the loop. (Figure 3).

Establishing a valid and stable PHIL interface is not trivial, and selecting an appropriate amplifier is a crucial step of the process, with several technical considerations including response times, slew rate, harmonic distortion, frequency resolution, and input/output ratings and impedances. Filtering is also a major technical consideration in order to reduce the impact of noise (introduced by voltage and current sensors, physical wiring, and electromagnetic coupling between devices) on the simulation. Any error introduced by noise can be amplified in the interface via positive feedback until hardware limits are exceeded. Filters for noise reduction introduce challenges of their own, including additional delays that may be added to the interface, so filter parameters should be determined carefully.

An interface like this can be used to investigate inverter behavior under a variety of conditions. For example, in this particular case, the inverter response to a simulated line to ground fault was investigated. Current plots obtained from the simulation show that the inverter disconnects from the grid after a fault with a 5-cycle duration was applied. Multitudes of tests can be run with varying fault severities and types, and interaction with the macrogrid, other DERs, and control behaviors simulated in detail. Gaps between the engineer's expectations for inverter performance and actual behavior can be identified. With a real time simulator, these tests can be run for even the least likely of conditions (providing that embedded device protection exists for these conditions), significantly or fully de-risking the installation of an inverter in a complex system. (Figure 4).

The UK'S Power Networks Demonstration Center (PNDC), which has unique testing abilities provided by an impressive on-site 11kV network, has performed significant inverter testing studies using a real time simulator. Seven off the shelf low-voltage PV inverters were tested with the goal of evaluating stability during disturbances resulting from transmission-level faults. Additionally, recently proposed engineering recommendations for the connection of small-scale embedded generators in Great Britain include more potent stability requirements for inverters under voltage shift conditions. The proposed test conditions were applied to these inverters and trip/no-trip boundaries for each device were determined. (Figure 5).

The results from these tests provided some insight into how varied the behavior of inverters really is. For example, faced with unsymmetrical faults, three-phase inverters from different vendors behaved entirely differently, with one tripping on none of the test faults, and another tripping for all of them.
Voltage shift stability varied significantly and showed that under typical faults, certainly some amount of inverter-connected energy generation would be lost. An inventory of all of the various types of inverters installed in the network would be a first step in attempting to accurately quantify the impact of a worst-case scenario.
Potential impacts of inverter behavior on other protection and control systems can also be investigated via real time simulation. For example, the PNDC's study found that some inverters remained connected but significantly reduced power output during voltage shift and reduction events.
This effect may magnify system rate of change of frequency and potentially negatively impact the stability of loss-of-mains protection following a major event.

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