Hardware in-the-loop testing for microgrids

Author: Kati Sidwall, RTDS Technologies, Canada

Regardless of the topology and components present in a given microgrid, we can expect that it will be required to operate both as a part of the larger surrounding grid and as an independent, self-contained network. Ensuring stable operation in both states and seamless transition between the two is a challenge for owners and operators. Power flow between the macro and microgrid depends on a variety of technical and economic factors and requires its own monitoring and control.
At a lower level, another odious technical challenge lies in wait. A microgrid might contain many distributed energy resources, each having its own conversion hardware. Inverters from multiple vendors with different operational standards, in an electrically close environment, present a major coordination challenge. Beyond steady state operation, achieving confidence in the interoperability of these devices during system transients is trying for engineers.
Given our expectations for microgrid performance, their relative novelty, and the multiple levels of tiered control required to manage these systems, the demand for detailed microgrid simulation and testing facilities is high.

The Unique Role and Potential for Real Time Simulation as a Microgrid Testing Tool
Modeling tools have played a longstanding role in power systems analysis, equipment development, and project implementation. Often, these programs run offline - they are software tools running on the user's PC to represent the behavior of the grid under steady state or transient conditions. Load flow, transient stability analysis, and electromagnetic transient simulation programs exist, offering various levels of detail and frequency reproduction.
Real time digital simulation was introduced to the power industry in the early 1990s.  At the time, it revolutionized the way engineers performed the factory acceptance testing of HVDC and FACTS devices. The technology takes advantage of powerful, specialized parallel processor computing hardware to run simulations much faster than a PC can. Running electromagnetic transient simulations in real time - that is, with subsequent instantaneous outputs separated by a constant, microsecond-scale duration of time in which all calculations required to reach the output are performed - has a distinct benefit. Real time operation means that physical devices can be connected to the simulated system in a closed loop. This closed loop interface can be made either by conventional analogue and digital inputs and outputs or via Ethernet-based communication protocols.
The interaction between the network and protection, control, and power devices during system transients can then be studied in great detail over a wide frequency bandwidth. Real time simulation is the only tool that offers this capability - hardware in the loop testing.

The efficiency of simulating in real time is also of interest. With a powerful, dedicated computing device, the time of interest to the user (whether that be the milliseconds following a fault, the seconds taken for a microgrid's frequency to stabilize, or events occurring over larger time scales) is equal to the time taken to complete the simulation. Scripting facilities to automate large numbers of simulations, in which system conditions or parameters may need to vary, make testing even more efficient.
Traditionally, real time simulation had a specific role for engineers, and was used for testing the operation of HVDC and FACTS controls and transmission-level protective relays. Over the last quarter century, as our power systems have changed, so too has the way this technology has been adapted and applied. Real time simulation has been used for wide area protection and control testing, digital substation automation, distribution automation, and inverter testing. At the time of writing, it has also been successfully applied to many microgrid projects. (Figure 2).

As applications have changed, real time simulator providers have had to furnish their technologies with capabilities and features to meet the needs of distribution engineers and those in the microgrid domain. For example, the communication protocols often used in microgrid control, such as MODBUS, must be "compatible" with any real time simulator that is to test them - that is, the simulator must have the innate capability to convert the power system data into a standard-compliant communication packet and send it to the device via LAN. Additionally, accurate and robust software models for renewable energy components and both common and custom-topology converters must be (and have been) developed. As new protocols and standards develop, simulator providers must be aware of their relevance. 
The potential for real time simulation in the microgrid domain is varied. The simulated microgrid can be connected to primary, secondary, and/or tertiary controls in order to assess their performance in managing power flow, stabilizing voltage and frequency, maintaining power quality, and/or preventing overcurrent in conversion devices. The simulated system could also be connected via a power interface to a particular inverter (and connected DER) of interest to evaluate its performance under contingency conditions, often in an environment with many other simulated converters. Full protective schemes can also be tested at secondary level through the use of power amplifiers.
Interesting use cases for real time simulation in the microgrids field are presented in this article.

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