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Posted on 07 October 2019

Hardware-in-the-Loop Solution for Test and Verification of Micro-Grid Converters

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Micro-grid converter control development, and testing made simple with ultra-high fidelity embedded Hardware-in-the-Loop system

By Andreas Dittrich - Enerdrive GmbH and Vlado Porobic - Typhoon HIL, Inc.

Micro-grid technology introduction

The power generation market is rapidly changing. With proliferation of micro-grid solutions and distributed energy generation (DER) evermore attention is being focused on utilizing smaller generators augmented with renewable energy sources and energy storage in order to provide high quality of service where grid is not available or to supplement grid operation. Consider that roughly 2 billion people do not have access to a reliable electricity supply and 300 000 houses in Europe have no access to the grid [1]. Hence, ensuring electricity supply to remote locations is particularly important and the use of renewable energy sources enhanced with energy storage solutions is increasingly attractive.

The main challenge in micro-grid converter and control design is achieving tight voltage and frequency control with constantly fluctuating generation/loading. One approach is to use energy storage and/or load control (match load to generation, define low priority loads), which usually requires cooperation of the local controllers. Another important challenge is to achieve the power sharing among different sources that feed a group of loads through a micro-grid. One approach is to use the so-called ‘droop control’. The basic idea is to reproduce the characteristic of the synchronous generators connected to a steam/water turbine regulated through a speed governor, which are controlled such that the frequency decreases as the fed active power increases and the voltage amplitude decreases as the fed reactive power increases. The droop control can be implemented by measuring frequency and voltage and imposing the active/reactive power set-point to each of the distributed units, figure 1. Furthermore, protection and fault detection and isolation is an important aspect of micro-grid control.

Micro-grid: voltage and frequency regulation by imposing appropriate level of active and reactive power

HIL solution for micro-grid control system development and comprehensive testing

HIL emulators provide a comprehensive environment for the design and testing of control systems for micro-grid power electronics converters. For real-time emulation of micro-grid systems it is critical to emulate power stage with ultra-high fidelity in order to capture all the system dynamics and interactions that span time-constants ranging from microseconds to seconds. These system interactions, if system control is not properly designed and tested, can give rise to unwanted oscillations, large-signal system instabilities, and even catastrophic failures. Typhoon’s ultra-high fidelity HIL systems provides needed flexibility - while behaving as close to the real system as it gets - to realistically explore and verify all the micro-grid operating conditions and scenarios including system faults (in control system, communication system, and power sections).

In addition, TyphoonLink-Gigabit per second serial link-enables seamless connection of multiple HIL602 units into one unified HIL system that can simulate 16 or more converters, as shown in figure 2. Whether you are testing a single converter control or multiple converter control (centralized or decentralized), HIL602 in the cluster configuration provides a unified environment that is as easy to use as if you were working with a single HIL unit. Furthermore, the real-time emulation process can be controlled in a fully automated way with Python scripts via Typhoon HIL API.

Complete solution of complex Micro-grid with networked HIL units

Features and Benefits of TyphoonLink-Gigabit connection are:

  • Connect 4 or more HIL602 units in HILCluster configuration.
  • Deploy 24 real-time computational cores as one unified HIL real-time emulation fabric.
  • Test one or multiple controllers in micro-grid environment for both centralized and decentralized micro-grid control configurations.
  • Test both low-level and application/micro-grid level control functionality.
  • Interface seamlessly your industrial controllers to multiple HIL units via HILConnect unit.
  • Automate test processes with Python scripts via Typhoon API.

Develop Micro-Grid Energy storage converter

An example of micro-grid system is given in figure 3. In this system, micro-grid model comprises diesel powered synchronous generator, intermittent photovoltaic power source, permanent magnet synchronous machine wind turbine generator, resistive load, nonlinear current load, energy storage grid-tie inverter, utility grid and main contactor that emulates both grid-connected and off-grid operational regimes.

The focus here is on the development and testing of grid-tie energy storage inverter and its controller operating in micro-grid environment. Synchronous diesel generator’s role is to supply the average power to the loads while energy storage converter can both take in and deliver peak power when load and supply are not matched (due to renewable intermittency and synchronous generator lag) thus helping support both voltage and frequency regulation in the micro-grid. Flexible and easy to use ultra-high fidelity real-time simulation environment enables fast and robust development and testing of control software for the battery energy storage inverter system.

An example micro-grid comprising: utility grid with islanding contactor, diesel synchronous generator, grid-tied inverter with energy storage, wind and PV intermittent source and load

Test, optimize, pre-certify

Control loops design and optimization for the energy storage inverter is done on an industrial controller platform directly interfaced with the HIL602 Cluster. All controller functions - i.e. PWM modulator, PLL (figure 4), current and voltage control loops (figure 5) etc. - as well as protection and high-level control functions (i.e. dynamic grid support) are tested for different operating conditions, such as imposed voltage harmonics (figure 6). Indeed, HILCluster is ideal for comprehensive and automated testing of micro-grid converter controllers. Automated test scripts can cover a spectrum of test cases (including fault conditions and system unbalance) that provide a cost effective solution for compete test and verification of control system performance. In addition, HILCluster provides easy to use development and test environment for system level micro-grid controller.

PLL steady-state response in distorted grid voltage conditions for different designed bandwidth

DC-link voltage loop, disturbance rejection test (DC-link voltage, phase voltage and phase currents)

Grid voltage sources configured to arbitrary waveforms in distorted grid voltage conditions

Summary

To support growing micro-grid market, efficient, reliable and compact power inverters, with unique set of functions and features that differ from standard grid connected converters, need to be developed. The HIL602 system is a compact and easy to use real-time emulator that provides ultra-high fidelity environment to simulate a number of converters, electrical machines, pv panels, that are controlled with real controllers. Controller performance can be tested both in grid-tie or in off-grid configuration. Even more importantly, HIL solution is distinguished by an unmatched ease of use and flexibility to choose the circuit topology and circuit parameters, and specify disturbances and fault conditions that are often difficult and expensive if not outright impractical to do in a power laboratory.

References
[1] Remus Teodorescu, Marco Liserreand and Pedro Rodríguez, Grid Converters for Photovoltaic and Wind Power Systems, © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-05751-3

 

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