A real-time simulator is a device that emulates the real-time behaviour of a system or apparatus at real-life speed. Real-time simulators comprise elements that are typically based on computers or similar digital computing devices that compute the apparatus-governing-equations, which typically include a set of Differential-Algebraic Equations (DAEs). Practically, the system/apparatus may comprise ‘controlled devices’, meaning that they normally work in conjunction with a controller. The controller has the objective of effectively controlling the apparatus within certain objectives and requirements. In real life, the performance of the device (also known as the ‘plant’) is controlled by accepting the commands of the controller. The controller adjusts its commands by reading the device parameters/states (e.g., currents, position, speed). Expediently, the controller and controlled devices are connected in a closed-loop. The study of the behaviour of a controlled device and a controller connected in a closed-loop is a complex subject. Although basic PID (Proportional Integral Derivative) control can be studied using analytical methods, non-linear behaviour, fault modes and protection considerations add to the complexity so much that it becomes significantly difficult to validate a controller analytically.
By using a real-time simulator, engineers can test and validate the control laws of the controller in a safe environment, without risk of injuries, by replacing the real apparatus by a virtual one (especially in high power applications such as ships, planes, electric plants or grids). Simulators are often used also in cases where it is not even possible to use a real plant, for example to test protection limits in borderline conditions.
The real-time simulator itself generally comprises two main parts: a computing unit/means (—CPU, FPGA, GPU, or a combination of these—) {FPGA is Field Programmable Gate Array}, running-models of a simulated apparatus and an input/output (I/O) interface. These I/Os connect the simulated controlled device to the controller under test. These I/Os are typically sets of analog inputs and outputs, and digital inputs and outputs. For example, the digital input will read the controller pulse that drives a switching converter (—power electronic converter—) simulated in the real-time simulator. Current and voltage values of the switching converter will be sent to the analog output of the simulator so that these values can be read by the controller. I/Os are required to close the loop between the real controller in the real world and the emulated controlled device. To synchronize the real-time simulator at real world time, the real-time simulator includes an internal clock, which can come from an I/O device or be generated by the operating system. It is also noted that the computing unit must be fast enough and/or use fast-enough algorithms to be able to compute and iterate the model states and outputs at the real-time pace. This is necessary to enable interaction with a real device connected to the I/O of the simulator.
Real-time simulation technologies are nowadays an integral part of the design and test process of many types of electric systems like large power grids, power converters and variable speed drives. These modern design approaches mitigate the risks through extensive use of technologies like Hardware-In-the-Loop (HIL) simulation and the model-based design approach. In HIL simulation, a plant controller is tested against a real-time simulated model of the plant. HIL simulation technologies enable more gradual integration, while diminishing the risk, and costs of such projects. Also, in HIL simulation, more elaborate test coverage can be achieved than is possible using analog prototypes because of the safe operational limits of real power electronic devices and power plants.