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Posted on 10 July 2019

Ultra low latency HIL simulator for Power Electronics Applications

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Software testing moves from the laboratory floor to the office desk

Digital simulation in power electronics (PE) is a standard tool of design but it has traditionally referred to "off-line" simulation in which simulation time on a PC may be orders of magnitude greater than the simulated events would be. For this reason, power electronic circuits are today still real-time (RT) tested with analogue simulators – basically scaled down versions of the real converter in a real power lab, because PCs are simply not as fast as power electronic switches. All this is about to change thanks to custom processors with one microsecond latency and their accompanying software.

By Eric Carroll and Ivan Celanovic, Typhoon RTDS GmbH

 

A brief History of RT Simulation

Interestingly, the first real time (RT) simulator was built, in fact, for power, (albeit for power transmission, not PE) in 1927 at MIT. The first RT digital simulator, Whirlwind, was built also at MIT in 1951 for air defence with data being sent to it, in RT, via telephone lines. In the 1980s, digital simulation was employed to emulate aeronautical systems and is today extensively used in the increasingly complex world of automobiles which contain up to 80 micro processors and the accompanying communication network between them. However, these simulators can only emulate relatively slow, mechanical systems and any PE response is effectively averaged since a typical computation cycle takes about 50µs (admittedly, with a very large number of simultaneous floating point operations - FLOPS).

Faster processors are bringing RT time steps down to about 30µs but semiconductor switching events occur within one microsecond, for the most part and that is about the time it takes to fail a semiconductor, so until now, RT simulation of converter circuits has remained the domain of analogue simulators: in 2010, this changes with the arrival of the first dedicated PE real time digital simulator (RTDS).

Digital versus analogue simulation

As mentioned, analogue systems have been successfully used for decades, so what is the reason for going digital?

The analogue system is usually a scaled down version of the real converter-under-test, which is still a real converter in its own right. It has to be built and verified before it can serve as a test vehicle. Moreover, it has to be tested in a power lab by trained technicians, with variable loads, variable power-supplies, instrumentation (oscilloscopes etc) while respecting the applicable safety regulations. When something goes wrong (and if it didn't there would be no need to test) there is often a loud “bang” and the simulator has to be repaired or rebuilt. In companies where several converters are in design or being upgraded with new controls, these test facilities and their personnel become bottle-necks in the development process.

A digital simulator, by contrast, is essentially a fast, dedicated desktop computer which can be used in the same office environment as the off-line simulator which supported the original design. Thus a recently designed controller, its software and firmware can be fully tested in the "Hardware in the Loop" (HIL) configuration before ever going into a power lab.

HIL testing allows:
• reduced development cycles
• safer and lower-cost testing
• better code coverage during tests
• easy testing of parameter changes
• elimination of costly failures and down-time
• automated test cycles which permit all operating conditions to be scanned with any malfunctions recorded and flagged
• comprehensive testing for improved safety and reliability (reduced commissioning times, fewer on-site repairs or recalls).

Testing in the power lab is not eliminated, as a final verification of a complete system remains necessary but it is now limited to type testing and out-going inspection rather than being an integral part of the development process.

Other applications of PE RTDS include customer and service personnel training as well as teaching of controls and power electronics.

The Typhoon RTDS Breakthrough

Today's RT digital simulators successfully emulate complex systems such as power system networks, trains, planes and automobiles in which massive computational power is required but the I/O (or loopback) latency is generally in the range of 50 to 100µs. PE represents a different challenge in that even a complex switching matrix such as, say, a back-to-back three-level inverter with braking choppers contain relatively little data (78 states, voltages and currents) but which change very quickly. Thus the processor required to emulate such a system needs to be very fast but does not necessarily need to handle many GFLOPS.

Where commercial processor development strives to achieve high levels of computing power, the Typhoon RTDS processor aims to be very fast operating at a fixed cycle time of 1µs by which it is understood that the combined I/O latency and calculation time is 1µs, irrespective of the circuit complexity. With such a short latency, the switches respond as quickly as in a real converter (turn-on and off times for a 1700V IGBT are about 1 and 2µs respectively).

T-RTDS Modelling Approach

To achieve “hard” real time digital simulation of switched dynamic systems, not only the processor has to be fast but the PE system representation has to be minimalist i.e. the algorithms must be "lean and mean" and the input/output boards (I/O), very fast. It is the combination of a custom processor, ideal switches and optimised algorithms which allows a completely deterministic simulation time step – the key to real-time simulation. As opposed to off-line simulations, where variable time steps are possible and time-reversals are used to correct zero-current crossings; with RT, there is no going back!

Typhoon Simulators

Currently, one standard product is available for simulating a 2-level/3- phase inverter with a diode rectifier input, according to the circuit of Fig. 1, in which all the passive components and motor parameters can be programmed and the supply voltage set and perturbed by harmonics, flicker or glitches. Fig. 1 also shows the graphic user interface (GUI) for selecting and scaling the o/p signals.

RTDS150 schematic editor and graphic user interface

The RTDS150 has 8 BNC outputs which can be scaled and programmed to display any state, voltage or current in the circuit of Fig. 1. Fig. 2 shows a typical output of four channels displayed on a Tektronix oscilloscope.

Typical simulator output

The RTDS800 is a more versatile machine scheduled for release in the autumn of 2010. It is based on the circuit of Fig. 3 and allows a degree of topology configuration achieving 16 variants of the circuit (single-phase, 3-phase, brake and boost choppers etc.) and allows the addition of up to 50 passive components.

The versatile RTDS800 has semi-variable topology

Beyond these two standard simulators, other semi-fixed topologies as well as multi-converter and/or multi-level configurations can be realised on a custom basis.

The RTDS150 which simulates the system of Figure 2

Outlook

The high speed platform having been firmly established, two major developments will follow in the next 2 – 3 years.

Firstly, Typhoon's present custom-processor technology allows a further reduction of latency below 1µs which will be needed for the emulation of switches in very high frequency converters (say, to 1MHz PWM). Secondly, Typhoon application-SW will be expanded with a suit of design-evaluation tools such as test-automation, oscilloscopefunctionality, junction-temperature calculation as well as efficiency and EMC evaluation.

Conclusions

The growth of PE in recent years has been remarkable, driven by the need for energy savings (e.g. with motor drives) and energy production from renewables (e.g. wind and solar) as well as by the rapid introduction of electric drives in automobiles. This increasing development speed puts new time-to-market constraints on both development and test engineers.

HIL testing makes controls development and upgrades faster, more thorough and bug fixes are far less costly when caught early in the process, which is the reason why HIL testing methodology is so widely adopted for testing complex systems, other than PE. With the latency barrier finally broken, full RTDS for stand-alone PE simulators and the embedding of PE RTDS into existing system simulators, is now not only, possible but also very cost-effective.

 

 

 

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