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Posted on 26 November 2019

The Revolutionary New Impedance Analyzer

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Originally conceived as a time domain analysis device the HIL is now entering the frequency domain which will revolutionize the way power electronics controllers are developed, tuned and tested.

By Elaina Chai Massachusetts Institute of Technology and Adrien Genic Typhoon HIL Inc.

Introduction

As the applications and complexity of power electronics systems continue to grow, there is an increasing demand for fast and accurate tools for improving power electronics controls design, test, and verification. Hardware-in-the-loop (HIL) is receiving increased attention as an indispensable tool for designing and validating complex controls systems. It enables control engineers to test real controller systems by directly interfacing them with a real-time emulation of a converter power stage. Since engineers are able to quickly test controllers under a wide range of operating and fault conditions without a highpower lab, the amount of time and resources required for the design, testing, and validation of these systems can be drastically reduced.

The HIL configuration enables repeatable, formalized testing for a wide selection of operating conditions (including faults), that are often impossible or too expensive to test with real hardware. The high power when digitized in the HIL opens a world of new possibilities.

Challenges of Power Electronics impedance analysis

In the design, integration and analysis of complex power electronics systems, it is often necessary to obtain the small-signal impedance characteristics of an existing power electronics component or subsystem at a given operating point. Stability analysis in power electronics power systems is a crucial task due to the nearly ideal control capability of many modern power converters. The excellent load regulation capability of converters is a desirable feature in many applications, but it also makes converters a constant-power load device, which is a potential cause of negative impedance instability.

In order to obtain the frequency-dependent characteristics through experiment, there are significant obstacles that have to be solved. First of all, the system has to be running in the laboratory. Secondly, periodic voltage or current disturbances have to be injected into the system while it is running. The magnitude of the disturbances at a specific frequency has to be large enough to be able to measure it which can be challenging. Measurements of the disturbed system are then taken and post-processed to determine the impedances at specific frequencies. These measurements, when done in high power systems, are dangerous and require expensive equipment.

Figure 1 illustrates a typical stand for developing and testing power electronics controls in a conventional way.

Power electronics test stand, the conventional way; 1 - digital controllers; 2 - high power systems; 3 - measurement and testing equipment

Impedance analyzing the HIL way Thanks to the ultra high fidelity of HIL emulators, impedance and stability analysis of power electronics converters is made simple. Since the power stage is digitalized inside the HIL device, there is no high power in the loop anymore. A very intuitive interface allows the user to easily inject any kind of disturbances into the emulated power stage, measure them and process them automatically in order to get the desired impedances or transfer functions. All these tests and measurements are done on a HIL device, within the comfort of an office desk, shown in Figure 2. Various types of automated tests and analysis can be done automatically using scripting.

Power Electronics test stand, the Typhoon HIL way

The compound of a digitalized power stage and advanced signal processing capabilities of the HIL gives its impedance analyzer limitless usage of known impedance analyzing methods in a completely new fashion.

Using HIL's powerful oscilloscope and its capture function capabilities with storage space of up to 32 Mpts, a detailed and accurate time and frequency domain analysis can be obtained.

Figure 3 represents a block diagram of HIL impedance analyzer interaction paths with the power stage model and the controller. Disturbances can be injected anywhere in the power stage by adding disturbance voltage or current sources. Additionally, any measurement can be fed back into the embedded impedance analyzer. Frequency range and/or the exact frequencies of interest can be defined and the step of the frequency sweep as well. Disturbances can also be fed directly into the controller in order to obtain the control to output transfer function.

Impedance analyzing, the HIL way

The work flow of the impedance analyzer is shown in Figure 4. Disturbance sources and required measurements are included in the model. The controller is started up, and set to the required operating point. The impedance analyzer starts to inject disturbances at specified frequencies in the operating model. Measurements are collected for each disturbance frequency and plotted. Post-processing will obtain the discrete Fourier transform (DFT) of the collected data. In order to get the specified impedance characteristic or transfer function, measured variables are compared.

The time needed for completing the impedance analyzing process depends on the level of details specified by user. On TFS (frequency sweep time), shown in Figure 4, the number of swept frequencies has the biggest impact, while on data acquisition time (TAC) the amount of data captured for each disturbance frequency has the biggest impact. The overall time needed for the complete process can vary from a few seconds to few minutes, if high requirements are specified.

Timeline of the HIL Impedance Analyzer

The input impedance is measured by injecting a disturbance into the input voltage, while measuring its effect on the input current. The disturbance is modeled as a small sinusoidal voltage source in series with the input voltage. Indeed, this proves the benefits of using a HIL emulator, there is no need for an isolation transformer or impedance matching in order to successfully inject disturbances into the power stage. Input impedance magnitude and phase characteristics are shown with green dots in Figure 5.

HIL impedance analyzer results for a buck converter; the upper diagram shows the magnitude while the bottom is the phase diagram; green dots represent the input impedance characteristics; red dots are output impedance characteristics; blue dots are the lineto- output transfer function

The output impedance of buck converter is measured by injecting a current disturbance into the output of the converter. The resulting current and voltage across the load is measured to calculate the frequency response. Characteristics shown with red dots in Figure 5 represent the magnitude and phase of output impedance.

Line-to-output transfer function is measured by injecting disturbances into the input voltage and measuring the resulting output voltage. Input and output voltages are processed to get the line-to-output transfer function. Line-to-output transfer function is represented with blue dots in Figure 5.

Conclusion

By removing high power from the loop and replacing it with digital HIL devices, completely new possibilities for developing and testing power electronics systems are unfolded. From the comfort of his office, an engineer is able to develop and test power electronics systems in such detail that cannot be done in a high power lab. HIL devices provide vastly improved test coverage and reduced time to market. With HIL digitalized power, testing is made simple.

 

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