Improved Power Hardware in the Loop Interface Methods via Impedance Matching
Sanaz Paran, Student Member, IEEE
Chris S. Edrington, Senior Member, IEEE
Florida State University Center for Advanced Power System Tallahassee, USA
[email protected]
Florida State University Center for Advanced Power System Tallahassee, USA
[email protected]
Abstract- The power hardware in the loop (PHIL) method is a reliable simulation and validation technology. It can be utilized
to
augment
the
test
of
electrical,
mechanical
or
electromechanical components or subsystems with high power ratings. In addition, it can also be used to characterize their behavior when connected to some complex electrical networks or mechanical environment. Reliable and safe performances are paramount in a PHIL system. Thus, the proper PHIL modeling and
power
interface
validation
precedes
any
experimental
implementation. In this paper, we focus on the stability and accuracy of the damping impedance method (DIM) and we propose the modified DIM method which increases the accuracy and the stability of the pmL through dynamic impedance matching of the load and the linking impedance component. Different cases are introduced in order to study the parameters which have effects on the PHIL system. First, the basic DIM method and the modified DIM architecture are introduced, and then two cases consisting of a constant load and a variable load will be presented, respectively.
I.
INTRODUCTION
Power hardware in the loop (PHIL) is a combination of computer simulation of part of the system in real time and the control and measurement of the hardware under test that is connected to that real-time simulation, operating at power signal levels. PHIL is a safe and economical method that can be utilized to test hardware devices and thus is extremely useful in the power system/power electronic areas in order to provide a more realistic environment not available in software simulation models alone. As a consequence it has gained particular attention as a technology de-risking methodology among the aerospace, naval, utility, commercial and industrial sectors. Some literature has been published on utilizing PHIL for the switched reluctance machine and motor drives [1], [2]. Stability is one of the most important factors that have to be taken into consideration in the PHIL. Different papers introduce methods for stabilizing the PHIL system such as: introduction of an additional current filter in the feedback path [3], proposing a new method for the inductor coupled system which change the inductors value for both the
simulated inductor and inductor of the input matrix converter filter [4]. Common methods for modeling the power interface include ITM (ideal transformer model), TFA (time-variant first order approximation), TLM (transmission line model), PCD (partial circuit duplication), and DIM (damping impedance method) [5], [6], [7], [8]. Among these methods, DIM exhibits higher stability and accuracy [9], and unlike other methods has the ability to adapt by updating the value of the linking impedance (Z*) (resistor series with the inductor for the inductive case). To achieve a more stable and precise PHIL system, we improve the DIM method by providing a strategy to achieve this adaptation. The key feature of this modified DIM method is the ability to track the load changes via the impedance, rendering the methodology as quasi-dynamic utilizing component updates in the PHIL interface (Z*) to match with the new value of the load. Therefore, the updated linking impedance component is not a constant value and highly depends on the load changes. This paper will expound on this new PHIL interface technique. This article is structured in the following way: section II introduces the basic DIM method and the modified DIM method. In section III, the simulation results are shown for two different scenarios and section IV gives some conclusions and outlook on future work. II. A.
The DIM method is a composite of the ITM and the PCD algorithms obtained by inserting a damping impedance Z'. The DIM method configuration can be seen in Fig. 1. It is observable from equation (1) that when Z' equals Zb (the equivalent impedance of the hardware under (HUT)); the PHIL experiment will be absolutely stable because the magnitude of the open loop transfer function becomes zero. There is however, the challenging issue of obtaining Zb because this would require a perfect model of the hardware; but there are ways to approximate Zb from prior simulation results [10]. Therefore, the DIM method shows inaccurate results when Z' is not equal to Zb' The modified method we
This project is funded by Office of Naval Research.
978-1-4673-5245-1I13/$3l.00 ©20 13 IEEE
DIM METHOD
The Basic Concept of the DIM Method
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found, calculates Zb and then this value is injected to Z*. We illustrate this concept in the next part by elaborating the idea and simulation results. The equations for the controllable current source and controllable voltage sources in Fig.1 are shown in (2). In the circuit shown in Fig. 1, in order to have the most accurate result, Zab should be very small. G OL_DIM -
Za(Zb - Z*) e (Zb + Zab )(Za + Zab + Z*)
_
At s"
(1)
III. A. Constant Load
The modified DIM method will be applied to interface the hardware and the software as shown in Fig. 3. For the purposes of simulation, both the software and the hardware are modeled in MatlabIPLECS. Consider the linear RL load circuit with the component parameters that are given in the Table I.
(2)
i1(K) = iz (K)
SIMULATION RESULTS
Considered as the Hardware under Test
V1(K) =V�(K) vz(k) = v� (k- 1)
TABLE I SPECIFICATIONS OF THE CONSTANT LOAD Parameter Symbol Quantity Sample time 50 fls T. Input source 24sin(2 * Tr * 60 * t) V It: 2[2 Input resistance R 22[2 Output resistance Rb 5 mH Output inductor Lb Software "'-1---' Hardware
Figurel. DIM configuration
B. Implementation of the Modified DIM Method From the previous part, it is concluded that DIM depicts stability and accuracy but it needs some modification and adaptation in order to yield the desirable results. The concept is that the linking impedance must be equal to the load impedance that will be achieved by impedance matching. For implementing the modified DIM method, the voltage and current of the load are measured and then the root mean square (rms) values of the signals are calculated by the rms block. Additionally, the phase difference between voltage and current are measured, see Fig. 2. With these calculations, the Iinkin� impedance, Z* is obtained, which in this case is R* and L since it is an inductive load. In equations (3) - (5) the phase difference computed in Matlab is defined as cpo The output voltage and current are v and i, respectively. The excitation frequency of the voltage source is w. The left side of (3) has to be equal to the right side yielding (4) and (5).
rms(v) --C-') (cos
