Regenerative AC Electronic Load with One-Cycle Control

In Wha Jeong, Mikhail Slepchenkov, Keyue Smedley, and Franco Maddaleno Power Electronics Laboratory

Dept. of Electrical Engineering and Computer Science University of California, Irvine

Irvine, CA 92697, USA

Abstract—Variable linear or nonlinear loads are critical for testing AC power supplies and power equipment in various operating conditions. At present, most testing loads are based on bulky resistors, capacitors, and inductors on which the testing energy is consumed to generate excessive heat, and moreover, the impedance value is limited by finite combination of these bulky loads. Some commercial electronic loads are available, which provide flexible impedance control but the energy absorbed by the load is still dissipated as heat. The work presented in this paper is a continuous effort of UCI Power Electronics AC load project. A Regenerative AC Electronic Load (RACEL) is developed with One-Cycle Controller (OCC), which doesn’t require any Digital Signal Processor (DSP) and software in the control loop. Using a reconfigurable Field Programmable Analog Array (FPAA), the OCC can be integrated into a single FPAA with easy reconfiguration and simple interface structure. The proposed RACEL can emulate any impedance load, linear or nonlinear as well steady or dynamic with the regeneration of the testing energy. Simulation and experimental results are presented to verify the proposed RACEL performance.

I. INTRODUCTION At present, passive dissipative loads are used for most

power equipment testing. In these situations, the testing energy is wasted. At high power levels, heat management of the bulky load becomes very challenging. Moreover, the testing impedance cannot be adjusted smoothly. When a different load configuration is required, reconnection of the load bank has to be performed to achieve different load parameters. Some commercial electronic loads are available for relatively low power rating in the range of 1-5kW, which provides flexible impedance control but the energy absorbed by the load is still dissipated as heat. What in great need is a Regenerative AC Electronic Load (RACEL). RACEL is a piece of power electronic equipment that emulates any physical impedance and sends the testing energy back to the power grid. The RACEL may be used in the burn-in test of AC power supplies, including UPSs, AC motor drives, and protection facilities.

Some researchers proposed RACELs with regenerative function based on hysteresis current control and repetitive control using Digital Signal Processors (DSPs) [1-2]. Those control methods require real time calculation and some complex circuit to implement the main controllers. The UCI Power Electronics Laboratory proposed a regenerative AC electronic load in [3-4]. In this effort, One-Cycle Control (OCC) was used to realize a three-phase AC electronic load. Due to time limitation of the MS thesis, the regenerative operation was not completed. The work presented here is the continuation of the effort in [3-4].

In this paper, a single-phase RACEL is implemented with One-Cycle Controller. Based on a back-to-back configuration, the proposed RACEL can emulate linear or nonlinear load in steady or dynamic condition with energy regeneration capability.

II. REGENERATIVE RACEL CONFIGURATION Fig. 1 shows the single-phase regenerative RACEL with

the OCC controller. The Equipment Under Test (EUT) is connected to the input rectifier, which is connected to the output inverter in a back-to-back configuration. The control objectives of the OCC controller are to draw the required active and reactive power from the testing AC power source for the input rectifier and make the output current synchronous with the grid voltage for the output inverter. The current reference generator produces the current reference signal Viref for the OCC controller to emulate the user-defined linear or nonlinear load.

The power stage of the single-phase RACEL based on OCC control is shown in Fig. 2. The input rectifier and the output inverter are all based on the standard H-bridges. The input rectifier emulates the specified load and the output inverter feeds back the absorbed energy to the utility grid with unit power factor. When the input of the RACEL is connected to the AC power supplies or the power equipment under test, the real impedance load can be emulated losslessly by controlling the amplitude and phase of the input current and feedback of the energy to the power grid.

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Figure 1. Regenerative RACEL implementation with OCC control.

Figure 2. Single-phase regenerative RACEL power stage configuration.

In addition, the proposed RACEL can be used in dynamic load emulation. Since the OCC controller has fast and stable response as well as simple circuit, it can perfectly be used in the RACEL applications.

III. OCC CONTROL OF RACEL The proposed RACEL has three working modes: linear

load mode, rectified load mode, and user-defined load mode. In linear load mode, the RACEL can exactly emulate the load whose impedance value can be adjusted continuously. In rectified load mode, the RACEL can emulate nonlinear rectified loads for a wide range of testing applications. Finally, in user-defined load mode, if user has a complicated load model and needs a specific dynamic test, the RACEL can provide unique capability to emulate any complex load. The proposed RACEL works in regenerative operation for all the operating modes.

The OCC controller of the single-phase RACEL requires one current reference signal (Viref) with the phase shift from the line voltage (Vs) of the EUT depending on the emulated load. The input rectifier accurately controls the input current to track the current reference signal and make the RACEL to function as the specified linear or nonlinear impedance load.

From [5], the active and reactive testing power processed by the input rectifier can be directly controlled by changing

the values of kh and kv, respectively. On the other hand, the output inverter of the RACEL can operate with the fixed amplitude of the saw-tooth carrier, Vm and the regenerated testing power can be controlled by only adjusting kh with unit power factor.

The proposed RACEL uses a FPAA-based OCC controller. Using a statically or dynamically reconfigurable Field Programmable Analog Array (FPAA), the OCC controller has a simple integrated structure and enough flexibility to implement variable internal parameters and multiple operation functions for the regenerative RACEL. The Anadigm 5V 44pin FPAA is used to realize the single-FPAA OCC controller with the help of AnadigmDesigner2 software.

IV. CURRENT REFERENCE GENERATOR OF RACEL Fig. 3 shows the Graphical User Interface (GUI) window

of the current reference generator for the regenerative RACEL using GUIDE, the GUI development environment of MATLAB. This current reference generator particularly has unique function to emulate any complex or dynamic load with the help of simulation tools, for example, Simulink. It can open the simulation software which includes the schematic diagram of the complicated or dynamic load model, execute the simulation program, and receive the simulated data of the dynamic load to accurately generate the current reference signals.

The stand-alone current reference generator of the regenerative RACEL uses a 340 x 240 resolution touch screen and an 8-bit microcontroller, ATmega328P. With the help of the user-friendly interface, any testing condition or any impedance load can easily be selected and realized by generating the proper current reference signals. In linear load mode, the RACEL can emulate the linear impedance load by the specified R, L, and C values or the testing active and reactive power values, P and Q.

For the single-phase regenerative RACEL, the current reference signal Viref of the OCC controller can be calculated by the following equations in each case.

Figure 3. GUI-type current reference generator of RACEL.

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Assuming that the input line voltage Vs and input current is from the EUT are sinusoidal, the input voltage Vs and current is of the input rectifier can be described as:

(1)

where VSM and ISM are the voltage and current amplitudes of the tested AC power supply, φ is the phase difference between the input voltage and current or the phase angle of the impedance load, and ω is the angular frequency of the tested AC power supply.

A. Resistive Load Emulation In this case, the RACEL appears pure resistive

characteristic. The current reference signal Viref can be described as follows:

(2)

where R is the required load resistance and Rs is the input current sensing resistance.

B. Inductive Load Emulation The RACEL works as a RL impedance load during this

test condition.

(3)

where R and L are the resistance and inductance of the impedance load, respectively.

C. Capacitive Load Emulation In this test condition, the RACEL shows a RC impedance

characteristic.

(4)

where R and C are the resistance and capacitance of the impedance load, respectively.

D. Active and Reactive Power Load Emulation From (1), the average active power PS and reactive power

QS can be represented as follows:

(5)

If the user wants to emulate the active power PSref and reactive power QSref of the specified impedance load, it is clear that the amplitude ISMref and phase angle φref of the input current reference can be calculated by (5).

(6)

E. Nonlinear Load Emulation The RACEL operates as a nonlinear rectified load or a

distorted load for the harmonic test. For three-phase applications, various unbalanced loads can also be emulated by the three-phase RACEL.

For the output inverter of the RACEL, the control goal is to make the output current io of the RACEL synchronous with the grid voltage Vo and contribute to the unit power factor injection of the recycled testing energy with the highest efficiency. In this condition, the control requirement is just realizing unit power factor operation and stabilizing the dc link voltage. Thus the dynamic performance of the output inverter is not strict.

V. SIMULATION RESULTS The single-phase regenerative RACEL with the OCC

controller has been modeled and tested by PSIM to verify the steady and dynamic performance of the proposed regenerative RACEL. In the simulation model, the input line voltage Vs and the output grid voltage Vo have the same amplitude, 208Vrms and phase angle. The dc link voltage Vdc is 400Vdc and the dc link capacitor CDC is 5mF.

In the input rectifier, the input inductor Ls is 2mH and the switching frequency is 16kHz. In the output inverter, the inductor Lo is 3mH and the switching frequency is the same value, 16kHz.

Fig. 4 shows the simulation result for the RL load emulation with 60° phase angle. The input current is has 40A peak amplitude and 60° phase delay compared to the input line voltage Vs, exactly following the current reference signal with the dc link voltage control. In addition, the output current io is in 180° phase shift with the grid voltage Vo. The output inverter feeds back the testing energy to the utility grid with unit power factor.

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Figure 4. Input and output waveforms with RL load, 60°.

Figure 5. Input waveforms with L load (20mH), 90°.

Figure 6. Current reference and input waveforms with diode rectified load.

Fig. 5 shows the input line voltage and current waveforms for the L load emulation with 90° phase angle. In this case, the RACEL works as a 20mH inductor which has a huge size and a massively expensive price for high power applications. Since there is the power dissipation in the semiconductor switches of the input rectifier, the pure inductor emulation may not be possible to realize by using the input current control of the input rectifier with 90° phase-shifted current reference signal. But since a real inductor has some resistance, the practical inductor emulation would not be purely reactive.

Fig. 6 shows the current reference signal, the input line voltage, and the input current waveforms when the RACEL emulates a nonlinear rectified load with a diode bridge rectifier, a 1.5mH dc link inductor, a 1mF dc link capacitor, and a 100Ω resistor load. The actual input current has a long tail between two sharp conduction periods but looks almost identical to the waveform of the current reference signal.

VI. EXPERIMENTAL RESULTS On the basis of the simulation results, a 5kVA single-

phase regenerative RACEL with the OCC controller is designed and built to verify the effectiveness and performance of the OCC based RACEL. The design specification of the RACEL is the same as the simulation condition.

A variable transformer is used to change the input line voltage Vs. To realize the OCC controller, a single FPAA, AN221E04 from Anadigm, is used to integrate all the OCC core circuit into a single IC with easy reconfiguration and simple interface structure.

Fig. 7 shows the current reference signal and the input voltage waveforms for the RL load emulation with 30° phase angle. In this test, the active power PSref is 1732W and the reactive power QSref is 1000VAR. Using (6), the input current is 13.6A peak and the required amplitude of the input current reference is 0.05x13.6A = 680mV which is almost the same as 620mV in Fig. 7.

Fig. 8 shows the experimental result for a nonlinear load emulation with a diode bridge rectifier. Since the peak amplitude of the input current reference is 780mV, the RACEL prototype can provide the specified test condition of the diode rectified load with 15.6A peak input current. This test condition can be applicable to the harmonic test for typical power equipment such as power transformers.

Fig. 9 and 10 show the input line voltage and current waveforms for the RL load emulation with 20° and 45° phase angle, respectively. The input current is has about 7A peak and 6A peak amplitude with 20° and 45° phase delay, respectively compared to the input line voltage Vs.

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Figure 7. Input voltage and current reference waveforms with RL load, 30°.

Figure 8. Input voltage and current reference waveforms with diode rectified load.

Figure 9. Input waveforms with RL load, 20°.

Figure 10. Input waveforms with RL load, 45°.

Figure 11. Input waveforms with distorted load.

Fig. 11 shows a nonlinear load emulation with harmonic distortion. This current distortion affects the AC power supplies or the testing power equipment, and may cause the testing power transformers to overheat from the harmonic loss. The proposed RACEL can provide special test conditions to emulate this kind of nonlinear loads for a wide range of testing applications.

Fig. 12 shows the experimental result for regenerative operation of the proposed RACEL. The input rectifier emulates the RL load with some phase delay and the output inverter feeds back the absorbed testing energy to the utility grid as seen in Fig. 12.

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Vs

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Vs

Io

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Vo

Figure 12. Input and output waveforms with regenerative operation.

VII. CONCLUSIONS In this paper, a RACEL is implemented with a FPAA-

based One-Cycle Control (OCC) to emulate any linear or nonlinear impedance load. For the proposed RACEL with energy recycling capability, the testing energy absorbed from the tested AC power equipment can be fed back to the utility grid with unit power factor. In addition, if necessary, the proposed RACEL can operate as a variable AC or DC power source by the back-to-back configuration.

The control method and controller design of the RACEL are simple and flexible with the use of the FPAA-based OCC controller instead of using separated integrator, comparator, and compensator. With the dynamic reconfigurability, the FPAA-based OCC controller can implement multiple operation functions and maintain precision operation as various AC loads. The proposed RACEL can easily be extended from single-phase to three-phase applications with the help of the three-phase OCC control. Furthermore, the

regenerative RACEL can emulate and change any complicated test load with the developed GUI program and user’s simulation tool. There is only small power dissipation in the semiconductor switches to realize the test load condition.

Simulation and experimental results based on the 5kVA single-phase RACEL prototype are presented to verify the regenerative RACEL performance as the linear and nonlinear loads. In the future, an OCC multilevel regenerative RACEL for high voltage and high power applications will be discussed and demonstrated with the emulation capability of various steady state and dynamic loads.

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[4] Simone Primavera and Giuseppe Rella, “Three-Phase Electronic Load,” MS Thesis, POLITECNICO DI TORINO, 2007.

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