An Investigation of Series and Parallel Configurations for Hybrid Power Amplifiers

Power Hardware-in-the-Loop (PHIL) is becoming increasingly popular for compartmentalized testing of electric power equipment in several areas such as in electric drive systems and distributed power generation systems. The fundamental idea of PHIL is to create flexible conditions for Devices under Test (DUT) to be properly assessed in real time and dynamic conditions with their rated power levels. Connected to the DUT is the Power Amplifier (PA), which is responsible for increasing the voltage and current levels, given from the Real-Time Simulator (RTS). The DUT is a physical equipment and high-complexity models are used to control the PAs to emulate necessary conditions for the DUT to be evaluated. One of the main benefits of PHIL is that it can provide a platform for conducting a number of severe tests without risking damaging the equipment that is being emulated, while testing the actual response of the DUT. It can also help with the preliminary design and performance assessment of new types of machines, drivers and controllers, thus significantly reducing the time to market of new equipment. The flexibility of PHIL is also one of its main assets, since the combination of the RTS and the PA can be used for various applications only by changing the model and/or parameters of the emulated element.
\nThis thesis will evaluate the main architectures, control strategies and PHIL applications of PAs. Linear Power Amplifiers (LPA) provide an overall great performance due to its high bandwidth but are expensive, mostly at increased power ratings. For high PAs with fast dynamic response and reduced waveform distortion, the Hybrid Power Amplifier (HPA) configuration provides a good cost-performance compromise. HPAs are built essentially with the association of a low-cost Switch Mode Power Amplifier (SMPA) and an LPA.
\n The first configuration to be investigated is the series connected HPA intended for high voltage systems. The SMPA consists of a Cascaded H-Bridge Multilevel (CHBM) converter for increased modularity. A single-pulse per H-bridge modulation technique called Nearest Level of Control (NLC) is used for minimizing the switching losses. However, this leads to unbalanced power consumption by the H-bridges when the SMPA provides relatively low output voltages, thus compromising the reliability and power quality of the SMPA. A new modulation technique called Split-Voltage Fist-In First-Out (SV-FIFO) that mitigates this issue is proposed. Its implementation requires the use of a supplemental, but simple, control loop based on the magnitude and frequency of the reference output voltage. Experimental results are presented to validate the design approach and demonstrate the high performance achieved with SV-FIFO.
\nThe parallel connected HPA is also evaluated in this thesis. In a similar way to the series connected HPA, the LPA provides high bandwidth (BW) and active power filtering while the bulk of the power is provided by the SMPA. The SMPA is realized with a three-phase Voltage Source Converter (VSC) and three single-phase LPAs. The contribution relies on proposing a new topology and current control strategy that aims to reduce the size of the required LPA, which is costly. This is achieved by using the reference current of the HPA for the current control loop of the LPA, and the actual HPA current as the reference for the SMPA current loop. By making the bandwidth of the current loop of the LPA higher than that the SMPA one, the first provides the fast transient components and harmonic filtering while the second, the bulk of the HPA current. 
\nAdditionally, this thesis also covers the evaluation of techniques for Amplitude, Phase Angle and Frequency (APAF) detection for single-phase systems. Amplitude, phase and frequency detection is a key feature for the control of the series HPA, but it is also useful for other important applications, such as the synchronization of renewable sources to Alternate Current (AC) grids, which is a largely growing practice. APAF for single-phase systems are more challenging since they require additional and more complex techniques to determine the phase angle. Usually, both single and three-phase systems are designed for a single and known frequency, usually the grid’s frequency. However, a wider range of frequencies is necessary for other applications such as HPAs. This thesis will examine two proposed techniques for APAF. The first is based on the combination of the integral and derivative actions and the second is based on the modification of a zero-crossing detection system. Both systems are discussed in detail and validated experimentally.

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