Design of an integrated hybrid configuration of a Modular Multilevel Converter (MMC) and an Impulse generator

Abstract

The penetration of power electronics into the grid is growing rapidly due to the increasing number of renewable generation systems. These power electronic converters introduce electrical stresses in the form of complex waveforms into the grid caused by their switching operations. The High voltage (HV) equipment is subjected to these stresses. To maintain the reliability of the power system it must be able to withstand these kinds of complex stresses. To test the HV equipment with such stresses and assess its correct functioning, it is important to be able to reproduce such complex waveshapes. A new Modular Multi-level Converter (MMC) based arbitrary waveshape generator with 67 sub-modules in each arm rated for 100 kV is being designed to target the testing of medium voltage (MV) class equipment. Since the MMC has its own limitations in generating superimposed Lightning impulse (LI) waveforms, an integrated hybrid configuration combining the conventional Marx generator circuit and MMC is needed to generate the superimposed LI. This thesis intends to design and demonstrate the feasibility of such an integrated hybrid configuration focusing on two major goals. Firstly, to investigate the capabilities of the Typhoon HIL to implement the control of MMC and compare this with the existing OPAL-RT controller, and secondly, to design an integrated hybrid configuration to produce superimposed waveforms. To achieve the first goal the Typhoon HIL hardware, software, MMC topology, and the various modulation techniques were studied in depth. Phase shifted carrier (PSC) modulation-based open loop control was implemented on the CPU and FPGA of the HIL device. The results were compared with the offline simulations, and it was found that the control implemented on the CPU is suitable for up to 1 kHz, and the FPGA control is recommended for higher frequencies. The performance of Typhoon HIL is better than OPAL-RT regarding generating accurate gate pulses at higher fundamental frequencies. The approach for the second goal used is to study the literature on existing composite test setup to understand the requirements to design the decoupling circuit between impulse generator and MMC. A downsized model of an integrated hybrid configuration was designed and validated by offline simulation. Next, a 300 V hardware prototype was implemented in a step-by-step manner. First, Rapid control prototyping was performed separately on the MMC comprising 2 sub-modules in each arm with the FPGA control developed using the Typhoon device. Thereafter, the impulse circuit was coupled with the MMC to test for both power frequency and DC superimposed impulses. It was observed that the real-time simulation results were identical to the offline simulations and the superimposed impulses could be generated with high accuracy. Finally, the output of the MMC for higher signal bandwidth was tested using the Typhoon FPGA control. It was found that the MMC could generate sinusoidal waveforms up to 3 kHz.