Modular Multilevel Converter Valve Research Articles

A failure mechanism of IGBT module in MMC and improvement

In modular multi-level converter valves, as the blocked mode happens, the failure in IGBT modules often causes a chain explosion reaction, which masks the initial failure mechanism. The root cause of the failure was that the voltage, consisting of the diode forward voltage and the induced voltage on the parasitic inductance, exceeded the reverse blocking capability of the anti-parallel IGBT. The methods against the failure were studied from three aspects: decreasing the diode’s peak forward voltage, increasing the reverse blocking capability of IGBT, and minimizing the parasitic inductance of the package. It was found that the doping concentration’s effect on the diode’s peak forward voltage is not monotonic. It can be explained from the carrier density distribution during forward recovery transient that lower doping concentration reduces the carrier injection, and higher doping concentration reduces the excess carrier concentration in the doping region. Then, the influence of two buffer designs on the reverse blocking capability of IGBT was compared. It was found that a higher reverse blocking voltage would be achieved when a multi-proton implantation buffer replaced the diffused buffer. And the new design has almost no degradation in forward blocking capability.

Analysis and suppression of voltage oscillation at DC circuit breaker terminal in DC fault transient process

High-frequency voltage oscillation occurs at the initial stage of DC short-circuit fault in the modular multilevel converter (MMC)-based high-voltage direct-current systems, which directly threatens the reliable operation of the secondary equipment in DC circuit breakers (DCCBs). Therefore, this paper focusses on the voltage oscillation characteristic at DCCB terminals, considered the stray parameters of connecting lines, the parasitic parameters of main equipment and the impedance of grounding grid, established equivalent circuit models of MMC-HVDC converter station at the initial stage of pole-to-pole and pole-to-ground fault, and obtained the analytical formula for the voltage oscillation frequency. Then, the comparison with the DC short-circuit test data of ±200 kV MMC-HVDC converter station verified the validity of the models and revealed the mechanism of voltage oscillation. Finally, the influence of the parameters of the converter station on the oscillation characteristics was discussed. It was proposed to install the DCCB at the MMC valve's terminal near the DC side, and the oscillation frequency is about 10–60 kHz, which is more than 5 times lower than that at the DC reactor's terminal near the DC pole bus, and the disturbance of oscillation to DCCB was alleviated.

Wireless Communication in Modular Multilevel Converters and Electromagnetic Interference Characterization

The wireless control of modular multilevel converter (MMC) submodules was recently proposed. The success of the control depends on specialized control methods suitable for wireless communication and a properly designed wireless communication network in the MMC valve hall while aiming for low latency and high reliability. The wireless communication in the hall can be affected by the electromagnetic interference (EMI) of MMC submodules, voltage and current transients. In this article, firstly, a wireless communication network based on 5G New Radio is designed for an example full-scale MMC valve hall. After that, the radiated EMI characteristics of the MMC submodules with different voltage and current ratings and two dc circuit breakers are measured. The effects of EMI on wireless communication in the multi-GHz frequency band are tested. The interference from the components is confined below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {500 \text {M} \text {Hz} }$ </tex-math></inline-formula> , and the wireless communication with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {5825 \text {M} \text {Hz} }$ </tex-math></inline-formula> center frequency is not affected by the interference.

A Novel Copper Layer-Based Field-to-Trace Coupling Model and EMS Evaluation Method for DSP-Based Control Circuit in MMC-HVDC System

Underlying control circuit failure problems, especially control signal output failure problems, widely occur in high-voltage large-current modular multilevel converter (MMC) valve towers. The failure mechanism for the digital signal processor (DSP)-based control circuit is studied in this article. This study is based on the combination of low-voltage-level integrated circuit (IC) immunity model and a new perspective on the field-to-trace coupling effect. A novel copper-layer-based field-to-trace coupling model which combines the power system and digital control circuit in the MMC valve tower is proposed. Therefore, the specific electromagnetic interference (EMI) value conducted into the IC can be calculated. Then a new DSP IC electromagnetic sensitivity (EMS) evaluation method is proposed, which can be a guideline in industrial applications. To show the validity and accuracy of the field-to-trace coupling model, an indirect laboratory experiment is designed, then a situation is considered in this article, and the results are compared with PSpice circuit simulations, which show acceptable relative errors. Moreover, based on the situation considered above, one application example of the novel evaluation method is described in detail.

Hybrid ultra‐HVDC system with LCC and cascaded hybrid MMC

Nowadays, the hybrid high-voltage direct current (HVDC) system consisting of line commutated converter (LCC) and modular multilevel converter (MMC) has become the most competitive candidate in multi-infeed power systems. Owing to the limitation of insulated-gate bipolar transistor's withstand voltage, the DC voltage rating of MMC is not compatible with the LCC valve. To increase the DC voltage level, the number of sub-modules of each arm should be increased drastically, leading to many technical problems in communication design, optical fibre instalment, cooler system design and so on. To overcome the above problems, a hybrid HVDC system consisting of LCC and cascaded hybrid MMC valves is proposed to realise ultra-HVDC transmission. It can switch two operating modes (single valve/double valve) according to the conditions so that the system can maintain a certain power transmission when one of the valves quit operation during faults or maintenances. Meanwhile, by implementing the hybrid MMC, the system can ride through DC faults. The design of the topology and controllers, the DC fault ride-through strategy and the online switching strategy for MMCs will be presented in this study. Finally, the performance during DC faults and online switching is verified by extensive simulations.

Type tests on MMC-based HVDC valve section with synthetic test circuits

Summary Electrical type tests are a key step to verify the design of voltage source converter valves for a specific modular multilevel converter (MMC) high-voltage direct current (HVDC) project. For a high-power handling capability of modern insulated gate bipolar transistor (IGBT), it is very difficult to balance between type test quality and test rig investment. In compliance with IEC 62501 and CIGRE 447 requirement, this paper presents a kind of synthetic test circuits (STCs), consisting of a steady-state test circuit, an IGBT overcurrent turn-off test circuit, and a short-circuit current test circuit, used for operational tests on MMC-based IGBT valve sections. Using the STCs, the current stress, the voltage stress, the junction temperature, di/dt, the performance of the valve's electronic circuits, and the protection capability of the parallel thyristor can be validated. Further, the parameters of the test circuits can be adjusted with the number of submodules of the MMC test valve. Thus, the STCs are an economical alternative for testing a representative part of an MMC valve. The paper describes and discusses the type tests, completely performed with the three test circuits, on MMC valves for a Xiamen MMC HVDC scheme, which is rated at 1000 MW/±320 kV. Copyright © 2016 John Wiley & Sons, Ltd.