Interoperability in HVDC systems could be supported with open upper-level control and protection (C&P) software, while hardware-near C&P functions stay black-boxed and proprietary. Methodologies like model-based systems engineering and graph theory can assist in defining the boundary between open and closed software. Most likely, partially open C&P software in HVDC is not hindered by legislation, but has to be addressed in contractual agreements. Also, a new responsibility matrix for testing is proposed.

Interconnecting power converters (IPCs) are the main elements enabling the interconnection of multiple high-voltage alternating current (HVac) and high-voltage direct current (HVdc) subgrids. To ensure stable operation of the resulting hybrid ac/dc systems, grid-following (GFL) and grid-forming (GFM) controls need to be carefully assigned to individual IPC terminals when using common IPC controls. In contrast, dual-port GFM control imposes a stable voltage on the ac and dc terminals and can be deployed on all IPCs regardless of the network configuration. In this work, we use hybrid ac/dc admittance models, eigenvalue sensitivities, and case studies to analyze and quantify the underlying properties of ac-GFM control, ac-GFL, and dual-port GFM control. Compared to common ac-GFM and ac-GFL controls, dual-port GFM control: 1) renders IPCs dissipative over a much wider range of frequencies and operating points; 2) significantly reduces the sensitivity of IPC small-signal dynamics to operating point changes; and 3) exhibits an improved dynamic response to severe contingencies. Finally, the results are illustrated and validated in an experimental scaled-down point-to-point HVdc system.

This article presents a novel method for accurate online extraction of semiconductor ON-state resistance in the presence of measurement noise. In this method, the ON-state resistance value is extracted from the measured ON-state voltage of the semiconductors and the measured load current. The extracted ON-state resistance can be used for online condition monitoring of semiconductors. The proposed method is based on the extraction of selective harmonic content. The estimated values are further enhanced through an integral action that increases the signal-to-noise ratio, making the proposed method suitable in the presence of noisy measurements. The efficacy of the proposed method is verified through simulations in the MATLAB/Simulink environment, and experimentally. The estimated ON-state resistance values from the online setup are compared to offline measurements from an industrial curve tracer, where an overall estimation error of less than 1% is observed. The proposed solution maintains its estimation accuracy under variable load conditions and for different temperatures of the device under test.

Commercially available semiconductor devices have a limited range of operating voltages. This operating voltage can be increased through series connection of the devices. In this paper, a novel active snubber circuit (ASC) is proposed that protects series-connected semiconductor devices from overvoltages during operation. The unique advantage of the proposed solution is that it does not use additional sensors or an external controller for voltage protection. That is, each ASC is equipped with sufficient components to protect its respective device. The proposed solution is mainly developed for cascaded H-bridge (CHB) and modular multilevel converters (MMCs) intended for flexible alternating current transmission systems (FACTS) and high-voltage direct current (HVDC) applications, which typically operate at low switching frequencies. Suitable extensions of the proposed design are provided for increased current capability and for possible fault-ride-through functionality. The efficacy of the proposed solution is verified by simulations in the MATLAB/Simulink environment.

This article presents a unique method of extracting the ON-state resistance of metal-oxide-semiconductor field-effect transistors (MOSFETs) used in power electronic systems. The proposed method uses ON-state voltage measurements of the device-under-test as well as the load current. Contrary to prior-art solutions, the proposed method does not require estimating the device currents, but rather directly uses the load current measurements. This reduces the computational effort for estimation without reducing the estimation accuracy. Moreover, it is shown that unlike time-based estimation techniques such as the recursive least-squares estimation method, the proposed solution does not require ON-state voltage and current measurements to be accurately synchronized. The extracted ON-state resistance can be used for online condition monitoring of semiconductors, as well as for estimating the junction temperature. The efficacy of the proposed method is verified experimentally under constant and variable load conditions. Moreover, the extracted resistance values from the online setup are compared to offline measurements from an industrial curve tracer, where an overall estimation error of less than 1% is observed.

Grid-forming converters can emulate the behavior of a synchronous generator through frequency droop control. The stability of grid-forming modular multilevel converters can be studied via the impedance-based stability criterion. This paper presents an ac-side impedance model of a grid-forming modular multilevel converter which includes a complete grid-forming control structure. The impact of different control schemes and parameters on the closed-loop output impedance of the converter is thoroughly analyzed and the learnings have been used in mitigating undesired control interactions with the grid. The results are verified through simulations in time- and frequency-domains along with experiments on a down-scaled laboratory prototype.

Series connection of semiconductor devices facilitates the development of higher-voltage submodules in modular multilevel converters (MMCs) using commercially available lower-voltage semiconductor devices. However, if left uncontrolled, series-connected semiconductors may experience unequal voltage sharing after each switching event. This unequal distribution lead to semiconductor damage and system failure. This paper proposes a novel method for the series connection of semiconductor devices. The proposed solution utilizes an active snubber circuit (ASC) and a unique control system that ensure controlled voltage sharing among the semiconductors. The solution is mainly intended for MMCs operating at low switching frequencies. The efficacy of the proposed solution is verified through simulations in the MATLAB/Simulink environment.