Paul D. Judge

Power loss and thermal characterization of IGBT modules in the Alternate Arm converter

Power losses in high power HVDC converters are dominated by those that occur within the power electronic devices. This power loss is dissipated as heat at the junction of semiconductor devices. The cooling system ensures that the generated heat is evacuated outside the converter station but temperature management remains critical for the lifetime of the semiconductor devices. This paper presents the results of a study on the temperature profile of the different switches inside a multilevel converter. The steady state junction temperatures are observed through the simulation of a 20 MW Alternate Arm Converter using 1.2kA 3.3 kV IGBT modules. A comparison of the Alternate Arm Converter is made against the case of both the half-bridge and full-bridge Modular Multilevel Converter topologies. Furthermore, the concept of varying the duty-cycle of the two alternative zero-voltage states of the H-bridge modules is introduced. Simulation results demonstrate that it can change the balance of electrical and thermal stress between the two top switches and the two bottom switches of a full-bridge cell.

Reliability Model of MMC Considering Periodic Preventive Maintenance

Periodic preventive maintenance (PPM) is a significant measure to ensure reliable operation of the modular multilevel converter (MMC), but to date has always been neglected in the reliability evaluation. A mathematical reliability model of MMC considering PPM is proposed in this paper. The model is derived rigorously by the reliability function and finally described by the indices equivalent failure rate and the forced outage rate. Two operation modes related to redundant submodules are considered and described by the standby model and k-out-of-n model, respectively. Employing this model, we can analyze the sensitivity of reliability to redundancy and maintenance interval and provide some reliability indices for planning, which is especially valuable for the offshore cases. The necessity of the considered PPM and the effectiveness of the proposed model are verified by case studies. A method to include PPM in reliability modeling is provided.

Lab-scale experimental multilevel modular HVDC converter with temperature controlled cells

It is important to be able to produce representative experimental results when researching HVDC grids and converters. This paper reports on a lab-scale multi-level converter build. The converter was built with a large number of cells to enable experiments with cell balancing effects and with temperature control to investigate thermal effects and appropriate responses. The converter is able to behave as both a traditional MMC (or CTLC which is functionally similar) and as the newer AAC converter. Results are presented that show the converter is able to charge up from the DC bus and is able to function well with both static and changing power references.

Alternate arm converter operation of the modular multilevel converter

A new operating mode of the Modular Multilevel Converter (MMC) using modified arm current waveforms inspired from the working principle of the Alternate Arm Converter (AAC) is presented in this paper. A reduction in the cell voltage deviation is observed at power factors close to unity at the cost of an increase in power losses, especially when reactive power is required. This gain in voltage margin is then used in further optimizations of the MMC performance, mainly focusing on either increasing the number of redundant cells or improving the overall power efficiency of the converter.

Reliability Analysis of MMCs Considering Submodule Designs with Individual or Series-Operated IGBTs

The half-bridge-based modular multilevel converter (MMC) has emerged as the favored converter topology for voltage-source HVDC applications. The submodules within the converter can be constructed with either individual insulated-gate bipolar transistor (IGBT) modules or with series-connected IGBTs, which allows for different redundancy strategies to be employed. The main contribution of this paper is that an analytical method was proposed to analyze the reliability of MMCs with the consideration of submodule arrangements and redundancy strategies. Based on the analytical method, the relative merits of two approaches to adding redundancy, and variants created by varying the submodule voltage, are assessed in terms of overall converter reliability. Case studies were conducted to compare the reliability characteristics of converters constructed using the two submodule topologies. It is found that reliability of the MMC with series-connected IGBTs is higher for the first few years but then decreases rapidly. By assigning a reduced nominal voltage to the series valve submodule upon IGBT module failure, the need to install redundant submodules is greatly reduced.

Dynamic thermal rating of a Modular Multilevel Converter HVDC link with overload capacity

The power rating of Modular Multilevel Converter based HVDC has increased rapidly over the past decade, with individual links in the gigawatt power range now technically feasible and further power increases on the horizon. Such large links may be required to provide ancillary services such as fast frequency response or emergency power re-routing in the event of a system disturbance. Providing such services may require converters to be designed with overload capacity. This paper examines how the thermal aspects of semiconductor devices may impact the operation of such converters and how the exploitation of short-term thermal dynamics may lead to dynamic overload rating.

An Analytical Approach to Evaluate the Reliability of Offshore Wind Power Plants Considering Environmental Impact

The accurate quantitative reliability evaluation of offshore wind power plants (OWPPs) is an important part in planning and helps us to obtain economic optimization. However, loop structures in collector systems and large quantities of components with correlated failures caused by shared ambient influences are significant challenges in the reliability evaluation. This paper proposes an analytical approach to evaluate the reliability of OWPPs considering environmental impact on failures and solve the challenges by protection zone models, equivalent power unit models, and common cause failure (CCF) analysis. Based on the investigation of the characteristics of OWPP and related failures mechanisms, the components are divided into three CCF subsets. With the aid of the protection zone model and equivalent power unit model merged with CCF, the faulty collector system state evaluation is applied to reduce the computational burden. The case studies present the necessity and improved performance of merging CCF analysis into modeling via the comparison with other two simplified methods. A sensitivity analysis is also carried out to account for inaccuracy of failure data. The results show that the assumption of independent failures in the conventional method might lead to over-optimistic or over-pessimistic evaluation depending on the CCF style.

Dimensioning and Modulation Index Selection for the Hybrid Modular Multilevel Converter

A hybrid modular multilevel converter, comprising a mixture of full-bridge and half-bridge submodules, provides tolerance to dc faults without compromising the efficiency of the converter to a large extent. The inclusion of full bridges creates a new freedom over the choice of ratio of ac-to-dc voltage at which the converter is operated, with resulting impact on the converters internal voltage, current, and energy deviation waveforms, all of which impact the design of the converter. A design method accounting for this and allowing the required level of derating of nominal submodule voltage and uprating of stack voltage capability to ensure correct operation at the extremes of the operating envelope is presented. A mechanism is identified for balancing the peak voltage that the full-bridge and half-bridge submodules experience over a cycle. Comparisons are made between converters designed to block dc-side faults and converters that also add STATCOM capability. Results indicate that operating at a modulation index of 1.2 gives a good compromise between reduced power losses and additional required submodules and semiconductor devices in the converter. The design method is verified against simulation results, and the operation of the converter at the proposed modulation index is demonstrated at the laboratory scale.

Thyristor-Bypassed Submodule Power-Groups for Achieving High-Efficiency, DC Fault Tolerant Multilevel VSCs

Achieving dc fault tolerance in modular multilevel converters requires the use of a significant number of submodules (SMs) that are capable of generating a negative voltage. This results in an increase in the number of semiconductor devices in the current path, increasing converter conduction losses. This paper introduces a thyristor augmented multilevel structure called a Power-Group (PG), which replaces the stacks of SMs in modular converters. Each PG is formed out of a series stack of SMs with a parallel force-commutated thyristor branch, which is used during normal operation as a low loss bypass path in order to achieve significant reduction in overall losses. The PG also offers negative voltage capability and so can be used to construct high-efficiency dc fault tolerant converters. Methods of achieving the turn-on and turn-off of the thyristors by using voltages generated by the parallel stack of SMs within each PG are presented, while keeping both the required size of the commutation inductor, and the thyristor turn-off losses low. Efficiency estimates indicate that this concept could result in converter topologies with power losses as low as 0.3% rated power while retaining high quality current waveforms and achieving tolerance to both ac and dc faults.

Soft-switching of the director switch in the alternate arm converter using blocked sub-modules

The presence of Director Switches (DS) in its arms enables the Alternate Arm Converter (AAC) to generate smooth current waveforms while using a reduced number of Sub-Modules (SMs) when compared to the Modular Multilevel Converter. To maximise the power efficiency of the AAC and simplify the DS design, it is highly beneficial to soft-switch the DS. This paper presents a method which reliably ensures their soft-switching operation using the blocked state of SMs during the non-conducting part of the cycle of an arm. Experimental results demonstrate the effectiveness of the proposed method over a wide range of operating conditions.