Converter Outage Research Articles

A coordinated control scheme for VSC-MTDC system with multiple renewable energy sites based on voltage margin and adaptive drooping control

This paper designs a novel coordinated control scheme to improve the control performance of voltage source converters (VSC) based multi-terminal high voltage direct current (MTDC) transmission network by merging the benefits of adaptive drooping and voltage margin controllers. In this proposed scheme, the droop coefficient is used to manage the DC voltage and maintain power sharing by shifting the control modes of the rectifiers and inverters in the presence of considerable power disturbances. The droop coefficient is planned based on the stability limit of DC voltage. The performance of the designed control scheme is compared to that of existing coordinated control and voltage margin droop control approaches. A ±400 kV five-terminal VSC-MTDC network is developed in PSCAD to verify the planned control method. The simulation results indicate that the proposed control scheme can achieve good performance for controlling the DC voltage and maintaining active power in the MTDC system.

Steady-state power distribution in VSC-based MTDC systems and dc grids under mixed P/V and I/V droop control

This paper proposes a steady-state power distribution derivation method for voltage source converter (VSC)-based multi-terminal HVDC (MTDC) systems and dc grids under mixed power/voltage (P/V) and current/voltage (I/V) droop control. P/V and I/V droop control are two commonly used control schemes, which can be combined to achieve co-regulation of powers & currents in MTDC systems and dc grids. The proposed method can be used to estimate the power distributions under different scenarios for MTDC systems and dc grids based on VSCs with mixed P/V and I/V droop control. After determining the initial operating point based on an estimation-correction algorithm, redistributed power due to power disturbances, current changes or converter outages is analyzed in detail considering converter overload. An excess power reduction strategy is further proposed to avoid violation of power limits after converter outage. The accuracy of the proposed method is validated through multiple scenarios in a modular multilevel converter (MMC)-based four-terminal dc grid. The comparison between the proposed method and other approaches in the current literature further demonstrates the advantages of proposed power distribution derivation method. • Determination of initial operating point for mixed P/V and I/V droop control by proposed estimation-correction algorithm. • Derivation of power redistribution after dc power/ current reference changes and converter outages. • Consideration of possible converter overloads caused by different system disturbances.

Projections of Cyberattacks on Stability of DC Microgrids—Modeling Principles and Solution

Microgrids relying on cooperative control are supported by communications, which are highly vulnerable to cyberattacks. A significant amount of research is already carried out on the detection and mitigation of cyberattacks to secure the operation of dc microgrids. Although cyberattacks are fully capable of causing cascaded converter outages leading to full/partial system blackouts, disturbing the system stability can also be a viable target by the adversaries, which has been overlooked so far. Hence, this article focuses on addressing the instability caused by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">stealth</i> cyberattacks, which can easily bypass the well-defined observability tests. In addition, this article also introduces a novel adaptive stabilization method to eliminate the unstable modes due to cyberattacks, which has been designed considering a previously defined cyberattack detection metric as an input. To investigate its feasibility, a detailed model of a stable dc microgrid is first developed. Then, considering <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">stealth</i> cyberattack as a nonlinear element, the describing function-based method is used to investigate system stability under attack conditions. Finally, theoretical analysis, simulation, and experimental results under various scenarios are presented to verify the effectiveness of the proposed stabilization scheme.

Adaptive drooping control scheme for VSC-MTDC system with multiple renewable energy sites based on variable droop constant

The generated power from multiple renewable energy sites can be collected and integrated into the conventional alternating current grid by using a multi-terminal high voltage direct current (MTDC) system based on the voltage source converter (VSC). Following the converter outage or large disturbance, this system with a fixed drooping control scheme suffers overloading of the power converter, resulting in the imbalance of active power-sharing and large deviation of direct current voltages. To improve control performance, this paper proposes a novel adaptive drooping control approach according to the variable droop constant to keep balance active power-sharing and manage the alternation of DC voltage in huge power disturbances. The designed scheme is implemented on the both rectifier side and inverter side converters. The variable droop constant is evaluated according to the converter headroom with DC voltage deviations. The performance of the planned control method is compared with the fixed and variable droop control techniques. A 400 kV four-terminal VSC-MTDC test network is developed to verify the effectiveness of the designed control method. The proposed control technique, based on the PSCAD simulation results, can achieve good performance in the rectifier and inverter converter outage and faults operating scenarios.

Zero-Inertia Offshore Grids: N-1 Security and Active Power Sharing

With Denmark dedicated to maintaining its leading position in the integration of massive shares of wind energy, the construction of new offshore energy islands has been recently approved by the Danish government. These new islands will be zero-inertia systems, meaning that no synchronous generation will be installed in the island and that power imbalances will be shared only among converters. To this end, this paper proposes a methodology to calculate and update the frequency droops gains of the offshore converters in compliance with the N-1 security criterion in case of converter outage. The frequency droop gains are calculated solving an optimization problem which takes into consideration the power limitations of the converters as well as the stability of the system. As a consequence, the proposed controller ensures safe operation of off-shore systems in the event of any power imbalance and allows for greater loadability at pre-fault state, as confirmed by the simulation results.

AC/DC fault handling and expanded DC power flow expression in hybrid multi-converter DC grids

Hybrid multi-converter dc grids are extended dc grids formed by combinations of different converter topologies such as the line-commutated converter (LCC), modular multilevel converter (MMC), alternate arm converter (AAC) and other various topologies. The stable and secure operation of such systems in steady-stage and transient conditions is critical. This paper investigates the ac and dc fault-ride-through (FRT) capability and the expanded expression of dc power flow in a hybrid multi-converter dc grid consisting of LCCs, MMCs and AACs. Fault handling schemes of single converters are combined, extended and coordinated to provide satisfactory transient response of the hybrid dc grid under both ac and dc faults. This paper proposes an expanded expression of dc power flow considering the initial power flow determination and the power flow after converter outages under mixed P/V and I/V droop control. The expanded dc power flow expression can be used in LCC and modular VSC-based hybrid multi-converter dc grids to derive the initial dc power flow and assess the system static security after converter outages. Simulation results for steady-state and transient operation based on a detailed equivalent model verify the accuracy of the proposed dc power flow expression, and validate the fault handling capability of the hybrid dc grid.

A novel control strategy based on a look-up table for optimal operation of MTDC systems in post-contingency conditions

Multi terminal VSC-HVDC systems are a promising solution to the problem of connecting offshore wind farms to AC grids. Optimal power sharing and appropriate control of DC-link voltages are essential and must be maintained during the operation of VSC-MTDC systems, particularly in post-contingency conditions. The traditional droop control methods cannot satisfy these requirements, and accordingly, this paper proposes a novel centralized control strategy based on a look-up table to ensure optimal power sharing and minimum DC voltage deviation immediately during post-contingency conditions by considering converter limits. It also reduces destructive effects (e.g., frequency deviation) on onshore AC grids and guarantees the stable operation of the entire MTDC system. The proposed look-up table is an array of data that relates operating conditions to optimal droop coefficients and is determined according to N-1 contingency analysis and a linearized system model. Stability constraints and contingencies such as wind power changes, converter outage, and DC line disconnection are considered in its formation procedure. Simulations performed on a 4-terminal VSC-MTDC system in the MATLAB-Simulink environment validate the effectiveness and superiority of the proposed control strategy.

Primary Frequency Support Through North American Continental HVDC Interconnections With VSC-MTDC Systems

This paper proposes a frequency control strategy for voltage source converter based multi-terminal HVDC systems (VSC-MTDC) to facilitate the exchange of primary frequency reserves among asynchronous AC systems, thus providing frequency support from each AC system to the others. The proposed frequency control utilizes a reference signal calculated from global measurements which reflects the overall frequency dynamics of all connected asynchronous AC systems. The proposed control outperforms the traditional frequency droop scheme by reducing impact on the DC voltage profile and improving frequency nadir. The performance of the designed frequency control with different DC voltage droop controls is evaluated. The adaptation of the proposed frequency control for converter outages is analyzed. The robustness of the proposed control to communication latency and the sensitivity of frequency support to power limits are also investigated. The VSC-MTDC model and the proposed control are implemented in the commercial grade software PSS/E and thus is suitable to study large-scale realistic systems and can be incorporated into the power system planning process at utilities and ISOs. The effectiveness of the proposed control is illustrated on a developed AC-MTDC test system and also on a large-scale realistic model combining the North American Western Interconnection (WI) and Eastern Interconnection (EI) with continental HVDC interconnections.

Adaptive droop control strategy for autonomous power sharing and DC voltage control in wind farm‐MTDC grids

This study presents a novel adaptive droop control (ADC) strategy for power-sharing in a multi-terminal high-voltage DC grid while maintaining a desirable DC voltage level. The ADC scheme can share the power without burdening any power converters during one or more converter outages and huge power imbalance situations. To improve the effectiveness of the power sharing and DC voltage deviation control, the ADC is also implemented for wind farm converters. This is achieved through a derated mode operation of wind farms. The performance and effectiveness of the proposed control approach are evaluated in several case studies based on the specific type of converter outage under the various outage scenarios. The proposed method is validated on a five-terminal CIGRE B4 DC grid test system.

Mission-Profile-Based System-Level Reliability Analysis in DC Microgrids

Mission profiles such as environmental and operational conditions together with the system structure including energy resources, grid and converter topologies induce stress on different converters and thereby play a significant role on power electronic systems reliability. Temperature swing and maximum temperature are two of the critical stressors on the most failure-prone components of converters, i.e., capacitors and power semiconductors. Temperature-related stressors generate electrothermal stress on these components ultimately triggering high potential failure mechanisms. Failure of any component may cause converter outage and system shutdown. This paper explores the reliability performance of different converters operating in a power system and indicates the failure-prone converters from wear out perspective. It provides a system-level reliability insight for design, control, and operation of multiconverter system by extending the mission-profile-based reliability estimation approach. The analysis is provided for a dc microgrid due to the increasing interest that dc systems have been gaining in recent years; however, it can be applied for reliability studies in any multiconverter system. The outcomes can be worthwhile for maintenance and risk management as well as security assessment in modern power systems.

Design of Optimal Droop Control for Multi-Terminal High-Voltage Direct Current Systems During Line Outages

This article investigates the required modifications to droop control design for optimum operation of Multi-Terminal High-Voltage Direct Current (MTHVDC) systems during line and converter outages. A typical 6-bus MTHVDC system is used in this study. Different scenarios are considered during the design process such as different outage types, violation of power ratings of system components, and variability of wind power injected into the system. A variable droop control gains technique is proposed to ensure minimum power loss and maximum annual energy collection. The study is extended to investigate the effect of employing constant droop control gains calculated during rated conditions on the system performance. Moreover, the results of the constant and the proposed variable droop gain schemes are compared. Simulation results show the effectiveness of the proposed techniques for designing optimum droop gains during abnormal conditions. Additionally, general guidelines are presented for the implementation of the proposed techniques.

Stability-Constrained Adaptive Droop for Power Sharing in AC-MTDC Grids

A secure operation of AC-multiterminal dc (MTDC) grids following an ( N $-$ 1) contingency (e.g., converter outage) is of a major concern for the system planners and operators. For the case of converter outage, the concept of adaptive droop control enables the healthy converters to share the burden of power mismatch in an effective manner. However, certain settings of the droop coefficients may lead to instability of the AC grid where multiple converters are connected and one of them goes out. This paper proposes a stability-constrained adaptive droop approach for autonomous power sharing following the outage of a converter in an MTDC grid. A trajectory sensitivity analysis-based approach is presented to impose the constraints on the adaptive droop gains that can be an outcome of dynamic security assessment performed by the system operators. The robustness of the proposed approach for post-contingency system is demonstrated through nonlinear time-domain simulation results using models developed in MATLAB/Simulink.

A Novel Load Flow Algorithm for Islanded AC/DC Hybrid Microgrids

This paper proposes a novel branch-based load flow approach for isolated hybrid microgrids. This evolving network configuration involves the application of a distinctive operational philosophy that poses significant challenges with respect to conventional load flow techniques. Hybrid microgrids are characterized by small-rating, droop-based distributed generators (DGs) and by variable but coupled frequency and dc voltage levels. In particular, the absence of a slack bus that results from the small DG rating impedes the application of traditional techniques such as branch-based methods. To overcome these limitations, a modified branch-based approach provided the basis for the development of the proposed algorithm. The new algorithm solves the load flow sequentially by dividing the problem into two coupled ac and dc subproblems. The coupling criterion is established by modeling and updating the power exchange between the subgrids, hence enabling an accurate and efficient formulation of the subproblems. For each subproblem, a modified directed forward-backward sweep has been developed to perform the load flow analysis based on consideration of the individual characteristics of each subgrid. Case study results demonstrate that the algorithm is applicable and effective for the steady-state analysis of several operational factors associated with an isolated hybrid microgrid system: 1) load changing; 2) converter outages; and 3) the probabilistic nature of renewable generation and loads.

Power imbalance sharing among the power converters in MTDC system

At the time of contingency like converter outage in multi-terminal DC grid, it is critical that the working converter stations share the power imbalance/load according to their respective capacity. In this study, several concepts of designing variable droop constant are proposed. These variants are formulated out of conventional fixed droop scheme considering operational parameters of converters and desired voltage and frequency regulation. The adoption of variable droop allows converters to share the imbalance power proportional to their rating and available headroom. A droop design method to reduce the voltage deviations at converter terminals is also proposed. Further, a variable droop control scheme using voltage deviation, frequency deviation of the non-synchronous AC area connected to DC terminal and available headroom of the converters is also discussed. This scheme modifies the DC power flow in such a way that the converter terminal with frequency decrease in AC system, allocates to supply more power to the AC system and vice-versa. A five terminal MTDC system is modelled and simulated to show the effectiveness of proposed droop control schemes. The droop scheme for proportional power sharing according to available headroom adjusts the converter power cooperatively to respond to the needs of frequency regulation.

Current Sharing in Multiterminal DC Grids—The Analytical Approach

This paper presents a new analytical approach for dc voltage control in dc grids. The control approach aims at improving the dc voltage droop control by generalized feedback which combines the local voltage signal available at the converter terminals with remote voltage signals at different locations in the dc system by means of communication. The main contribution is an analytical approach to derive the generalized feedback gain allowing to differentiating the system response for different converter outages. The goal of the system response is to share the power, in the event of disconnection of a converter, between the remaining converters according to a predetermined scheme. The local voltage feedback control is used for fast and reliable system response, and remote signals to achieve the goal. The analysis of the feasibility space the power shares is also presented, which is a direct function of the grid layout. Simulation results show the validity of the proposed control scheme.

Selective Power Routing in MTDC Grids for Inertial and Primary Frequency Support

The possibilities of selectively providing inertial and primary frequency support as an emergency support service to asynchronous AC areas through multiterminal direct current (MTDC) grid with minimal-communication are explored. Under nominal condition, only voltage droop is considered to allow the AC systems operate asynchronously as they are meant to. When an AC system needs support, the corresponding converter transmits basic information via a distress signal through the DC lines or existing communication channels used for common DC voltage-droop control. Following AC-side disturbances, the frequency droop is activated with values that are predetermined based on our proposed design procedure, which ensures primary frequency support through selective converter participation. Model predictive control, which aims to achieve the best possible frequency dynamics with a single-point actuation is also implemented at the converter station connected to the affected area to provide inertial support. Following a converter outage, MTDC power references are modified to ensure minimal impact on the AC-side frequencies. Simulation studies showing the effectiveness of the proposed strategies are presented.

Potential of Wind Farms Connected to HVdc Grid to Provide DC Ancillary Services

DC grids will require ancillary services to compensate for uncertainties, such as disturbances and converter outages. The appropriate equipment behavior and the sources of these services consequently need to be defined. This paper describes the reaction, activation, and provision intervals of ancillary services for dc grids to maintain the dc voltage in a certain range. It also discusses how wind power plants and synchronous generators that are exclusively connected to a dc grid can comply with these requirements. The theoretical descriptions are validated using simulations.

Robust Droop and DC-Bus Voltage Control for Effective Stabilization and Power Sharing in VSC Multiterminal DC Grids

Voltage-source converter (VSC)-based multiterminal dc grids are receiving widespread acceptance as an enabling technology to integrate renewable energy sources, energy storage units, and modern dc-type loads into existing ac grids. Droop control is a common power-sharing strategy to facilitate autonomous power sharing among different terminals in dc grids. However, the dynamics and stability of a gird-connected VSC with dc power sharing droop control can be affected by several important factors that are not addressed in the current literature. Important among these are: 1) ignoring the effect of the outer droop loop on the dc-link voltage dynamics when the dc-link voltage controller is designed, which induces destabilizing dynamics particularly under variable droop gain needed for optimum economic operation, energy management, and successful network operation under converter outages and contingencies; 2) uncertainties in the dc grid parameters (e.g., passive load resistance and equivalent capacitance as viewed from the dc side the VSC); and 3) disturbances in the dc grid (i.e., power absorbed or injected from/to the dc grid), which change the operating point and the converter dynamics by acting as a state-dependent disturbance. To overcome these difficulties, this paper presents a robust power sharing and dc-link voltage regulation controller for grid-connected VSCs in multiterminal dc grids applications. A detailed dynamic model that considers the droop controller dynamics and the impact of the effective dc-side load parameters and disturbances imposed on the VSC is developed. Then, a robust controller that preserves system stability and robust performance is developed. Theoretical analyses, including the MIMO dynamic analysis of a multiterminal dc grid with the proposed controller, as well as simulation and experimental results are provided to show the effectiveness of the proposed controller.

Optimized Power Redistribution of Offshore Wind Farms Integrated VSC-MTDC Transmissions After Onshore Converter Outage

The increasing penetration of renewable sources, particularly wind energy, has provided the power system with large-scale clean electricity. For integrating gigawatt offshore wind energy, voltage source converter (VSC) based multiterminal HVDC transmission is the prospective technology with flexible control characteristics. Most of the papers so far have dealt with improving the frequency performance caused by an ac system disturbance, and only a few deals with the influence of dc system converter outage on frequency performance. However, the converter outage will bring successive power impulses to the onshore ac grids. This paper presents an optimized power redistribution method to enhance the frequency performance of onshore ac grids. The two scenarios of power redistribution after converter outage are analyzed: self-distributed type and nonself-distributed type. For the self-distributed scenario, the optimized power redistribution method involves three steps: sensitive cluster identification, optimized power redistribution calculation, and dc grid operating point setting. For the nonself-distributed scenario, a combined offshore wind farm control strategy with emulating inertia control and auxiliary pitch angle control is proposed to alleviate the adverse effect of the transferred surplus power on asynchronous grids. Simulation verifications are carried out in a modified New England 39-bus system with seven terminal VSC-HVDC transmission system.

The Effects of Converter and DC line Outages on Load Flow Analysis of VSC-MTDC based Hybrid AC/DC Distribution System

The Effects of Converter and DC line Outages on Load Flow Analysis of VSC-MTDC based Hybrid AC/DC Distribution System