Multi-terminal DC Grid Research Articles

Stability Criterion for Inertial and Primary Frequency Droop Control in MTDC Grids With Implications on Ratio-Based Frequency Support

Multiple strategies have been proposed to exchange frequency support amongst asynchronous AC areas through Multiterminal DC (MTDC) grid. Majority of these strategies rely on basic droop control mechanisms involving either inertial or primary frequency droop, or both. This paper addresses the requirements on those droop coefficients, which ensure small-signal stability. To that end, a stability criterion is hypothesized based on certain observations. Using a reduced Nth-order model, the hypothesis is analytically proved for the cases when either inertial droop or primary frequency droop control is active. Moreover, implication of this hypothesis on a recently proposed ratio-based frequency support method is analyzed to derive new stability constraints over and above the existing performance requirements. To support the stability hypothesis, numerically constructed stability boundaries in the droop coefficient space are studied. The stability boundary obtained from small-signal analysis of the full-order nonlinear model is shown to approximately match the boundary obtained from the Nth-order model, which further strengthens the hypothesis. Finally, various time-domain simulation studies are performed on full-order model to validate the stability hypothesis.

Frequency regulation of multiple asynchronous grids using adaptive droop in high‐voltage direct current system

Frequency stability control in multiple asynchronous grids is a challenging and complex issue. An adaptive droop control strategy to improve the frequency regulation of asynchronous AC areas connected by a multi-terminal DC grid is proposed here. The droop coefficients are adjusted to share the active power adaptively among multiple asynchronous AC grids based on the characteristics of frequency deviation and rate of change of frequency. This results in a cogent allocation of imbalance power in multiple asynchronous grids from the frequency variation perspective. The performance of the proposed scheme is evaluated in a modified multi-machine power system using DIgSILENT Power Factory. Simulation results under significant frequency disturbances caused by credible contingencies are presented to demonstrate the effectiveness of the proposed approach. It is found that the proposed adaptive control ensures an excellent and robust frequency response under different operative conditions.

Diode-Bridge Multiport Hybrid DC Circuit Breaker for Multiterminal DC Grids

Hybrid dc circuit breaker (HCB) is currently the most promising dc circuit breaker. However, since it requires hundreds of semiconductor switches to withstand high transient interruption voltage, the implementation cost is high. Besides, in the multiterminal dc grid, three or more HCBs may be required at a dc bus, which worsens the disadvantages of HCBs in economy and volume. In order to solve this problem, this article designs a diode-bridge multiport HCB (DB-MPB) with its control procedures. By integrating the n HCBs at the dc bus to a single n-port HCB, the number of semiconductor switches and the footprint can be reduced, while the good current interruption performance can be still maintained. Besides, for the possibility of ultrafast disconnector (UFD) failure, the proposed DB-MPB has the function of UFD failure protection. The effectiveness of the designed DB-MPB under different scenarios is verified by simulations and experimental tests. In addition, the performance comparisons in economy and functionality of the proposed DB-MPB with existed solutions are presented to highlight the advantages of the proposed solution.

A Reliable Protection Scheme for Fast DC Fault Clearance in a VSC-Based Meshed MTDC Grid

A multi-terminal high voltage DC (MTDC) grid, is an optimum and cost-effective transmission network to minimize the energy crisis worldwide largely. However, the core demand for a fast DC protection scheme with an extraordinary strict fault clearance time of a few milliseconds, is a key research gap in this network, holding back its development and scalability. To bridge this gap, this paper proposes a novel protection scheme for a meshed MTDC grid. The main goals of the scheme include accurate discrimination of a faulty line, rapid fault detection, fault location estimation, significant fault current reduction, and fully selective isolation of only the faulted line, while continuing normal power flow in the healthy grid zones. Reliability of the scheme for long and extra-long-distance power transmission is increased by aiding the differential protection and Type-D traveling wave (TW)-based algorithms utilizing the distributed optical current sensing technology with the other auxiliary methods and backup plans. These auxiliary methods include independent discrete wavelet transform (DWT), current derivative polarity principles with a minimum sample (short time) window, overcurrent relays, and AC circuit breakers (ACCBs). A faulty segment of a transmission line is accurately discriminated from the healthy ones by measuring a series of multi-point differential currents on it. A faulty line at a particular DC node is accurately discriminated using the differential protection by measuring the current flowing into or out of each line from each side at every node to obtain the algebraic sum. The current sum of a real-time local transient data and a delayed remote data is compared to a preset threshold level. DC fault current is significantly reduced below the breakable levels by coordinating bidirectional hybrid DC circuit breakers (HDCCBs) with the active and passive fault current limiters (FCLs) and the half bridge-VSC-based modular multilevel converters (MMCs). The proposed concepts are successfully verified by the simulation results under a variety of fault scenarios and are found to be accurate.

Small-Signal Stability Analysis of the Interactions Between Voltage Source Converters and DC Current Flow Controllers

Small-signal stability analysis of the voltage source converter (VSC) based multi-terminal DC (MTDC) grids has been mainly focused on the dynamics of AC/DC converters while there is a lack of consideration of DC networks with DC power-flow controllers. In this paper, an integrated small-signal model is established to investigate the interactions between VSC and DC current flow controllers (CFC). The impacts of the control system of CFC on small-signal stability of the integrated system and the impacts of the control system of VSC on the DC networks with CFC are discussed via eigenvalue analysis. The above interactions are demonstrated through dynamic simulations against both small and large disturbances using real time digital simulator (RTDS).

A DC Line Protection Scheme for MMC-Based DC Grids Based on AC/DC Transient Information

With the rapid development of renewable energy, multi-terminal flexible DC grids composed of modular multilevel converters have been widely used in power systems. To ensure the safe and stable operation of a DC grid, this paper proposes a DC line protection scheme for MMC-based DC grids using AC/DC transient information. In the proposed scheme, the difference between the transient voltage root mean square ratios (DTVR) on both sides of the current-limiting inductor of DC lines is used to identify the DC line fault, and the instantaneous zero sequence voltage (IZSV) of the AC side of the converter station is used to discriminate the faulted poles. According to the theoretical analysis result, a fault criterion setting method suitable for faulted pole discrimination is proposed. The proposed scheme is based on local information and tested in a PSCAD/EMTDC model of a four-terminal MMC-based DC grid. The simulation results show that the scheme can fast and accurately identify faults.

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.

A Multi-Domain Co-Simulation Method for Comprehensive Shifted-Frequency Phasor DC-Grid Models and EMT AC-Grid Models

To accurately capture the dynamics of a large-scale ac/dc system as a whole and the interactions between its individual components, a simulation method with high precision and efficiency is in great demand. For this purpose, we develop a multi-domain co-simulation method, in which the target system is partitioned into multiple dc and ac subsystems, represented by our proposed shifted-frequency phasor (SFP) models and the traditional EMT models, respectively. SFP models can be simulated with a much larger time step, leading to a significant improvement in simulation efficiency under a given requirement of precision. Further, a new interface model, namely, hybrid multi-domain transmission-line model (HMD-TLM), is developed to reflect the interactions between SFP models and EMT models, with the additional benefit of producing instantaneous and phasor waveforms simultaneously. Thus, the multi-domain co-simulation is implemented based on the efficient SFP models, the HMD-TLM, and a designed time sequences of simulation. The performance (efficiency and accuracy) of the proposed method has been validated on the well-known CIGRE ac/dc system as well as a practical system integrating large ac grids and modular-multilevel-converter-based multi-terminal dc grids.

Cosimulation of Shifted-Frequency/Dynamic Phasor and Electromagnetic Transient Models of Hybrid LCC-MMC DC Grids on Integrated CPU–GPUs

To effectively capture interactions of large-scale ac–dc systems integrating line commutated converter (LCC) and modular multilevel converter (MMC) based multiterminal dc grids, a numerically accurate and efficient simulation method is desirable. To achieve this objective, a cosimulation method is proposed in this article, where the target system is decoupled into the shifted-frequency phasor (SFP) subsystem, the dynamic phasor (DP) subsystem, and electromagnetic transient (EMT) subsystem, respectively. The MMCs are included in the SFP subsystem and implemented on massively paralleled graphics processing units (GPUs). Thus, the simulation efficiency is greatly improved by adopting a much larger time step, the model order reduction technique, and GPU acceleration. The LCCs are represented by DPs and are included in the DP subsystem. The majority of ac grids are covered in the EMT subsystem. Further, the interactions between SFP and EMT subsystems are reflected by the proposed multidomain transmission line model, which can produce instantaneous and phasor values simultaneously. The interface model between DP and EMT subsystems is modeled as a special controlled voltage and current circuit. Finally, the overall cosimulation method is realized by the respective SFP/DP and EMT models, among which their interactions are reflected by the proposed interface models and the time sequences of simulations. The performance of the proposed method has been fully validated on a practical large-scale ac–dc system.

Variable Time-Stepping Modular Multilevel Converter Model for Fast and Parallel Transient Simulation of Multiterminal DC Grid

The efficiency of multiterminal dc (MTDC) grid simulation decreases with an expansion of its scale and the inclusion of accurate component models. Thus, the variable time-stepping scheme is proposed in this paper to expedite the electromagnetic transient computation. A number of criteria are proposed to evaluate the time-step and regulate it dynamically during simulation. Meanwhile, as the accuracy of results is heavily reliant on the switch model in the modular multilevel converter, the nonlinear behavioral model with a greater accuracy is proposed in addition to the classic ideal model, and their corresponding variable time-stepping schemes are analyzed. Circuit partitioning is effective in accelerating the MTDC grid simulation via fine-grained separation of nonlinearities. A subsequent large number of identical circuits enabled a massively parallel implementation on the graphics processing unit, which achieved a remarkable speedup over the CPU-based implementation. The inclusion of variable time-stepping schemes eventually makes the simulation of MTDC grid with highly detailed nonlinear switch models feasible. The results are validated by commercial device-level and system-level simulation tools.

Novel EPR-insulated DC cables for future multi-terminal MVDC integration

A new twist on a legacy EPR turned it into a MVDC cable insulation with superior space charge resistance to polarity reversal, well suited for multi-terminal DC grid for renewal integration and harsh environmental and marine electrifications.

Exact Nonlinear Micromodeling for Fine-Grained Parallel EMT Simulation of MTDC Grid Interaction With Wind Farm

Detailed high-order models of the insulated-gate bipolar transistor (IGBT) and the diode are rarely included in power converters for large-scale system-level electromagnetic transient (EMT) simulation on the CPU, due to the nonlinear characteristics albeit they are more accurate. The massively parallel architecture of the graphics processing unit (GPU) enables a lower computational burden by avoiding the computation of complex devices repetitively in a sequential manner and thus is utilized in this paper to simulate the wind farm-integrated multiterminal dc (MTdc) grid based on the modular multilevel converter (MMC). Fine-grained circuit partitioning is proposed so that the nonlinear switching elements are physically separated with the smallest circuit unit. By implementing these subsystems with the same attributes as a GPU program and computing it in a massively parallel manner, it is demonstrated that the GPU is able to achieve a significant speedup over multicore CPUs and its computation time incremental is much smaller when the MMC level scales up. The improved insight and accuracy of the proposed modeling methodology and the designed GPU program are validated at the system- and device-level by off-line commercial simulation tools.

Active and reactive power control of hybrid offshore AC and DC grids

The future ‘SuperGrid’ may requires the benefit of both offshore AC network and multi-terminal DC grid. AC cable limits the power transfer capability from the larger offshore wind farm, however, HVDC transmission system is economical viable for large power wind farm integration with the grid. Another approach to develop the offshore network infrastructure is by forming an offshore AC grid connecting several offshore wind farms. Then, this offshore AC network is connected with different onshore grid using HVDC system. This enhances the trade among the countries as well as provide an economical solution for wind energy integration. In this article, operational and control concept of voltage source converter is presented to integrate an offshore AC grid with an offshore DC grid. The article presents the control principle of offshore AC network frequency and voltage with respect to active and reactive power distribution in the AC network. Later, the principle of multi-terminal HVDC system is discussed with respect to power distribution using DC voltage droop control. Power distribution criteria are defined with respect to operator power-sharing requirement and network stability. In the end, a hybrid AC/DC offshore grid is modelled and simulated in MATLAB/SIMULINK to validate the distribution criteria.

Model Predictive Control Based AC Line Overload Alleviation by Using Multi-Terminal DC Grids

The increasing power/load demand is becoming a new concern for the grid operators as it can potentially overload transmission lines. Among advantages of a multi-terminal dc (MTDC) grid, its high controlability makes it a promising asset for ac line overload alleviation. This paper proposes a model predictive control based strategy, which regularly updates the power setpoints of the MTDC converter stations and ac generators to alleviate ac line overloads. To compute and update the power setpoints, necessary measurements are received by the controller within regular time intervals. The proposed controller ensures that subsequent to any ac line overload, changing the power setpoints of the MTDC converter station does not cause any operation violation in the MTDC grid. Furthermore, it ensures that the ac voltage limits and voltage stability criteria are met to avoid voltage instability. Performance and effectiveness of the proposed controller are evaluated by simulation studies on two systems, i.e., the 39-bus New England system integrated with a 5-bus MTDC system and the IEEE 118-bus system connected to a 6-bus MTDC system. Simulation results confirm that the MTDC grid can play a major role in controlling active powers of most of the ac transmission lines.

A Novel Current-Commutation-Based FCL for the Flexible DC Grid

In the flexible dc grid, the dc fault propagates extremely fast. It requires the dc protection and the dc circuit breaker (DCCB) can identify and isolate the fault line very quickly, for the healthy network ride-through, especially in the multi-terminal dc grids. However, the existing protection and DCCB techniques for flexible dc grid can still not match the dc fault propagation speed. Therefore, a novel current-commutation-based fault current limiter (FCL) is proposed in this paper. It can limit the dc fault current effectively, to guarantee the converter continuous operation and healthy network ride-through after dc faults, so the requirement on the protection and DCCB acting speed can be decreased. Compared with the existing fault current limiting methods, the proposed FCL has tiny negative influence on the system transient response speed, operation stability, and DCCB isolating speed. The experiment tests and simulation case studies are carried out to verify the working principle and superiorities of the proposed novel FCL.3

Optimum Selection of Circuit Breaker Parameters Based on Analytical Calculation of Overcurrent and Overvoltage in Multiterminal HVDC Grids

This paper proposes a time-domain method to calculate the fault response in multiterminal dc (MTdc) grids and the performance of hybrid dc breakers. The proposed method, based on traveling waves: 1) provides a sound representation of fault performance by considering all created traveling waves; 2) introduces a new approach to estimate the reflection coefficients; and 3) provides an approximation of the worst case fault location. Then, based on the analytical results, three parameters of the hybrid dc circuit breaker, i.e., current limiting reactor, arrester rated voltage, and time delay are optimally selected with respect to maximum overcurrent, maximum overvoltage, fault clearance time, and energy absorption in arresters through multiobjective optimization. Accuracy and performance of the proposed method are evaluated and verified by time-domain simulation studies in the power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) environment using frequency-dependent models. The results confirm reasonable accuracy of the proposed fault performance calculation and represent a further step towards optimized design of hybrid dc circuit breakers.

Data modelling of converters for the automation and monitoring of MTDC grids

Multi-terminal DC (MTDC) grids based on voltage source converters (VSCs) are a promising option to integrate the increasing share of generation from the distributed energy resources (DERs) both in the transmission and distribution grids. However, with the broader use of VSCs for the transmission and distribution grid, there is an absence of a standardised data model for the VSC. In this work, the authors propose an IEC 61850 based data model for VSCs and an existing IEC 61850 model is extended for the dual-active bridge (DAB) converter as an interlink between various DC terminals and the modular multilevel converter (MMC). Furthermore, a generic data modelling guideline is proposed for intelligent electronic devices (IEDs), which are responsible for the interoperability among converters in the MTDC grid. The implementation of the real-time monitoring system of a hybrid AC-DC MTDC grid with the proposed IED is presented as an exemplary case. Through experimental tests of this MTDC grid, it is shown that the proposed IED meets the latency requirements for P2 through P6 class of monitoring, protection, and control applications according to the IEC 61850 standard. The automation architecture presented in this work is also validated for interoperability within the MTDC grid.

Integrated Massively Parallel Simulation of Thermo-Electromagnetic Fields and Transients of Converter Transformer Interacting With MMC in Multi-Terminal DC Grid

A computationally efficient model to study the transient interaction of the finite-element (FE) converter transformer with modular multi-level converter (MMC) can provide the advanced knowledge of the filed-circuit interactions that can be utilized for the design and test of equipment; however, the runtime of existing simulation tools developed for CPU execution usually takes days or even weeks, which is prohibitively long. In this paper, an integrated thermo-electromagnetic model is proposed for the transient simulation of FE-based transformer interacting with the MMC in a multi-terminal dc gird, with the magnetic field, thermal field, and electrical networks fully coupled. The transmission-line modeling solution is employed for the nonlinear FE problem, and each MMC is split into a number of minimum possible circuits, so that the codes can be sufficiently parallelized and implemented on the graphics processing unit with thousands of Cuda cores to be runtime friendly. The integrated model can provide the transient field distributions within the transformer and the device-level information of the MMC such as switching transients and junction temperatures. Compared with a commercial software package, the execution time of $10^5$ time-steps decreased from several days to only hours with a speedup of more than 47 times while maintaining high accuracy.

Adaptive Droop Control for DC Voltage Deviation and Power Sharing in Multi-terminal DC(MTDC) Grids

Voltage source converter multi-terminal DC system(VSC-MTDC) have been used in many applications due to their many advantages. With conventional droop control, the voltage and power of the stable point are on a straight line with a specific slope. When subjected to a power disturbance, the voltage quality may be deteriorated or the power distribution effect of the converter station may be deteriorated. In this paper, a new adaptive control strategy is proposed for VSC-MTDC. The control system focuses on voltage quality at low loads and power distribution at high loads. It not only ensures voltage quality but also meets power distribution requirements. Simulation results are provided to verify the validity and effectiveness of the proposed method.

Fault Current Estimation in Multi-Terminal HVdc Grids Considering MMC Control

DC faults are critical events for the multi-terminal HVdc protection systems, where knowing the critical fault time is fundamental in the protections design. Existing literature analyzes the critical fault time with simulations or with complex analysis, requiring large computational effort and they are not robust to parameters variations. Furthermore, standard approaches rely on simplified modular multilevel converter (MMC) models neglecting the influence of converter control loops dynamic. As result, the prediction of the node voltages and especially the dc currents become increasingly inaccurate as bigger is the distance from the fault. This paper introduces a fault current estimation method in MMC-based multi-terminal dc (MTdc) grids, which improves the current differential equation based methodologies including the dynamics of power and dc voltage controllers. The proposed method gives the possibility of analyzing the fault current under different grid conditions (e.g., fault resistance and limiting inductor value). Furthermore, this method is extended to bipolar MTdc systems in case of pole-to-pole and pole-to-ground faults, and it is validated by means of simulations in PSCAD environment and standard CIGRE MTdc grid.