Multi-terminal Flexible DC Research Articles

The Improved Topology and Control Strategy for the HCLC in the Multiterminal Flexible DC Grid

The dc fault-current-limiting technique is extremely important for operation security and power supply reliability of the flexible dc grid. Generally, the dc reactor can effectively limit the dc fault current; however, it will result in negative influences on the system operation stability and dc circuit breaker (DCCB) isolating speed. The hybrid current-limiting circuit (HCLC) can effectively cooperate with the DCCB and avoid the negative influence of dc reactor on the fast fault current clearing, thus being considered as a promising technique in dc grid. In this article, the improved HCLC (I-HCLC) is proposed. It can effectively limit the dc fault current without time delay and eliminate the conduction losses of the used power electronic switches. Compared with the traditional HCLC, the investment of the I-HCLC is significantly reduced for multiport application in multiterminal dc grid. In addition, the improved control strategy, which can avoid the negative influence of dc reactor on the system operation stability, is also researched. Also, this problem has not yet been considered when the HCLC was initially proposed. The experiment tests and simulation cases are carried out to verify the feasibility and superiorities of the proposed I-HCLC and of the improved control strategy.

Research on Control Method of Multi-terminal Flexible DC Transmission System Based on Voltage Droop Control

Flexible DC transmission system has the advantages of small line loss, high reliability and strong controllability, and can reduce the frequency of system inactivity. In this paper, the voltage droop control method of flexible DC transmission system is studied in detail. Firstly, the principle and characteristics of the series structure, parallel structure and hybrid structure of multi-terminal flexible DC transmission system are introduced in detail. Based on this, the principle and characteristics of the control strategy of DC voltage droop control of the system are analyzed, and an improved voltage droop control strategy is proposed. Finally, a three-terminal flexible DC transmission system model is built on the Matlab/Simulink platform. The simulation results show that the control strategy plays a certain role in stabilizing DC, AC voltage, AC current and power balance control.

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.

Modular Multi-terminal Flexible DC Micro-grid and Its Hierarchical Control Method

In the traditional DC micro-grid, the structure is complex, the control is difficult and there is no fixed networking mode. Thus the construction of micro-grid is expensive and the output is slow. In this paper, a hierarchical control method of modular multi-terminal flexible DC micro-grid is proposed, which simplifies the structure and control method of DC micro-grid and is beneficial to the construction of micro-grid. The structure of flexible multi-terminal DC micro-grid and the modularized method of network construction are discussed. The feasibility of the study is confirmed by a specific example of DC micro-grid. On the basis of the above research, the hierarchical control method of DC micro-grid, the specific control method and control logic in each layer are determined. PSCAD/EMTDC simulation is established to verify the effectiveness of the proposed theory and control method.

A Novel Single-Ended Transient-Voltage-Based Protection Strategy for Flexible DC Grid

The fast dc line protection is one of the key techniques for the multi-terminal flexible dc grid whose main technical difficulty lies in quick and reliable fault identification. The traditional protection principles for the ac system are not suitable for the flexible dc grid, and those for the conventional dc transmission system can also not be applied directly. Therefore, the frequency-domain traveling-wave boundary characteristics after faults are analyzed first in this paper to find the critical condition for distinguishing the internal and external faults as well as the forward and backward faults. And then, a novel single-ended transient-voltage-based protection strategy for flexible dc grid was proposed, which mainly consists of the fault identification criterion and the direction criterion. The protection scheme used wavelet transform to extract transient voltage energy. In order to eliminate the influences of the wavelet transform boundary effect and converter blocking, the principle to select the proper data window was proposed. Compared with the existing traveling-wave-based dc protection, the proposed method has a higher reliability and stronger ability to endure high transition resistance, which was verified by lots of simulation cases based on the PSCAD/EMTDC.

Single-Terminal Quantity Based Line Protection for Ring Flexible DC Distribution Grids

The fast and reliable identification of DC faults is of great significance for the development of multi-terminal flexible DC distribution networks. With regard to MVDC lines in a multi-terminal ring DC distribution network with modular multilevel converters and two-level voltage source converter, a fault identification approach based on the voltage of the additional inductor was proposed. First, the calculation method of the initial voltage across the additional inductor after faults was elaborated based on modulus networks. Subsequently, the difference of the initial voltage across the additional inductor between internal and external faults was utilized to accomplish the fault identification. Furthermore, the faulty pole is discriminated based on the difference between the initial voltage across the additional inductor of the faulted pole and that of the healthy pole. The proposed scheme can identify the fault quickly and reliably. Finally, the proposed calculation method and the feasibility of the protection scheme are verified via PSCAD/EMTDC-based simulation.

Protection Scheme of Multi-terminal Flexible DC Distribution Network Based on Current-Limiting Inductor Voltage

The rapid development of power electronic devices has promoted the rapid development of multi-terminal flexible DC distribution networks, indicating the direction of DC technology and urban power distribution systems. However, there are several technical problems in multi-terminal flexible DC distribution network that still need to be solved. In relay protection, the main difficulty faced by DC distribution network is the rapid and reliable identification of DC faults. This paper proposes a multi-terminal flexible DC distribution network protection scheme based on current-limiting inductor. First, the system configuration of the multi-terminal flexible DC distribution network is introduced. Then, the DC-side fault characteristic of the flexible DC distribution network are analysed. Based on this, a protection scheme based on current-limiting inductor voltage is proposed. Next, a complete protection scheme is designed. Finally, the proposed protection scheme is verified by real time digital simulator (RTDS). The simulation results show that the scheme is not affected by the fault resistance and fault distance.

Study on DC Breaker Fault Current and Its Limiting Method of Multiterminal Flexible DC Distribution System

The multiterminal flexible DC (MTDC) distribution system has become a hot research topic for its advantages such as new energy load accessibility and high reliability of power supply. However, the fault current during short-circuit faults will be more complicated and serious because of the multiple operating modes and specific converter structure in MTDC distribution system. The high fault current will affect the safety of equipment and system operation. As a result, it is necessary to study the fault current generation mechanism and current limiting method of the DC distribution system. In this paper, the generation mechanism, transient characteristics, and the influencing factors of the fault current in the DC breaker during the short-circuit faults are analyzed by theoretical analysis and simulations separately based on a ±10 kV three-terminal flexible DC distribution system. The current limiting devices are analyzed, and its configuration position and parameter calculation method are proposed. On this basis, the parameters of the current limiting reactor and its effects on each position are analyzed by electromagnetic transient simulations. The current limiting scheme is proposed and its reliability is verified. The study results can provide some reference for overcurrent analysis and current limiting measures of the multiterminal DC distribution system.

Rapid modelling for flexible DC distribution network considering droop control

With the growth of DC electricity load, the difficulty to improve the power quality, and the increasing penetration of renewable power, flexible DC distribution network, which contains complex renewable energy generation equipment and a large number of power electronic converters, become an important research direction. The modelling of the DC distribution network is an essential step for the research on the coordination control. However, the traditional modelling will be a burden for simulation software. This article models all electrical devices as equivalent power sources based on the proportional–integral control and the power–voltage curve, ignoring the complex internal control processes. Based on the simplified models of power, load, DC/DC converters, and AC/DC converters considering droop control, a simplified model of a multi-terminal flexible DC distribution network is derived. Taking the impact of droop control into account, the simulation of DC distribution power flow under the different distribution of generation and load is done. Results show that the simplified model of DC distribution network with droop control can accurately and rapidly simulate the transient process and steady states of the testing network.

Analysis of the fault current limiting requirement and design of the bridge‐type FCL in the multi‐terminal DC grid

In the multi-terminal flexible dc grid, it is important for the healthy network to ride through the dc fault. The converters should operate continuously after dc faults, staying unblocked. But the existing selective protection and isolation method may fail to react swiftly enough, because the propagation speed of the dc fault current is extremely fast. This paper discussed the fault current limiting requirements and proposed a calculation method to determine the required dc reactor value, for the purpose of the converter continuous operation during dc faults. On this basis, the bridge-type fault current limiter (FCL) was proposed for using in the dc grid, due to merits including minor negative influence on dc grid normal operation, fast response to dc faults and efficient coordination with the dc circuit breaker (DCCB). The parameter design principle of the bridge-type FCL for dc grid was also discussed. Then the scaled-down dc experiment test circuit was built to verify the working principle and performance of the bridge-type FCL. Finally, the simulation cases based on the PSCAD/EMTDC were carried out to verify the feasibility of the theoretical calculation method and the superiority of the proposed bridge-type FCL for using in the dc grid.

Improved Voltage Control Strategy for Multi-terminal Flexible DC System

This paper presents an improved Voltage Droop control strategy, which updates the reference of DC link voltage by adding the difference between reference and actual voltage, which also adopts a dynamic P-V droop coefficient to improve the DC voltage level. By applying this method, we can eliminate the steady-state deviation of DC link voltage caused by ignoring the power loss in converter. In this paper, in order to demonstrate the reduction of deviation of DC link voltage, an improved control strategy is implemented for a three-terminal flexible DC power system modelled in PSCAD/EMTDC.

The Model and Parameters Based on the Operation Mode of a 500kV Multi-terminal Flexible DC Power Grid

The operating conditions of 500kV Zhangjiakou-Beijing Demonstration Project at all operation modes were studied regarding a multi-terminal flexible DC system with DC switches and a true bipolar wiring method. Based on the actual AC-DC hybrid method, this paper concluded 34 kinds of N-1 operation modes for keeping the system stable, simulated the multi-level converter valve (MMC) bank by using RTDS small-step elements, adopted an ideal voltage balancing method for the treatment of sub-units and established a MCC system simulating structure in compliance with the site. The operation modes of the system were simulated by selecting the parameters which are consistent with the line, using such parameters as the actual configurations and parameters of key elements such as the converter transformer, filter, bridge arm reactor, converter, etc. adopted in the project, and establishing a flexible DC control system with a layered structure. According to the result of the simulation, the corresponding modes of the system were provided and references were given for the feasibility study and implementation of the project.