Hybrid Hybrid High Voltage Direct Current Research Articles

Power Flow Calculation for AC/DC Power Systems with Line-Commutated Converter–Modular Multilevel Converter (LCC-MMC) Hybrid High-Voltage Direct Current (HVDC) Based on the Holomorphic Embedding Method

To harness the strengths and mitigate the limitations of line-commutated converter (LCC) and modular multilevel converter (MMC) HVDC systems, the LCC-MMC hybrid HVDC system has been developed. This paper proposes a holomorphic embedding (HE)-based power flow calculation method for AC/DC power systems with the LCC-MMC hybrid HVDC system. Firstly, the methodology involves establishing the mathematical model of the LCC-MMC hybrid HVDC system and its control modes. Subsequently, the HE formulation for the constructed LCC-MMC model is derived using the HE method. The proposed method simplifies the HE formulation using substitute variables, making it adaptable to various control modes. The designed HE formulation facilitates the derivation of the germ solution and the calculation of high-order power series coefficients. A sequential solution flow chart compatible with AC power flow algorithms is constructed to enhance the practicality of the HE method for real-world large-scale systems. The accuracy and effectiveness of the proposed method are verified through numerical tests on the modified IEEE 14-bus, IEEE 57-bus, and large 2383-bus system. The proposed method provides a reliable and efficient solution for power flow analysis in hybrid HVDC systems.

A novel linear regression density peak clustering‐based transmission line protection for line‐commuted converter‐voltage source converter hybrid high voltage direct current system

AbstractLine‐commuted converter (LCC)–voltage source converter (VSC) hybrid high voltage direct current (HVDC) transmission system is an innovative technology, the existing LCC and VSC protection require setting, and it is difficult to be directly applied in the LCC–VSC hybrid HVDC, which is a serious problem in practical engineering. A novel linear regression density peak clustering (LRDPC) approach is introduced for transmission line protection. LRDPC employs Least Squares linear regression to compute the fault current slope, followed by density peak clustering for fault type identification. The proposed protection structure is straightforward and setting‐less, eliminating the need for fault pole selection elements and classification thresholds. Validation on the Kun‐Liu‐Long LCC–VSC HVDC RTDS system demonstrates the method's effectiveness in identifying diverse faults under varying conditions, including fault types, locations, resistances, and signal‐to‐noise ratios. Notably, it remains robust against fault impedance (600 Ω) and noise interference (20 dB).

A Multi-Terminal Control Method for AC Grids Based on a Hybrid High-Voltage Direct Current with Cascaded MMC Converters

The hybrid high-voltage direct current (HVDC) transmission system with cascaded MMC converters has become a promising alternative for possessing the technical merits of both line-commuted converter (LCC) and voltage source converter (VSC), resulting in favorable characteristics and potential control of good prospect. This paper pays heightened attention to the feasible power and DC voltage control modes of a hybrid HVDC system; characteristics of master–slave control show higher flexibility than the LCC-VSC HVDC system, which demonstrates that the exceptional potential can serve to stability support the AC power grids. To optimize the control effect, besides damping level to attenuate power oscillations, the robustness suitable for various faults is also considered to obtain a multi-objective control problem. A detailed solution is proceeding using the TLS-ESPRIT identification algorithm and H2/H∞ hybrid robust control theory. This motivates multi-terminal controllers in the LCC rectifier and MMC inverters, which immensely improve the stability of both sending and receiving girds at the same time. According to the parameters of the actual hybrid HVDC project, the simulation model is established in PSCAD v4.6.2 software, and proposed control methods have been verified to satisfy damping objectives and perform well in multiple operating scenarios.

Single-ended protection method for hybrid HVDC transmission line based on transient voltage characteristic frequency band

Hybrid high-voltage direct current (HVDC) transmission has the characteristic of long transmission distance, complex corridor environment, and rapid fault evolution of direct current (DC) lines. As high fault current can easily cause irreversible damage to power devices, rapid and reliable line protection and isolation are necessary to improve the security and reliability of hybrid HVDC transmission system. To address such requirement, this paper proposes a single-ended protection method based on transient voltage frequency band characteristics. First, the frequency characteristics of the smoothing reactor, DC filter, and DC line are analyzed, and the characteristic frequency band is defined. A fault criterion is then constructed based on the voltage characteristic frequency band energy, and faulty pole selection is performed according to the fault voltage characteristic frequency band energy ratio. The proposed protection method is verified by simulation, and the results show that it can rapidly and reliably identify internal and external faults, accurately select faulty poles without data communication synchronization, and has good fault-resistance and anti-interference performance.

Coordination Control of Wind Farm Integrated Cascaded Hybrid HVDC System in Weak Grids

Combing the benefits of line-commutated converter (LCC) and modular multilevel converter (MMC), the hybrid high-voltage direct-current (HVDC) system is a viable option for the long-distance transmission of large-scale wind power. However, as renewable energy becomes more prevalent, the inertia of power networks will decrease, possibly leading to instability problems. In this scenario, the conventional grid following (GFL) control scheme is no longer applicable, and suitable coordination control strategies are required. This paper proposes two coordinated grid forming (GFM) control strategies for the cascaded hybrid system and evaluates the control performance of different control types. Firstly, the conventional control scheme of the cascaded hybrid HVDC system and problems under weak AC power systems integration are introduced. Then, coordinated grid forming control strategies are proposed, and the control performance of the GFL and GFM control schemes are compared. Finally, the cascaded hybrid HVDC system's electromagnetic transient model is constructed in PSCAD/EMTDC to confirm the efficacy of the proposed coordinated control strategies.

A New Communication-Free Grid Frequency and AC Voltage Control of Hybrid LCC-VSC-HVDC Systems for Offshore Wind Farm Integration

We propose frequency and voltage control method for a hybrid high-voltage direct current (HVDC) system for integrating an offshore wind farm into transmission networks. Conventionally, DC voltage level is maintained as constant regardless of the wind velocity and the grid frequency, so AC voltage fluctuates with active power variation in a hybrid HVDC system due to the innate reactive power absorption of inverter side line-commutated converter (LCC). However, in the proposed method, the DC voltage changes according to the wind velocity. Furthermore, DC voltage is simultaneously regulated to participate in frequency control in a communication-free manner. These two characteristics can be achieved by using five droop control methods proposed here. Additionally, by determining droop constants using the proposed determination methods, AC voltage can be maintained as constant during active power fluctuations. Therefore, using the proposed method, an offshore wind farm can be stably connected to mainland transmission networks and successfully used as frequency supporting resources, because voltage characteristics of the AC network are not destabilized. A small-signal state-space (SS) model of the target system was also developed to investigate stability of the proposed control. The utility of the proposed method is demonstrated by case studies using PSCAD and SS models.

Combinatorial Multi-Objective Optimization of Resistive SFCL and DC Circuit Breaker in Hybrid HVDC Transmission System

To cope with the DC fault isolation issue of a hybrid high-voltage direct-current (HVDC) system, a promising option is to coordinate superconducting fault current limiter (SFCL) and DC circuit breaker (DCCB). In this paper, a multi-objective Pareto optimization method is proposed for the combinatorial use of a resistive-type SFCL and a DCCB in a hybrid HVDC. Firstly, theoretical modeling and coordinated action process of the SFCL and DCCB are expounded. Then, a multi-objective function that collaboratively minimizes the SFCL resistance and the breaking capability of the DCCB is established, and a non-dominated sorting genetic algorithm-II (NSGA-II) is utilized to acquire the satisfying Pareto optimal solution set. The optimizing calculation is done in a 320 kV hybrid HVDC for validating the approach efficacy. Besides, transient simulations are implemented by PSCAD/EMTDC, and comprehensive performance analyses are executed to evaluate the rationality of the combinatorial optimal schemes. The findings indicate that the DC fault current is reduced and interrupted with the minimal rating of the resistive-type SFCL and the DCCB, and also, the HVDC system's safe and stable operation will be ensured. Therefore, the proposed approach is adequately verified for its availability and feasibility.

Research on DC Protection Strategy in Multi-Terminal Hybrid HVDC System

Multi-terminal hybrid high-voltage direct current (HVDC) systems have been developed quickly in recent years in power transmission area. However, for voltage-source converter (VSC) stations in hybrid HVDC systems, no direct current (DC) filters are required. In addition, the DC reactor is also not installed at the line end because the DC fault can be limited by the converter itself. This means that the boundary element at the line end is absent, and the single-ended protections used in line commutated converter (LCC) based HVDC (LCC-HVDC) systems or VSC-HVDC systems cannot distinguish the fault line in multi-terminal hybrid HVDC systems. This paper proposes a novel single-ended DC protection strategy suitable for the multi-terminal hybrid HVDC system, which mainly applies the transient information and active injection concept to detect and distinguish the fault line. Compared with the single-ended protections used in LCC-HVDC and VSC-HVDC systems, the proposed protection strategy is not dependent on the line boundary element and is thus suitable for the multi-terminal hybrid HVDC system. The corresponding simulation cases based on power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) are carried out to verify the superiority of the proposed protection.

Fault Diagnosis with Wavelet Packet Transform and Principal Component Analysis for Multi-terminal Hybrid HVDC Network

In view of the fact that the wavelet packet transform (WPT) can only weakly detect the occurrence of fault, this paper applies a fault diagnosis algorithm including wavelet packet transform and principal component analysis (PCA) to the inverter-side fault diagnosis of multi-terminal hybrid highvoltage direct current (HVDC) network, which can significantly improve the speed and accuracy of fault diagnosis. Firstly, current amplitude and current slope are used to sample the data, and the WPT is used to extract the energy spectrum of the signal. Secondly, an energy matrix is constructed, and the PCA method is used to calculate whether the squared prediction error (SPE) statistics of various signals that can reflect the degree of deviation of the measured value from the principal component model at a certain time exceed the limit to judge the occurrence of the fault. Further, its maximum value is compared to determine the fault types. Finally, based on a large number of MATLAB/Simulink simulation results, it is shown that the PCA method using the current slope as the sampled data can detect the occurrence of a ground fault with small transition resistance within 2 ms, and identify the fault types within 10 ms, without being affected by the sampling frequency.

Research on steady state control strategies of wind farm integration by VSC-LCC hybrid HVDC transmission

Wind farm is integrated by a hybrid high voltage direct current (HVDC) system with line commutated converter (LCC) and modular multilevel converter (MMC). Power sources at the sending end include wind farm and AC system, which can complement each other. Basic control strategies of each converter of the system are proposed. Considering the capacity limitation of MMC, basic control strategy under fluctuating wind power is made and the system is not able to transmit rated power under the strategy when the wind power is low. In order to enlarge the range of wind power when the system can transmit the rated power, an improved control strategy that lower the voltage of MMC is devised. Under the improved strategy, performance of the system is improved. Operation characteristics are compared under different short circuit ratio (SCR) of AC system at the sending end, and additional control is introduced when SCR is low.

DC Current and Voltage Droop Control Method of Hybrid HVDC Systems for an Offshore Wind Farm Connection to Enhance AC Voltage Stability

A new DC current-voltage droop control method for a hybrid high-voltage direct-current (HVDC) system featuring a voltage-sourced converter (VSC) rectifier and a line-commutated converter (LCC) inverter is presented. The proposed method utilizes DC voltage droop control to reduce the AC voltage fluctuation at the inverter side, to exploit the hybrid HVDC system for connecting offshore wind farms to weak grids without necessitating continuous reactive power compensation facilities. Additionally, an optimal droop coefficient calculation method is proposed to keep the AC voltage constant for wind power fluctuations. To evaluate the stability of the proposed method, a small-signal state-space (SS) model of the target system is derived and the root locus analysis is given. Simulation case studies are performed using an electromagnetic transient model in PSCAD and an SS model in MATLAB. The results demonstrate that a hybrid HVDC system with the proposed method successfully integrates offshore wind energy and weak grids without additional facilities because the AC voltage can be maintained at a near constant during wind fluctuations.

Operation and control of hybrid HVDC system with LCC and full‐bridge MMC connected in parallel

In this study, a new kind of hybrid high-voltage direct current (HVDC) system is proposed. Each terminal of the proposed system consists of one line commutated converter (LCC) and one full-bridge modular multilevel converter (FB-MMC). The LCC and FB-MMC are connected in parallel so that they can share the same transmission line. The active-reactive power capability of the hybrid HVDC system is extended compared with the conventional LCC-HVDC system, and power reversal control without power interruption can be achieved by the coordination control of LCC and FB-MMC. Besides, the proposed hybrid HVDC system is capable of handling DC fault, because both LCC and FB-MMC have DC fault blocking capability. Moreover, the power rating of FB-MMC can be designed to low value while keeping the bulk-power transmission capability of LCC. A two-terminal bipolar hybrid HVDC system is built in PSCAD/EMTDC. The simulation results verify the effectiveness and feasibility of the proposed hybrid topology and corresponding control strategies.

Reactive Power Control Method for the LCC Rectifier Side of a Hybrid HVDC System Exploiting DC Voltage Adjustment and Switched Shunt Device Control

We present a new reactive power control method for hybrid high-voltage direct-current (HVDC) systems that features a line-commutated converter-based (LCC) rectifier and a voltage-sourced converter-based (VSC) inverter. Our principal objective is to regulate the reactive power of the LCC rectifier side via co-operative control of the DC voltage and switched shunt devices (SSDs) such as capacitors and reactors. To achieve these objectives, we used three operative modes: reactive power, voltage, and power factor control. For each mode, we show how to determine the DC reference voltage. Also, to maintain the voltage within operational bounds and enhance the range over which reactive power is controllable, our method features the cooperation of SSDs. Furthermore, we show how to determine the required SSD capacities. MATLAB and PSCAD simulations confirmed that our method improves reactive power controllability at the LCC rectifier side of a hybrid HVDC system.

AC fault ride through control strategy on inverter side of hybrid HVDC transmission systems

Hybrid high-voltage direct current (HVDC) transmission systems employ a new type of HVDC transmission topology that combines the advantages of the line-commutated converter system and the voltage-source converter system. They can improve the efficiency and reliability of long-distance power transmission. However, realizing alternating-current (AC) grid-fault ride through on the inverter side of a hybrid HVDC transmission system is a challenge considering that a voltage-source converter based HVDC (VSC-HVDC) is used on the inverter side. In this study, a control strategy for an overvoltage fixed trigger angle based on the power-balance method is developed by fully utilizing the operation characteristics of a hybrid HVDC transmission system. The strategy reduces the inverter-side overvoltage of the HVDC system under a fault in the inverter-side AC system. Simulations based on Gezhou Dam are conducted to validate the effectiveness of the proposed strategy.

Electromechanical Transient Modeling and Control Strategy of Decentralized Hybrid HVDC Systems

This paper studies the electromechanical transient model and the control strategy of line commutated converter (LCC) and modular multilevel converter (MMC) based decentralized hybrid High Voltage Direct Current (HVDC) Transmission systems. The decentralized hybrid HVDC system is a new type of topology, and the related electromechanical transient model and control strategy have not been studied well. In this paper, the electromechanical transient model of a decentralized hybrid HVDC system is devloped through mathematical deduction. This model can be easily implemented in electromechanical transient simulation software and meet the time domain simulation requirements of large-scale systems. Then, in order to ensure the safe absorption of the DC power under various conditions, an optimal power flow model considering the decentralized hybrid HVDC system is proposed. Finally, the electromechanical transient model proposed in this paper is verified by the electromagnetic transient model, and the control strategy is validated in a modified New England 39-bus system.

DC-side harmonic analysis and DC filter design in hybrid HVDC transmission systems

Hybrid high-voltage direct current (HVDC) transmission is a new transmission structure that uses line commutated converter (LCC) at the rectifier side and voltage source converter (VSC) at the inverter side. This study proposes a method to calculate DC side harmonic currents of hybrid HVDC transmission systems and design filters to mitigate DC-side low-frequency voltage pulsations. First, an equivalent harmonic analysis model of modular multilevel converter (MMC) is deduced according to the relationship among different harmonic components. The harmonic voltage expressions at DC side of 12-pulse LCC are deduced by combining the three-pulse harmonic voltage source model and FFT decomposition. On this basis, the expressions for harmonic currents are derived by multiplying the harmonic voltage transfer function and transfer admittance equation of DC network and are solved for the inverse Laplace transform. Two different types of DC filters are designed according to the requirements of DC filter on decaying specific order harmonic currents. The influence of partial element decay on the transfer characteristic of filter is analyzed, and appropriate partial component dissipation parameters are applied. The decay pole near the harmonic resonance frequency is configured to achieve reasonable DC-filtering results. Simulation results of a hybrid HVDC transmission system demonstrate the effectiveness of the proposed analysis model and filter design method.

Travelling wave directional pilot protection for hybrid HVDC transmission line

As known, hybrid high-voltage direct current (HVDC) is wide applied in power system. Because two converter stations are line commutated converter based HVDC (LCC-HVDC) type and voltage source converter based HVDC (VSC-HVDC) type respectively, the HVDC transmission line (HVDC-TL) has special travelling wave (TW) propagation characteristic. Especially, due to the capacitance installed at VSC-HVDC station, the relay at VSC-HVDC side cannot detect voltage TW (VTW) signal. Meanwhile, TW protection is widely applied in HVDC transmission system, but it has no fault direction recognition ability. Hence, the paper proposes a novel TW directional pilot protection. Firstly, the paper analyzes the special propagation characteristics of TW along HVDC-TL. Secondly, the paper proposes novel fault direction discrimination criterions based on the integration of instantaneous power and current at LCC-HVDC side and VSC-HVDC side, respectively. Meanwhile, faulty pole selection criterion is also proposed based on the ratio of positive pole current to negative pole current. Thirdly, based on PSCAD/EMTDC, a simulation model of ±400 kV LCC-VSC-HVDC transmission system is established. And the simulation results prove that the proposed protection method is immune to fault location, fault resistance and fault type.

Static voltage sharing technology of multi‐break mechanical switch for hybrid HVDC breaker

The hybrid high-voltage direct current (HVDC) breaker combines mechanical and power electronics switching that enables it to interrupt power flows within a few milliseconds. Mechanical switch is a key component of hybrid HVDC breaker and has a number of serially connected interrupter units which ideally would divide the voltage equally. The static voltage distribution characteristics and voltage sharing design of a multi-break mechanical switch were discussed in this study. A finite-element model was developed to study the static voltage distribution characteristics and capacitance parameters of multi-break mechanical switch (which actually consists of resistance and capacitance parameters under direct current) as a preliminary study. Comparisons were made under the simulation of vertical and U-shaped arrangement forms. The results indicate that the static voltage distribution of the high-voltage terminal is at least more than 65%, whereas the severe non-uniform voltage distribution can be well improved by means of the method proposed in this study.

Hybrid ultra‐HVDC system with LCC and cascaded hybrid MMC

Nowadays, the hybrid high-voltage direct current (HVDC) system consisting of line commutated converter (LCC) and modular multilevel converter (MMC) has become the most competitive candidate in multi-infeed power systems. Owing to the limitation of insulated-gate bipolar transistor's withstand voltage, the DC voltage rating of MMC is not compatible with the LCC valve. To increase the DC voltage level, the number of sub-modules of each arm should be increased drastically, leading to many technical problems in communication design, optical fibre instalment, cooler system design and so on. To overcome the above problems, a hybrid HVDC system consisting of LCC and cascaded hybrid MMC valves is proposed to realise ultra-HVDC transmission. It can switch two operating modes (single valve/double valve) according to the conditions so that the system can maintain a certain power transmission when one of the valves quit operation during faults or maintenances. Meanwhile, by implementing the hybrid MMC, the system can ride through DC faults. The design of the topology and controllers, the DC fault ride-through strategy and the online switching strategy for MMCs will be presented in this study. Finally, the performance during DC faults and online switching is verified by extensive simulations.

Applicability analysis of travelling wave protection for the hybrid HVDC system

The hybrid high-voltage direct current (HVDC) system, in which the rectifier is made up of the line commutated converter (LCC) and the inverter is composed of the modular multilevel converter (MMC), has quite different fault characteristics from the symmetrical LCC–HVDC and MMC–HVDC systems. Therefore, it is necessary to analyse the applicability of the existing line protection principles to the hybrid HVDC system. First, the fault characteristics of the travelling wave on both ends of the hybrid HVDC line are obtained by analysing the boundary characteristics. The applicable conditions of travelling wave protection to the hybrid HVDC line are given based on the criteria of travelling wave protection and the fault characteristics. Second, an electromagnetic transient simulation model of the hybrid HVDC system is established in Power System Computer-Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) in order to verify the theoretical analysis results. A large amount of simulation results show that the travelling wave protection has good applicability in the hybrid HVDC system, but it has poor sensitivity for high-resistance grounding faults; an appropriate sampling rate should be chosen according to the parameters of the boundary element for a better performance of the protection.