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.

Integration of E-core DC reactor based on thyristors with hybrid HVDC breaker

In general, the fault current in high-voltage direct current (HVDC) grids has two characteristics: the rapid increase and the lack of a zero-crossing point. Therefore, a large reactor is necessary to fulfill DC fault current limiting. But this can have disadvantages on the dynamics and stability of the HVDC grid. In this paper, the integration of an E-core DC reactor based on thyristors with a hybrid HVDC breaker (RTHCB) is suggested as a solution. The proposed model reduces the rising rate and peak of the fault current and protects the LCS through the primary inductance of the reactor. In RTHCB, several capacitors and thyristors are placed in parallel with the secondary winding of the reactor to control the gradual and soft placement of the large inductance in the path of the fault current. This integration reduces the electrical stresses caused by the sudden disconnection of conventional HCBs. Due to the presence of the capacitors, the energy absorbed by the arrester decreases. In addition, it decreases semiconductor switches and absorbs fault energy in a short time. After theoretical analysis, the RTHCB is modeled on the 3D space of the COMSOL Multiphysics. Finally, an MMC-HVDC grid has been simulated in the PSCAD/EMTDC to validate the performance of RTHCB.

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 protection method for LCC-VSC hybrid HVDC system based on boundary transient power direction

In a line-commuted converter–voltage source converter (LCC-VSC) system, in the hybrid multi-terminal high-voltage direct-current (HVDC) sending end, a line-commuted converter (LCC) converter is used, while the receiving end uses a full-half hybrid modular multi-level converter (MMC), with a fault self-clearing capability. The DC line faults can be self-cleared within 1–2 ms. Therefore, there is an inevitable problem of short availability of faulty information since only 1-ms complete fault data are available to identify faults. Thus, it is necessary to develop a fast and reliable protection scheme for an LCC-VSC system. To address this need, this paper proposes a DC line protection method based on the boundary transient power direction. First, the characteristics of boundary transient power direction of the internal and external faults are analyzed. For an internal fault, the boundary transient power direction is positive, whereas for an external fault, this direction is negative. Therefore, this paper uses the sum of the boundary transient power directions to structure internal- and external-fault identification elements and a pole detection element. The proposed method is validated by real-time digital simulations using Kun-Liu-Long LCC-VSC three-terminal hybrid HVDC–RTDS test system. Numerous tests have demonstrated that the proposed protection method is effective in detecting different fault types under various fault distances, fault transition resistance values, and signal-to-noise ratios in the period of 1 ms. Compared with the existing methods, the proposed protection principle has a better performance in withstanding fault transition resistance (600 Ω) and noise interference (10 dB).

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.

Travelling wave fault location approach for hybrid LCC-MMC-MTDC transmission line based on frequency modification algorithm

Line-commutated-converter (LCC) based and modular-multilevel-converter (MMC) based hybrid high-voltage direct-current (HVDC) transmission technology is already applied in actual power grid, which has the ability to avoid commutation failure issue. Besides, aiming to acquire high transmission capacity, the multi-terminal direct-current (MTDC) approach is adopted. Although the travelling wave (TW) fault location approach is widely applied in actual power grid, its adaptability for hybrid MTDC topology and transmission distortion are not well considered. Therefore, a novel TW fault location approach for LCC-MMC-MTDC transmission line based on frequency modification algorithm is proposed in this paper. Firstly, the complete frequency modification algorithm suitable for MTDC transmission system is described. Secondly, the novel TW fault location approach and its implementation scheme are given. Eventually, via PSCAD/EMTDC, a series of case studies are illustrated based on typical ±400 kV hybrid LCC-MMC-MTDC simulation model, which implying high robustness against fault location and grounding resistance issues.

Diode rectifier‐based hybrid high‐voltage direct current converter for offshore wind farms

This paper presents a diode rectifier (DR)-based hybrid converter topology for high-voltage direct current (HVDC) transmission system with offshore wind farms (OWFs). The hybrid converter consists of a DR and an auxiliary converter. A high conversion ratio ac/dc converter is designed as the auxiliary converter, which can form the offshore grid voltage for OWFs to achieve black-start. Under steady state, all the active power output from OWFs is delivered through the DR. Meanwhile, the auxiliary converter is able to compensate the reactive power and harmonic currents of the system. Comparing with the conventional modular multilevel converter (MMC), the number of submodules (SM) of the proposed hybrid converter is significantly reduced, which decreases the construction cost and size of the system. The topology and operation principle of the hybrid HVDC converter are introduced. Then the corresponding control strategy is designed to achieve energy balance and power transmission. Based on the mathematic model, a design method for the parameters of the auxiliary converter is proposed. Finally, the feasibility of the proposed hybrid HVDC converter is verified by both the simulation and hardware-in-the-loop experiment results.

Novel travelling wave directional pilot protection approach for LCC-MMC-MTDC overhead transmission line

Currently, line commutated converter (LCC) and modular multi-level converter (MMC) based hybrid high voltage direct current (HVDC) transmission technology is utilized in actual power grid. Aiming to acquire large transmission capacity and avoid commutation failure, LCC and MMC converters are applied in rectifier and inverter stations in multi-terminal direct current (MTDC) topology structure, respectively. Generally, LCC-MMC-HVDC transmission line adopts travelling wave (TW) protection principle based on one-ended signals. However, it cannot identify fault direction and exists threshold issue. Besides, the different structures of LCC and MMC stations have not been considered. In order to overcome these shortcomings, the paper proposes a novel TW directional pilot backup protection for hybrid LCC-MMC-MTDC transmission line. First, TW propagation process in hybrid multi-terminal transmission system is analysed. Afterwards, fault direction identification criterion is proposed. Eventually, using PSCAD/EMTDC, a ±400 kV hybrid three-terminal LCC-MMC-MTDC simulation model is established. And the simulation results prove that the novel protection is not affected by different fault locations, different fault resistances and different fault types.

Performance of a protection system for DC grids

The development of High Voltage Direct Current (HVDC) transmission technology is still challenging due to unresolved technical issues, mostly in terms of protecting the system. The overcurrents and voltage drop produced by fault conditions are severely dangerous to the Voltage Source Converters’ (VSC) electronic components. Hence, fast protection systems are needed. This paper proposes a fullselective protection system in conjunction with hybrid HVDC circuit breakers. This protection system employs the rate-ofchange-of-voltage and the differential-current algorithms for primary and backup fault protections. This protection scheme is applied to a HVDC grid and an analysis of its performance is presented. This system is evaluated with several fault scenarios that include different fault characteristics. The rate-of-change-ofvoltage algorithm’s fast operation and the differential-current algorithm’s high selectivity are verified.

Dual-Circuit-Based Test Bench Design for HVDC Circuit Breaker Verification

High-voltage direct current (HVDC) circuit breaker development and deployment strongly depend on the testing process, which ensures that the HVDC circuit breakers will satisfy design requirements. This article presents an HVDC circuit breaker test bench circuit configuration that can provide controllable large output currents to simulate different fault conditions for the current breaking test and high output voltage for the dielectric withstand test. The current breaking test circuit is based on multiple cascaded power converters connected in parallel to provide the necessary output current capability. Each cascaded power converter is composed of multiple cells that are operated by a phase-shifted pulsewidth-modulated signal for greater controllability and higher quality of the output waveform. The dielectric withstand test circuit is a simple high-voltage source with a low power rating that can also be used to charge the test bench and the internal circuitry of the circuit breaker that is to be tested. The proposed test bench ensures that fault conditions can be replicated accurately and offers greater flexibility by being able to test mechanical, semiconductor-based, or hybrid HVDC circuit breakers with different current and voltage ratings on the same hardware without any changes. The idea and the operating principle of the proposed test bench are verified experimentally on a downscaled system that consists of three cascaded power converters connected in parallel with three cells per cascaded power converter and with a total equivalent switching frequency of 92.5 kHz.

Optimization and Configuration of Control Parameters to Enhance Small-signal Stability of Hybrid LCC-MMC HVDC System

This paper investigates the small-signal stability of the hybrid high-voltage direct current (HVDC) transmission system. The system is composed of line commutated converter (LCC) as rectifier and modular multi-level converter (MMC) as inverter under weak AC grid condition. Firstly, the impact of short-circuit ratio (SCR) at inverter side on the system stability is investigated by eigen-analysis, and the key control parameters which have major impact on the dominant mode are identified by the participation factor and sensitivity analysis. Then, considering the quadratic index and damping ratio characteristic, an objective function for evaluating the system stability is developed, and an optimization and configuration method for control parameters is presented by the utilization of Monte Carlo method. The eigenvalue results and the electromagnetic transient (EMT) simulation results show that, with the optimized control parameters, the small-signal stability and the dynamic responses of the hybrid system are greatly improved, and the hybrid system can even operate under weak AC grid condition.

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.

A hybrid HVDC converter based on M2C and diode rectifiers without DC capacitors for offshore wind farm integration

In this paper, a new topology of high-voltage direct current (HVDC) converter system for transmitting the power from offshore wind farms (OWF) to the mainland grid is proposed. The presented HVDC converter is composed of a hybrid series connection scheme of voltage-source converters (VSC) based on M2C (modular multilevel converter) and twelve-pulse diode rectifiers (12PD) without DC capacitors integrating with the WFs in the offshore station and a full-scaled M2C in an onshore station. In this scheme, the HVDC-link voltage is controlled by the M2C in the onshore station, while the M2C in the offshore side is operated as controlled-voltage source to maintain the offshore AC voltages. A feedback linearization technique is applied to the M2C in the offshore station for regulating the offshore AC voltages, which offers a better performance for a nonlinear model of the voltage-controlled VSC when compared to schemes based on the linear control theory. Furthermore, with the utilization of the diodes and elimination of DC capacitors at the output terminal of the 12PD, the proposed HVDC converter topology offers advantages over the full-scaled M2C HVDC systems in terms of efficiency, capital investment costs, maintenance, and footprint. The proposed topology is proved through the simulation tests for a 525-kV 600-MW HVDC system integrating with wind turbine generators. Fundamental operation of the converter is also demonstrated by the experimental results for a reduced-scale HVDC system in laboratory.

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.

HVDC Breaker Power Loss Reduction by Bridge-Type Hybrid Breakers

Several types of high voltage direct current (HVDC) breakers have been introduced and commercialized. Each of them has advantages and disadvantages. Among them, the hybrid HVDC breaker is highly successful. One of the most important concerns that the hybrid HVDC breaker has faced is high power loss throughout its fault current breaking process. The hybrid HVDC breaker comprises a high voltage bidirectional main HVDC breaker. A significant number of electronic switches need to be connected in a series where anti-parallel diodes are essentially embraced. During fault inception, a number of series solid-state switches and a number of series diodes dramatically increase the power loss of the main breaker. This study, firstly, studies the power loss of the hybrid HVDC breaker and later develops a structure of a full-bridge hybrid breaker (FBHB) to reduce the losses of the current structure both in the normal and fault protection states. In this paper simulations are done based on PSCAD. In addition to the analytical study and simulations, we show that the developed structure substantially decreases the amount of power lost during the normal operation and fault current breaking stage.

Interoperability of Modular Multilevel and Alternate Arm Converters in Hybrid HVDC Systems

This paper demonstrates the interoperability of an emerging alternate arm converter (AAC) with the state-of-the-art modular multilevel converter (MMC) in high-voltage direct current (HVDC) systems based on a hybrid VSC-HVDC system. The paper also showcases the parameter derivation of the hybrid HVDC system and its detailed control structure. The study provides preliminary steps towards detailed analysis of AAC interoperability in complex hybrid dc grid configurations. A detailed set of results based on the 800 MVA hybrid voltage source converter (VSC)-HVDC system showcases the interoperability performance of the AAC under different operating scenarios and verifies its associated control functions.

Communication Requirements for a Hybrid VSC Based HVDC/AC Transmission Networks State Estimation

The communication infrastructure of the modern Supervisory, Control and Data Acquisition (SCADA) system continues to enlarge, as hybrid High Voltage Direct Current (HVDC)/Alternating Current (AC) networks emerge. A centralized SCADA faces challenges to meet the time requirements of the two different power networks topologies, such as employing the SCADA toolboxes for both grids. This paper presents the modern communication infrastructure and the time requirements of a centralized SCADA for hybrid HVDC/AC network. In addition, a case study of a complete cycle for a unified Weighted Least Squares (WLS) state estimation is tested on a hybrid HVDC/AC transmission network, based on Voltage Source Converter (VSC). The cycle estimates the elapsed times from the sensors up to the SCADA side, including the data acquisition and the WLS processing times. The case study is carried out on the Cigre B4 DC test case network with 43 virtual Remote Terminal Unit (RTU)s installed and 10 data concentrators, all connected through a fiber-based communication network. It is concluded that the time requirements can be fulfilled for a hybrid HVDC/AC network.