DC line faults are major issues for a multiterminal high-voltage direct current (HVDC) system based on voltage-source converter (VSC). The fault current increases quickly along with a large peak, and ...complete isolation of the faulted system is not a viable option. Therefore, protection with high selectivity and accuracy is essential. In this paper, a new protection scheme for dc line in multiterminal VSC-HVDC system is proposed, which consists of a main protection and a backup protection. Both the protection principles are based on the supplemental inductor placed at each end of the dc line. Fault identification can be achieved by calculating the ratio of the transient voltages (ROTV) at both sides of the inductor. The main protection is able to detect the fault quickly without communication, while the backup protection is a pilot method based on the ROTVs at both ends of dc line, which is employed to identify the high-resistance faults and offer a backup in case the former fails. Comparison with some previous protection methods shows that the performance of the proposed protection scheme is promising. Numerous simulation studies carried out in PSCAD/EMTDC and real-time digital simulator (RTDS) under various conditions have demonstrated that fault identification with high selectivity and strong robustness against fault resistance and disturbance can be achieved by employing the proposed protection scheme.
The increasing wind penetration in today’s power grids has led to growing interest in the frequency control capabilities of wind generation. Several publications have proposed a variety of methods ...both on the levels of a single turbine and of a wind farm. This paper focuses on the role of wind generation in a system’s primary frequency control. Wind turbine control methods that enable frequency support and control are presented. The advantages and disadvantages of each method are discussed.
The high fault current of dc line is a major threat to multiterminal voltage-source-converter-based HVDC (VSC-HVDC) system. However, dc circuit breaker (DCCB) with large capacity and fast breaking ...speed is still under development. Therefore, fault current limitation is vital for the multiterminal VSC-HV DC system. This paper proposes a simple and easily applied hybrid current-limiting circuit (HCLC) at dc side, which consists of a current-limiting inductor (CLI) and an energy dissipation circuit (EDC) in parallel with the CLI. The CLI is designed to reduce the requirement for the DCCB's capacity and breaking speed. The EDC, which consists of thyristor-controlled resistors, is proposed to reduce the stress on energy absorption element (metal oxide arrester) in DCCB and to accelerate the fault current interruption. The design and discussion about the HCLC parameters are performed in detail. By employing the proposed HCLC, dc line fault in the multiterminal system can be isolated effectively with existing DCCB technology, and fast system restoration without power interruption of healthy part can be achieved. Numerous simulations with real-time digital simulator and comparisons with traditional schemes have demonstrated the promising performance of the proposed HCLC. The effectiveness of the HCLC's topology has also been verified by a simplified and scaled test circuit.
Explicit fault analysis is the basis of protection for the bipolar HVDC Line. The characteristics of the initial values of traveling waves under various internal fault conditions are investigated on ...the basis of the symmetrical component analysis. The criteria of fault classification and faulty-pole selection are put forward based on the zero- and positive-sequence backward traveling waves, and an integrated traveling wave-based protection scheme is proposed. The simulations based on real-time digital simulation show that the proposed scheme can detect faults rapidly, determine the fault type effectively, and select the faulty pole correctly.
The virtual synchronous generator (VSG) improves the robustness of the inverter-interfaced distributed generator (IIDG) against instability by introducing a virtual inertia. However, the transient ...response of the active power and the angular frequency conflict with each other for the IIDG with fixed inertia control. It is necessary to adopt adaptive control to improve overall performances of power and frequency as the operating condition changes. This paper analyzes the impact of the inertia on power and angular frequency. A dual-adaptivity inertia control strategy is proposed to offer a responsive and stable frequency support and also achieve the balance between power regulation and frequency regulation according to different operating conditions. The principle of parameter design is given to obtain the range of adaptivity. Quantitative assessment considering the cumulative effect of the output deviation and its duration is also presented to evaluate the proposed strategy intuitively. The strategy is further verified based on PSCAD/EMTDC and a hardware-in-loop experiment platform based on RTDS. Results confirm that the proposed strategy not only achieves rapid frequency response with slight dynamic deviations under disturbances but also strikes a balance between the frequency and power and leads to improved overall control.
The relation between the parameters of dc transmission line and the variation of transient energy has been analyzed under various fault conditions in this paper. According to that, a new transient ...energy protective scheme is proposed. It is developed based on the distributed parameter line model in which the transient energy distribution over the line can be obtained from the voltage and current measurements at both terminals and the fault can be recognized from the calculated value simply. The test system is modeled based on the CIGRE benchmark and considered the distributed parameters of the dc transmission line. Comprehensive test studies show that the performance of transient energy protection scheme is encouraging. It can not only identify internal fault and external faults correctly and quickly, but can also respond to the high ground resistance fault. Finally, two main factors, including fault resistance and transmission distance, that affect the performance of the protection are also discussed.
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.
Based on the inherent characteristic of the carrier-based pulse-width modulation converters, the response of the first carrier frequency harmonic (FCFH) current in the voltage source converter ...(VSC)-based high-voltage direct current (HVDC) transmission system is analysed under different fault conditions in this paper. A new protection scheme for the VSC-based HVDC transmission systems is proposed based on the FCFH currents. By extracting the harmonic currents at both the endings of the DC transmission cable, the fault type can be identified. The VSC-based HVDC test system is modelled in the PSCAD/EMTDC and the proposed protection scheme is evaluated with a variety of values of the fault resistance and the fault locations. Comprehensive test studies show that the performance of the proposed protection scheme is inspiring. It can recognise the internal and the external faults correctly.
The complex energy conversion and the volatility of renewable energy/load bring great challenges to the operation of the park‐level integrated energy system (PIES). To overcome this challenge, this ...paper proposes a multi‐timescale flexible dispatching method to fully exploit the flexibility of PIES in the energy cascade utilization mode. The cascade utilization model for energy flow is firstly established to analyse the coupling and complementary of heterogeneous energy. On this basis, the supply‐demand general equations of multi‐energy flexibility are proposed, which accurately quantify the ability to cope with uncertainty through mutual flexibility. Through coordinated complementation and mutual exchange of multi‐grade flexibility, the system realizes the suppression of random power fluctuations. The scheduling model includes day‐ahead dispatch and intraday multi‐time scale dispatch, which can satisfy the adjustment speed requirements of different energy. Numerical results demonstrate that the proposed method effectively enhances the flexibility and economy of system operation. The flexible demand for energy of all grades can be satisfied. Compared with the flexible dispatch in the triple energy supply structure, the operating cost is reduced by 9.07%.
Distribution networks (DNs) gradually become more active and flexible with the massive integration of renewable resources. The conventional protection methods are unsuitable for the promising active ...DNs. This paper proposes a protection scheme on the basis of measured impedance, which includes an impedance differential method and an inverse-time low-impedance method. The former approach identifies fault instant and provides time reference for the exchanged data. As primary protection, it is able to clear different types of faults as soon as possible. The latter method operates in an automatic coordination way with inverse-time characteristic. It can be used as backup protection or provide protection for single-end lines alone. Both of the methods are independent of operation modes and are able to clear faults with high sensitivity. The efficiency of the protection scheme is validated by PSCAD/EMTDC.
