Weixing Lin

An Investigation on the Active-Power Variations of Wind Farms

Variation is one of the most important inherent characteristics of wind power. Very few common methods exist to quantify the variations. This presents difficulty for the security and efficiency operations of power systems if large-scale wind power is included. Based on the analysis results from the large amounts of field measurements from nine aggregated wind farms, it is found that the t location-scale distribution and the Laplace distribution are able to cope with this problem. A systemic methodology is proposed accordingly. Therefore, the characteristic of the wind-power variations could be appropriately represented.

A Three-Terminal HVDC System to Bundle Wind Farms With Conventional Power Plants

There is a concern over large, fluctuating wind power injected into a power system, which could affect stability, frequency and power quality of the power system. A method of using three-terminal HVDC system to bundle large scale wind farms with conventional power plants is proposed to overcome this problem. The wind power fluctuation is canceled by regulating the power from the conventional power plants. Topology of the scheme is described. A control strategy particularly suitable to cope with wind power fluctuation is designed. Novel fault ride through mechanisms are designed to enable safe operation of the system. Simulations using PSCAD/EMTDC verify the successful operation of the system during starting-up, steady state, wind power variations and faults. The proposed scheme provides a novel way to deal with the integration of large scale wind power.

Multiport DC–DC Autotransformer for Interconnecting Multiple High-Voltage DC Systems at Low Cost

This paper proposes a multiport dc-dc autotransformer (multiport dc auto) that is used to interconnect multiple HVDC systems with different voltage levels. The multiport dc auto is able to reduce 50-80% the converter cost compared with conventional dc-ac-dc technology. Different from the conventional dc-ac-dc technology using magnetic coupling at the ac sides of the converters, there is direct electrical connection between the interconnected dc systems in the multiport dc auto. Such direct electrical interconnection significantly reduces the used power converters in the multiport dc auto. Taking interconnecting a ±250, ±320, and ±400 kV dc system with the rated exporting/importing dc power at each of the dc system being 500, 1000, and 1500 MW as an example, the conventional multiport dc-ac-dc technology requires a total of 3000 MW power converters while only 775 MW power converter is required in the multiport dc-dc autotransformer. The required power converter in the multiport dc-dc autotransformer is only 26% of the converter used in the conventional multiport dc-ac-dc technology. Cost and operating power loss is therefore significantly reduced.

LCC based MTDC for grid integration of large onshore wind farms in Northwest China

In recent years, many large onshore wind farms are constructed or under construction all over the world. In northwest region of China, the planned capacity of onshore wind farms has reached over 20GW in total. However, the locations of those wind farms are very remote and far away from load centers, and their distance to load centers are beyond 1000km. Therefore, its of primary importance to address the issues concerned with bulk wind power transmission over long distance. This paper is aimed at development and application of using the line-commutated converter based multi-terminal HVDC (LCC-MTDC) technology for grid integration of large remote onshore wind farms located at Northwest China region. The studies have firstly focused on the design of LCC-MTDC system configuration and its parameters, taking into consideration the practical operation requirements for wind farm grid integration. The control strategy for the LCC-MTDC operation is then proposed, and the comparison in control strategy with and without coordinated operation is also carried out and verified by PSCAD/EMTDC simulation. Various operation scenarios such as ac fault on rectifier and inverter sides are simulated to investigate the system performance during disturbances. Results show that the proposed LCC-MTDC configuration and its control strategy are effective and the LCC-MTDC system is well controlled over the whole operating range.

An Investigation on the active power variations of wind farms

Variation is one of the most important inherent characteristics of wind power. Very few common methods exist to quantify the variations. This presents difficulty for the security and efficiency operations of power systems if large-scale wind power is included. Based on the analysis results from the large amounts of field measurements from nine aggregated wind farms, it is found that the t location-scale distribution and the Laplace distribution are able to cope with this problem. A systemic methodology is proposed accordingly. Therefore, the characteristic of the wind-power variations could be appropriately represented.

DC–DC Autotransformer With Bidirectional DC Fault Isolating Capability

This paper studies the control and dc fault isolation of a dc-dc autotransformer topology (DC AUTO). The operating principle and power flow analysis of a DC AUTO are studied. Internal dynamic study shows that dynamics of the common bus ac voltage is a purely algebraic equation. A control strategy of using VSC2 to control common ac bus voltage, and VSC1 (3) to control the transferred dc power is then proposed. System responses to dc fault are analyzed. Corresponding design methods that enable bidirectional dc fault isolating are then proposed and their impacts on component cost are analyzed. A family of possible DC AUTO topologies is also proposed. Extensive simulations on power step change, dc and ac faults confirmed the theoretical studies of a DC AUTO employing the modular multilevel converter topology. Taking a ±320-kV/±500-kV DC AUTO transferring 1000-MW dc power as an example, conventional dc-ac-dc technology requires 2000-MW total converter rating with power loss ratio of 1.8%, while the DC AUTO technology only requires 1020-MW total converter rating with power loss ratio of about 0.8%. The DC AUTO is able to achieve exactly the same functions as a dc-ac-dc with significantly reduced investment and operating cost under low- and medium-dc voltage stepping ratio.

Equivalent Electromagnetic Transient Simulation Model and Fast Recovery Control of Overhead VSC-HVDC Based on SB-MMC

Self-blocking modular multilevel converter (SB-MMC) is able to block dc fault current with relatively lower number of power semiconductors and lower valve power loss compared to other dc fault tolerant MMC topologies. Operating principle, design, reliability, and redundancy analysis of SB-MMC have been studied while the equivalent modeling and application of SB-MMC at overhead VSC-HVDC have not been fully investigated. Since operating principle of SB-MMC is different from half-bridge MMC, the existing equivalent modeling method of half-bridge MMC cannot be directly applied to modeling of SB-MMC. As dc fault at overhead VSC-HVDC are typically temporary, how to securely and quickly restart a VSC-HVDC after isolating dc fault also needs detailed studies. To solve the above two challenges, an equivalent model (EM) and a fast recovery control of overhead VSC-HVDC based on SB-MMC are proposed. Accuracy of the proposed model is verified by extensive simulations in PSCAD/EMTDC. The proposed fast recovery control further exploits the control capability of SB-MMC and enables fast and secure recovery of overhead VSC-HVDC based on SB-MMC. The proposed model and controllers also provide guidance for the modeling and application of other MMC topologies that are able to block dc fault current.

Dynamic phasor modelling and operating characteristic analysis of half-bridge MMC

A unified dynamic phasor model for modular multilevel converter (MMC) under the rotating frame is developed. The model takes the AC voltage in the rotating frame and the DC voltage as electrical inputs, the fundamental frequency and second harmonic modulation indices as the control inputs. By separately modelling the controller, the electrical dynamics of MMC and the dynamics of external electrical system, the proposed dynamic phasor model is able to seamlessly interconnect with models of controllers and external electrical system. The dynamic phasor model of MMC is therefore normalized. By setting the derivative terms in the dynamic phasor model to be zero, static phasor model of MMC is achieved. By linearizing the dynamic phasor model around the steady-state operating point, small signal model of MMC is obtained. As a result, the proposed dynamic phasor model of MMC unifies the dynamic phasor model, static phasor model and small signal model of MMC. Based on the obtained static phasor model of MMC, an open loop circulating current suppressing control (CCSC) strategy and a method of analytically analyzing the operating characteristic of MMC are proposed. Simulations on PSCAD/EMTDC verified the accuracy of the dynamic phasor model, the efficacy of the open loop CCSC strategy and the different constraints on the PQ operation zone of a MMC.

Series VSC-LCC converter with self-commutating and dc fault blocking capabilities

This paper proposes a new topology of converter that possess self-commutating and dc fault blocking capabilities. The converter (SVLC) is composed of series connection of LCC and VSC converters. If connected to wind farms, the VSC converter is used to maintain the ac voltage of wind farm and provide commutation voltage for the LCC while the LCC is used to control the dc voltage of the VSC so as to maintain power balance of the wind farm and the power transmitted by the SVLC. The SVLC combines advantages such as high power, high voltage, low loss, relatively low cost of LCC and the self-commutating ability of VSC. Meanwhile, the SVLC is able to achieve dc fault blocking capability. Simulations in normal operation and during dc faults in PSCAD/EMTDC using detailed switching model verified the feasibility of the proposed topology.

Equivalent electromagnetic model of self-blocking MMC with DC fault isolation capability

Self-blocking MMC (SB MMC) is able to isolate DC fault current and is promising for use in overhead HVDC transmission. Each converter arm of a SB MMC is consisted of half the self-blocking sub-module (SBSM) and half the half bridge (HB) sub-module. This hybrid design achieves the DC fault isolation capability without significantly increase of the cost and power loss. The detailed electro-magnetic simulation of SB MMC with hundreds of voltage levels is time consuming. Since a SBSM can be negatively inserted, the dynamics of SB MMC are different from HB MMC under DC fault condition. The conventional equivalent models of HB MMC are not sufficient for SB MMC. To solve the above challenges, an equivalent electromagnetic model for SB MMC is proposed and verified in this paper.