Program Topic: HVDC
- F3-2 – Friday 13:20-15:00
- 13:20 – In-Situ Test Assessment of Thyristor Level in the Converter Valve of HVDC Power Transmission
- 13:40 – Self-synchronisation of Wind Farm in MMC-based HVDC System
- 14:00 – Planning and Design of a European HVDC Grid Divided Into Feasible Protection Zones
- 14:20 – Droop Gains Selection Methodology for Offshore Multi-Terminal HVDC Networks
- 14:40 – Dynamic Performance Control of Modular Multilevel Converters in HVDC Transmission Systems
F3-2 – Friday 13:20-15:00
13:20 In-Situ Test Assessment of Thyristor Level in the Converter Valve of HVDC Power Transmission
The thyristor level is the basic unit of HVDC converter valve, which is the core equipment in the HVDC power transmission system. Because the converter valve performance directly affect the reliability, stability and efficiency of the whole power system, each thyristor level in the converter valve needs to be test routinely during the maintenance period of the converter valve equipment. Currently there is no specific and detailed test scheme for the thyristor level with thyristor control unit (TCU). Therefore, it is particularly urgent to study the in-situ test technology for the thyristor level in the HVDC converter valve. In this paper, the in-situ test principle, content and methods for the thyristor level with TCU are presented based on the analysis of the thyristor level working principle and the IEC 60700-1-2008 standard in order to offer technical support for the operation and maintenance of the HVDC converter valve. In general, the reverse recovery period of thyristor level is the most vulnerable stage of thyristor operation process, so a transient high-voltage pulse is applied to the thyristor level during its reverse recovery period in order to test the characteristics of thyristor level. Then, the in-situ test method for the thyristor level is applied to the converter valve test of ±800kV HVDC power transmission project, and the practical test result verifies the reasonability and validity of the proposed in-situ test method.
13:40 Self-synchronisation of Wind Farm in MMC-based HVDC System
The stability of a offshore wind power network connected through a high voltage dc (HVDC) transmission line is a critical problem since there is no direct connection to a strong ac collection (ACC) bus. Field experience has shown that subsynchronous oscillation (SSO) and harmonic resonance can occur between the wind farms and the HVDC systems. The oscillations can appear in the presence of background harmonics and is arguably resulting from the controller interaction of the Wind Energy Conversion System (WECS) converter controller and the HVDC converter controller. The design of the synchronization unit (Phase-Locked-Loop) has shown to have a significant impact in achieving satisfactory performance. However, both slow and very fast synchronization units could directly affect the control performance and degrade system stability. This paper proposes a controller design without PLL for the WECS grid-side converter which does not have a dedicated synchronization unit (PLL) in the controller. This controller is implemented on the WECS converters of the ACC side to synchronise them to the Modular Multi-level Converter (MMC)-based HVDC system. A detailed analysis is presented and the results are compared with the widely used decouple d-q control structure. The impedance frequency responses for both the d-q frame control and the synchronverterbased control are presented to show a comparison of the system performance. The time domain simulation results are presented to show how the self-synchronisation impacts on the system performance compared to the classical control solution.
14:00 Planning and Design of a European HVDC Grid Divided Into Feasible Protection Zones
While recent advances in voltage source converter technology boost the number of installed offshore DC links and interconnectors, the possibility to build HVDC-grids only arises, if existing technical barriers can be removed. In this work the planning of an HVDC overlay grid is introduced. Using a European transmission network model, a future scenario of 2020 is considered to identify HVDC terminals and connect them in an optimal way. Based on the determined HVDC network, strategies are applied to split the network into protection zones subsequent to contingencies. Transient simulations proof that DC faults can be handled and a global outage is avoided.
14:20 Droop Gains Selection Methodology for Offshore Multi-Terminal HVDC Networks
This paper presents a methodology for selecting the droop gains of the voltage source converters (VSCs) in multi-terminal high-voltage direct current (MT HVDC) transmission system. The droop gains are selected to improve the DC voltage transient and steady state dynamics performance. The proposed methodology relies on improving the small signal stability of the HVDC network, which is performed by selecting the droop gains values that increase minimize the real part of the system critical eigenvalues, while maintaining the steady state voltage deviation within limits. The proposed methodology has been tested on the CIGRE B4 DC grid test system. Furthermore, the simulation results confirm the effects of selecting the proper droop gains on the DC voltage dynamics.
14:40 Dynamic Performance Control of Modular Multilevel Converters in HVDC Transmission Systems
This paper focuses on dynamic performance control of modular multilevel converters (MMC) in high-voltage direct current (HVDC) transmission systems. To achieve this objective, a new mathematical model including six state variables of ac-currents and dc-link voltage of MMC, and circulating currents of converter arms are proposed for MMC in d-q reference frame. In addition, a robust control technique with three sub-control loops is designed to provide the stable operation of MMC. In the overall structure of the proposed controller, three outer, central and inner loops have the duties of 1) making the state variables error zero with changeable convergence rate, 2) adding robustness characteristic to the proposed controller, and 3) generating the appropriate reference values for MMC’s currents, respectively. The effectiveness of the proposed control algorithm is investigated via MATLAB simulation. The simulation results highlight the capability of the proposed control algorithm in offering an accurate active and reactive power tracking through the control method of MMC, a stabilized dc-link voltage, capacitor voltage balancing of submodules, and minimization of circulating currents of converter arms during dynamic transitions and steady state operation.