Control and operation of MMC-HVDC system for connecting offshore wind farm

Student thesis: Doctoral Thesis

Abstract

For connection of large offshore wind farms over distances 80-100 km, modular multi-level converter (MMC) based high voltage DC (HVDC) system emerges as a more suitable solution than HVAC due to its flexible control and transmission distance not affected by the cable charging current. For HVDC connected wind farms, the offshore wind farm AC networks are established by the offshore MMC stations which exhibit significant difference compared to conventional onshore networks. Consequently, this thesis focuses on the offshore AC voltage control, system stability, and fault analysis of the offshore wind farm system connected with MMC-HVDC transmission.To control the offshore AC voltage and frequency using the offshore MMC, fixed frequency control is the most common approach. However, offshore system with fixed frequency control presents a slow response, especially during large transients. Thus, in order to improve the voltage controllability and system performances, a PLL based enhanced voltage and frequency control is proposed for the offshore MMC station. The performance of the proposed control is validated through time-domain simulations.The stability of the offshore system with the proposed control is further analysed using a developed small-signal offshore system model, including the offshore MMC and lumped wind turbine grid side converters. Bode plots and pole/zero maps are utilized to investigate suitable control parameter ranges and interactions between the power and AC voltage.For the offshore AC network, system control and response during offshore AC faults need to be carefully considered. During an offshore fault, both the offshore MMC and WT converters have to ensure their safe operation by limiting the currents to be within their maximum ranges, and in the meantime, enable satisfactory operation of the overcurrent protection relays. Considering the requirements of overcurrent relay, a fault current providing control for the offshore MMC is proposed which ramps up current during faults over a predefined profile.This ensures adequate fault current for the relays while avoids excessive overcurrent during WT string faults. The proposed fault current providing control is validated with offshore AC faults at different locations in a wind farm (e.g. clusters and strings).Onshore faults which lead to the rapid reduction of power transition capability of the onshore MMC is another challenge to MMC-HVDC connected offshore wind farm system. A DC voltage dependent AC voltage control is thus applied at offshore MMC station which reduces the offshore AC voltage once DC overvoltage is detected. The reduced offshore AC voltage results in the automatic reduction of power generated by the wind farm such that the power imported to and exported from the HVDC link can be rebalanced.Therefore, DC overvoltage is alleviated and when the onshore fault is cleared, the offshore AC voltage returns to the nominal value, and normal power generation and transmission can be quickly restored. Different onshore fault conditions with voltage drops of 100%, 50% and 20% are tested to show the satisfactory onshore fault ride-through control of the system.
Date of Award1 Oct 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsChina Electric Power Research Institute & University of Strathclyde
SupervisorLie Xu (Supervisor) & Derrick Holliday (Supervisor)

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