Multiple-Terminal Grid Interconnected Offshore Wind Farms: Development of Transient Behavioural Simulation Models and Protection Schemes

Abstract

Offshore wind farms (OWF) are the highly penetrative energy sources in the electric power systems due to the continuous increase in energy demand. The generated offshore wind power cannot be supplied directly to the customers due to the variable generations and installed locations; instead, it can be integrated with the AC grid using high voltage alternating current (HVAC) or high voltage direct current (HVDC) link. The HVDC link is mostly preferred for the long-distance bulk power transmission due to its various advantages such as low losses, possible to control the power flow and perform an asynchronous operation, no charging current and stability issues. Voltage source converters (VSC)-based HVDC transmission system is a favorable option to interconnect the remote renewable sources with the AC grids since it has fault current blocking capability, operation with weak AC grids and maintains constant DC voltage even if power direction changes. Multi-terminal (MT) HVDC network becomes more attractive over two-terminal configurations due to the reduced number of terminals and sustains the power flow even under fault in a DC line. The research work in this thesis built transient behavioural model of the multi-terminal VSC-based HVDC link connected offshore wind farms, and also different case studies are carried out to evaluate the performance of the MT VSC-based HVDC system under power system disturbances. One of the main limitations of VSC is its vulnerability to DC faults. The protection of the DC line is more challenging due to low impedance, no natural zero crossing, low rise time and high steady-state fault currents. DC faults in a multi-terminal VSC-based HVDC transmission system gives very high peak fault current within a few milliseconds. The protection unit installed in the AC grid can address only steady-state faults in the DC grid. Protection unit has to be developed before semiconductor-based device damages due to very less overload capability. Two-end measurement gives certain time delay for long transmission lines which will slow down the protection decision speed. This thesis presents the development of a single-ended protection scheme for DC faults in multi-terminal VSC-based HVDC transmission system both without and with current limiting reactors. In this protection scheme, the protection iii starting unit uses the under-voltage criterion to detect the faults. The fault discrimination is done by using three conditions such as rate of change of DC voltage and current, and increment of transient energy. Current limiting reactors are designed and connected in series with the DC circuit breaker (CB) to maintain the DC fault current within the breaker capacity until protection unit isolates the faulty line. With the penetration of VSC-based HVDC system into the AC grid, the challenges in the distance relaying of AC transmission line has increased. When the fault occurs in a line close to the point of common coupling (PCC) in an AC grid with VSC-based HVDC transmission system, Zone-2 distance relay overestimates the fault distance due to the fast control of VSC. This makes distance relay to treat Zone-2 fault as Zone-3 fault, whereas Zone-3 fault is pushed out of the protection zones, leads to protection miscoordination. Therefore, the research work in this thesis intends to investigate the impact of VSC-based HVDC system on distance protection of AC transmission lines.