The basic architecture of voltage source converter operation is presented for both high voltage direct current transmission system and offshore wind generation applications. The voltage source converter has ability to operate as constant current source by synchronizing with the connected network, or work as a constant voltage source by imposing network frequency on the network. In both operational characteristics, the control system has current closed loop to improve the system stability as well as to be able to limit the VSC current during short circuit which is particularly difficult in VSC direct voltage control method for grid forming operation.
Moreover, the method of selecting controller gains have been proposed by applying small signal stability analysis. The condition of stability according to each control loop is derived and controller are tuned for desired perfor- mance by graphical methods such as bode or root locus analysis. Although the electrical network is a nonlinear system, the derive linearized model suffi- ciently demonstrate the VSC dynamic behavior and enables the application of linear control theory for control system design. The control structure presented in this chapter is applied in the further extension of VSC control system in next chapters.
3 Offshore Network having Grid
Forming VSC-HVDC System
In this chapter, a concept of an offshore grid is introduced to interconnect several offshore wind power plants with different onshore grids using VSC- HVDC transmission system. The VSC grid forming control presented in previous chapter has been extended to form an offshore AC network. Frequency and voltage droop schemes have been implemented to control the sharing of active and reactive power among VSC-HVDC transmission systems. Furthermore, the voltage droop control provides additional degree of freedom to manage the reactive power in the offshore AC network. Later in the chapter, an optimization algorithm is developed to optimize the reactive power flow in order to minimize the losses in the network2,3.
3.1 Introduction
Renewable energy is becoming a universally adopted primary source of energy. A breakthrough in advanced power transmission system technologies, specially in the power electronics devices, have enabled the installation of renewable generation units at remote location. Wind energy among other renewable sources has been a main focus in the last decade, particularly offshore. Up to the mid of 2016, 3344 offshore wind turbines with a combined power of 11.54 GW are installed in European waters [98].
2
This chapter is based on the following publications:
M. Raza, K. Schoenleber, and O. Gomis-Bellmunt, “Droop control design of multi-VSC system for offshore network to integrate wind energy”. Energies (MDPI), 9(10):826, oct 2016, doi: 10.3390/en9100826.
3
M. Raza, C. Collados, and O. Gomis-Bellmunt, “Reactive power management in an off- shore ac network having multiple voltage source converters”. Applied Energy (Elseiver), 206:793-803, nov 2017. ISSN 0306-2619. doi: 10.1016/j.apenergy.2017.08.182.
The most common export system of an offshore wind power plant is the high voltage alternating current (HVAC) cables link with an onshore grid, typically at the voltage level of 150 kV. The HVAC export system is well established technology as well as it provides grid forming interconnection to offshore network. However, the HVAC cables have high effective capacitance that limits the transmission of large active power over the long distance through Sea, typically limited up to 90 km for 100 MW [21]. The voltage source converter (VSC) based high voltage direct current (HVDC) transmis- sion system eliminate the power export limitation due to the cable capacitive effect and it has an ability to form the offshore network. However, the current rating of the switching devices (IGBTs) restrains the power transfer capacity.
Many experts are foreseeing the need of having offshore grid for better trade and integration of large offshore wind energy generation. In [99], the impact of an offshore grid on the European energy market has been studied considering several technical concepts for grid connection. One of the concept in the offshore grid is to create a multiterminal (MT) HVDC system to connect the several offshore wind power plants and integrate them with different onshore grids. However, the DC circuit breaker based protection scheme in the MTDC system increases the overall development cost [60, 65]. On the contrary, an offshore AC network formed by connecting wind power plants using HVAC cables and then using several VSC-HVDC systems in a point-to-point configuration to link with different onshore grids provides advantage to not to employee DC circuit breaker and the protection scheme can be applied similar to conventional AC system. An offshore AC network act as a mediator among several wind power plants and different countries grids. In [22], several options of forming offshore AC network are considered by connecting wind power plants center collection points with each other using medium or high voltage AC cable (normally, at 150 kV). This offshore AC network is then connected with the main grids of neighboring countries via VSC-HVDC system. Offshore wind power plants that are far from shore and are within the vicinity of 20 km from each other are economically suitable to form an offshore AC network [100].
3.2 Offshore AC Network Control and Operation