3.3 Modularisation with Parallel Configuration and Redundancy
3.3.3 Including Redundancy
If an n+1 redundancy was included in a modularised system, and a fault occurred in one module, the system rating would remain at 100%. If the maximum system rating can be maintained and the odd fault is covered by redundancy, the repair of this fault can be delayed to a more convenient time and operation can continue as normal. This assumes the fault can be cut-off and isolated.
Every VSC should include at least one redundancy for the whole system to cover an n−1 fault and then benefit from full rating availability. The total number and cost of adding redundancies is higher if n+1 redundancy is needed for every distributed fully rate WT converter. However, only at least one redundant module is needed for cover a fault for a centralised VSC system that is shared by a cluster of WTs.
The alternative notion to n+1 redundancy is increasing the rating of each module and a scale factor of n/(n−1) for maintaining the maximum wind power throughput in case of a module failure. If one module is lost (n−1), the power is distributed across the remainder modules and the increased power throughput per module is supported by the higher module rating. For example, the modules need to be over- rated by 20% higher for an n=5 case, which is the same overall rating for an n=5 case with n+1 redundancy.
3.4
Chapter Summary
• Power electronics tend to have higher failure rates than traditional electrical components in the electrical system. Centralising the power electronics com- ponents in a station for each network cluster can provide a central and direct access for faster group maintenance and repair. Elimination of the converter from the WT improve the compactness of the WT nacelle.
• Centralising and modularising a VSC system can significantly improve its availability in case a VSC module fails. Although the difference in the avail-
ability is not very significant between modularising the distributed VSCs in the WTs and a centralised VSC in the substation platform, the latter topology benefits from easier and faster access to the VSCs for maintenance and repair. A centralised converter scheme would also need fewer redundancies to main- tain full availability in case of module failure, than including redundancies for each VSC if they were distributed inside individual WTs.
• A star network topology is robust and can provide higher availability with a centralised converter network topology. The WT will have its own cable connection to the substation platform, there is capacity to operate at the low-medium voltage level. Such topology at such voltage levels also presents opportunity to eliminate the transformer for further WT weight reduction. However, technical challenges need to be addressed for such topology. The relatively higher current needed per connection will increase the losses, the cost of the conductors, the reactive power consumption and voltage drop across the cable. Although low voltage cable terminals and insulation is low cost and robust due to lower electrical stresses, the total cable length requirement for the star network is too high.
• The cost of installing a cable in the harsh offshore environment is more ex- pensive than the cable itself, therefore an interconnected WT topology with a shorter total cable length is more ideal. As converter systems become more re- liable, the need will reduced for a such a star network topology with centralised converters.
• Transformers add inductance to the system, and potentially contribute to very high downtimes and cost of repair if a failure occurred offshore, because they are heavy and bulky. However, it is a relatively mature technology that is robust and reliable. The transformer is also necessary for isolation from high voltage and electrical zero sequence components. The large voltage step gains also enable the WT to employ cheaper LV devices and still be connected to an efficient MV collector network. A transformer-less connection would require more expensive MV devices or a reduction in the voltage level of the
3.4. Chapter Summary 85 distribution cable connection to the substation, which is also expensive in terms of copper requirements.
• The variable frequency AC network topology in Section 3.2.3 can have both centralised frequency converter and network cable interconnection. The max- imum power point tracking is not optimal but could be improved if combined with Type B limited variable speed WTs to individually fine-tune the tip speed ratio for maximum wind power extraction. When operating below the rated wind speed and power, the voltage must reduce with frequency to ensure that the transformer does not saturate at low frequencies. The frequency and the voltage output of the VSC can be highly controllable for this application. Further investigation is needed to study the dynamics characteristics of this control.
AC Voltage Controller for the
Offshore Wind Farm Network
Connected by VSC-HVDC
This chapter discusses an AC voltage control of the offshore AC network topology connected by VSC-HVDC. The offshore wind farm connection scheme used in this study assumes a traditional AC collection network connected by VSC-HVDC, as shown in Figure 4.1. Although the graphical representation only shows two wind turbines, there would be hundreds interconnected in a radial, ring or star network cable arrangement [103]. Each could represent a group of turbines without the centralized converter arrangement. The entire offshore network will be islanded and fully decoupled from the onshore grid frequency. The AC voltage and frequency for the network are established by the VSC-HVDC converter station.
Long distance HVDC Offshore AC Network Onshore AC Grid Wind Turbines (DC voltage control) (AC voltage control) (Machine Control)
Figure 4.1: A basic representation of a typical concept of a large scale offshore wind farm electrical system connected by VSC-HVDC.
4.1. Direct-Quadrature Coordination System 87 The traditional AC collection network topology is chosen for this study, because AC technology is mature and readily available. Although conceptual DC collection networks have cost saving benefits of having efficient transmission transformers-less connection with reduced and number of converter stages, AC systems on the other hand are maturer, simpler, and more reliable. Circuit breaking protection is simpler and faster, due to the fact that the AC waveform has inherent zero crossing once every half cycle [5]. AC grid connection voltage and frequency is standardised in industry, and therefore an AC network topology will not discriminate against any WT manufacturers that are already using AC.
4.1
Direct-Quadrature Coordination System
Three-phase abc electrical magnitudes can be transformed into two-component vec- tors relative to a stationary alpha-beta (αβ) frame or a direct-quadrature (dq) dy- namic reference frame that is rotating at ω rad.s−1 relative to the stationary αβ frame, as shown in Figure 4.2.
a
b
c
α
β
q
d
ω(t)
f (t)
f (t)d f (t)qθ (t)
Figure 4.2: The dq reference system relative to the abc and αβ stationary reference frame.
The d-axis is typically synchronised with the angle position θ of the AC grid voltage or the synchronous generator rotation speed, depending on the application. Components of any AC electrical magnitude f (t) will be sinusoidal functions, in the steady-state, when referred to the αβ reference frame. However, they will appear as DC magnitudes when referred to a dq reference frame if its angular speed ω is equal to the grid frequency [136].
Therefore control with these DC components in the dq frame is simpler and the control gains can be smaller. In comparison, a control system in the αβ ref- erence frame needs higher control gains in order to keep up to speed with the AC fundamental frequency [137].