Harmonic voltage. With the reduced 0.5 pu capacitor and

Harmonic Filter Reduction of Offshore
Wind Farms Connected with a Diode Based HVDC Link Report


and industrial relevance

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Even though LCC-HVDC links have benefits
in terms of efficiency, cost and power capacity, they are not used for
connecting large off-shore windfarms to mainland grid due to the requirement of
very large filter banks. This paper shows that using the additional control
flexibility provided by using turbines having fully rated converters, the
filter bank size can be considerably reduced. Hence, making it a feasible,
efficient, reliable and cost-effective solution for connecting off shore farms
to AC-grid.




In the system, consisting of off shore
parts (windfarm, ac grid, rectifier) and on shore parts (ac grid and inverters)
each turbine has a fully rated back-to-back converter connected to a PMSG, effectively
controlling the frequency and voltage of the off shore grid using a distributed
control algorithm. They also provide protection to the HVDC link and off shore
grid from short circuit. So, a controlled rectifier is no longer needed, this
enables us to replace the controlled SCR rectifier with uncontrolled diode
rectifier. The need for large and heavy capacitor and filter banks (the MVA
rating being typically around 0.4-0.6 pu), to provide reactive power and reduce
harmonics, makes the system bulky and expensive. Additionally, the filter
design depends on the network harmonic impedance which requires lengthy
analysis of the network. Reactive power compensation can be provided by the turbine
frontend converter, thereby reducing the capacitor and filter banks to approximately
0.1 pu, which is one-fifth of its original size. As, the wind turbine front-end
converters are needed to produce 0.4pu reactive power, we need to increase the
power ratings of the converters and transformers by 7.7%.    

Filter bank reduction has
considerable effect on off shore ac currents and voltage. With the reduced 0.5
pu capacitor and filter bank, the diode rectifier current harmonics increase
marginally as the distortion of windfarm ac voltage is small, below 0.03 pu, within
planned level specified in IEC-1000-3-6. Nonetheless, it has significant effect
on current through the capacitor bank. The harmonics rise from 0.05 pu to a max
of 0.28 pu. This rise in distortion must be taken in consideration while
defining capacitor bank’s ratings. The conduction loss at rated power of front
end converter increases by approx. 16% due to its increase in current rating by
7.7%, The wind turbine transformer loss, consisting mainly of of  and Eddy current losses
is increased to 0.605%( 0.1 pu bank) from 0.6%, and the increase in harmonic
loss is found to be negligible, 0.7%.

To conclude, with the control
algorithm we can use diode rectifier, with much reduced filter bank
requirements, causing only small effect on system losses. The need for
switching filter banks according to the power transmitted is also eliminated
thereby, improving reliability and reducing costs.


How the paper relates to H54PE2