protection notice / copyright noticefor presentation in ewec 2010 hvdc solution for offshore wind...
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Protection notice / Copyright noticeFor presentation in EWEC 2010
HVDC Solution for Offshore Wind Park Comprising Turbines Equipped with Full-Range Converters
Presenting Author - Ranjan Sharma
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 2
Why High Voltage Direct Transmission (HVDC) ?
Future offshore wind-farms Longer transmission distance Higher installed power capacity
The amount of charging current associated with HVAC cable increases with distance, resulting in
Reduced active power transmission capacity
Added losses
Reactive power compensation becomes necessary (refer Fig. 1)
Gernerally compensated from the two ends of the transmission cable
Placing the compensation units between the cable sections in the offshore is not feasible economically
Fig. 1 Transmission capacity of HVAC cable with distance
It is sometimes a challange finding a strong grid in the close proximity of nearby on-land transmission system
A VSC can operate in all four quadrant – absorve or release both active and reactive power, which helps to connect to a weaker grid
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 3
Description of the System Under Consideration
Individual wind turbines in a wind-farm has a configuration as in Fig. 2.
Wind Turbine rating – 3.6MVA Generator – Induction generator Power electronics – back-to-back
full-range converter
The wind-farm collection network is a medium voltage AC grid – 33kV
The offshore platform consists of a AC-DC conversion unit (Voltage source converter)
The DC power transmission is via sea cables – 200kV
Fig. 2 Structure of the wind turbine
Fig. 3 Structure of the transmission system
The DC-AC conversion unit connects the wind-farm to the on-land transmission system
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 4
Principle of Operation
Under normal operating conditions, the following control strategy is implemented:
Wind-farm side VSC operates with fixed AC voltage and frequency
Controlled to act as an infinite voltage source
Condition is similar to if the wind-farm is connected to the HVAC system
No major changes are required in the wind turbine control
Grid-side VSC controls DC voltage and the reactive power
Total active power generated from the wind farm is thus transfered to the grid.
Fig. 3 Structure of the transmission system
Fig. 4 Block diagram of converter control
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 5
Principle of Operation
Fig. 3 Structure of the transmission system
Fig. 5 Without fault control
Fig. 6 With fault-ride-through
Fault in the super grid
Options
Direct fast communication between the converters
Use of DC chopper
Use the DC voltage rise to detect fault and control the wind farm side AV voltage
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 6
Principle of Operation
The fault-ride-through mode of the turbines can be directly utilized to control the power balance
The effect of the grid side fault (or power imbalance) is the increase in DC side voltage
Upon the detection of overvoltage in the DC transmission
The grid side VSC is set to support the grid with reactive current and limit the active current
The VSC at the wind-farm side is set to actively control the DC voltage by reducing the AC voltage
Fig. 7 Control sequence of the wind farm side converter
During fault in the super grid
The upper and the lower DC voltage threshold are defined
When the upper threshold level is exceeded, the grid-side converter is set to control the DC voltage
To avoide any possible conflict, the grid side VSC releases its DC control during fault
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 7
Principle of Operation
The reduced AC voltage (in the collection network) will activate the fault-ride through mode on the individual wind turbines, hereby reducing power output
Once the fault is cleared, the grid side VSC starts delevering power to the grid, causing the DC voltage to drop
When the DC voltage crosses the lower threshold value, the operation of the system shall revert to normal
Fig. 8 Conditions at the grid side converter
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 8
Summary
The grid conditions are thus reflected at the wind-farm collector network From the wind turbine prospective, the situation is similar as with the HVAC transmission system
No major modificaitons required in the structure of the wind turbine
Turbines in a wind-farm can still have individual speed control Wind velocity seen by all the wind turbines in a wind-farm may not be
same due to the large area that a wind-farm has to cover
The proposed control strategy provides flexibility to connect different generator converter topologies to the same VSC platform offshore
Prepared for EWEC 2010Ranjan SharmaSiemens Wind Power A/SPage 9
References:
As provided in the published paper:’ HVDC solution for offshore wind park comprising turbines equipped with full-range converters’
Thankyou for your attention!!!