1. 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

2. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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

3. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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

4. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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

5. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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

6. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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

7. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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

8. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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

9. Prepared for EWEC 2010 Ranjan Sharma Siemens Wind Power A/S Page 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!!!