Mehrdad Boloorchi, P.Eng. September2010

OFFSHORE NETWORK APPROACH IN GRID CONNECTION OF OFFSHORE WIND PARKS

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General

Objective: To Discuss network topology or grid connection

configuration when connecting several offshore wind parks located in the same area to the transmission grid.

Assumption: There is no concern about available transfer capability, in

other words, transmission grid is capable of reliably transferring the wind power generated in the offshore area to other areas of consumption over all transmission lines.

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Grid connection of an offshore wind park

The electrical connection system of an offshore wind park includes:

a. A collector system which transfers WTG power to the offshore collection point. The voltage level of the collector system is normally at 34.5kV.

b. Transmission Link to the mainland. Depending on the conditions stated below HVAC or HVDC systems will be used for Transmission Link.

The type of Transmission Link depends on

(1) Wind Park size, and

(2) Distance from sea shore.

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Impact of Wind Park Size and Distance from Seashore on Transmission LinkI. Wind Parks larger than 40MW may need an offshore transformer station to increase the

voltage level of transmission link beyond the collector system voltage level which is normally kept at 34.5kV. The higher voltage level will reduce the power losses in transmission link on one hand and the voltage drop across the transmission link on the other.

II. In long distances from the seashore the capacitance of the cable will increase and cause charging current of HVAC cable to increase. This will reduce the current carrying capability of the cable.

III. An increase in system voltage to minimize voltage drop and line loss will also increase the charging current.

IV. The capacitance of the long run cable can also cause overvoltage, harmonic currents, and unwanted resonance and may require circuit breakers with high capacitance current switching capability and power conditioning.

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Impact of Wind Park Size and Distance from Seashore on Transmission LinkV. The effects of cable capacitance in the long run, and charging current will not be the

same concerns in HVDC cables, as they are in HVAC cables, except for a short moment during the energization of cable.

VI. In summary, an increase in the length of HVAC transmission link will result in higher capital cost, higher construction & maintenance cost, an increase in transmission link loss, an increase in complexity of design to maintain operational reliability of the cable system, and a decrease in the megawatt energy transmitted to onshore grid .

VII. If the size of the wind park and length of the transmission link exceed HVAC system limits, HVDC transmission system shall be employed. This will require two AC/DC converter stations, one offshore and another onshore, hence result in higher capital costs. Part of this extra cost will be offset by reducing the losses and cable cost.

VIII. By isolating two AC systems, i.e. wind park and transmission grid, HVDC Transmission Link will provide better control of reactive and active power, will let the wind power and grid operate asynchronously, will prevent resonance between cable capacitance and grid inductance, and also will not let the short circuit current to be transferred.

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Cost comparison of HVAC and HVDC Transmission Link The current information indicate that HVAC is the most economical

option for distances shorter than 50 km.

For distances between 50 and 80 km, HVAC and HVDC are expected to be similar in cost.

For distances longer than 80 km, HVDC systems will likely cost the least.

Slide 7 gives a better idea on the area of application of HVAC and HVDC transmission link considering both size of wind park and length of transmission link .

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Cost comparison of HVAC and HVDC Transmission Link (Continued)

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Cost comparison of HVAC and HVDC Transmission Link (Continued)

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Table below shows increase in grid connection cost as a function of distance to seashore and type of transmission link and as a percentage of total capital cost of wind park .

Slides 10 and11, shows the idea of connecting a 100MW wind park using HVAC and HVDC transmission link.

Slide 12 shows the idea of connecting a 200MW wind park using HVDC transmission link

Table below shows increase in grid connection cost of different types of transmission link for different distances from seashore as a percentage of total cost, and investment cost of wind park.

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Cost comparison of HVAC and HVDC Transmission Link (Continued)

Connecting a 100MW wind park using HVAC and HVDC Transmission Link

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Connecting a 100MW wind park using HVAC and HVDC Transmission Link

Connecting a 200MW wind park using HVDC Transmission Link

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Grid connection of several offshore wind parks in the same area

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Slides 14 & 15 illustrate the idea of connecting several offshore wind parks to a transmission grid.

Diagram 1 shows three wind parks individually connected to a transmission grid with individual right of way (ROW) for transmission link.

Diagram 2 is the same method of connection as given in diagram 1 but all three transmission links are routed in the same ROW.

The idea of offshore network is given in diagram 3 where the three wind parks are connected to the same bus and connected to the transmission grid utilizing a dual HVDC link in a common ROW.

Diagram 4 illustrates the generalization of offshore network concept.

The cable system in the upstream of the offshore network bus is similar to cable system for collector system.

Grid connection of several offshore wind parks in the same area

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Grid connection of several offshore wind parks in the same area

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Comparison of various methods of connections of several wind parks

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