Table 11 Evaluation for OWFs connection technologies

HVAC

[21, 48, 159]

Transmission distance under 60 km

Small and medium capacity OWFs

High reliability

Mature technology

Simple structure

NO black start capability

Large reactive power loss

***

***

***

**

**

HVDC

LCC-HVDC

[58, 61, 181]

Onshore station of OWFs transmit system

Higher voltage levels Transmission

Low line cost and loss

Asynchronous operation enhanced

Mature technology

Large Harmonics

Commutation failure

NO reactive power capability

Large converter station area

Difficult in offshore station building

**

***

**

**

**

VSC-HVDC

[12, 29, 85, 94, 182]

VSC-HVDC (two-port)

Two or Three level VSC

Large-scale integration of OWFs

Transmission distance over 100 km

Multi-terminal power grid

Distributed power integration

Ultra-high voltage levels transmission

Distributed energy connection

Independent power control

Power flow inversion realized

Converter station area reduced

No commutation failure problem

Higher power losses

Expensive

***

***

****

****

***

MMC-HVDC

Reactive power absorption absented

Higher-power capability

Better harmonic performance

Inexpensive cable

Black start

Short MMC lifetime

Huge submodule capacitors

Expensive MMC submodule

****

***

****

***

***

MTDC-VSC

MTDC (two level)

Flexible power flow control realized

Low Loss

High grid security and supply security

Renewable power balance improved

Congestion is reduced

Pole-to-pole and pole-to-ground faults

High fault current

***

****

***

***

***

MMC-MTDC

Device efficiency utilized

Investment and operational costs are reduced

System redundancy increased

High flexibility

Facilitate access to DC load

Poor dynamic response of voltage adjustments

Irrational power allocation

Complex control system

****

***

***

***

****

Hybrid-HVDC

[108, 109]

Ultra-high voltage and flexible power transmission

Large-capacity and high DC failure rate OWFs

Lower switching loss

Smaller investments

Power flow reversed problem

Start-up difficult

Commutation failure

****

****

***

***

****

DR-HVDC

[118,119,120]

Large-capacity OWFs

Transmission distance over 100 km

Volume and weight of offshore platform reduced

Transmission capacity increased

Transmission losses and costs decreased

AC voltages less set up

No start-up powers

No reactive power provisions

Lack of control possibilities

****

***

****

****

****

ALL-DC

[127,128,129, 136,137,138]

Series-connection WTs

Medium and large capacity OWFs

Transmission distance over 100 km

Offshore platform eliminated

Capital cost reduced

Insulation trouble

Strong power-voltage coupling

*****

**

****

***

****

Parallel-connection WTs

High reliability

Flexible operation

Cost decreased

High DC/DC convert gain

Difficult voltage control

*****

**

****

****

****

LFAC

[30, 31]

Medium capacity OWFs

OWFs integration at a range of 80 km–180 km

Cost reduced

High transportation efficiency

Low system complexity

Size of offshore transformer

Technology is not mature enough

*****

***

****

***

****