1

Abstract—Due to the potential future energy crisis and

ever increasing population, wind power, as a renewable

energy source producing no emissions and with a sufficient

wind resource in many parts of the world, is attracting

increasing interest and growing rapidly. Offshore wind

strength is relatively much stronger than onshore levels

and many large-scale wind farms (greater than 1GW) are

planned to be constructed in offshore location and must

transmit power over long distances. Voltage-source

converter based High Voltage Direct Current (VSC HVDC)

transmission system, which enables fast active and reactive

power control and has relatively lower losses than

conventional AC transmission, is a potential solution for

offshore power transmission.

Offshore wind farms usually have widely dispersed

locations in a strong wind area. Furthermore, VSCs have a

limited transmission capacity due to limitations on IGBT

and capacitors ratings. For these reasons, a multi-terminal

HVDC (MTDC) transmission system, which can extract

and deliver power from and to several terminals and

provide power to more than one terminal, is an attractive

method for offshore wind power transmission. In addition,

MTDC has been proposed in other fields such as urban

sub-transmission and offshore oil/gas. A detailed

description of a MTDC control scheme is presented and its

operation demonstrated. The paper concludes with an

overview of future research in this field.

Index Terms—Multi-terminal HVDC, MTDC, Voltage

Source Converter, Vector Control, Voltage Margin.

I. INTRODUCTION

s the global population grows, power engineers must

establish alternative energy sources to gradually replace

fossil-fuelled sources like coal and oil, which emit greenhouse

gases that are widely believed to result in climate change.

Energy supplies for the future are facing a severe shortage and

require increased levels of security. For these reasons,

renewable energy, investment in which is attracting public

funds and other financial incentives, has been significantly

developed in the past decade.

In the future, proposed wind farms at distances of over 60

km from the shore will be connected to the mainland grid only

through DC links [1], DC being used due to the large

capacitive current losses associated with AC links. Typically,

existing and potential offshore wind farms may be located as

far as 100-150 km from the shore. For this reason, high

voltage direct current (HVDC) transmission technology

becomes a feasible and economical solution compared to

HVAC transmission. . Compared to HVAC, VSC HVDC

transmission is able to flexibly control active and reactive

power, and can alleviate the propagation of voltage and

frequency fluctuations due to wind variations in wind strength.

The fact that HVDC transmission lines can be routed underground eliminating hazards such as corona makes

HVDC attractive and environmentally friendly. For this reason,

they are sometimes known as ―the invisible transmission

lines‖ [2]. DC can also transport relatively more power at the

same voltage/insulation level as AC. Therefore, HVDC

transmission is considered an effective way of connecting

offshore wind farms to the main grid.

Two techniques, the classical line commutated converter

(LCC) and the voltage-source converter (VSC), have been

used for HVDC applications. Compared with the LCC HVDC,

VSC HVDC has many advantages [4][5]. It is able to control

the active and reactive power independently and supply a

passive network. Furthermore, power flow reversal can be

realised by reversing DC current direction without reversing

DC voltage polarity. There is no need for communications

between the converters at each node, and this is an important

advantage that can facilitate the creation of a multi-terminal

HVDC system.

A VSC multi-terminal HVDC (MTDC) system has

superiority over a two-terminal HVDC system, in that it

facilitates gradual expansion of distributed networks, the input

and output power can be controlled flexibly in order to

increase the total power transportation capacity. Initially, a

double-input-single-output HVDC was proposed, which

would connect two wind farms to the AC grid through one DC

link, and this has been studied in terms of system control and

stability [6]. MTDC systems have also been proposed for

urban sub-transmission [7], oil and gas platforms [8], and

―premium quality power parks‖ [9]. The objective of this

paper is to discuss MTDC with respect to the following

aspects:

Motivations and historical proposals for MTDC

transmission;

Local control for each terminal of a MTDC system;

Master control for DC voltage regulation and power

coordination within an overall MTDC system.

Future Multi-Terminal HVDC Transmission

Systems using Voltage Source Converters Jiebei Zhu

Institute for Energy and Environment

University of Strathclyde, Glasgow, UK, G1 1XW

E-mail: zhu.jiebei@eee.strath.ac.uk

Campbell Booth Institute for Energy and Environment

University of Strathclyde, Glasgow, UK, G1 1XW

E-mail: c.booth@eee.strath.ac.uk

A

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II. MOTIVATIONS FOR MULTI-TERMINAL VSC HVDC

A. High Voltage AC System

HVDC transmission system offers significant potential

benefits for both long-distance power transmission and

distribution applications. Though HVDC systems require

significant investment, the investment can be effective,

particularly as the transmission distance increases.

B. VSC HVDC Basic Concept

Figure 1. VSC HVDC system components

A basic VSC HVDC, as shown in Figure 1 consists of two

converter stations which use series-connected fast-switching

IGBTs to transform three-phase AC voltage to DC and vice

versa at each end of the DC link. With the SPWM (Sinusoidal

Pulse-Width-Modulation) controlled VSC, it is possible to

deliver virtually any phase angle and voltage amplitude to the

AC grid, by changing the PWM modulation depth and relative

phase displacement respectively.

C. MTDC Motivation and Proposals

In addition to transmitting power over long distances,

HVDC has also been used to interface independent AC

systems and to enable voltage and frequency support to be

provided from one system to another. Due to its versatility and

fast control capabilities, it also has the potential to be utilised

for the interconnection of wind farms. It can also be used to

provide overall (HVDC and connected AC) grid power flow

control functions as is the case with conventional AC

generators. However, there is considerably higher cost (when

compared with AC systems) associated with VSC HVDC due

to the requirements for converter stations at each end of a

single HVDC link.

In order to reduce these converter station costs, to

effectively utilise the existing assets and to provide greater

flexibility, MTDC HVDC systems, which can reduce the

number of required converter stations, have been proposed in

many papers [6][7][8][9][11]. Voltage source converter (VSC)

is the possible solution for the inverters used in MTDC as

most of the AC systems connected to MV/LV substations are

passive AC networks [7].

1) DISOC HVDC system for offshore wind farms

Figure 2. Single-input-single-output HVDC links (SISOC-HVDC)

Figure 3. Double-input-single-output HVDC links(DISOC-HVDC)

A multi-terminal HVDC topology, termed double

-input-single-output HVDC system, has been studied in [6][12]

as a means to economically connect two neighbouring

independent wind farms to the AC grid. Paper [12] compared

multi-terminal DISOC (Double-input-single-output) HVDC

system with two SISOC (Single-input-single-output) HVDC

links, focusing on a qualitative analysis of the behaviour of each topology, and determined the general performance of

DISOC-HVDC. Paper [6] investigated the electrical response

of a DISOC-HVDC under different fault conditions by

simulation. However, as acknowledged by the authors, a

detailed analysis of the system response and the control

strategy was not carried out and research in this area is still

required.

2) MTDC for oil and gas platforms

Figure 4. Three multi-terminal HVDC used for platforms

Offshore oil and gas platforms sometimes use gas turbines

to generate electrical power for operation. These gas turbines

may contribute to large emission of greenhouse gases [13].

There is therefore an opportunity to consider providing power to these platforms from offshore wind farms. In [14], an

interconnection between offshore platforms and wind farms

has been proposed by utilizing a three-terminal HVDC

transmission arrangement.

The voltage margin control method [15], which will be

discussed in the next section, was employed in the simulation

presented in the paper. The results showed that the proposed

simulation with the control strategy resulted in system stability,

under both steady state and dynamic operation. Without any

communication between each terminal, the MTDC system,

using voltage margin control methods, can maintain DC

voltages even after disturbances by returning the DC voltage to desired levels [14].

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3) MTDC for urban distribution

Figure 5. Urban MTDC distribution system

Presently, HVDC transmission technology has rarely been

utilised in urban networks [7]. The development of power

VSCs, capable of controlling and supplying active and rective

power independently, makes MTDC potentially feasible for

urban distribution (with no generation to supply reactive power). There are potential economic and performance

benefits associated with using MTDC for such power

distribution applications.

In [7], the authors have studied the behaviour of a proposed

two-input-eight-output MTDC system and the results show

that the system can operate effectively under a variety of

operating conditions. Specially, a fault occurring on the AC

network of one inverter terminal had little impact on the other

AC systems supplied via other terminals. The paper concludes

that MTDC for urban is a potential alternative to a classical

AC distribution system in urban areas of large cities.

III. CONTROL METHODS OF MTDC

MTDC control systems consist of two elements: local control

and master control [15][16][17]. The local controllers control

the specific converters by calculating the PWM pulses for the

converter bridges. The master control optimises the overall

performance of the MTDC by regulating the DC side voltage.

A. Local Control

There are four control modes that can be adapted in MTDC

network local control: constant active power; constant DC

voltage; constant DC current; and constant AC voltage. For

most circumstances where the inverters supply passive networks, the constant AC voltage mode should be chosen for

inverter control in order to maintain AC supply voltage

magnitude and frequency at constant or near-constant levels.

Due to the strongly coupled nonlinear system of MTDC,

close-loop dq reference frame control, which implements

decoupled active and reactive power control, has to be applied

to control the local control systems for each local inverter.

Figure 6. Control system of HVDC VSC

The control system of a HVDC VSC can be demonstrated by referring to Figure 6. The AC voltage and current of

HVDC VSC terminal at reference point X for are measured

and separate into d- and q-axis voltage and currents in

abc-to-dq transformation block in order to implement

independent active and reactive control. Frequency is also

measured by a phase-locked loop whose output is the angular

frequency ωs and time integral θ=ωst[20] as the reference for

the abc-to-dq and the dq-to-abc transformations.

The outer controller, which constitutes subtraction blocks

and proportional integral (PI) controllers, is used to achieve

the reference currents for the desired active and reactive power

and voltage levels. The inner current controller calculates the d- and q-axis modulation indices from the reference (target)

and actual (measured) values. The indices are then transferred

to the abc reference frame for the PWM operation on all three

phases.

1) AC Grid side control

Figure 7. Schematic diagram of grid side inverter

The operation of an AC grid side inverter has been

comprehensively studied in [18] and this paper will only

provide a brief overview. From the Figure 7, point X is the

reference point for measuring voltage, active and reactive power. Based on the abc reference frame, the following

relationship can be derived:

𝑉𝑐𝑎𝑏𝑐 − 𝑉𝑥𝑎𝑏𝑐= 𝐿

𝜕𝑖𝑎𝑏𝑐

𝜕𝑡+ 𝑅𝑖𝑎𝑏𝑐 (1)

The dq reference frame transformation is processed [14]

and the equivalent equation (1) is given by:

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𝑉𝑐𝑑𝑞 − 𝑉𝑥𝑑𝑞= 𝐿

𝜕𝑖𝑑𝑞

𝜕𝑡+ 𝑗𝜔𝐿𝑖𝑑𝑞 + 𝑅𝑖𝑑𝑞 (2)

The phase locked loop (PLL) which is phase locked with

reference point X provides a real-time phase angle reference

for the abc-dq-abc transformation. The d axis in the dq

reference frame is aligned with the voltage phasor of phase a

at reference point X in abc stationary reference; this leads to

Vq=0 and Vd=Vx. Therefore, equation (2) can be further expanded as:

𝑑

𝑑𝑡 𝑖𝑑𝑖𝑞 =

−𝑅

𝐿𝜔

−𝜔 −𝑅

𝐿

𝑖𝑑𝑖𝑞

+1

𝐿 𝑉𝑐𝑑 − 𝑉𝑥𝑑

𝑉𝑐𝑞 − 𝑉𝑥𝑞

(3)

The apparent power (S), active and reactive power (PAC and

QAC) exported to the AC grid are given as:

𝑆 =3

2(𝑉𝑥𝑑 ∙ 𝑖𝑑 + 𝑉𝑥𝑞 ∙ 𝑖𝑞) (4)

𝑃𝐴𝐶 =3

2𝑉𝑥𝑑 ∙ 𝑖𝑑 (5)

𝑄𝐴𝐶 =3

2𝑉𝑥𝑞 ∙ 𝑖𝑞 (6)

As mentioned previously, the inverter on the grid side

operates in constant AC mode. The control of the active power

output is implemented by varying the d-axis current id with an

inner current control loop. By pulse width modulation control,

the amplitude of the output AC voltage with DC voltage is

determined by the modulation index M (value of 0 to 1) which

determines the amplitude of output AC voltage by varying the

"width" of the IGBT switching time:

𝑉𝐴𝐶 =𝑀

2∙ 𝑉𝐷𝐶 (7)

Substitute the modulation index M in equation (6) with

equation (7), the relationship between AC and DC quantities

are given:

𝑑

𝑑𝑡 𝑖𝑑𝑖𝑞 =

−𝑅

𝐿𝜔

−𝜔 −𝑅

𝐿

𝑖𝑑𝑖𝑞

−1

𝐿 𝑉𝑥𝑑

𝑉𝑥𝑞

+𝑉𝑑𝑐

2𝐿 𝑀𝑑

𝑀𝑞 (8)

Where Md and Mq are defined as the d- and q-axis modulation indices.

The current control loop evaluates equation (8) by

following the equation (9) and (10), which is effectively a of

proportional and integral (PI) controller:

∆𝑖𝑑 =𝑑𝑖𝑑

𝑑𝑡= 𝑘𝑝1 𝑖𝑑𝑟𝑒𝑓

− 𝑖𝑑 + 𝑘𝑖1 (𝑖𝑑𝑟𝑒𝑓− 𝑖𝑑)𝑑𝑡 (9)

∆𝑖𝑞 =𝑑𝑖𝑞

𝑑𝑡= 𝑘𝑝1 𝑖𝑞𝑟𝑒𝑓

− 𝑖𝑞 + 𝑘𝑖1 (𝑖𝑞𝑟𝑒𝑓− 𝑖𝑞 )𝑑𝑡 (10)

Where kp1 and kp2 are referred to as the proportional and

integral gains of the current controller, and ∆icd and ∆icq are

variables of the current controller output. Therefore, the

control variables Md and Mq are derived by equations (8), (9)

and (10) which also include the control logic of the inner

current controller:

𝑀𝑑 =2𝐿

𝑉𝐷𝐶(∆𝑖𝑑 +

𝑅

𝐿𝑖𝑑 +

1

𝐿𝑣𝑥𝑑 − 𝜔 ∙ 𝑖𝑞) (11)

𝑀𝑞 =2𝐿

𝑉𝐷𝐶(∆𝑖𝑑 +

𝑅

𝐿𝑖𝑑 +

1

𝐿𝑣𝑥𝑑 +𝜔 ∙ 𝑖𝑞 ) (12)

The AC grid-side inverter also fulfils the function of DC

voltage regulation. Based on the power balancing equation and equation (4), the power relationship between the AC and DC

side of the inverter is given as:

𝑃𝐷𝐶 = 𝑉𝐷𝐶 ∙ 𝐼𝐷𝐶

𝑃𝐴𝐶 = 𝑃𝐷𝐶 − 𝑃𝑐𝑎𝑝 3

2 𝑉𝑥𝑑 ∙ 𝑖𝑑 + 𝑉𝑥𝑞 ∙ 𝑖𝑞 = 𝑉𝐷𝐶 ∙ 𝐼𝐷𝐶 − 𝐶

𝜕𝑉𝐷𝐶

𝜕𝑡∙ 𝑉𝐷𝐶 (13)

Thus, by combining equations (8) and (13), an expression

for DC voltage variation expression with respect to the d- and

q-axes modulation indices for DC voltage regulation is

expressed as: 𝜕𝑉𝐷𝐶

𝜕𝑡=

1

𝐶∙ 𝐼𝐷𝐶 −

3

4𝐶(𝑀𝑑 ∙ 𝑖𝑑 +𝑀𝑞 ∙ 𝑖𝑞 ) (14)

In a similar fashion to the AC current control loop, the DC

voltage PI controller is also expressed as:

∆𝑉𝐷𝐶 =𝜕𝑉𝐷𝐶

𝜕𝑡= 𝑘𝑝2 𝑉𝐷𝐶𝑟𝑒𝑓

− 𝑉𝐷𝐶 + 𝑘𝑖2 (𝑉𝐷𝐶𝑟𝑒𝑓− 𝑉𝐷𝐶)𝑑𝑡 (15)

Where kp2 and ki2 are the proportional and integral gains in

the DC voltage controller (regulator).

Combining equations (14) and (15), the reference current as

a function of the control current of equations (9), (10), (11),

and (12) is calculated as follows:

𝑖𝑑𝑟𝑒𝑓=

4𝐶

3𝑀𝑑(−∆𝑉𝐷𝐶 +

1

𝐶𝐼𝐷𝐶 −

3

4𝐶𝑀𝑞 ∙ 𝑖𝑞 ) (16)

Therefore, the DC voltage is directly controlled by the

d-axis current controller.

2) Wind farm side control

The wind farm side control of a HVDC rectifier has been studied in [19]. The function of a wind farm side rectifier as a

DC regulator is not considered, as in a multi-terminal HVDC

application, may individual wind-farms are relatively small

power sources and would not be capable of performing

regulation of the DC voltage.

Figure 8. Equivalent schematic diagram of wind farm side rectifier

The equivalent schematic diagram of the wind farm side

rectifier in the dq reference frame in Figure 8 is used to design

voltage and current controllers. The relation of voltage and

current at the sides of reference point X is given as:

𝑉𝑤 = 𝑉𝑐 − 𝑖𝑐𝑅 − 𝐿𝜕𝑖𝑐

𝜕𝑡− 𝑗𝜔𝐿𝑖𝑐 (17)

𝑖𝑐 = 𝑖𝑓 − 𝑖𝑤 − 𝑗𝜔𝐿𝑖𝑐 = 𝐶𝑓𝜕𝑉𝑤

𝜕𝑡− 𝑖𝑤 − 𝑗𝜔𝐿𝑖𝑐 (18)

5

Transferring these equations into the dq reference frame:

𝑑

𝑑𝑡 𝑖𝑐𝑑𝑖𝐶𝑞

= −

𝑅

𝐿𝜔

−𝜔𝑅

𝐿

𝑖𝑐𝑑𝑖𝐶𝑞

+1

𝐿 𝑉𝑤𝑑

𝑉𝑤𝑞

−𝑉𝐷𝐶

2𝐿 𝑀𝑑

𝑀𝑞 (19)

By the similar principle employed by the grid side current

controller, the wind farm current controller is designed as:

∆𝑖𝑐𝑑 =𝑑𝑖𝑐𝑑

𝑑𝑡= 𝑘𝑝1 𝑖𝑐𝑑𝑟𝑒𝑓

− 𝑖𝑐𝑑 + 𝑘𝑖1 (𝑖𝑐𝑑𝑟𝑒𝑓− 𝑖𝑐𝑑 )𝑑𝑡 (20)

∆𝑖𝑐𝑞 =𝑑𝑖𝑐𝑞

𝑑𝑡= 𝑘𝑝1 𝑖𝑐𝑞𝑟𝑒𝑓

− 𝑖𝑐𝑞 + 𝑘𝑖1 (𝑖𝑐𝑞𝑟𝑒𝑓− 𝑖𝑐𝑞 )𝑑𝑡 (21)

Where kp1 and kp2 are the proportional and integral gains of

the current controller, and ∆icd and ∆icq are variables of the

current controller output.

Thus, the d- and q-axis modulation indices Md and Mq for

the PWM are derived from by equation (19) and equations (20)

and (21):

𝑀𝑑 =2𝐿

𝑉𝐷𝐶(∆𝑖𝑑 +

𝑅

𝐿𝑖𝑐𝑑 −

1

𝐿𝑣𝑤𝑑

− 𝜔 ∙ 𝑖𝑐𝑞 ) (22)

𝑀𝑞 =2𝐿

𝑉𝐷𝐶(∆𝑖𝑑 −

𝑅

𝐿𝑖𝑐𝑑 −

1

𝐿𝑣𝑤𝑞

+𝜔 ∙ 𝑖𝑐𝑞 ) (23)

As doubly-fed-induction-generation (DIFG) based wind

farms are able to control the amplitude and frequency of the

AC voltage, the PLL module is not required [19]. The wind

farm side rectifiers should be coordinated with the DFIG

control of the wind farm.

B. Master control

As MTDC systems typically span significant distances,

using communications-based control systems may be

expensive and possibly prone to concerns over reliability.

Master control, as presented in Figure 9, is a control scheme

that can be included in individual terminal controllers and

consists of a small set of terminal control functions (e.g.

startup, close and power flow reversal) and does not require

communication. The voltage margin method for overall

control [15], enables DC voltage regulation to be carried out

by one terminal and with other terminals only carrying out power flow corrdination.

Figure 9. Control system of MTDC

In a MTDC system, when there is a disturbance or short

circuit on either the AC or DC sides, the energy stored in the

DC link capacitors may be released and this may result in DC

voltage fluctuations and current surges. In order to avoid

over-voltage and under-voltage and to ensure stable operation,

the voltage margin method, which implements power

interchange among all terminals, is included in both the active

power control block and the DC voltage control block of the

outer controller.

Figure 10. Active power limits and DC reference voltage characteristic of

one terminal

Based on the voltage margin method, each converter will

maintain the DC voltage (Uref1 in Figure 10) as long as

transmitted power remains within its upper and lower limits

are not reached. It ensures that the system load is supplied by

the appropriate wind/AC grid balance and that changes in

wind power output will not impact on AC grid-side voltage.

Other converters which operate at upper or lower limits (operate at full power output or input) will act as constant

active power terminals. For example, consider a MTDC

consisting of an offshore wind farm, the AC grid and the

onshore load, as shown in Figure 11:

Figure 11. Voltage margin method for the wind farm, the grid and the local

load terminals of a MTDC

The specific characteristics of the voltage margin method

for the MTDC master control for the system shown is preset as

shown in Figure 11 would be defined prior to installation. Two

cases of operation are now briefly described. Case 1 refers to

the upper dotted line in Figure 11 (DC voltage relatively

higher), while Case 2 refers to the lower dotted line on Figure

11 (DC voltage relatively lower). Load power is equal in both

cases. 1) Case 1

At the time when the available wind strength is relatively

weak, the power output of the MTDC rectifier for the wind

farm is Pw1, which is below the specified upper power flow

limit. Thus this terminal will determine the magnitude of the

system DC voltage using its DC reference voltage and act as a

DC reference bus. For the grid side terminal, the upper power

flow limit is reached. Therefore, the grid terminal operates as

a rectifier that inputs the power PG1 up to upper limit into the

6

MTDC. The load consumes constant power PL1 and the DC

reference voltage also follows DC voltage magnitude at the

operating point. The power balance rule (i.e. sum total of

power generated into the DC grid to the loads/AC grid) is

obeyed.

2) Case 2 Consider a time when the wind farm is able to operate at

maximum power output. Based on the characteristic, the upper

power flow limit of the terminal for the wind farm is reached

by PW2 so that this terminal can no longer regulate the DC

voltage and this function will be shifted to another terminal.

The grid side terminal extracts PG2 which is between the upper

and lower limits and its value satisfies the power balance

equation (24). Thus the grid side terminal, which is now acting

as an inverter (i.e. exporting power from the DC grid to the

AC grid), determines the DC voltage that the other two

terminals must follow. For the load terminal, the power drawn

from the DC grid is maintained constant.

IV. CONCLUSION

In this paper, the advantages of VSC HVDC has been

discussed and compared to conventional HVAC and LCC

HVDC. The basic concept of VSC with PWM has been

introduced. Based on the fast and flexible control capabilities

of VSC HVDC, the multi-terminal HVDC (MTDC) becomes a realistic possibility. Three proposals for MTDC systems

applied to wind power transmission, for gas/oil platform

supplies and for urban power distribution have been

highlighted.

Prospective MTDC control systems were also reviewed.

This includes classical HVDC terminal control (local control),

the operation of which has been described through a

description of the logic and equations used in the VSC

controllers. AC voltage, DC voltage, active (current) and

reactive power control modes can be implemented with these

equations. The concept of master control, which is based on

the voltage margin method, was also described and this can play a key role in the coordination of the electric power flows

within the MTDC system and external to the system by

making one (master) terminal responsible for DC voltage

regulation and the other terminals responsible only for

providing active power. MTDC will undoubtedly provide

useful solutions in many fields in the future and the control

schemes will be continuously optimised and developed. The

Future research may consider analysing the performance,

operation, protection and control of such systems, using

detailed models and case study simulations.

REFERENCES

[1] N. M. Kirby, X. Lie, M. Luckett and W. Siepmann, "HVDC transmission

for large offshore wind farms," Power Engineering Journal, vol. 16, pp.

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