Current Matching Control System for Multi-Terminal DC

Transmission to Integrate Offshore Wind Farms

J. Zhu, C. Booth, G.P. Adam Department of Electrical and Electronic Engineering,

University of Strathclyde, Glasgow G1 1XW, UK.

Email: zhu.jiebei@eee.strath.ac.uk

Keywords: HVDC, voltage source converter, multi-terminal

DC, control.

Abstract

The inherent features of Voltage Source Converters (VSCs)

are attractive for practical implementation of Multi-Terminal

HVDC transmission systems (MTDC). MTDC can be used

for large-scale integration of offshore wind power with

onshore grids. However, many of the control strategies for

MTDC that have been proposed previously for offshore wind

farm integration depend on local control of the wind turbine

generators.

This paper proposes a new control strategy, termed Current

Matching Control (CMC), which can be used with any

number of converter terminals, and is independent of the

types of wind turbines used within each wind farm. The

proposed CMC matches the current reference of the grid side

converter to that of the wind farm side converters. In order to

achieve such current matching, a telecommunication system

will be required to facilitate calculations of the grid side

current references to be carried out in real time. To validate

the performance of the proposed control strategy, a generic

four-terminal MTDC network, which integrates two offshore

wind farms with two mainland grids, is simulated and results

relating to several steady state and transient scenarios are

presented.

1 Introduction

There has been a tremendous pace of development of large-

scale offshore wind farms in recent years. It is anticipated that

there will be an approximate increase of 26.6 GW in

aggregate generation capacity over the period from 2009/10

to 2016/17 in the UK, 11.7 GW of which will be contributed

by wind power [1]. More broadly, the European Wind Energy

Association has, in its “high wind” scenario, a target of 180

GW from wind energy sources in by 2020, of which 35 GW

will be sourced from offshore wind installations. This

capacity target for offshore wind increases to 120 GW by

2030 [2].

These targets, if they are achieved, will have great impact on

power transmission design, planning, construction and

operation. Many offshore wind farms will require long-

distance power transmission systems. AC may not be suitable

due to high power losses over longer distances. Classical line-

current-commutated (LCC) HVDC transmission has been

used for many HVDC installations [3], but is not well suited

to MTDC, in comparison to VSC. A summary of the

advantages of VSC over LCC is listed below:

VSC has a smaller footprint which facilitates offshore

installations of reduced platform size [3];

LCC requires large filtering components;

VSC provides additional reactive power support and AC

voltage regulation for wind farms connected to weak AC

systems, and possesses black-start capability;

VSC improves wind farm AC fault ride-through

capability and facilitates Grid Code compliance at

reduced costs [4];

Power reversal can be achieved in VSC without changing

the DC voltage polarity, facilitating the realisation of a

flexible MTDC transmission system.

MTDC transmission systems have attracted much attention

for wind farm integration [5][6][7]. Firstly, MTDC reduces

converter numbers when compared to numerous point-to-

point HVDC solutions. Secondly, it is conceivable that, due to

limited correlations of weather systems in geographically

dispersed wind farm locations, the overall variability of wind

power may be reduced by interconnecting many

geographically-dispersed wind farm systems via a large-area

MTDC system, thus increasing the overall availability of

energy. In future, energy storage devices may be integrated

with MTDC system [1], which further supports energy

availability and quality of supply. MTDC is also being

proposed as the means of interconnecting independent large-

scale AC power systems, (e.g. European super-grid proposal

[5] to promote the interconnection of Norwegian hydro,

French nuclear, Sahara solar and North sea wind power into a

common MTDC) to resolve local power shortages or

congestion, to enable international power sharing, and to

provide an excellent level of overall power system reliability.

As discussed in [5], there are many challenging obstacles to

the introduction of MTDC. The control system for an MTDC

must be robust, coordinated and reliable, as problems with

one terminal have the potential to affect the entire MTDC

network. A number of control strategies are proposed [6]

[8][9] which will be introduced later. These strategies remain

at the modelling and testing stages of development. Critical

concerns about these strategies are the controllability and

reliability of MTDC systems, as most of the proposed

strategies manage an MTDC system without use of

communications between terminals. The proposed current

matching control strategy employs minimal (in terms of

required traffic and bandwidth) telecommunications between

terminals in an MTDC network. While there may be concerns

Fig.1 The test MTDC system configuration

over use of communications, modern telecommunication

technologies are increasingly highly developed, reliable and

redundant [10]. Furthermore, risk can be mitigated by

employing redundancy, through continuous monitoring of

telecommunications channels, and ensuring operation can

continue, albeit in a less efficient fashion, if

telecommunications is lost. Operation of the scheme is based

on measurement (discretely, with a step of 1-2ms in this

example) of the total DC current provided by wind farm side

rectifiers (i.e. WFR1 and WFR2 in Fig.1) and allocation

(matching) of this current across the inverters, according to a

pre-determined sharing factor. This is described in more

detail in Section 4. The scheme also provides further

protection for the entire MTDC system by monitoring the DC

voltage. Finally, the system can operate if the

telecommunication system fails, but accurate sharing of the

inverter currents may not be possible.

2 MTDC system configuration

MTDC topology design may vary depending on specific

situations (e.g. the locations of grid connection points and

offshore wind farms, available undersea cable routes). Fig.1

presents a four terminal MTDC system for wind farm

integration, which utilises bi-polar cables R5 with nominal DC

voltage of 200 kV (± 100 kV). On the offshore side, two wind

farms are connected via two voltage source neutral-point

clamped rectifiers (WFR1, WFR2). On the onshore side, two

grid-connected inverters (GCI3, GCI4) feed power to two

independent 2000 MVA equivalent AC power systems. All

VSCs are rated at 200 MVA. Targets of converter control

differ for WFR1 and WFR2, implementing frequency and AC

voltage control at the point of common connection (PCC)

with wind farms, while GCI3 and GCI4 are equipped with a

current controller and DC voltage regulator respectively, in

addition to AC voltage/or reactive power control.

2.1 AC/DC interaction for a VSC

Instead of presenting an in-depth study of the VSC control

system formulae, such as those presented [9] and [7], this

paper focuses on the dynamics of AC and DC interaction,

which is dictated by the converter control. As the control

system for VSC employs vector control in the synchronous

rotating reference frame dq, the current references id_ref and

iq_ref which are derived by the commanded active power Pcomm

and reactive power Qcomm, are given in equation (1). In the

rotating reference frame, the d-axis voltage Vd, aligned with

one of three phases, is equal to the magnitude of AC voltage,

and the q-axis voltage Vq is zero.

,ref ref

comm commd q

d d

P Qi i

V V (1)

Once the current references id_ref and iq_ref are generated, the

inner current control loops adjust the actual id and iq values to

be in accordance with the computed reference values. This

process takes a short period to complete and is determined by

the natural frequency of the converter control dynamic in

Laplace equation (2), which contains the proportional gain

(kp) and the integral gain (ki) of the proportional-and-integral

(PI) controller, reactor inductance (L) and resistance (R):

2

p i

p iref

k kdq L L

R k kdq

L L

i

i s s

(2)

From the DC side perspective of the VSC in Fig.2, the DC

voltage udc across the converter or the output capacitors, the

DC current idc injected by the converter, and the current ic

conducted by the DC cables, are related as shown in equation

(3). The capacitors are charged (or discharged) to possess a

certain DC voltage. The current idc injected to the MTDC

network by the converter is calculated from AC side PCC

currents id and iq, pulse-width-modulation index M and

converter terminal voltage angle with respect to the PCC

voltage, using equation (4):

dcdc c

duC i i

dt (3)

cos sindc d qi Mi Mi (4)

2.2 Equivalent MTDC circuit

As demonstrated in Section 2.1, the DC property of individual

VSCs in the MTDC can be represented as a “controlled”

current source, shown in the equivalent circuit in Fig.(2). The

extremely small inductance and capacitance of the DC

network with respect to direct current are neglected.

As the focus of this section is on the analysis of DC network

behaviour, it is essential to analyse the effect of the variation

in DC current idc from one converter station, on either its DC

voltage and/or the DC voltage at other stations. Taking WFR1

as an example, a control action to increase WFR1 current idc1

will quickly charge its DC capacitors and boost its DC

voltage udc1 to a higher value, based on equation (3). The

higher udc1 with respect to other DC node voltages acts to

supply the conducted current ic1 into the MTDC network. The

increased current ic1 charges capacitors at other nodes, until

the voltage levels at all the nodes reach a new higher

equilibrium value. The rectifier DC voltage is slightly higher

than the inverter voltage so that current flows from rectifier

node(s) to inverter node(s). The magnitude of individual

converter DC voltage depends on two elements: (a) the

conducted current though the node; (b) the resistances

between the nodes.

Thus, it can be concluded that a temporary current mismatch

between rectifier and inverter in the MTDC results in an

overall DC voltage variation. As the converter control

systems use DC voltage information to function, it is

desirable to quickly address any DC current surplus (by

increasing exported current) or DC current shortage (by

reducing exported current), so that a stable DC voltage

operating point for the MTDC system will be realised.

Communications between rectifier and inverter nodes in the

system is therefore critical to the operation of this scheme.

3 Previously reported MTDC control

strategies

Historically, there have been two distinct control strategies

used to facilitate power dispatch from DC to AC systems,

namely voltage margin control [9] and voltage droop control

[8] [6].

3.1 Voltage margin control (VMC)

In VMC control, one node’s DC voltage is controlled by a DC

voltage controller (DCVC). This effectively acts as a DC

voltage slack bus, with other VSCs operating in current

control mode as illustrated in equations (1) and (2).

VMC equips all converters with DCVCs, but the DCVC of

only one converter station must be activated. Considering the

V-I characteristic of Fig.3(a), GCI4 has its DC node voltage

controlled at udc4 ref by the activated DCVC, represented as the

solid line. This acts to balance the current flows from

rectifiers WFR1 and WFR2 to inverter GCI3, by automatically

“sliding” its current output along the constant DC voltage

udc4_ref. Inverter GCI4 has an inherent current limit. If this

limit is exceeded (e.g. strong wind pushing more current

though rectifiers into the MTDC), GCI4 will not be able to

maintain the DC voltage, and will operate at its maximum

current output. According to the analysis in Section 2.2, the

DC network voltage will continuously rise in line with current

“surplus” in the MTDC network. The voltage will ultimately

rise to a new level (udc3_ref) that activates the back-up DCVR

in the other inverter GCI3. In this case, GCI3 begins

maintaining the MTDC voltage at udc3_ref, the dashed line in

Fig.3(a). The term “voltage margin” refers to the difference

between udc4 _ref and udc3_ref in Fig.3(a).

3.2 Voltage droop control (VDC)

VDC basically has multiple activated DCVCs (in both of the

inverters in this example). The two DCVCs are controlled at

different levels for inverter current dispatch, as shown in

Fig.3(b). The V-I droop characteristic is obeyed by GCI3 to

share the total current with GCI4. To demonstrate the droop

operation, for example, in order to increase the current of

GCI3 and decrease GCI4 based on the droop characteristic, the

DCVC in GCI3 converter control must lower the voltage

reference udc3_ref and then its current output “slides” along the

droop to the right hand side, to output more current.

4 Proposed current matching control strategy

As discussed in Section 2.2, a stable DC network operating

point can be achieved by quickly acting to reduce any

mismatch between rectifier and inverter DC currents. As both

WFR1 and WFR2 inject all the power generated by wind farms

into MTDC network, they will operate in current control

Fig.2 DC equivalent circuit for the MTDC

Fig.3 MTDC control strategies: (a) voltage margin (b) voltage droop

mode. GCI3 and GCI4 will operate using the proposed CMC

in order to address the shortcomings of the VMC and VDC

control, regarding DC current mismatch that may arise during

changes in wind power generation. The detailed operation of

the scheme is now presented.

4.1 Converter operating states

To facilitate the development of the proposed control

strategy, it is important to understand VSC operating states

with their control references in the MTDC system. The

following equations (5) and (6) are given, referring to Fig.2:

1 1 1 2 2 2S dc c dc cu u R i u R i (5)

3 3 3 4 4 4S dc c dc cu u R i u R i (6)

uS is the sending end voltage and uR is the voltage at the

receiving end of the DC link. ic1 to ic4 are the rectifier and

inverter currents as shown in Fig.2. R5 is given by:

5 5S R cu u R i (7)

ic5 is the current through the DC link as shown in Fig.2.

Kirchhoff’s current law dictates that:

1 2 3 4 0dc dc dc dci i i i (8)

As demonstrated in VMC and VDC, GCI4 has its DCVC

activated to control DC voltage at udc4; the other converters

WFR1, WFR2 and GCI3 control their currents at idc1 idc2 and

idc3 respectively. Therefore, by combining equation (5), (6),

(7) and (8), the following converter operating state matrix,

which incorporates DC network resistance, can be derived:

1 11 4 5 4 5 4

2 24 5 4 5 4

3 34 4 3 4

4 4

1

1

( ) 1

1 1 1 0

dc dc

dc dc

dc dc

c dc

u iR R R R R R

u iR R R R R

u iR R R R

i u

(9)

4.2 Current matching control principle

Fig.4 shows the communicated variables of the proposed

CMC for an MTDC. The green blocks in Fig.1 and Fig.4 are

telecommunication feedback signals “idc1”and “idc2” from the

rectifiers WFR1 and WFR2, based on equation (4). Feedback

signal “udc4” from GCI4 is used for over- or under-voltage

protection. The current reference for GCI3 converter is

continuously updated by the proposed CMC and transmitted

to the GCI3 vector control, via the communicated control

signal “idc3_ref”.

Modern wireless communication system has been proposed in

HVDC application to secure power reliability [12] and it is

here used here to favour the CMC strategy for the MTDC

system. Fig.5 illustrates the CMC inner control logic, where

the total rectifier current from WFR1 and WFR2 is the sum of

feedback signals “idc1”and “idc2”. Rectified current is divided

between the inverters GCI3 and GCI4 by applying a sharing

factor KS. KS represents the portion of the expected power to

be exported from the MTDC network through GCI4, given by

equation (10). Accordingly, the reference current idc3 for GCI3

is given by equation (11):

4 1 2( )dc S dc dci K i i (10)

3 1 2(1 )( )dc S dc dci K i i (11)

DC current reference for idc3 is transmitted from the CMC to

the GCI3 converter control system, to produce a commanded

active power reference, given in equation (12):

3 3 3comm dc dcP i u (12)

In this way, GCI4 with activated DCVC maintains the current

balance in the MTDC network, but GCI3 also acts to

effectively preserve the current matching by adjusting its

output active power, using the data relating to the total

rectified current. By setting a proper sharing factor KS,

accurate current allocation between GCI3 and GCI4 is

achieved. For example, a setting of KS=0.4 will allocate 40%

of the total current to GCI4, with the remaining 60% allocated

to GCI3.

Additionally in Fig.5, there is an over-voltage and under-

voltage protection function placed within the main control

loop in Fig.5. It will detect MTDC over-voltage or under-

voltage by monitoring the feedback signal DC voltage “udc4”

at GCI4, and will trigger the back-up DCVR in GCI3 if udc4

exceeds an upper or lower constraint value (set to 180 kV and

220 kV in this simulation).

In the event of telecommunication failure, which could be

detected by the loss of data, or by use of a standard

communications health monitoring signal, GCI3 also adopts a

triggering voltage which is higher (230 kV) than the higher

DC voltage protection constraint of GCI4 (220 kV). If this

voltage is exceeded, the converter control system in GCI3 will

trigger its back-up DCVR in any case. With the CMC strategy,

the converter operating states can be ascertained with

reference to equation (13):

1 11 4 5 4 5 4

2 24 5 4 5 4

3 1 24 4 3 4

4 4

1

1

(1 )( )( ) 1

1 1 1 0

dc dc

dc dc

dc S dc dc

c dc

u iR R R R R R

u iR R R R R

u K i iR R R R

i u

(13)

Fig.4 The Central CMC with communicated variables

Fig.5 CMC and the additional protection loop

5 Performance Evaluation

For the performance evaluation of the proposed CMC

strategy, a generic four-terminal MTDC network with each

converter station rated at 200MVA is simulated in Matlab

SimPowerSystems [14], as shown in Fig.1. The central CMC

unit is placed in an independent block from the converter

current control systems of each of the four converters. DC

cable resistances are obtained from typical HVDC cable

parameters [15] and copper resistivity at 0 in [16]. This

results in modelled resistance values of 1.61 for R2 and R4,

0.32 for R1 and R3, and 1.94 for R5. The performance of

the MTDC using the proposed CMC is examined, under

steady state and fault conditions. Several events have been

simulated, and details are listed in Table 1.

Fig.6 shows the direct current injected into the DC network

from the wind farm rectifiers WFR1 and WFR2, while Fig.7

illustrates the direct current flow from the DC network into

the grid connected inverters GCI3 and GCI4 (the red dashed

line represents the reference current idc3ref for GCI3, calculated

by the CMC). Initially, GCI3 and GCI4 share the current flow

based on the specified sharing factor KS=0.6, that is 60% for

GCI4, and 40% for GCI3. At t=1s, when KS is changed from

0.6 to 0.4, a new current reference is assigned to GCI3 to

increase its DC current, and the current quickly tracks the

reference change. GCI4 is observed to decrease its current

from 60% to 40%.

At t=3s, due to the simulated increase in wind power

production (a gust simulated by a step change in wind speed),

the active power references for WFR1 and WFR2 change and,

as shown in Fig.7, their DC current input to the MTDC rises

to 0.7 and 0.5 pu respectively. This increased input current is

exported and shared correctly by GCI3 and GCI4, based on KS.

Fig.8 presents the DC voltage of WFR1, WFR2, GCI3 and

GCI4. At t=5s, there is a severe AC voltage dip due to a

100ms duration three-phase-to-earth fault at PCC4. In this

case, GCI4 contributes limited current to the fault to support

the grid voltage at PCC4 until the fault is cleared. It can be

noticed that a transient DC over-current occurs not only at

GCI4 but also at WFR1, WFR2 and GCI3. This is due to the

temporary reduction in the power transfer capability of GCI4

as the voltage magnitude at PCC4 collapses. That is because

of DC voltage interactions across all converters. The DC

over-current is effectively limited by the converter current

control system to no greater than 1.8 pu; this current is

exported by the CMC and returns to normal values as soon as

the fault is cleared.

At t=7s, inverter GCI4 is tripped, and the total rectifier current

mismatches the inverter GCI3 current output (sharing only

60% of total current based on KS=0.6) during a short period,

leading to significant over-voltage in the MTDC network as

shown in Fig.8. The protection control loop, depicted in the

lower part of Fig.5, detects the over-voltage when feedback

signal udc4 reaches the upper constraint level (220 kV) at

t=7.3s. Immediately, the CMC triggers the back-up DCVC in

GCI3’s converter controller via communicating the control

signal “Trigger_DCVC_3” (highlighted in orange in Fig.1

and Fig.5), and GCI3 begins controlling the DC voltage to a

higher target level using its DCVC (220 kV in this case). This

is to allow the DC capacitors to absorb the additional power

that cannot be temporarily transferred to the AC side through

GCI3. The CMC therefore can continue to operate the MTDC

after tripping of inverter GCI4.

It should be noted that inverter GCI3 is directly controlled by

the CMC, so plays an important role in continuously adjusting

Table.1: Simulation event description and timescales

Time (s) Events

1 Sharing factor KS changes from 0.6 to 0.4

3 PWFR1 changes from 0.3 to 0.5 pu

PWFR2 changes from 0.4 to 0.7 pu

5 3-ph-earth fault at GCI4 (100 ms)

7 Permanent trip of GCI4

Fig.6 Direct current idc1 and idc2 from WFR1 and WFR2

Fig.7 Direct current idc3 and idc4 from GCI3 and GCI4

Fig.8 DC voltage udc of WFR1,WFR2,GCI3 and GCI4

its DC current export to ensure the DC network current

balance, as shown by the red dashed line in Fig7. Inverter

GCI4, with its DCVC activated, acts as the DC side “slack

bus” to maintain voltage stability in the MTDC system, and it

also contributes to power dispatch in conjunction with

inverter GCI3. There is minimal DC voltage variation

throughout the entire simulation process, with amplitudes of ±

10 kV, until the activation of the DCVC at GCI3 at t=7.3s due

to the loss of GCI4. From this point forward, the DC current

through GCI3 is not controlled by the CMC, and it

automatically acts to balance the MTDC network current, as

shown by the GCI3 DC current reference (red dashed line)

and actual DC current in Fig.7.

From the AC side, the converters are controlled by their

individual vector control systems, as introduced in Section

2.1. Fig.9 shows that the AC-side currents associated with the

converter PCCs under the proposed CMC control are stables

and coordinated. The output AC current/power of inverter

GCI3, which is primarily dictated by the DC current reference

from the CMC in equation (12), is observed to be

continuously varied to achieve the DC current matching

function.

6 Conclusions

This paper has presented the theory and examples of

operation of a novel current matching control (CMC) scheme

for MTDC networks, where the total rectified (input) current

to the MTDC network is used as basis for actively controlling

the inverted (output) current from the network to supplied AC

grid systems. Power sharing and ability to protect against

voltage violations are also features of the scheme. The

scheme requires communications, but can still operate in the

event of loss of communications facilities. The CMC scheme

can be used with any number of converter terminals, and

independent of the types of the wind turbines used within

each wind farm.

The theoretical study and simulation results prove that, with

coordination between the CMC and local converter control

systems, the passiveness of previous control strategies –

voltage margin and voltage droop – is avoided. This enables a

stable and secure DC network operating environment, allows

flexibility of power allocation across inverters, and provides

an effective restriction of DC over-voltages. Future work is

being conducted to analyse the performance of this system

under other scenarios, with different control targets (e.g. to

provide voltage support to connected grid AC systems) and to

more extensively compare performance with other existing

and emerging MTDC control strategies.

Acknowledgement

The authors gratefully acknowledge the kind support of the

Engineering and Physical Sciences Research Council and

Rolls-Royce plc.

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Fig.9 AC current output at PCCs of WFR1,WFR2,GCI3 and GCI4