1. 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 used for many HVDC installations [3], but is not well suited
DC, control. to MTDC, in comparison to VSC. A summary of the
advantages of VSC over LCC is listed below:
Abstract  VSC has a smaller footprint which facilitates offshore
installations of reduced platform size [3];
The inherent features of Voltage Source Converters (VSCs)  LCC requires large filtering components;
are attractive for practical implementation of Multi-Terminal  VSC provides additional reactive power support and AC
HVDC transmission systems (MTDC). MTDC can be used voltage regulation for wind farms connected to weak AC
for large-scale integration of offshore wind power with systems, and possesses black-start capability;
onshore grids. However, many of the control strategies for  VSC improves wind farm AC fault ride-through
MTDC that have been proposed previously for offshore wind capability and facilitates Grid Code compliance at
farm integration depend on local control of the wind turbine reduced costs [4];
generators.  Power reversal can be achieved in VSC without changing
the DC voltage polarity, facilitating the realisation of a
This paper proposes a new control strategy, termed Current flexible MTDC transmission system.
Matching Control (CMC), which can be used with any MTDC transmission systems have attracted much attention
number of converter terminals, and is independent of the for wind farm integration [5][6][7]. Firstly, MTDC reduces
types of wind turbines used within each wind farm. The converter numbers when compared to numerous point-to-
proposed CMC matches the current reference of the grid side point HVDC solutions. Secondly, it is conceivable that, due to
converter to that of the wind farm side converters. In order to limited correlations of weather systems in geographically
achieve such current matching, a telecommunication system dispersed wind farm locations, the overall variability of wind
will be required to facilitate calculations of the grid side power may be reduced by interconnecting many
current references to be carried out in real time. To validate geographically-dispersed wind farm systems via a large-area
the performance of the proposed control strategy, a generic MTDC system, thus increasing the overall availability of
four-terminal MTDC network, which integrates two offshore energy. In future, energy storage devices may be integrated
wind farms with two mainland grids, is simulated and results with MTDC system [1], which further supports energy
relating to several steady state and transient scenarios are availability and quality of supply. MTDC is also being
presented. proposed as the means of interconnecting independent large-
scale AC power systems, (e.g. European super-grid proposal
1 Introduction [5] to promote the interconnection of Norwegian hydro,
French nuclear, Sahara solar and North sea wind power into a
There has been a tremendous pace of development of large- common MTDC) to resolve local power shortages or
scale offshore wind farms in recent years. It is anticipated that congestion, to enable international power sharing, and to
there will be an approximate increase of 26.6 GW in provide an excellent level of overall power system reliability.
aggregate generation capacity over the period from 2009/10 As discussed in [5], there are many challenging obstacles to
to 2016/17 in the UK, 11.7 GW of which will be contributed the introduction of MTDC. The control system for an MTDC
by wind power [1]. More broadly, the European Wind Energy must be robust, coordinated and reliable, as problems with
Association has, in its “high wind” scenario, a target of 180 one terminal have the potential to affect the entire MTDC
GW from wind energy sources in by 2020, of which 35 GW network. A number of control strategies are proposed [6]
will be sourced from offshore wind installations. This [8][9] which will be introduced later. These strategies remain
capacity target for offshore wind increases to 120 GW by at the modelling and testing stages of development. Critical
2030 [2]. concerns about these strategies are the controllability and
These targets, if they are achieved, will have great impact on reliability of MTDC systems, as most of the proposed
power transmission design, planning, construction and strategies manage an MTDC system without use of
operation. Many offshore wind farms will require long- communications between terminals. The proposed current
distance power transmission systems. AC may not be suitable matching control strategy employs minimal (in terms of
due to high power losses over longer distances. Classical line- required traffic and bandwidth) telecommunications between
current-commutated (LCC) HVDC transmission has been terminals in an MTDC network. While there may be concerns

2. Fig.1 The test MTDC system configuration
over use of communications, modern telecommunication paper focuses on the dynamics of AC and DC interaction,
technologies are increasingly highly developed, reliable and which is dictated by the converter control. As the control
redundant [10]. Furthermore, risk can be mitigated by system for VSC employs vector control in the synchronous
employing redundancy, through continuous monitoring of rotating reference frame dq, the current references id_ref and
telecommunications channels, and ensuring operation can iq_ref which are derived by the commanded active power Pcomm
continue, albeit in a less efficient fashion, if and reactive power Qcomm, are given in equation (1). In the
telecommunications is lost. Operation of the scheme is based rotating reference frame, the d-axis voltage Vd, aligned with
on measurement (discretely, with a step of 1-2ms in this one of three phases, is equal to the magnitude of AC voltage,
example) of the total DC current provided by wind farm side and the q-axis voltage Vq is zero.
rectifiers (i.e. WFR1 and WFR2 in Fig.1) and allocation
(matching) of this current across the inverters, according to a Pcomm Q
pre-determined sharing factor. This is described in more idref  , iqref  comm (1)
Vd Vd
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 Once the current references id_ref and iq_ref are generated, the
telecommunication system fails, but accurate sharing of the inner current control loops adjust the actual id and iq values to
inverter currents may not be possible. 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
2 MTDC system configuration Laplace equation (2), which contains the proportional gain
MTDC topology design may vary depending on specific (kp) and the integral gain (ki) of the proportional-and-integral
situations (e.g. the locations of grid connection points and (PI) controller, reactor inductance (L) and resistance (R):
offshore wind farms, available undersea cable routes). Fig.1
presents a four terminal MTDC system for wind farm idq
kp
 Li
k
integration, which utilises bi-polar cables R5 with nominal DC  L
Rk p
(2)
s2  s
idqref ki
voltage of 200 kV (± 100 kV). On the offshore side, two wind L L
farms are connected via two voltage source neutral-point
clamped rectifiers (WFR1, WFR2). On the onshore side, two From the DC side perspective of the VSC in Fig.2, the DC
grid-connected inverters (GCI3, GCI4) feed power to two voltage udc across the converter or the output capacitors, the
independent 2000 MVA equivalent AC power systems. All DC current idc injected by the converter, and the current ic
VSCs are rated at 200 MVA. Targets of converter control conducted by the DC cables, are related as shown in equation
differ for WFR1 and WFR2, implementing frequency and AC (3). The capacitors are charged (or discharged) to possess a
voltage control at the point of common connection (PCC) certain DC voltage. The current idc injected to the MTDC
with wind farms, while GCI3 and GCI4 are equipped with a network by the converter is calculated from AC side PCC
current controller and DC voltage regulator respectively, in currents id and iq, pulse-width-modulation index M and
addition to AC voltage/or reactive power control. converter terminal voltage angle  with respect to the PCC
voltage, using equation (4):
2.1 AC/DC interaction for a VSC dudc
C  idc  ic (3)
Instead of presenting an in-depth study of the VSC control dt
system formulae, such as those presented [9] and [7], this idc  Mid cos   Miq sin  (4)

3. Fig.3 MTDC control strategies: (a) voltage margin (b) voltage droop
namely voltage margin control [9] and voltage droop control
[8] [6].
3.1 Voltage margin control (VMC)
Fig.2 DC equivalent circuit for the MTDC
In VMC control, one node’s DC voltage is controlled by a DC
voltage controller (DCVC). This effectively acts as a DC
2.2 Equivalent MTDC circuit
voltage slack bus, with other VSCs operating in current
As demonstrated in Section 2.1, the DC property of individual control mode as illustrated in equations (1) and (2).
VSCs in the MTDC can be represented as a “controlled”
current source, shown in the equivalent circuit in Fig.(2). The VMC equips all converters with DCVCs, but the DCVC of
extremely small inductance and capacitance of the DC only one converter station must be activated. Considering the
network with respect to direct current are neglected. V-I characteristic of Fig.3(a), GCI4 has its DC node voltage
controlled at udc4 ref by the activated DCVC, represented as the
As the focus of this section is on the analysis of DC network solid line. This acts to balance the current flows from
behaviour, it is essential to analyse the effect of the variation rectifiers WFR1 and WFR2 to inverter GCI3, by automatically
in DC current idc from one converter station, on either its DC “sliding” its current output along the constant DC voltage
voltage and/or the DC voltage at other stations. Taking WFR1 udc4_ref. Inverter GCI4 has an inherent current limit. If this
as an example, a control action to increase WFR1 current idc1 limit is exceeded (e.g. strong wind pushing more current
will quickly charge its DC capacitors and boost its DC though rectifiers into the MTDC), GCI4 will not be able to
voltage udc1 to a higher value, based on equation (3). The maintain the DC voltage, and will operate at its maximum
higher udc1 with respect to other DC node voltages acts to current output. According to the analysis in Section 2.2, the
supply the conducted current ic1 into the MTDC network. The DC network voltage will continuously rise in line with current
increased current ic1 charges capacitors at other nodes, until “surplus” in the MTDC network. The voltage will ultimately
the voltage levels at all the nodes reach a new higher rise to a new level (udc3_ref) that activates the back-up DCVR
equilibrium value. The rectifier DC voltage is slightly higher in the other inverter GCI3. In this case, GCI3 begins
than the inverter voltage so that current flows from rectifier maintaining the MTDC voltage at udc3_ref, the dashed line in
node(s) to inverter node(s). The magnitude of individual Fig.3(a). The term “voltage margin” refers to the difference
converter DC voltage depends on two elements: (a) the between udc4 _ref and udc3_ref in Fig.3(a).
conducted current though the node; (b) the resistances
between the nodes. 3.2 Voltage droop control (VDC)
VDC basically has multiple activated DCVCs (in both of the
Thus, it can be concluded that a temporary current mismatch
inverters in this example). The two DCVCs are controlled at
between rectifier and inverter in the MTDC results in an
different levels for inverter current dispatch, as shown in
overall DC voltage variation. As the converter control
Fig.3(b). The V-I droop characteristic is obeyed by GCI3 to
systems use DC voltage information to function, it is
share the total current with GCI4. To demonstrate the droop
desirable to quickly address any DC current surplus (by
operation, for example, in order to increase the current of
increasing exported current) or DC current shortage (by
GCI3 and decrease GCI4 based on the droop characteristic, the
reducing exported current), so that a stable DC voltage
DCVC in GCI3 converter control must lower the voltage
operating point for the MTDC system will be realised.
reference udc3_ref and then its current output “slides” along the
Communications between rectifier and inverter nodes in the
droop to the right hand side, to output more current.
system is therefore critical to the operation of this scheme.
3 Previously reported MTDC control 4 Proposed current matching control strategy
strategies As discussed in Section 2.2, a stable DC network operating
point can be achieved by quickly acting to reduce any
Historically, there have been two distinct control strategies mismatch between rectifier and inverter DC currents. As both
used to facilitate power dispatch from DC to AC systems, WFR1 and WFR2 inject all the power generated by wind farms
into MTDC network, they will operate in current control

4. 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 Fig.4 The Central CMC with communicated variables
following equations (5) and (6) are given, referring to Fig.2:
uS  udc1  R1ic1  udc 2  R2ic 2 (5)
uS  udc3  R3ic3  udc 4  R4ic 4 (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:
uS  uR  R5ic5 (7)
ic5 is the current through the DC link as shown in Fig.2.
Kirchhoff’s current law dictates that: Fig.5 CMC and the additional protection loop
idc1  idc 2  idc3  idc 4  0 (8)
idc3  (1  KS )(idc1  idc 2 ) (11)
As demonstrated in VMC and VDC, GCI4 has its DCVC
activated to control DC voltage at udc4; the other converters DC current reference for idc3 is transmitted from the CMC to
WFR1, WFR2 and GCI3 control their currents at idc1 idc2 and the GCI3 converter control system, to produce a commanded
idc3 respectively. Therefore, by combining equation (5), (6), active power reference, given in equation (12):
(7) and (8), the following converter operating state matrix, Pcomm3  idc3udc3 (12)
which incorporates DC network resistance, can be derived: In this way, GCI4 with activated DCVC maintains the current
 udc1   R1  R4  R5 R4  R5  R4 1   idc1  balance in the MTDC network, but GCI3 also acts to
u    
 dc 2    R4  R5 R4  R5  R4 1   idc 2 
 effectively preserve the current matching by adjusting its
 udc3  
(9)
R4 R4 ( R3  R4 ) 1   idc3  output active power, using the data relating to the total
    
 ic 4  
   1 1 1 0  udc 4 
  rectified current. By setting a proper sharing factor KS,
accurate current allocation between GCI3 and GCI4 is
4.2 Current matching control principle achieved. For example, a setting of KS=0.4 will allocate 40%
of the total current to GCI4, with the remaining 60% allocated
Fig.4 shows the communicated variables of the proposed to GCI3.
CMC for an MTDC. The green blocks in Fig.1 and Fig.4 are
telecommunication feedback signals “idc1”and “idc2” from the Additionally in Fig.5, there is an over-voltage and under-
rectifiers WFR1 and WFR2, based on equation (4). Feedback voltage protection function placed within the main control
signal “udc4” from GCI4 is used for over- or under-voltage loop in Fig.5. It will detect MTDC over-voltage or under-
protection. The current reference for GCI3 converter is voltage by monitoring the feedback signal DC voltage “udc4”
continuously updated by the proposed CMC and transmitted at GCI4, and will trigger the back-up DCVR in GCI3 if udc4
to the GCI3 vector control, via the communicated control exceeds an upper or lower constraint value (set to 180 kV and
signal “idc3_ref”. 220 kV in this simulation).
Modern wireless communication system has been proposed in In the event of telecommunication failure, which could be
HVDC application to secure power reliability [12] and it is detected by the loss of data, or by use of a standard
here used here to favour the CMC strategy for the MTDC communications health monitoring signal, GCI3 also adopts a
system. Fig.5 illustrates the CMC inner control logic, where triggering voltage which is higher (230 kV) than the higher
the total rectifier current from WFR1 and WFR2 is the sum of DC voltage protection constraint of GCI4 (220 kV). If this
feedback signals “idc1”and “idc2”. Rectified current is divided voltage is exceeded, the converter control system in GCI3 will
between the inverters GCI3 and GCI4 by applying a sharing trigger its back-up DCVR in any case. With the CMC strategy,
factor KS. KS represents the portion of the expected power to the converter operating states can be ascertained with
be exported from the MTDC network through GCI4, given by reference to equation (13):
equation (10). Accordingly, the reference current idc3 for GCI3  udc1   R1  R4  R5 R4  R5  R4 1  idc1 
u    
is given by equation (11):  dc 2    R4  R5 R4  R5  R4 1 
 idc 2 
 udc3    (1  K S )(idc1  idc 2 ) 
(13)
idc 4  KS (idc1  idc 2 ) (10)   
R4 R4 ( R3  R4 ) 1
 
 ic 4  
   1 1 1 0 
 udc 4 


5. 5 Performance Evaluation Table.1: Simulation event description and timescales
Time (s) Events
For the performance evaluation of the proposed CMC
1 Sharing factor KS changes from 0.6 to 0.4
strategy, a generic four-terminal MTDC network with each
converter station rated at 200MVA is simulated in Matlab PWFR1 changes from 0.3 to 0.5 pu
3
PWFR2 changes from 0.4 to 0.7 pu
SimPowerSystems [14], as shown in Fig.1. The central CMC
5 3-ph-earth fault at GCI4 (100 ms)
unit is placed in an independent block from the converter
7 Permanent trip of GCI4
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 Fig.6 Direct current idc1 and idc2 from WFR1 and WFR2
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 Fig.7 Direct current idc3 and idc4 from GCI3 and GCI4
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 Fig.8 DC voltage udc of WFR1,WFR2,GCI3 and GCI4
over-current is effectively limited by the converter current t=7.3s. Immediately, the CMC triggers the back-up DCVC in
control system to no greater than 1.8 pu; this current is GCI3’s converter controller via communicating the control
exported by the CMC and returns to normal values as soon as signal “Trigger_DCVC_3” (highlighted in orange in Fig.1
the fault is cleared. and Fig.5), and GCI3 begins controlling the DC voltage to a
higher target level using its DCVC (220 kV in this case). This
At t=7s, inverter GCI4 is tripped, and the total rectifier current is to allow the DC capacitors to absorb the additional power
mismatches the inverter GCI3 current output (sharing only that cannot be temporarily transferred to the AC side through
60% of total current based on KS=0.6) during a short period, GCI3. The CMC therefore can continue to operate the MTDC
leading to significant over-voltage in the MTDC network as after tripping of inverter GCI4.
shown in Fig.8. The protection control loop, depicted in the
lower part of Fig.5, detects the over-voltage when feedback
It should be noted that inverter GCI3 is directly controlled by
signal udc4 reaches the upper constraint level (220 kV) at
the CMC, so plays an important role in continuously adjusting

6. 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.
Fig.9 AC current output at PCCs of WFR1,WFR2,GCI3 and GCI4 References
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