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CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 3, NO. 4, DECEMBER 2017
Modeling, Control, and Protection of Modular
Multilevel Converter-based Multi-terminal HVDC
Systems: A Review
Lei Zhang, Member, IEEE, Yuntao Zou, Jicheng Yu, Student Member, IEEE, Jiangchao Qin, Senior Member,
IEEE, Vijay Vittal, Fellow, IEEE, George G. Karady, Life Fellow, IEEE, Di Shi, Senior Member, IEEE,
and Zhiwei Wang, Member, IEEE
Abstract—Multi-terminal direct current (MTDC) grids provide
the possibility of meshed interconnections between regional
power systems and various renewable energy resources to boost
supply reliability and economy. The modular multilevel converter (MMC) has become the basic building block for MTDC
and DC grids due to its salient features, i.e., modularity and
scalability. Therefore, the MMC-based MTDC systems should
be pervasively embedded into the present power system to improve system performance. However, several technical challenges
hamper their practical applications and deployment, including
modeling, control, and protection of the MMC-MTDC grids. This
paper presents a comprehensive investigation and reference in
modeling, control, and protection of the MMC-MTDC grids. A
general overview of state-of-the-art modeling techniques of the
MMC along with their performance in simulation analysis for
MTDC applications is provided. A review of control strategies
of the MMC-MTDC grids which provide AC system support is
presented. State-of-the art protection techniques of the MMCMTDC systems are also investigated. Finally, the associated
research challenges and trends are highlighted.
Index Terms—DC circuit breaker (CB), DC-fault blocking,
DC voltage droop control, detailed switching model, embedded
HVDC, equivalent circuit model, high-voltage direct current
(HVDC), meshed DC grids, modeling of MMC-MTDC, modular
multilevel converter (MMC), multi-terminal direct current
(MTDC), power oscillation damping.
I. I NTRODUCTION
T
HE voltage-source converter based high-voltage direct
current (VSC-HVDC) system has been widely investigated due to its salient features such as interconnection of
weak AC systems or even passive networks, reactive power
support, improved reliability, and integration of various energy
resources [1]. Its capability of changing power flow direction
Manuscript received April 16, 2017; revised October 3, 2017; accepted
October 29, 2017. Date of publication December 30, 2017; date of current
version November 10, 2017. This work was funded by SGCC Science and
Technology Program under project Research on Electromagnetic Transient
Simulation Technology for Large-scale MMC-HVDC Systems.
L. Zhang, Y. T. Zou, J. C. Yu, J. C. Qin (corresponding author, e-mail:
jqin@ asu.edu), V. Vittal, and G. G. Karady are with the School of Electrical,
Computer and Energy Engineering, Arizona State University, Tempe, AZ
85287-5706, USA.
D. Shi and Z. W. Wang are with GEIRI North America, Santa Clara, CA
95054, USA.
DOI: 10.17775/CSEEJPES.2017.00440
without reversing the voltage polarity enables the construction
of multi-terminal HVDC systems and DC grids [2]. Multiterminal direct current (MTDC) grids can provide the possibility of meshed interconnections between regional power
systems and various renewable energy resources, which potentially enhance reliability of the AC and DC systems, improve
flexibility and economy of power dispatching and efficiently
utilize converters and cables [3]–[11].
The modular multilevel converter (MMC) has become the
most attractive converter topology for high power and medium/
high voltage applications, especially for VSC-HVDC systems [12]. In comparison with the two-level VSC and other
multilevel converter topologies, the salient features of the
MMC include: 1) its modularity and scalability to meet any
voltage level requirements, 2) reduced voltage ratings and
dv/dt stress of switches and capacitors, 3) high efficiency, 4)
improved power quality for filter-free applications, 5) inherent
fault-tolerance capability and 6) fault-blocking capacity to
improve fault interruption performance of the MMC-based
HVDC systems. Therefore, the MMC has become the basic
building block for MTDC systems and DC grids.
Although the line commutated converter (LCC)-based
HVDC technology dominates long-distance, bulk power transmission and several LCC-based MTDC systems have been
commissioned or are under construction [13]–[18], VSC (especially MMC) is more suitable for building DC grids through
providing more grid services [1]. The Italy-Corsica-Sardinia
(SACOI) HVDC project is the first three-terminal LCC-MTDC
system. The first two converter stations were completed in
1967 and were rated at 200 kV, 200 MW and then upgraded to 300 MW in 1992. The third station was installed
in 1988 [14]–[16]. The Québec-New England LCC-HVDC
system is the first five-terminal HVDC transmission system in
the world [13]. The North-East Agra multi-terminal ultrahighvoltage direct current (UHVDC) system is under construction
in India, which is the world’s first multi-terminal UHVDC
transmission link based on LCC [17], [18]. However, due to
the advantages of VSC (especially MMC) over conventional
LCC, the MMC technology is more promising and has been
applied to the Nan’ao three-terminal HVDC system, which
is the world’s first multi-terminal VSC-HVDC transmission
system [19]. The Zhoushan HVDC project is the first five-
2096-0042 © 2017 CSEE
ZHANG et al.: MODELING, CONTROL, AND PROTECTION OF MODULAR MULTILEVEL CONVERTER-BASED MULTI-TERMINAL HVDC SYSTEMS: A REVIEW
341
TABLE I
M ULTI - TERMINAL HVDC S YSTEMS A ROUND THE W ORLD
Name of System
Italy Corsica Sardinia (SACOI)
Québec-New England
Nan’ao
Zhoushan
North-East Agra
Zhangbei
Terminals
3
5
3
5
4
4
Commissioning Year
1967, 1988, 1992
1990–1992, 2016
2015
2016
Under construction
Under construction
terminal MMC-HVDC system in the world [20], [21]. In
Europe, meshed DC grids have been planned in the North Sea
Super Grid (NSSG) and European Super Grid projects [22]–
[27]. Table I lists the details of some multi-terminal HVDC
projects.
Over the past few years, there has been a significant
effort towards addressing the technical challenges associated
with operation, control, modeling, and protection of MTDC
systems [3], [16], [28]. As compared with LCCs or two-level
VSCs, the MMC is more suitable for building MTDC systems
and DC grids due to its salient features. However, there are
some technical challenges when applying the MMC to DC
transmission systems. The main intention of this paper is to
provide a comprehensive analysis and review in modeling,
control, and protection of the MMC-based MTDC systems.
This paper also highlights the associated research challenges
and trends.
The rest of this paper is organized as follows. Section II
gives an overview of the features of the MMC-based MTDC
systems. Section III introduces the state-of-the-art modeling
techniques of the MMC along with their simulation performance for MTDC applications. This is followed by a review
of control strategies of the MMC-MTDC systems in Section
IV. Section V presents the protection techniques for the MMCMTDC systems. Section VI concludes the paper.
II. OVERVIEW OF F EATURES OF MMC-MTDC
The challenges of a general VSC-MTDC system have been
presented in [3], [16], [26]. When employing the MMC in
MTDC systems, it provides new features as compared with
conventional two-level and multilevel VSCs.
A. Modeling Issue
The MMC is the most attractive device for building MTDC
systems and DC grids due to its advanced characteristics [12].
A schematic diagram of an MMC is shown in Fig. 1. The
MMC based on the half-bridge (HB) submodules (SMs) is
the dominant topology for HVDC applications due to its low
cost and low loss. However, in case of a DC-side short-circuit
fault, the HB-MMC cannot block the fault currents fed from
the AC grid. Various SMs have been proposed and investigated
to improve fault-blocking performance of the MMC-HVDC
systems, including the full-bridge (FB), the unipolar-voltage
full-bridge (UFB), the clamp-double (CD) and the threelevel/five-level cross-connected (3LCC/5LCC) SMs [29]–[31].
Different SM circuits and MMC configurations have different
operating behaviors and design considerations.
Rated Capacity (MW)
200/50/200
2,250/2,138/690/690/1,800
200/100/50
400/300/100/100/100
6,000
3,000/3,000/1,500/1,500
+
Rated DC Voltage (kV)
+200
±450
±160
±200
±800
±500
Converter Type
LCC
LCC
MMC
MMC
LCC
MMC
SM
SM 1
SM 1
SM 1
SM N
SM N
SM N
Rarm
Rarm
Rarm
Larm
Larm
Larm
Vdc
Half-bridge SM
Full-bridge SM
Larm
Larm
Larm
Rarm
Rarm
Rarm
Clamp-double SM
SM 1
SM 1
SM 1
SM N
SM N
SM N
+
+
−
Three-level crossconnected SM
Fig. 1.
Schematic representation of an MMC with various SMs.
In a large-scale MMC-embedded power system, as shown in
Fig. 2, it is essential to investigate dynamic performance, fault
and protection, control design, reliability and stability based
on modeling and simulation analysis to satisfy planning and
operational criteria [32]–[35]. The use of a detailed switching
model (DSM) of the MMC is time consuming and infeasible
for large-scale MTDC system simulation due to a large number
of semiconductor switches. The main challenge associated
with modeling and simulation of the MMC-MTDC systems is
lack of high efficient and accurate models of the MMCs, which
are expected to cover various behaviors of the MMCs, based
on different SM circuits/configurations and can be applied to
various simulation platforms.
B. Reliability and Stability Issues
1) Reliability
A large number of components in an MMC lead to low reliability of the MMC [36]. However, the MMC has inherent faulttolerance capability which potentially improves its reliability
with proper design and control. Several methods have been
proposed for reliability improvement of the MMC, including
redundancy design, fault detection/location, post-fault control
and periodic maintenance [36]–[48]. Nevertheless, it is still
challenging to accurately estimate and cost-efficiently improve
the reliability of the MMC-based systems.
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104
G1 101
AC Grid 1
109
108
G3 103
106
105
102 G2
MMC-MTDC systems with coordination of circuit breakers
(CBs), converters and other protection devices.
110
III. M ODELING OF MMC-MTDC
107
A. Modeling of the MMCs
111
DC
Bus 4
DC
MMC 1 Bus 1
MMC 4
Wind Farm 1
DC Grid
DC
Bus 2
Wind Farm 2
DC
DC Bus 5
Bus 3
MMC 5
MMC 2
MMC 3
Ga 201
204 205
206
208
202
Gb
203
Gc
207
AC Grid 2
209
Fig. 2.
As discussed in Section II-A, due to a large number of semiconductor switches, the DSM is time consuming and infeasible
for large-scale MTDC system simulations. To address this
challenge, several modeling techniques have been developed
to accelerate electromagnetic transient (EMT) simulation.
1) Averaged models: In [60], [61], an averaged model has
been proposed only for the HB-MMC configuration
under normal operating conditions, without considering
DC fault operating conditions, as shown in Fig. 3.
Similar models are also presented for the HB-MMC
and FB-MMC in [35], [62], [63]. However, they are not
applicable for investigating DC-side transient and fault
conditions.
210
+
vau
vbu
vcu
Rarm
Rarm
Rarm
Larm
Larm
Larm
Larm
Larm
Larm
Rarm
Rarm
Rarm
val
vbl
vcl
Topology of an MMC based MTDC system.
2) Stability
To improve system performance, it is important to investigate the stability of the MMC-MTDC systems and enhance the
stability of both AC and DC systems [49]–[55]. In an MMC,
the SM capacitors can provide energy storage capability, which
can be utilized to damp power oscillation [56]–[58]. Consequently, by employing the MMC in MTDC systems, it can
potentially provide enhanced and flexible ancillary services to
improve system stability with inherent energy storage, proper
design and coordinate control and operation between AC and
DC systems.
3) Protection Issue
To reliably operate the MTDC systems, the protection strategy is very critical, involving proper coordination of circuit
breakers (CBs), converters and other protection devices. The
protection systems of the MTDC systems must seamlessly satisfy the requirements including sensitivity, selectivity, speed,
reliability and robustness [3].
The HB-MMC based MTDC systems have the similar DC
fault characteristics with that of the conventional two-level
VSC-MTDC systems. As discussed in [31], [59], due to the
fault currents flowing through the freewheeling diodes, the
HB-MMC and the two-level VSC cannot block the fault
currents during a DC-side short-circuit fault. Therefore, the
protection systems need to be properly designed and deployed
in the HB-MMC-MTDC systems.
The fault-blocking capability can be embedded into the
MMC by employing the fault-blocking SMs to block fault
current flowing through the converter [31]. Another solution
is to use DC CBs to isolate the faulty lines. However, both solutions increase cost and loss. Therefore, it is still challenging
to identify the most cost-efficient protection scheme for the
V dc
Grid
voltages
iarm
Fig. 3.
−
Arm voltage
calculation
The averaged model of the MMC.
2) Detailed equivalent circuit models: To investigate DC
fault and transient conditions, a detailed equivalent circuit model (ECM) is proposed in [64] for the HB-MMCbased MTDC system, which is able to estimate the
capacitor voltage for each SM. Its configuration is shown
in Fig. 4. The instantaneous capacitor voltages and arm
voltages can be determined by differential equations
through numeric methods. However, it is not applicable
for the SMs with fault-blocking capability (e.g., the FB
SMs).
3) Equivalent circuit models with fault-blocking capability:
To consider the MMC with embedded fault-blocking
capability, a detailed ECM has been proposed for selfblocking MMC in [65]. Reference [66] proposes an
ECM for hybrid MMC configurations based on various
SM circuits, which is applicable for normal and fault
operations. In [66], a detailed ECM is derived, along
ZHANG et al.: MODELING, CONTROL, AND PROTECTION OF MODULAR MULTILEVEL CONVERTER-BASED MULTI-TERMINAL HVDC SYSTEMS: A REVIEW
Equivalent
arm branch
+
+
_
vau
+
_
vbu
+
_
vcu
Rarm
Rarm
Rarm
Larm
Larm
Larm
Larm
Larm
Larm
Rarm
Rarm
Rarm
V dc
Grid
voltages
iarm
S SM (1:N)
Fig. 4.
−
+
_
val
+
_
vbl
+
_
vcl
Arm voltage
calculation
The detailed equivalent circuit simulation model of the MMC.
with considering the individual capacitor in each SM and
voltage balancing. To further accelerate the simulation
model of the MMC-based system, a reduced-order ECM
can be derived by assuming that all capacitor voltages
of each arm are well balanced and become a one state
variable.
The DSM, detailed ECM, and reduced-order ECM are compared by a 21-level HB-MMC-HVDC system in a PSCAD/
EMTDC simulation program, as shown in Fig. 5. The run
time of the three simulation models is compared and listed in
Table II. Based on Table II, the reduced-order ECM has the
highest simulation efficiency.
343
B. Modeling of the MTDC Systems
To evaluate the system-level dynamic performance, stability
and interaction between AC and DC grids, modeling and
simulation of the MTDC systems are required.
In [67], [68], the modeling of AC grids with embedded
MT-DC systems is analyzed and a unified method for power
flow calculation is proposed. A generalized VSC-MTDC dynamic model is proposed and extended in [69], [70]. In [71],
three dynamic models are compared and investigated for the
stability evaluation of the MTDC systems. Reference [72]
presents a dynamic model for the VSC-MTDC systems based
on the Newton-Raphson method, while considering a more detailed VSC model for positive-sequence dynamic simulations
of large-scale power systems. The aforementioned references
consider the general VSC-MTDC systems. Reference [73]
presents two EMT simulation models of the MMC-MTDC
systems based on the equivalent circuit of the MMC. However,
the existing models are not eligible for various SM circuits and
MMC configurations under different operating conditions.
IV. C ONTROL S TRATEGIES OF MMC-MTDC
To properly and efficiently control the VSC-MTDC systems,
two control tasks should be considered: 1) DC voltage control
and 2) AC-side auxiliary control. DC voltage control is used to
stabilize DC grid operations. The DC voltage of each converter
is related to the balance of active power, and determines the
power flow and sharing in the DC networks [74], [75]. On
the other side, the VSC-based converter stations have features
of power decoupling and fast response, which could provide
auxiliary support to the AC systems and potentially, enhance
performance and stability of the AC systems. The existing
control strategies of the VSC-MTDC systems are applicable
for the MMC-MTDC systems.
A. DC Voltage Control and Coordination Strategy
(a)
(b)
Time (s)
(c)
Fig. 5. The phase-a capacitor voltages of the HB-MMC HVDC systems
based on (a) DSM, (b) detailed ECM, and (c) reduced-order ECM.
TABLE II
C OMPARISON OF S IMULATION M ODELS FOR
21- LEVEL MMC-HVDC S YSTEMS
Model
DSM HB-MMC
Detailed ECM HB-MMC
Reduced-order ECM HB-MMC
Simulation
Time (s)
1
1
1
Simulation
Step (µs)
10
10
10
Running
Time (s)
1,656.21
30.67
11.52
The commonly used control architecture of a VSC in the
MTDC systems is shown in Fig. 6, which consists of inner and
outer control loops and is also applicable for the MMC-based
converter station. The DC voltage control is implemented by
the outer control loop. Unlike the frequency of the AC system
as a global control parameter, the DC voltage varies in the DC
grid due to power flow controlled by the difference between
bus voltages of the DC network [75], [76]. Based on this
basic control architecture, various DC voltage control and
coordination strategies have been developed.
Currently, there are primarily three methods for DC voltage control: master-salve control, voltage margin control and
voltage droop control [76]–[78]. These methods can also be
categorized as centralized and decentralized control methods.
A brief introduction of these three DC voltage control methods
is presented as follows:
1) Master-slave control: For this control, a master converter
is selected and responsible for controlling the voltage
profile while other converters control power [76]. This
method has been implemented in the Nan’ao five terminal MTDC system [19]. However, this method has
instability issues since only the one master converter
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CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 3, NO. 4, DECEMBER 2017
U
E
L1
R1
uabc
iabc
abc
θ
PLL
θ
abc
dq
dq
ud uq
id iq
Modulation
dq
Active power
control
Pref
ud
DC voltage
control
ed
idref
I
id
Vdc
Reactive
power
control
Qref
Q
L
L
iq
P
eq
iqref
I
AC voltage
control
Vac,ref
abc
P
P
Vdc,ref
θ
uq
Vac
Fig. 6.
DC voltage and power control scheme of a VSC.
control has the similar instability issue of master-slave
control due to only one regulating converter existing at
one time [79]. In worst case scenarios, master converter
shifting will cause oscillation of the DC voltage and may
lead to collapse of the DC grid [76]. Additionally, the
dynamic response of the voltage margin control is slow
due to its controller architecture [80].
3) Voltage droop control: Voltage droop control, as a decentralized strategy and as shown in Fig. 8, is very different
compared with the two aforementioned centralized control methods. In the voltage droop control, DC voltage
is controlled by multiple converters to maintain voltage
stability and power balance [76], [79]. This approach
cannot ensure proper operations if the master converter
fails [76], [78]. Additionally, the master converter should
be connected to a very strong AC grid to ensure fast
conditioning of the DC grid and avoid negative effects
on the AC grid [5].
2) Voltage margin control: Voltage margin control is considered as an improved master-slave voltage control, as
shown in Fig. 7. There are some reserved converters to
ensure the capability of the DC voltage regulation when
power limitation is exceeded [79]. If the master converter
is off-line or reaches its power limitation, one of the
reserved converters will immediately take over the role
of DC voltage regulation [5]. However, voltage margin
Vdc
Vdc
*
Vdcfl
Pmax Inverter
Fig. 7.
*
Vdcsl
Rectifier −Pmax P1*
P2* Pmax Inver ter
Pmax Inverter Rectifier −Pmax
Characteristics of voltage margin control.
Vdc
Rectifier −Pmax
Fig. 8.
*
Vdcsu
*
Vdcfu
Vdc*
Rectifier −Pmax
Vdc
Vdc
Pmax Inver ter Rectifier −Pmax
Characteristics of voltage droop control.
Vdc
Pmax Inverter Rectifier −Pmax
Pmax Inver ter
ZHANG et al.: MODELING, CONTROL, AND PROTECTION OF MODULAR MULTILEVEL CONVERTER-BASED MULTI-TERMINAL HVDC SYSTEMS: A REVIEW
originated from power-frequency droop control of the
AC system. Compared to other methods, voltage droop
control has high reliability and does not cause oscillations. Reference [76] presents a generalized voltage
droop control and power sharing strategy based on a
two-layer hierarchical structure.
Based on these three basic methods, some modified and
improved strategies have been proposed. These emerging
methods can be recognized as consisting of two categories.
One category focuses on the design and optimization of the
coefficients of the voltage droop control, such as adaptive
droop control for available headroom of power limitation to
improve stability [81], integration of renewable generation and
energy storage with trusted stability [74], [77] and optimal
DC power flow [75], [80], [82]. The other category focuses
on modification and combination of these three basic methods
for better performance. In reference [79], a master-auxiliary
coordinated control is proposed by combining voltage margin
and voltage droop control methods for offshore wind farms.
In reference [70], a control approach is proposed for twoterminal systems and extended to multiple-terminal systems,
which involves master-slave controls. In reference [83], the
conventional active power control and voltage control are
modified and combined to form a new method.
B. Power Flow and Sharing Between Multiple Converters
Power flow and sharing of the VSC-MTDC systems is
controlled by the DC voltage of each terminal, as shown in
Fig. 9. It is essential to consider stability region, limitation and
optimal parameter determination of voltage control approaches
to calculate the power flow and sharing.
0
Station
#1
0
Station
#2
0
Station
#3
0
Station
#4
0
Station
#5
0
Station
#3 out
0
Station
#4
0
Station
#5
Operating
point
0
Station
#1
Fig. 9.
0
Station
#2
Characteristics of power sharing in a five-terminal MTDC system.
In reference [82], a DC voltage and power sharing control
strategy is proposed based on a combination of an optimal
DC power flow algorithm and voltage-droop method for the
optimal operation of the MTDC grids. Reference [75] also
combines droop strategy and an optimal power flow algorithm in a hierarchical control architecture for the MTDC
systems. Reference [84] proposes an improved analytic model
for the steady-state analysis of droop-controlled VSC-MTDC
systems. Based on the bisection algorithm and superposition
principle, this analytic model can estimate the results of power
distributions, DC voltage deviation and power loss variations
345
of the MTDC systems under converter outage and overload
conditions. Additionally, reference [78] gives an analytical
expression to estimate the distribution of balancing power
under voltage droop control. In reference [5], a generalized
algorithm is proposed to solve DC-power flow of the MTDC
systems with various nonlinear voltage droops.
C. Power Oscillation Damping and AC System Performance
Enhancement
The MTDC systems can also offer additional control functions to improve system dynamic performance, such as power
oscillation damping [85], transient stability and fault recovery [52], [86], [87] and subsynchronous damping enhancement [88]–[90].
Low frequency inter-area power oscillations are a common
phenomenon in power systems [91] and have caused a few
wide-scale blackouts [92], [93]. The inter-area oscillation is
the main reason for power system separation and cascading
failure [94]. Neither the damper windings of the synchronous
machines nor the modern digital electro-hydro control systems
without global signal measurement can effectively attenuate
the inter-area oscillations [95], [96]. Therefore, the attenuation
of the inter-area power oscillations is important and has
remained a challenge for a long time.
The power oscillation control can be integrated into the DC
power control of the MTDC systems. The basic concept is
to compensate the oscillations of AC systems by exchanging
extra power and modifying the DC power reference of Fig. 6.
The extra power utilized for damping the oscillation can be
calculated based on the optimal control strategy with the
inputs: 1) generator rotor speed difference [96], 2) temporary
frequency difference among the converter stations when these
stations are within the same AC network [97] and 3) instantaneous AC power flow at the converter stations.
Reference [98] analyzes and presents an inter-area oscillation damping solution to a two-machine MTDC system by
using an active-power modulation. Reference [99] proposes a
coordinated control strategy for VSC-HVDC integrated with
offshore wind farms, which can provide more synthetic inertia.
Reference [100] proposes an oscillation damping method
based on frequency and power differences in accordance with
a communication latency compensator to overcome the signal
delay. In reference [101] and [102], the MTDC grids are used
to damp power oscillation of an AC system by active power
injection. In reference [103], a time-optimal power injection
control strategy for the MTDC systems is proposed based on
Lyapunov theory and can enhance the rotor-angle stability of
the AC system. Reference [104] proposes a generic inertia
emulation control strategy for the VSC-MTDC systems and
emulates the behaviors of synchronous generators by utilizing
DC-link capacitors of the MTDC grids to exchange energy and
contribute an inertial response to AC system disturbances.
Reference [105] shows that the VSC-MTDC systems can
improve power transfer capability and provide extra dynamic
security under low wind conditions of the wind farm. In
reference [106], a control strategy is proposed to stabilize
the interconnected AC-MTDC system and converge the AC
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frequencies to the nominal frequency. Furthermore, this control
strategy can optimize the operation of the MTDC systems by
minimizing the quadratic cost functions of voltage deviations
and generation power. Reference [107] uses the MTDC grid to
minimize frequency deviation of the interconnected AC system
by autonomous power sharing control.
Although these aforementioned control strategies for the
VSC-MTDC systems are applicable for the MMC-MTDC
systems, there are a lack of literature on investigating the
interaction of the internal dynamics of the MMCs, the DC
networks and the AC grids. Compared to the conventional
two-level VSC, the MMC needs more complicated internal
controllers to properly control the internal dynamics. Additionally, the internal energy storage of the MMC may provide
a potentially enhanced capability to stabilize the AC systems
as well as the DC networks.
V. P ROTECTION T ECHNIQUES OF MMC-MTDC
A. DC Fault Analysis, Detection, and Location
Under DC fault conditions, the HB-MMC has similar behavior as two-level VSC and cannot block the fault currents,
as shown in Fig. 10. Consequently, the DC fault analysis,
detection and location methods for the VSC-MTDC systems
are also applicable for the HB-MMC based MTDC systems.
1) DC fault analysis: Reference [59], [108]–[112] comprehensively investigate the DC fault types and characteristics for general VSC-MTDC systems, which are also
suitable for MMC-MTDC. In reference [113], the behaviors of the conventional two-level VSC-HVDC system
and the HB-MMC HVDC system are analyzed under DC
line-to-earth faults. It shows that the system’s behaviors
depend on the earthing configuration and the HB-MMC
HVDC system has a lower fault current due to the
reduced size of the DC-link capacitance. Reference [114]
investigates DC network partition and minimum fault
protection cost for the MTDC systems.
2) DC fault detection and location: Fast fault detection and
location are critical for protection of the MTDC systems.
Reference [115] presents a fault detection and location
method by using the DC current direction to identify
the potential faulty line. However, its limitation is that
the faulty line can only be identified in the process
of clearing the fault. References [116]–[119] present
several fault detection and location methods based on
transient voltages and currents during fault condition,
which are not sensitive to high-resistance faults [120].
Reference [121]–[125] proposes a class of wavelet and
traveling-wave based fault detection and location methods, which can accurately identify fault type and location. To improve the robustness of fault detection and location, reference [126], [127] employ several intelligent
methods in the process of fault detection and location,
such as artificial neural network (ANN) and support
vector machines (SVMs). Reference [128] presents a
model-based fault detection and location method based
on Kalman filters.
LSC
idcf
+
l
vupa
iupa
RSC
ia
vta
vtb
vtc
Vdc
ilowa
vlowa
o
l
m
Fig. 10. Equivalent circuit of the HB-MMC system during a DC-side shortcircuit fault [31].
B. DC Fault Interruption
In reference [115], AC-side CBs coordinating with DC fast
switches are employed to clear DC grid faults. However, due
to the slow response of AC CBs, this solution cannot meet
the fast response requirements of DC grids. The MMCs with
embedded fault-blocking capability provide more choices for
DC fault interruption. Consequently, for the MMC-MTDC
systems, three options are feasible for clearing DC grid faults.
The first one is to use DC CBs to isolate the faulty lines while
continuing to operate the rest of the DC grid. The second one
is to use converter topologies with embedded fault-blocking
capability. The third one is to coordinate converters, CBs, and
other protection devices.
1) DC CBs
The basic operational principles and configurations of the
DC CBs have been investigated and reviewed in [129]–[134],
as shown in Fig. 11. The DC CBs need to create a zerocurrent switching to interrupt the fault current and dissipate the
energy stored in the system. The DC CBs can be categorized
as follows:
1) Mechanical DC CBs: The basic configuration of the
mechanical DC CBs includes a normal current path,
commutation path and energy dissipation path, as shown
in Fig. 11(a) [130]. During normal operation, the current
flows through the mechanical switch. When the fault
is detected, the mechanical switch is opened. An arc
voltage will commutate current from the mechanical
switch into the commutation branch, which can generate
current oscillation. When the arc is extinguished, the
fault current charges the capacitor to the system voltage.
Meanwhile the dielectric strength of the mechanical
switch recovers and is able to withstand the system
voltage. When the voltage is higher than the threshold
voltage of the varistor, the varistor conducts and absorbs
the energy stored in the line inductance.
2) Solid State DC CBs: The solid state DC CBs are
primarily implemented by various combinations of semiconductor switches and ancillary circuits with an ultrafast operating speed but high on-state losses [132].
3) Hybrid DC CBs: The hybrid DC CBs combine the con-
ZHANG et al.: MODELING, CONTROL, AND PROTECTION OF MODULAR MULTILEVEL CONVERTER-BASED MULTI-TERMINAL HVDC SYSTEMS: A REVIEW
L
Io
Circuit breaker
Is
Normal current path
Commutation path
Energy absorption path
Cc
Lc
Ic
Ia
Surge arrestor
(a)
L
Ic
Io
Circuit
breaker
Is
Ia
Commutation
element
Fast
disconnector
Fig. 13, all semiconductor switches in the FB-MMC system
are turned off under DC fault conditions, the fault current flows
through the freewheeling diodes and is blocked by the SM
capacitor voltages. In steady-state conditions, the amplitude
of the AC-side line-to-line voltage is less than the sum of the
capacitor voltages in the short-circuit loop, as shown in Fig.
13. Therefore, the fault-blocking capability of MMC can be
applied to AC and DC grids to interrupt fault.
Semiconductor
branch
D1
D1
D3
+
_
S1
Energy
absorption branch
Io
Ic
Ultra fast
disconnector
D3
+
_
S1
D4
D2
S4
S2
Load commutation switch
S3
D4
D2
(b)
L
347
S4
S2
(a)
(b)
D6
Circuit
breaker
Is
S1
Main dc breaker
(c)
S3
+
_ C1
S5
+
_
C2
S2
S4
D7
Fig. 11. (a) Basic configuration of mechanical DC CB, (b) general configuration of hybrid DC CB, and (c) typical specific configuration of hybrid DC
CB.
trollable semiconductor switches with fast mechanical
breakers. The general configuration and a typical configuration of the hybrid DC CBs are shown in Fig. 11 (b)
and (c), respectively. The commutation path is controlled
by the semiconductor switches and only operates during
the interruption interval [131], [134]. Reference [133]
analyzes the operational principles of the hybrid DC CBs
and summarized various configurations of the hybrid
DC CBs. In reference [135], a fast hybrid DC CB with
modular and scalable structure is presented.
The mechanical DC CBs are able to provide low loss
conduction paths and withstand the system voltage when they
are isolated. However, they take a long time (generally 10 to
100 ms) to clear fault [132]. The solid state DC CBs have
faster switching speed, which are able to clear faults in less
than 1 ms [131]. However, under normal operating conditions,
the power loss is much higher than that of the mechanical DC
CBs. The hybrid DC CBs have faster fault clear speed than
that of the mechanical DC CBs and lower power losses than
that of the solid state DC CBs.
2) MMCs with Fault-blocking Capability
The MMC with the HB SMs is the dominant topology
for HVDC applications. In case of a DC-side fault, the fault
current flows from the AC side to the DC side through the
freewheeling diodes. To solve this problem, various faultblocking SMs have been proposed and investigated, including
FB, UFB, CD, 3LCC, and 5LCC SM circuits [29]–[31].
Subsequent to a DC-side fault, when all of the semiconductor
switches of Fig. 12 are turned off, the capacitors can generate
reversed voltages to block the fault current flowing from
the AC side to the DC side. For instance, as shown in
(c)
S5
D6
S3
S1
+
C1 _
+
_
C2
S2
S4
SM2
SM1
(d)
Fig. 12. The fault current paths of various SMs: (a) FB, (b) UFB, (c) CD,
and (d) 3LCC SM circuits.
Fig. 13.
The fault current path of the FB-MMC [31].
348
CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 3, NO. 4, DECEMBER 2017
However, although the MMCs with fault-blocking SMs can
block the DC fault current, the increased number of semiconductor switches leads to extra power losses. Reference [31]
presents a hybrid design strategy to optimize the MMC design
in terms of cost, losses and fault-blocking capability.
3) Coordination of the MMCs with the DC CBs
The MMCs with fault-blocking SMs are not the most costeffective solution due to their high cost and loss [31], [136]. To
reliably isolate the faulty lines in the MTDC systems, DC CBs
are still required. However, interrupting the DC fault current
only by DC CBs is also not a cost effective solution due to the
large number of DC CBs required in meshed DC grids [137].
Therefore, to coordinate the MMCs with the hybrid DC
CBs is a potential solution. Reference [136] compares various
fault-blocking SMs and proposes a new topology of hybrid
DC CB. Reference [137] presents an assembly of DC CBs as
well as the corresponding control strategy. In reference [138],
a coordinated operation with the mechanical DC CBs and the
FB-MMC is proposed for the MTDC systems under DC fault
conditions.
VI. C ONCLUSION
The salient features of the MMC enable it to become the
primary building block for MTDC and DC grids. The MMCMTDC systems embedded into the present power system
will significantly enhance system reliability and efficiency,
support renewable energy integration and improve flexibility
and economy of power dispatching. Over the past few years,
the MMC-MTDC systems have become the most attractive
technology for construction of DC grids. However, there are
still several technical challenges in modeling, control, and
protection of the MMC-MTDC systems.
Although several modeling techniques have been investigated, such as averaged model and equivalent model, they are
not applicable for all MMC configurations and SM circuits
under various operating conditions. For large-scale power
systems, more efficient models need to be developed with
fast simulation speed. However, for the study of dynamic
behaviors, the models need to accurately simulate more state
variables with small time step, which leads to a high computational load. Therefore, there is a tradeoff between accuracy
and efficiency. The future control strategy of the MMC-MTDC
systems could coordinate the droop control with other controls
to improve both AC and DC system performances by addressing the challenges from contingencies in the AC system,
converter outages and power oscillations. Multi-functional
control configurations would be very promising as the overall
control strategy of the MMC-MTDC systems. Additionally, it
still needs effort to investigate the interaction of the internal
dynamics of the MMCs, the DC networks, and the AC grids
to optimize the operations of the MMCs and AC-DC systems.
The protection scheme for the MMC-MTDC systems is
still ambiguous and requires more investigation. Three major
protection strategies have been proposed to isolate the DC
faults in the MTDC grids. The protection strategies purely
based on DC CBs or purely based on fault-blocking MMCs do
not appear to be cost effective. Coordinating the MMCs with
the DC CBs becomes another trend. However, it is still not
clear which protection strategy has the superior performance
compared to the other available strategies. Therefore, there is
still a plenty of room for further development to address the
DC grid protection issues.
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352
CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 3, NO. 4, DECEMBER 2017
Lei Zhang (M’16) received his B.Eng. in electrical
engineering from Shandong University of Science
and Technology, Qingdao, China, in 2012. He received his M.Eng. degree in electrical engineering
from Shandong University, Jinan, China, in 2015.
From 2015 to 2016, he joined the School of Electrical and Electronic Engineering, Nanyang Technological University, where he worked as a Research
Associate. Currently, he is working toward his Ph.D
degree at Arizona State University, USA.
His research interests include modular multilevel
converters (MMC), energy storage systems and hybrid AC-DC microgrids.
Yuntao Zou received his B.S. degree in electrical
engineering from Northwest A&F University, Yangling, China in 2014 and his M.Eng. degree in electrical engineering from Xi’an Jiaotong University,
Xi’an, China in 2017. He is currently pursuing his
Ph.D. degree in electrical engineering at Arizona
State University, Tempe, Arizona, USA.
His research interests include system modeling
and control of MMC-MTDC systems, dynamic performance and stability of MTDC systems and integration and control of renewable energy sources.
Jicheng Yu (S’11) received his B.S. degree in
electrical engineering from Huazhong University of
Science and Technology, Wuhan, China in 2010
and his M.S. degree in electrical engineering from
Arizona State University (ASU), Tempe, Arizona,
USA in 2013. He is currently pursuing his Ph.D.
degree in electrical engineering.
His research interests include system modeling
and controls of HVDC systems, dynamic performance studies for power networks, application of
power electronic devices in energy systems and
virtual laboratory software development for remote education.
Jiangchao Qin (M’14–SM’17) received his Ph.D.
degree in electrical and computer engineering from
Purdue University, West Lafayette, IN, USA, in
2014.
He spent the summer of 2012 in the HighPower Energy Conversion Lab, GE Global Research,
Niskayuna, NY, USA, as a Research Intern. After
graduation, he joined the Georgia Institute of Technology, Atlanta, GA, USA, as a Postdoctoral Fellow.
He is currently an Assistant Professor at Arizona
State University, Tempe, AZ, USA. His research
interests include power electronics, applications of power electronics in power
systems and electric machines and drives.
Vijay Vittal (M’82–SM’87–F’97) received his B.E.
degree in electrical engineering from the B.M.S.
College of Engineering, Bangalore, India, in 1977,
his M.Tech. degree from the Indian Institute of Technology, Kanpur, India, in 1979, and Ph.D. degree
from Iowa State University, Ames, IA, USA, in
1982.
He is the Ira A. Fulton Chair Professor in the
Department of Electrical, Computer and Energy Engineering at Arizona State University, Tempe, AZ,
USA. He currently is the Director of the Power
System Engineering Research Center (PSERC) which is headquartered at
Arizona State University.
Dr. Vittal is a Member of the National Academy of Engineering.
George G. Karady (SM’70–F’78–LF’98) received
his masters and doctoral degrees in electrical engineering from the Technical University of Budapest.
He was appointed Salt River Chair Professor at
ASU in 1986. Previously, he was with EBASCO
Services where he served as chief consulting electrical engineer, manager of electrical systems and
chief engineer of computer technology. He was an
electrical task supervisor for the Tokamak Fusion
Test Reactor project in Princeton. Dr. Karady has
graduated dozens of doctoral and masters students.
He is an IEEE
Fellow and has published a book, several book chapters, and hundreds of
journals and conference papers. His expertise is in power electronics, highvoltage engineering, and power systems.
Di Shi (M’12–SM’17) received his B.S. degree from
Xi’an Jiaotong University, Xi’an, China, in 2007,
and his M.S. and Ph.D. degrees from Arizona State
University, Tempe, AZ, USA, in 2009 and 2012, all
in electrical engineering.
He currently leads the Advanced Power System
Analytics Group at GEIRI North America, Santa
Clara, CA, USA. Prior to that, he was a researcher
at NEC Laboratories America, Cupertino, CA, and
Electric Power Research Institute (EPRI), Palo Alto,
CA. He has published over 60 journals and conference papers and holds 14 US patents/patent applications. The One Energy
Management and Control (EMC) technology he developed has been successfully commercialized by NEC Corporation. He served as senior/principal
consultant for eMIT, LLC and RM Energy Marketing, LLC.
Zhiwei Wang (M’16) received his B.S. and M.S.
degrees in electrical engineering from Southeast
University, Nanjing, China, in 1988 and 1991, respectively. He is President of GEIRI North America, Santa Clara, CA, USA. His research interests
include power system operations and control, relay
protection, power system planning and WAMS.