U
I
EE529
VSC HVDC
Sessions 46
 The majority of existing HVDC systems are based on

LCC.
VSCs are have advantages in several applications
»
»
»
»
Independent control of active and reactive power injection
Provide dynamic voltage support to the ac system
Less harmonic filters requirement
Easier for multiterminal HVDC
 Also disadvances
» Losses
» Lower Vdc and MW ratings (so far)
» DC faults
Spring 2015
VSC HVDC Station
U
I
Converter station
Voltage Source(d)
Converter - VSC
Phase
Reactor
EE529
Strong or Weak Systems
Sessions 46
Dynamic Voltage Control
Underground Transmission
Up to ±150kV, 550MW
Up to ±300kV, 1100MW
Transmission Cable
AC bus
Dry DC Capacitor
AC filters
DC Capacitor
Control
system
IGBT Valves
Spring 2015
1
U
I
HVDC Converter Arrangements
EE529
Sessions 46
Thyristor Module
Conventional
HVDC
Single Double Quadruple
Valve Valve
Valve
Thyristors
Submodule
Chip
StakPak
VSC Based
HVDC
IGBT Valve Stacks
Cable Pair
Spring 2015
U
I
Underground Cable Systems
with VSC HVDC

EE529
Sessions 46
Economic

No distance limitation
Full utilization – no reactive power
 Two cables v three cables for AC




Land


Light, flexible and simpler design
Timely permitting
No induced circulating currents
Half the losses
Easier transport and installation
Sea
Spring 2015
2
Underground Cable
Systems (cont)
U
I

Sessions 46
Reliability






Land
EE529
No cable overloads possible
Dynamic reactive power support
Congestion relief
Isolate disturbances
Share ROW without increasing exposure
Black-start capability
Sea
Spring 2015
U
I
Control of VSC Based HVDC
Transmission (most existing)
EE529
Sessions 46
AC Line Voltages OPWM
u DC1
uAC-ref1
-
uAC1
u DC2
i
i
u
uDC-ref1DC-ref2
-
+
AC
voltage
control
qref1
+
PWM
internal
current
control
uAC2 uAC-ref2
-
DC
voltage
control
AC
voltage
control
+
DC
voltage
control
pref1 pref2
PWM
internal
current
control
q ref2
Principle control of HVDC-Light
Spring 2015
3
U
I
Comparison of Reactive Power EE529
Sessions 46
Characteristics
Spring 2015
7
Comparison of Reactive Power EE529
Sessions 46
Characteristics
P-Q Diagram
Active Power (p.u.)
U
I
Operating Area
Reactive Power (p.u.)
HVDC VSC Operating Range
8
Spring 2015
4
U
I
Topology Options
EE529
Sessions 46
 Simplest three-phase VSC
topology to build
 Consist of six IGBTs with six antiparallel diodes.
 Produce two voltage levels
 Use PWM switching technique to
control dc voltage and ac phase
voltage.
Two-Level, Six-Pulse Bridge
VSC
Spring 2015
U
I
Multilevel Converters
EE529
Sessions 46
 Additional lGBTs/diodes
 Plus clamp diodes
 Produce three voltage levels (or





more)
Use PWM switching technique
Less harmonics’ amplitudes
More efficient than a Two-Level
Bridge VSC
Less device stress
More complex and expensive than
a Two-Level Bridge VSC
Three-Level, Neutral Point Clamped
VSC
Spring 2015
5
U
I
EE529
Modular Multilevel ConverterSessions 46
 Preferred VSC topology in new orders
 Consist of 100s of single phase VSC
modules in series that switch in and
out to control voltage magnitude and
phase
Half-Bridge
Sub-Module
Two Sub-Modules MMC
Spring 2015
U
I
Advantages of MMC
EE529
Sessions 46
 Operate with lower switching frequency
 Scalable to higher voltage and power levels by changing the number
of modules
 Lower harmonic distortion
» Potential implement without filters
 Very high efficiency that can reach up to 99%
Spring 2015
6
U
I
EE529
Sessions 46
 What are MTDC systems?
 Advantages of Parallel configuration
over series configuration
» Transmission losses
 Disadvantages of Parallel
configuration
» Need DC circuit breaker to isolate faulted
lines
» Otherwise shut down entire system with AC
protection
Series MTDC System
Configuration
 Potential for grids
Parallel MTDC System
Configuration
Spring 2015
U
I





EE529
Sessions 46
VSCs using PWM and MMCs can control two variable independently
Control was done in synchronous dq frame to improve response
Inner current regulators and out control
Current cross coupling term may have small effect
Impact on ac systems
Spring 2015
7