Download 2 - Considerations on the Operation of SCDC Cables Fed by

EPRI SCDC Cable Stability Studies
Report 1 – Draft 01
Considerations on the Operation of a VSC Based Multi-Terminal
SCDC System
Date: December 15, 2008
Revision No. 0.2
Authors: Dr. Paulo Ribeiro, Dr. Thomas L. Baldwin, and Thomas Overbye
Produced by PowerWorld Corporation
PowerWorld Corporation
2001 South First Street
Champaign, IL 61820
Phone: (217) 384-6330
Fax: (217) 384-6329
Table of Contents
1 - Executive Summary.................................................................................................................................. 3
2 - Considerations on the Operation of SCDC Cables Fed by Multiple Voltage Source Converters ............. 5
Introduction .............................................................................................................................................. 5
Transient Oscillations of DC Cables ......................................................................................................... 6
Ramp Rate Limits for Long Cables ........................................................................................................... 6
3 - Simplified PSCAD Modeling / Simulation Results .................................................................................... 8
4- Basic Case Definitions and System Parameters for Stability Studies...................................................... 11
Overall Test System Description ............................................................................................................ 11
Proposed Cases for evaluation of SCDC Cable performance ................................................................ 11
Cases for the Seven-Node Test System ................................................................................................. 14
Cases for the Six-Node Test System....................................................................................................... 16
5 - VSC-HVDC Controls ................................................................................................................................ 18
Introduction ............................................................................................................................................ 18
Dynamic Model of the VSC-HVDC Converter Station ........................................................................... 19
Model of VSC-HVDC Control System ..................................................................................................... 20
Transient Damping Control .................................................................................................................... 23
Operation Modes ................................................................................................................................... 25
AC System Topologies - City Bulk Power Networks .............................................................................. 28
6 - Review of Exponent EMTP Study Results .............................................................................................. 29
7 - VSC Functional Specs for SCDC Multi-Terminal Operation .................................................................... 30
7.1. SCOPE .............................................................................................................................................. 30
7.2. APPLICABLE CODES AND STANDARDS ........................................................................................... 31
7.3. REQUIREMENTS .............................................................................................................................. 31
8 - Appendix – PSCAD Modeling and Simulations ...................................................................................... 44
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9 - References ............................................................................................................................................. 49
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1 - Executive Summary
A medium voltage (~100kV) superconducting DC line fed by multiple voltage source converters
capable transporting several Giga Watts of power over a couple thousand miles may become a
reality in the future, as both high temperature superconducting and power electronics
technologies advance and the cost of implementing such a system may become within
reasonable reach.
However, several technical, reliability and economic issues continue to challenge such projects.
Among them one could list:
1 – Complexity of controls systems for multi-terminal operation;
2 - Need for high power capacity dc circuit breakers;
3 – Mitigation of transients associated with faults and system events;
4 – Improved efficiency of forced commutation converters and higher power handling
capabilities;
5 – Improved power electronics reliability and lower costs
The control systems for a multi-terminal VSC based SCDC cable system needs to be fully
assessed for all possible system conditions, events and AC and DC faults. Although there
seems to be good reasons to believe they will perform much better than current source linecommutated converters, more detailed modeling and simulations with the actual current and
power levels proposed for the SCDC need to be carried out. Similarly DC circuit breakers can
be constructed, but for the current levels proposed by the SCDC project advanced technologies
will be required.
The transients associated with the superconducting low ac resistance of the cable, though
challenging can be achieved by superior controls plus active and passive filtering. Alternatively,
advances in power electronic s will allow soon VSC converter with ratings within the GW range
by the use of modular-multi-level technologies. However, efficiency, reliability and cost will
continue to be a challenge.
The first objective of this study was to perform preliminary analyses and develop possible
topologies for an SCDC multi-terminal system and determine the parameters necessary for
subsequent dynamic electro-mechanical stability study. Two basic topologies with 6 and 7
nodes / stations were proposed for stability system studies.
Other objectives included investigation of operational and control issues, review the results of
the electromagnetic transient studies conducted and provided by Exponent, Inc., develop a
functional specification for the converter and perform any additional simulations to support /
substantiate findings.
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Section 2 presents some considerations on the operation of long DC cables fed by multiple
voltage source converters and ramp rates of long and large DC cables. Add a couple of
sentences - Section 3 describes a PSCAD modeling and simulation results of a simplified model of a
multiple VSC DC cable system fed by separate AC systems. The modeled setup to study the
interaction and dynamics of a SCDC system. Results indicate that severe transients need to
be dealt with and may impose significant limitations on the operation of a multi-terminal system.
Section 4 and 5 proposes two basic system topologies, parameters and control models for
stability studies to be performed by PowerWorld. A West Coast and a Mid-West topologies for
both 5 and 10 GW are considered. The system proposed was modified and used by
PowerWorld for the stability simulations presented on Report ###.
Section 6 reviews the EMTP simulations performed by Exponent. The results seemed to be
consistent, but as expected only addresses the electromagnetic transients of an individual
converter. Detailed modeling of the operation of multiple terminals will be required to determine
the feasibility of the integrated operation with very little damping on the DC side.
Section 7 presents a preliminary function specification for a typical VSCs operating in a multiterminal SCDC configuration. Detailed controls for integrated operation will have to be
developed by the manufacturer. Section 8 presents an Appendix with additional PSCAD
simulation results.
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2 - Considerations on the Operation of SCDC Cables Fed by Multiple
Voltage Source Converters
Introduction
Voltage sourced converter (VSC) based HVDC transmission has come into use in the last ten
years. These systems are based on a modular design, reducing the installation time. Such a
system could apply high temperature superconducting transmission cables and expanded into
new arenas where the cable losses in a standard VSC transmission system would have cost
disadvantages [1].
Multi-terminal HVDC, however, is rarely used due to the cost of the additional converter
stations, complexity of the control system, the need for DC circuit breakers, etc.
The traditional line-commutated current source converters are very difficult to coordinate without
communications and hard to guarantee a stable response to a disturbance if the communication
is out. Voltage source converters work better and maybe able to overcome some of the
difficulties of the line-commutated technology.
The biggest challenges, however, are the need for some sort of DC circuit breaker and the
control issues. BPA tested HVDC breakers on the Pacific Intertie in the late 80's and it can be
done, but probably not with present technologies for the current levels discussed for the
superconducting SCDC super-grid studies. However, recently the manufacturers for VSC
HVDC have marketed their systems as working well for mesh connected multi-terminal systems.
Converter control, however, is seen as a significant problem for the multi-terminal systems, that
is how to adjust the system when converters hit their limits. The existing systems have one
converter act as a the master voltage controller (for the parallel system) or the master current
controller (for the series system). But they struggle with handling converter limits and get into
complicated modes of operation and mode switching.
A distributed voltage control system based on voltage droop for the converters acting as
rectifiers has been proposed and it seems that it would work better for a superconducting
system with a mesh connected or parallel system since all of the terminals will have the same
steady-state DC voltage. However, detailed modeling of a truly multi-terminal system with
power and current levels proposed for the SCDC system will need to be developed.
If power converter costs continue to fall and will be much less significant by the time the
superconducting cables could be built, plus forced-commutated converter losses can be
reduced significantly (close to line-commutated converters) with the use of Silicon Carbide
devices such system could be realizable.
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Other issues discussed below cover transient oscillations and ramp rates on DC cables of large
magnitudes
Transient Oscillations of DC Cables
Long DC cable system (>1000 km)
The behavior of the converters is affected by the propagation delays introduced by the
cables. The propagation time constant is similar to other cable systems with a small
frequency-dependent ac resistance provided by SCDC cables. Slower transient signals
(<1 kHz) have little attenuated along the cable length. Normal mismatches between the
converter impedance and the cable characteristic impedance cause most of the
transient signal energy to reflect back into the cable.
Ramp Rate Limits for Long Cables
A proper voltage profile is maintained at the converter with voltage control.
The voltage at a converter with current control sags and swells due to the inductance of
the SCDC cable. The propagation delay and mismatch of the cable’s characteristic
impedance with the converter’s impedance results in a decaying oscillation
The ramp rate of the current affect the magnitudes of the voltage sags, swells, and
ringing. See Table 2.1 below.
Table 2.1
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Impact of Cable Length
Figures 2.1 to 2.3 show the DC voltage ripple for different cable lengths. The simulation
parameters considered were: 80 kV, 10 kA, two-terminal SCDC cable, current control
terminal: 2 kA / sec ramp rate.
Figure 2.1 -
Figure 2.2 -
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Figure 2.3 -
3 - Simplified PSCAD Modeling / Simulation Results
A simplified model of a two/three VSCs (one operating as a rectifier and the others as an
inverters) connected by a DC cable ad fed by two separate AC systems was modeled to study
the interaction and dynamics of the system. The resistivity of the DC cable was varied to
investigate the impact of a superconducting cable on the response of the system to system
events and faults. Figure 3.1 shows the basic topology of the VSC and some of the controls
configuration.
Figures 3.2 and 3.3 shows the PSCAD simulations of the operation of the VSC converters. The
results indicate that the system to work adequately as far as the control of voltage is concerned.
However the current transients on the DC line for near superconducting conditions after an AC
fault are extremely severe and needs to be dealt with creative solutions in order to achieve a
stable operation. Additional simulation results are presented in the Appendix.
(a)
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Figure 3.1 – VSC (a) Topology (b) –Controls
Figure 3.2 – DC Voltage and Current –Typical HVDC Cable
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Figure 3.3 – Voltage and Current – Cable Near Superconducting Conditions
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4- Basic Case Definitions and System Parameters for Stability
Studies
Based on realistic topologies of the WECC and the Midwest areas, two basic test systems are
proposed. Several basic cases are proposed to investigate the impact of the superconducting
DC cable on the associated and interconnected AC network. Two basic generation conditions
(5GW and 10GW) are considered. Conclusions that may be drawn from the use of this test
system, however, should not imply actual behaviors on the WECC or Midwest areas.
Considerations on transient oscillations of dc cables, transient analysis and a functional
specification are included.
Overall Test System Description
Figures 4.1 and 4.2 show the generic test system and corresponding one-line diagram for the
WECC case seven-node multi-tap DC test system with seven ac sub-systems. Figures 4.3 and
4.4 shown the Midwest alternative with six-node multi-tap DC test system and ac sub-systems.
Proposed Cases for evaluation of SCDC Cable performance
For both the seven-node and six-node test systems there are several ac sub-systems, which
can strongly or weakly interconnected on the AC side. The proportion of the generation import
and export by each system are indicated with the majority of the DC transmission capacity being
delivered to the final delivery point. All ac sub-systems are assumed to have local generation
and voltage control capability. In order to reduce the complexity and size of the simulation each
of the ac sub-system should be represented by a reduced number of nodes (15 to 20
transmission buses) with an adequate representation of generation dynamics and load
characteristics. The initial basic cases involve three phase faults on the AC system and loss of
all or half of the inverters at each DC/AC stations. Other cases will look at DC faults and opencircuit DC Cable cases.
Other possible AC system topologies are also suggested.
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Figure 4.1. Seven-node test system.
Sub-System 1
Sub-System 2
Sub-System 3
Sub-System 4
Sub-System 5
AC Sub-System
Sub-System 6
VSC Rectifier/Inverter Station
SCDC
Sub-System 7
Figure 4.2. One-line Seven-Node Test System Topology
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Figure 4.3 – Six-Node Test System
Figure 4.4. One-line Six-Node Test System Topology
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Cases for the Seven-Node Test System
Table 4.1 shows the basic cases proposed. The cases are divided according to SCDC
Transmission Capacity (5GW or 10GW), Three Phase Faults on the AC Sub-Systems, and
Converter Substation Contingencies.
Table 4.2 shows the SCDC Line Data for the topology selected, Table 4.3 shows the SCDC
Power Delivery Distribution among the 6 converter stations / ac systems, Table 4.4 shows the
SCDC Delivery Capability versus Load System Characteristics of each corresponding system,
Table 4.5 lists the Local Load Composition and Generation Characteristics and Table 4.6 lists
the Terminal Station Model Parameters.
Table 4.1 – Basic Cases
Case #
Description
Observations
Sub-System 1 sending power
Sub-Systems 2 to 7 receiving or injecting power under normal
conditions and with Three Phase Faults
Sub-System 1 sending power
Systems 2 to 6 receiving or injecting power under normal
conditions with Three Phase Faults
Sub-System 1 sending power
Sub-Systems 2 to 6 receiving or injecting power under normal
conditions Case 1 with Loss of Inverter Station
Sub-System 1 sending power
Sub-Systems 2 to 7 receiving or injecting power under normal
conditions with Loss of all or fraction of Inverter
1
2
3
4
5GW Condition
10 GW Condition
5GW Condition
10 GW Condition
Table 4.2. SCDC Line Data
Line Identifier
Distance
1-2
350 miles
2-3
Inductance uH/m
Capacitance
Resistance
0.2
TBD
TBD
150 miles
0.2
TBD
TBD
3-4
150 miles
0.2
TBD
TBD
4-5
550 miles
0.2
TBD
TBD
5-6
100 miles
0.2
TBD
TBD
6-7
350 miles
0.2
TBD
TBD
Table 4.3. SCDC Power Delivery Distribution
Location
Percentage of Total SCDC
Transmission Capacity
Absorption
Generation
5 or 10 GW
Sub-System 1 (Local Generation)
80%
Sub-System 2 (Local Generation)
20%
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Sub-System 3 (Local Generation)
10%
Sub-System 4 (Local Generation)
10%
Sub-System 5 (Local Generation)
15%
Sub-System 6 (Local Generation)
15%
Sub-System 7 (Local Generation)
50%
Table 4.4. Local Load Composition / Generation Characteristics
5-20
40%
25%
35%
5-20
40%
25%
35%
5-20
40%
25%
35%
5-20
50%
25%
20%
5-20
60%
20%
20%
5-20
60%
20%
20%
5-20
0.070.12
0.070.12
0.070.12
0.070.12
0.070.12
0.070.12
0.070.12
Turbine
Time
Constant
0.150.25
0.150.25
0.150.25
0.150.25
0.150.25
0.150.25
0.150.25
Gov Time
Constant
Xd
Inertia
Resistive
Load
Basic Generation Characteristics
Xd
System 1
(Generation)
System 2
(Local Generation)
System 3
(Local Generation)
System 4
(Local Generation)
System 5
(Local Generation)
System 6
(Local Generation)
System 7
(Local Generation)
Induction
Motor
Location
Electronic
Load
Load Composition
0.2-0.3
0.5-0.6
0.2-0.3
0.5-0.6
0.2-0.3
0.5-0.6
0.2-0.3
0.5-0.6
0.2-0.3
0.5-0.6
0.2-0.3
0.5-0.6
0.2-0.3
0.5-0.6
Figure 4.5. Model of the Terminal Station
Table 4.5. Terminal Station Model Parameters
Parameter
Value
DC Bus Terminal
Capacitance
T.B.D.
Inverter
Number of Inverters
MVA Rating
2
100 %
(based on station rating)
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Nominal Operating
MVA
50 %
Transformer
MVA Rating
150 %
(based on station rating)
Resistance
1%
Reactance
7.5 % to 10 %
Cases for the Six-Node Test System
Table 4.6 shows the basic cases selected. The cases are divided according to SCDC
Transmission Capacity (5GW or 10GW), Three Phase Faults on the AC Systems, and
Converter Substation Contingencies.
Table 4.7 shows the SCDC Line Data for the topology selected, Table 4.8 shows the SCDC
Power Delivery Distribution among the 5 converter stations / ac systems, Table4.9 shows the
SCDC Delivery Capability versus Load System Characteristics of each corresponding system,
Table 4.10 lists the Local Load Composition and Generation Characteristics and Table 2.6 lists
the Terminal Station Model Parameters.
Table 4.6 – Basic Cases
Case #
Description
Observations
Sub-Systems 1 and 3 sending power
Sub-Systems 2 & 4 to 6 receiving or injecting power under
normal conditions with Three Phase Fault s
Sub-Systems 1 and 3 sending power
Systems 2 & 4 to 6 receiving or injecting power under normal
conditions with Three Phase Faults
Sub-Systems 1 and 3 sending power
Sub-Systems 2 & 4 to 6 receiving or injecting power under
normal conditions with Loss of Inverter Station
Sub-Systems 1 and 3 sending power
Sub-Systems 2 & 4 to 6 receiving or injecting power under
normal conditions with Loss of all or fraction of Inverter Station
1A - E
2A - E
3A - E
4A - E
5GW Condition
10GW Condition
5GW Condition
10GW Condition
Table 4.7. SCDC Line Data
Line Identifier
Distance
1-2
220 miles
2-3
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Inductance uH/ m
Capacitance
AC Resistance
0.2
TBD
TBD
160 miles
0.2
TBD
TBD
3-4
130 miles
0.2
TBD
TBD
4-5
250 miles
0.2
TBD
TBD
5-6
180 miles
0.2
TBD
TBD
16
Table 4.8. SCDC Power Delivery Distribution
Percentage of Total SCDC
Transmission Capacity
Absorption
Location
Generation
5 or 10 GW
System 1 (Local Generation)
60%
System 2 (Local Generation)
20%
System 3 (Local Generation)
40%
System 4 (Local Generation)
20%
System 5 (Local Generation)
20%
System 6 (Local Generation)
40%
Table 4.9. Local Load Composition / Generation Characteristics
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0.150.25
0.070.12
0.2-0.3
0.5-0.6
520
0.150.25
0.070.12
0.2-0.3
0.5-0.6
520
0.150.25
0.070.12
0.2-0.3
0.5-0.6
Turbine
Time
Constant
Gov Time
Constant
Resistive
Load
35%
Xd
25%
Xd
40%
Basic Generation Characteristics
Inertia
System 1
(Generation)
System 2
(Local
Generation)
System 3
(Local
Generation)
System 4
(Local
Generation)
System 5
(Local
Generation)
System 6
(Local
Generation)
Electronic
Location
Induction
Motor
Load Composition
50%
25%
25%
520
0.150.25
0.070.12
0.2-0.3
0.5-0.6
60%
20%
20%
520
0.150.25
0.070.12
0.2-0.3
0.5-0.6
60%
20%
20%
520
0.150.25
0.070.12
0.2-0.3
0.5-0.6
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5 - VSC-HVDC Controls
Introduction
The principal characteristic of VSC based HVDC stations is its ability to independently control the
reactive and real power flow at each of the AC systems. In contrast to line-commutated HVDC
transmission, the polarity of the DC link voltage remains the same with the DC current being reversed to
change the direction of power flow.
A typical VSC–HVDC station is shown in Figure 5.1. The HVDC station consists of a VSC connected to dc
cables. Today a maximum power handling of the system of at least 1000 MW at 300 kV dc is available
from Siemens and ABB. The VSCs are three-phase, two-level, six-pulse bridges, employing IGBT power
semiconductors. The relative ease with which the IGBT can be controlled and its suitability for highfrequency switching, has made this device the better choice over GTO and thyristors. The converters are
connected to phase reactors, which are connected to the ac system through conventional power
transformers. The reactors are used for controlling the active and reactive power flow by regulating the
current through them and for reducing the high frequency harmonic content of the ac line current
caused by the switching of the VSCs. Tuned shunt filters are needed to reduce the high frequency
switching ripple on the ac voltage and current. The transformers transform the ac system voltage to a
value suitable for the converter. The dc capacitors provide an energy buffer to keep the power balance
during transients and for reducing the voltage ripple on the DC side.
PAC  jQAC
I DC

P
I AC VSC
U DC
AC

VAC

VVSC
VSC
Figure 5.1. One-line diagram of a VSC-HVDC converter station.
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The dc capacitor size is characterized as a time constant , defined as the ratio between the stored
energy at rated dc voltage and the rated apparent power of the converter

2
0.5 C DC U DC_Nominal
S Nominal
(1)
where denotes the nominal dc voltage and is the nominal apparent power of the converter.
Dynamic Model of the VSC-HVDC Converter Station
The converter and controls presented in this report are for dynamic/electro-mechanical transient
analysis. The appropriate time step size for these models are in the range of 1 to 10 milliseconds. This
work provides a simplified control model, which handles the higher level controls that solve for the AC
terminal voltage phasor of the voltage-source converter. Algebraic equations map the dynamic voltage
phasor to the active and reactive power flows on the AC side of the converter and the direct current and
voltage on the other side.
The consequence of using a simplified model is primarily the elimination of lower level architecture such
as the firing control. Details such as the phase-lock loops, dq-axis representation, and pulse width
controls are condensed into equivalents.
A simplified one-line diagram of a VSC-HVDC converter station is shown in Figure 1. Its dynamic
equivalent circuit is shown in Figure 5.2. At the system interface bus, voltage, VAC is the fundamentalfrequency phasor. On the secondary side of the transformer, the VSC’s AC terminal bus has the output
voltage phasor VVSC.
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PAC  jQAC
VAC  AC

I AC
X VSC
RVSC
PVSC
VVSC  AC   
RDC 2
U DC
CDC
RDC 2
LDC 2
I DC
LDC 2
Figure 5.2. Dynamic equivalent circuit of a VSC-HVDC converter station.
VVSC is the fundamental phasor of the converter’s output voltage. AC, is the phase angle of voltage VAC. 
is the phase angle between VVSC and VAC (also denominated as the shift or power angle relative to VAC).
PAC and QAC are the active and reactive power absorbed from the AC side of the system by the converter.
PDC is the active power injected onto the DC side of the converter. Assuming no losses in the converter,
it is equal to PVSC. IAC is the phasor current injecting from the converter to the AC system. RVSC and XVSC
are the resistance and reactance of the interfacing transformer. CDC is the capacitor on the DC side of the
converter. RDC and LDC represent the resistor and inductor of the DC transmission line.
Model of VSC-HVDC Control System
Based on the PWM control technology for HVDC-Light, the amplitude and phase angle of the VSC output
voltage are regulated independently and rapidly by the modulation ratio M and the power angle SVC.
With these two control variables, the VSC can control the active and reactive powers in all four
quadrants.
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AC
Voltage
Control
VAC_ref
QSch
QAC
Reactive
Power
Control
Inner
Current
Control
Converter
Voltage
Limit
DC
Voltage
Control
Phase
Current
Limit
Q/VAC Mode
UDC_ref
Active
Power
Control
PSch
PAC
P/UDC Mode
Figure 5.3. Hieratical Control Diagram for the VSC-HVDC Converter.
The output voltage, VVSC, is dependent on the DC bus voltage, VDC, and the timing of the sinusoidal PWM
switching sequence. The following relationship models the AC output voltage of the converter.
VVSC  VSC 
M
2
VDC  AC   
(2)
From the calculated converter voltage phasor, the AC system can be solved. The active power injection
to the converter is mapped to the DC side of the converter. From the measured DC voltage, the DC
current can be computed.
There are two sets of optional control objects for each VSC-HVDC converter station. They are (i) the
active power, PAC, or the DC bus voltage, UDC and (ii) the reactive power, QAC, or the amplitude of the AC
bus voltage VAC. Under normal operation conditions, each converter can control its reactive power
independent of the other converter stations. However, the active power inject into the VSC-HVDC inner
DC system must be balanced. Active power flowing out from the DC network must equal the active
power flowing into the DC network less any losses in the network. Any difference in flows will cause the
DC voltage to increase or decrease. In order to achieve the active power balance automatically, one of
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the converters must select the DC-voltage mode as the control object. The other converters can control
active power at any value within the capacity limits.
The transfer-function block diagram of the four primary controllers for the converter are all based on
the proportional integral (PI) regulator. These controllers are shown in Figure 4. Four measurements and
four set points provide input to the controls. Measurements and their corresponding set points include:

Active power flowing on the AC side and the scheduled power flow,

Reactive power flowing on the AC side and the scheduled power flow,

Voltage magnitude on the AC side and the reference AC voltage, and

Voltage magnitude on the DC side and the reference DC voltage.
For the primary control functions, there are four reference values, PSch, QSch, Uref and Vref, for the
corresponding control objects. Time constants, TPmeas, TQmeas, TUmeas and TVmeas, model the time delays for
the measurements. The PI controls use proportional factors: KP, KQ, KU , and KV, and integral time
constants: TP, TQ, TU, and TV. The remaining parameters are associated with the active power damping
during transient events, and are described next. Suggested values for the parameters are provided in
Tables 1 to 3.
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Damping Control Enable Switch
0
1
1  sTdamp
1
sTW
1  sTW
K VSC
 1  sT1 


 1  sT2 
n
Pmax
Pmin
Active Power Damping Control
–
1
1  sTP meas
PAC
–
1
1  sTU meas
1
1  sTQ meas
VSC
KU 
–
+
1
1  sTV meas
VAC_Ref
1
sT U
DC Bus Voltage Control
KQ 
–
+
1
sT Q
P/Vdc Control
Mode Switch
1
0
Reactive Power Control
QAC_Sch
VAC
1
sT P
Active Power Control
UDC_Ref
QAC
KP 
+
PAC_Sch
UDC
+
M
KV 
–
+
1
sT V
1
0
Q/Vac Control
Mode Switch
AC Bus Voltage Control
Fig
Figure 5.4. Transfer-function block diagram of the simplified VSC-HVDC controls.
Transient Damping Control
In order to improve the damping characteristic on low frequency oscillations, it is necessary to add
another damping controller to the VSC-HVDC. This control adjusts the active power transmitted by the
VSC-HVDC during transient events. The active power of the AC line at one of the converter stations is
selected to provide the damping control. The parameters of the damping controller are shown in Table
2.
Table 5.1. Active Power/DC Bus Voltage Control Parameters.
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Parameter
Value
TPmeas
0.02
KP
0.052
TP
0.115
TUmeas
0.005
KU
0.5
TU
0.04
Table 5.2. Active Power Damping Control Parameters.
Parameter
Value
Tdamp
0.005
Tw
10.0
KVSC
1.5E3
T1
0.55
T2
0.2
n
2
Pmax
0.5
Pmin
0.5
Table 5.3. Reactive Power/AC Bus Voltage Control Parameters.
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Parameter
Value
TQmeas
0.02
KQ
0.03
TQ
0.20
TVmeas
0.01
KV
0.03
TV
0.20
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Operation Modes
The VSC-HVDC converter station can be operated in several modes; examples include:
1. Reactive power control. Each converter operates independent of each other. The reactive power
order could be selected to best suit the ac system, or could be modulated with an auxiliary input
from some external control.
2. AC voltage control. Each converter operates independent of each other. The ac voltage order is
normally constant, but may also be modulated with an auxiliary input from some external
control.
3. Active power modulation. One converter acts as an active damping control, frequency control
etc.
4. Passive net operation. Operation on a passive system or at black start with no other voltage
source present.
For the test cases of the SCDC cable project, the following initial operating modes have been defined.
Both the  and M modes of operation are defined for each converter station. As needed one of the
converter stations may be used to damping oscillations by activating the active damping controls.
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Table 5.4. Control Modes for the Converters Stations in the Six-Node Test System.
Converter
-Mode
M-Mode
Sub-system 1
V_DC
Q_AC
Sub-system 2
P_AC
Q_AC
Sub-system 3
P_AC
Q_AC
Sub-system 4
P_AC
Q_AC
Sub-system 5
P_AC
V_AC
Sub-system 6
P_AC
Q_AC
Sub-system 7
P_AC
Q_AC
Table 5.5. Control Modes for the Converters Stations in the Six-Node Test System.
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Converter
-Mode
M-Mode
Sub-system 1
V_DC
Q_AC
Sub-system 2
P_AC
Q_AC
Sub-system 3
P_AC
Q_AC
Sub-system 4
P_AC
V_AC
Sub-system 5
P_AC
V_AC
Sub-system 6
P_AC
Q_AC
26
Super Cable Model 1
Super Cable Model 2
Algebraic Equations
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AC System Topologies - City Bulk Power Networks
General Characteristics
Typical Metropolitan Area
Per Unit Base: 1 million residents
Land Area: 400 square miles per 1 million residents
Typical Power Facts
Utility Customers: 480,000 per 1 million residents
Average Power Consumption: 2000 MW per 1 million residents
Network Facts
Bulk Transmission Level: 345 kV, 500 kV, or 765 kV
4 to 8 Bulk Power Substations and/or Generation Stations per
2000 MW of load that have step down transformers to service
the metropolitan area transmission level
Metropolitan Area Transmission Level: 120, 138, or 161 kV
60 Transmission Substation per 2000 MW of load that service sub-transmission
or distribution networks
5 to 10 of the 60 substations serve as primary switching stations
Basic transmission configurations
ring networks
mesh networks
Scalable simplified city models
per 1 million residents
with 10 to 20 major buses
Examples
San Antonio’ ring network
AEP point-to-point network
Figure 4.10 – Alternative System Topologies
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6 - Review of Exponent EMTP Study Results
The results PowerPoint presentation made by Exponent at the November 2008 meeting during
the Superconductivity Utility Applications Conference (full report has not been provided) showed
a variety of results regarding the converter AC and DC sub-systems, models, filters / harmonics
etc during normal operations. Specifically the following tasks were carried out:
1 - Steady state operation: Start-up and shutdown
2 - DC cables: Fault, and Transients
3 - DC filter: Harmonic Mitigation and Damping, Cable Loss Minimization
4 - Sensitivity analysis: Different cable length, Systems with different short circuit
capacity
The results seem to corroborate some of the simulations performed by this team and indicate
similar concerns.
However the model is simplified (as expected) and only considers two converters (sending and
receiving) separated by a long cable rather than multiple converters, which is one of the major
challenges for the operation and control of such a system.
Thus, the results are valid for the verification and design specification of local converter
transient requirements. However, for integrated operation with multiple converters it will be
necessary to include a more detailed system model to fully assess the impact of SCDC cable
and investigate the controllability issues mentioned in the introduction.
Regarding the harmonic distortion results for the DC currents are very consistent but presented
only the harmonic distortion for the AC currents. Harmonic voltage distortion on the AC system
is also necessary as it provides a more representative parameter when investigating the impact
of the converters on the AC system.
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7 - VSC Functional Specs for SCDC Multi-Terminal Operation
7.1. SCOPE
This document defines the performance and design requirements for a Voltage Source Power Conversion
Subsystem (VS-PCS), as part of a Superconducting Cable DC System (SCDC). The SCDC system is
intended to transport and absorb and deliver power at several points which will be interconnected with the
AC Transmission Grid. The system will also provide continuous reactive power for utility network
support.
This specification is intended to convey to those familiar with utility operations the unique requirements
for operation of a VS-PCS in the utility environment. However, the specification is not intended to
convey all of the requirements for the VS-PCS Subsystem. The VS-PCS Subsystem supplier is
responsible for the design of the VS-PCS and is expected to do all that is necessary to ensure that the VSPCS is fit for its intended purposes. The control system and special active and passive filtering capability
for the operation of multiple VSC connected together via superconducting cable will need a special
feature of these converters.
The VS-PCS includes all components between the dc power terminals on the superconducting cable and
the connections to a 3-phase, 60 Hz line. The VS-PCS shall be self-commutated, and shall contain a
voltage source ac-to-dc converter, ac filters, shorting switches and buses, internal wiring and buses,
instrumentation, and a controller as specified herein.
The VS-PCS shall provide independent four-quadrant (active and reactive) compensation for variations in
the network voltage and frequency and SCDC cable power variations. The VS-PCS shall measure the
voltage, current, frequency, and frequency rate of change on the ac line, and shall compute appropriate
levels of active and reactive power to inject or absorb from the line in each of several modes of operation.
Operator selection of VS-PCS operating modes, and transmission of VS-PCS operating status will be
accomplished by discrete or analog signals transmitted between the VS-PCS and the utility Supervisory
Control and Data Acquisition (SCADA) system via a utility provided Remote Terminal Unit (RTU).
The VS-PCS shall be designed and constructed for integrated power utility operation and minimum life
cycle cost. Life cycle cost shall include the costs of capital, operating inefficiency, downtime, and
maintenance.
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7.2. APPLICABLE CODES AND STANDARDS
The following Codes and Standards are part of this specification to the extent referenced herein.
2.1 Codes and Standards
Unless otherwise specified, all VS-PCS equipment, components, and services shall meet the design, test,
system performance, and quality control requirements of this specification and applicable ANSI, IEEE,
NEMA, NESC, NEC IEC, OSHA, ASTM, AEIC, ICEA, UL, ASME, EIA, FCC, and NFPA .
7.3. REQUIREMENTS
3.1 Item Definition
The VS-PCS includes the functions specified herein, and interconnecting cables, wiring, and
ancillary parts as required to meet the performance requirements when connected to the magnet, a
utility provided SCADA RTU, and the utility power grid.
3.1.1 Item Diagram
The VS-PCS shall contain the major elements shown on the Electrical Interconnection Diagram
(TBD).
3.1.2 Subsystem Interfaces
3.1.2.1 Utility AC Line Interface
a. The VS-PCS shall have a physical interface with the utility grid at the secondary side of a 230 kV
(primary) / 115 kV (secondary) interface transformer. The utility interface transformer and
breakers will be provided by the utility.
b. Utility line characteristics and VS-PCS performance shall be measured at a Point of Common
Coupling (PCC) on the 230 kV side of interface transformer provided by the utility. Current and
potential measurement transformers will be provided by the utility
3.1.2.2 Auxiliary Power Interface
a. The utility shall supply auxiliary power for the at 4160 V, 480 V, 240 V, and 120 V.
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b. The VS-PCS shall have an Uninterruptible Power Supply (provided by VS-PCS supplier) to allow for
safe disconnect from the utility interface transformer and controls operational (for at least 45
minutes).
c. The utility will also supply DC power for the trip mechanism of the ac breakers.
3.1.2.3 Command Interface
The VS-PCS controller shall interface to a utility provided SCADA RTU as a collection of signal
discrete or analog. The RTU shall transmit operator initiated mode commands as a unique
combination of dry contact switch closures. In turn, the VS-PCS shall continuously indicate
operating mode by a similar unique combination of signal discretes transmitted to the RTU
interface.
3.1.2.4 SCDC Cable Interfaces
a. The VS-PCS shall include the dc buses and switchgear required to connect to the magnet dc
terminals.
b. Redundant signals shall be transmitted from the SCDC Cable System to the VS-PCS.
c. The inductance of the Cable will be TBD Henrys, and the maximum energy stored TBD MJ.
3.1.2.5 Site Interface
a. The location of VS-PCS units shall be as shown on diagram TBD. The VS-PCS dc buses, high-voltage
switchgear and cooling units shall be located outdoors, and an indoor area shall be provided which
accommodates the VS-PCS components, their installation and subsequent maintainability.
b. The environment provided within the VS-PCS facility shall be within the limits to be defined. The
VS-PCS shall not require cooling fluids or special environmental control from the facility.
c. The VS-PCS grounding shall be integrated to the substation grounding grid.
3.1.2.6 Data Interface
The VS-PCS shall have a front panel connector for accessing a VS-PCS generated data stream. The
data stream shall consist of VS-PCS measurements and status data.
3.1.2.7 Refrigeration System (RS)
The VS-PCS shall have an interface with the RS via a Warning signal discrete. The VS-PCS shall
automatically cancel active power interchange on receipt of the Warning signal indicating excessive
dewar pressure from the SCDC Cable Refrigeration System (RS).
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3.1.2.8 Cable Monitoring System (CMS)
The VS-PCS shall have an interface with the CMS via an Emergency signal discrete.
3.2 Characteristics
3.2.1 Performance
3.2.1.1 Rated Power
The VS-PCS shall be capable of continuously delivering to the utility grid, at nominal voltage, rated
power of TBD MVA in a four-quadrant form. Due to large ratings of power injected or absorbed
by the AC Sub-Systems the VS-PCS may be composed of several modules and divided into separate
groups feeding the two separate cables.
3.2.1.2 Efficiency
a. The VS-PCS shall have a one-way efficiency of not less than 95% at rated active power during
discharge or charge, including losses on both the VS-PCS and auxiliary power systems.
b. The VS-PCS shall have a one-way efficiency of not less than TBD %.
3.2.1.3 Utility AC Line Characteristics
3.2.1.3.1 Voltage
The VS-PCS shall be capable of continuous operation on the utility line (at reduced power) for
voltage deviations between +10/-20% of nominal, and shall tolerate without disconnect a deviation
of -50% for at least 150 cycles under transient conditions. It should be understood that the transient
voltage conditions represent fault or switching events in the ac network, which can affect one, two
or all three phases to a varying degree. These events are typically associated with significant
voltage distortion. The VS-PCS shall provide support to the ac network within the inverter
allowable transient voltage and current limits. If the VS-PCS operation ceases during the network
transient, the VS-PCS system must respond as required in subsection 3.2.1.5.1 upon return of the
voltages to within the +10 to -20% band as specified herein.
3.2.1.3.2 Frequency
a. The VS-PCS shall be capable of continuous operation on the utility line with frequency deviations
of 60 ± TBD Hz. The VS-PCS shall automatically synchronize to the utility line.
b. The VS-PCS shall be capable of continuous operation on the utility line with a frequency rate of
change not exceeding TBD Hz/sec. Active power exchange will not occur if the rate of frequency
change is greater than TBD Hz/sec. Note that the specified frequency changes represent the
electromechanical modes of the power system. Fault and switching operations will cause sudden
phase changes in the system voltages, which shall not be interpreted as a rate of change of
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frequency as specified in this subparagraph. Distorted voltage wave forms arising from
geomagnetic induced currents (GIC), transformer energizations or other switching events shall not
effect the frequency measurements.
3.2.1.3.3 Voltage/Current Harmonics, Unbalance, and Subsynchronous Oscillations
a. The total harmonic voltage and current distortion introduced by the VS-PCS on the 230 kV (or
above) line shall not exceed the recommendations of ANSI/IEEE 519. These limits shall be applied
at the point of common coupling (PCC) with the utility, for voltage unbalance up to 3%.
b. The VS-PCS shall be designed to preclude exciting subsynchronous resonance and/or oscillations in
the utility system.
c. Protection systems shall consider unbalances where may arise for many reasons including those
created by geomagnetically induced currents (GIC). Also consideration to carrier frequency
interference should be given.
d. The VS-PCS shall be capable of continuous operation on the utility line with voltage unbalances up
to TBD%, and provide correction (within VS-PCS capabilities) for unbalances.
3.2.1.3.4 AC Line Protection
a. The VS-PCS shall include an ac circuit breaker capable of interrupting the maximum fault power at
the utility line physical interface.
b. Ground fault protection shall be provided by the utility on the high-voltage windings of the utility
interface transformer.
c. The utility interface transformer shall include sudden pressure, over-temperature, over-current,
and differential relay sensors. Over-current and differential relay fault conditions shall cause a VSPCS shutdown.
d. The VS-PCS shall contain provisions to protect against transient voltage surges from switching,
lightning, and similar causes. The utility interface transformer shall contain a grounded
electrostatic shield between the high voltage and low voltage windings to prevent the capacitive
let-through of steep waveforms caused by lightning and switching surges.
e. The VS-PCS shall detect a reverse phase sequence and disconnect or not connect if a reverse
phase sequence is detected.
3.2.1.4 SCDC Cable Characteristics
TBD
3.2.1.4.1 Voltage, Current, and Grounding
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a. The rated operational dc voltage across the magnet terminals shall be 100 kV. For insulation
protection purposes an absolute maximum voltage of 250 kV or TBD. The operating voltage limit
shall be software programmable at a level less than the rated level.
b. The SCDC Cable impedance has a number of resonances which can amplify harmonic voltages and
currents. Due to the SC nature of the capable very little damping is provided by the cable.
Adequate DC filtering will be required. The component of the VS-PCS voltage (at the resonance
frequencies) applied to the SCDC Cable should not exceed TBD V.
c. The DC current shall not exceed TBD kA when the SC Cable is fully loaded, or be less than TBD kA.
The SC Cable shall be considered fully loaded when the current is within TBD of the maximum
current.
d. The maximum allowable DC voltage at SC Cable terminal to ground shall be TBD. The VS-PCS shall
include a sensor to monitor the coil potential with respect to ground.
3.2.1.4.2 DC Isolation and Over-Current Protection
a. Two-pole un-fused no-load-break disconnect switches shall be provided for isolating the VS-PCS
from the SCDC Cablel and shorting switch for VS-PCS maintenance. The isolation switches shall
be operable manually or by the VS-PCS controller.
b. The design of the VS-PCS shall ensure that the current in the SCDC Cable does not exceed the
maximum value.
c. Faults within the VS-PCS, including commutation failures, shall be cleared by the VS-PCS overcurrent protection device.
d. The VS-PCS DC link capacitor used in the voltage source converter shall be a self-fusing type for
protection and shall be capable of being rapidly charged and discharged as required when subject
to the ac system voltage transients The capability of the capacitors to survive a short circuit from
the maximum DC voltage shall be demonstrated in a type test.
3.2.1.4.3 SCDC Cable Protection
a. The VS-PCS shall contain a switch connected in series with an energy dump resistor. The switch
shall be a normal-open fault-to-close type, with a reaction time of less than TBD seconds after
receipt of a detection signal from the SCDC Cable supervision system to damp the energy on the
cable.
b. The switch shall have a DC voltage rated more than TBD kV, and a life of more than 100 switching
cycles.
3.2.1.5 Operating Modes (TO BE MODIFIED TO FIT SCDC APPLICATION)
3.2.1.5.1 Mode Description
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The VS-PCS shall operate in the following modes in response to commands as specified in
3.2.1.7.2:
a. Mode 1 - Variable VAR Compensation (Operator Selected)
When this mode is selected, the VS-PCS shall operate as a variable source of reactive power with no
real power exchange (except for losses). The VS-PCS shall calculate and apply reactive power,
within its operating range,to minimize the voltage deviation from a preprogrammed voltage
setpoint.
Alternatively, the setpoint at the VS-PCS front panel can be a reactive power setpoint. These limits
shall be user programmable parameters. The reactive power delivered by the VS-PCS for voltages
within the range for continuous operation (-20% to +10%) as defined in 3.2.1.3.1 shall be
constrained only by the maximum and minimum voltage and current limits for inductive and
capacitive reactive power operation.
For voltage deviations between -20% to -50% the VS-PCS shall remain connected to the grid but
will not respond to voltage deviations.
In this operating mode (variable var compensation), the VS-PCS shall apply independent voltage
control to each of the three phases to minimize any unbalance in the three phase voltages
(minimize negative sequence components).
b. Mode 2 - Frequency Support (Automatic Mode, Operator Enabled/Disabled)
When this mode is enabled, the VS-PCS shall automatically determine the need for frequency
support in the event of a network disruption.
This mode of operation shall interrupt operation in Modes 1, 3 or 6. This mode shall terminate
when spinning reserve is able to maintain the line frequency at a software programmable level.
The VS-PCS shall initiate frequency support when system frequency falls below a threshold that is
software programmable (but not less than TBD Hz) and the frequency rate of change is less than
TBD Hz/sec. Power shall be supplied to the line at a level determined by the VS-PCS until the
frequency recovers to a value that is also software programmable (but not greater than 5TBD Hz).
The time to initiate frequency support after crossing the low-frequency threshold shall not exceed
TBD msec. Also, this response time shall be met after recovery of the ac system from disturbances
in the ac network.
When the VS-PCS active power output is less than maximum rated power, the VS-PCS shall, while
maintaining the active power transfer, use available capacity of the VS-PCS for reactive power
control as defined in Variable VAR Compensation Mode.
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The VS-PCS shall inject or absorb power into the SCDC system at a TBD charging rate.
c. Mode 3 - Combined Operation (Operator Selected)
When this mode is selected, the VS-PCS shall operate with independent control of active and
reactive power in a four-quadrant form up to the rated apparent (MVA) power.
The VS-PCS shall calculate and apply the real and reactive power levels required to stabilize the
line frequency and voltage. The frequency deviation range and frequency rate of change for
which the VS-PCS will respond for frequency stabilization purposes shall be software
programmable.
In order to stabilize the system frequency, the VS-PCS shall inject or absorb real power in an
steady or oscillatory form.
The VS-PCS shall automatically switch to Variable Var Compensation Mode on receipt of a
Warning from the SCDC supervisory system.
d. Mode 4 - Disconnect / Reconnect (Automatic)
The VS-PCS shall automatically disconnect from the utility bus (secondary side of the utility
interface transformer) for voltage or frequency deviations in excess of TBD or from an internal
faults. After the voltage or frequency return to within the limits the VS-PCS shall automatically
reconnect to the line after a (which is software programmable within the limits of TBD seconds) to
support the system in the mode which was selected before the disconnect.
This mode of operation shall interrupt any mode, except Mode 9.
e. Mode 5 - Startup / Restart (Manually Selected from VS-PCS Console)
When this mode is selected, the VS-PCS shall connect to the secondary side of the utility interface
transformer. The VS-PCS shall inject power into the SCDC system.
f.
Mode 6 - Standby (Operated Selected)
When this mode is selected, the VS-PCS shall close the electronic shorting switch and remain
connected to the utility line but shall not provide active or reactive power in response to system
deviations.
g. Mode 7 - Manual Disconnect (Operated Selected)
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When this mode is selected the VS-PCS shall discharge all the energy on the SCDC Cable system to
the utility grid at a programmable power level, disconnect from the secondary side of the utility
interface transformer when the maximum current is reached, discharge the dc capacitor and close
the mechanical coil shorting switch.
h. Mode 8 - Manual Shutdown, Discharge (Manually Selected from VS-PCS Console)
When this mode is selected, the VSPCS shall discharge the SCDC Cable and then shall close the
mechanical shorting switch and disconnect from the utility line. The discharge shall be at an
initial programmable constant power output.
i.
Mode 9 - Emergency Shutdown, Discharge (Automatic or Operator Selected)
In response to a signal from SCDC Cable supervisory system the VS-PCS shall automatically activate
the dumping of the SCDC Cable energy.
3.2.1.5.2 SCDC Cable Discharge Requirements
a. The VS-PCS shall be capable of tracking the SCDC Cable current and controlling the DC discharge
voltage to produce a constant power output. The power level shall be variable up to the rated
power.
c. In case of operation in Manual Shutdown, Discharge (Mode 8), the VS-PCS shall calculate and
apply discharge power profiles to minimize frequency and voltage deviations in the utility system.
The VS-PCS shall discharge the coil with a power level not to exceed TBD MW and with a droop
characteristic between TBD% and TBD % that shall be software programmable.
3.2.1.5.3 SCDC Charge Requirements
a. During initial coil charging, the charging rate and the value of current at the end of a charging
interval shall be programmable at the VS-PCS front panel and limited to values specified in
3.2.1.4.1.
b. During SCDC Cable injection, the VS-PCS controller shall automatically initiate and carry out the
power transfer required. The charging rate and current shall be software programmable and
limited to values specified in 3.2.1.4.1.
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3.2.1.5.4 Operating Mode Changes
a. The time to initiate any mode after selection, or to change from any selected mode to another
selected mode, shall not exceed TBD seconds unless otherwise specified.
b. The VS-PCS shall sustain operation in the selected mode until a command is received to change to
a new mode, or until initiation of Frequency Support, Emergency Shutdown, or a SCDC Cable
supervisory system
3.2.1.6 Auxiliary Power
a. Auxiliary power at required voltage levels will be supplied for VS-PCS components such as fans and
the control system. The power required to operate the VS-PCS system at rated load shall not
exceed TBD kW on a continuous basis as specified in the ICD. The VS-PCS system shall not be
damaged by loss of auxiliary power for an indefinite period.
b. The VS-PCS shall safely disconnect from the utility line in the event that auxiliary power is lost.
3.2.1.7 Control and Instrumentation
3.2.1.7.1 General
a. Master control of the SCDC system facility will be implemented through the utility provided RTU,
which is connected to the VS-PCS. A slave capability for mode selection shall be provided at the
VS-PCS controller front panel.
b. VS-PCS-generated status discrete shall be transmitted to the RTU interface, and shall be accessible
at the VS-PCS controller front panel. VS-PCS data displays shall be adequate to present system
status and fault conditions, and to permit timely detection and isolation of problems and
identification of faulty units at the lowest field-repairable level. The VS-PCS front panel shall
conform to IEEE Standards.
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c. The VS-PCS controller shall prevent any commands from operating the VS-PCS in a manner that is
unsafe, or potentially damaging to the VS-PCS, the SCDC Cable system, or the utility.
d. All SCDC Cable operation and other parameter values necessary for verification of operation and
performance shall be stored in non-volatile memory such that the VS-PCS can resume operation
without reloading data after a loss of line or auxiliary power, or after a shutdown due to a fault.
Also logging the time and specific operator inputs shall be required for diagnostic and performance
analysis..
e. The design of the VS-PCS (including wiring configuration) shall be such as to prevent power circuits
from interfering with control and logic circuits. Logic or control printed circuit boards shall not
contain high voltage or high current circuits. Wiring associated with logic functions shall be twisted
pair(s) shielded cables.
3.2.1.7.2 VS-PCS Controls
a. The VS-PCS shall provide for manual selection of all modes. A key lockout of all front panel
functions shall be provided.
b. All control functions shall be entered with a keyboard at the VS-PCS front panel. Operator
initiated mode commands and variable var compensation (var or voltage levels) shall be received
by way of the SCADA RTU interface.
3.2.1.7.3 VS-PCS Instrumentation and Displays
a. The VS-PCS shall measure the line voltage, current, frequency and frequency rate of change with
sufficient accuracy that SCDC Cable system response to disruptions shall be capable of maintaining
the line frequency and voltage within the programmable limits.
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b. The VS-PCS shall measure, display on the VS-PCS front panel and make accessible at least the
following data:
1) DC current at SCDC Cable terminals
2) Average dc voltage at SCDC Cable terminals
3) Average voltage across link capacitor
4) AC voltages
5) AC currents
c. The VS-PCS shall transmit the following:
1) Current VS-PCS operating mode discretes.
2) Go/No Go (Pass / Fail) discrete
3) ac and dc switchgear discretes.
d. Overall resolution of measurement accuracy shall be TBD % of full scale reading. The time
resolution and range of the measurements shall be consistent with transient and trend analysis,
considering the expected time variations and parameter ranges.
e. Digital meters shall be used to continuously display voltage and current at the storage coil
terminals, and lights on the front panel shall be used to display the status of the ac and dc
switchgear. Other status and diagnostic data shall be displayed on CRT screens that collect data of
similar type and function. Operating mode and status data shall be available continuously on a
single screen.
f.
All status indicator lights shall include a circuit to permit positive testing via a single front panel
push-button.
g. The instrumentation signals shall be accessible during operation to portable diagnostic test
equipment (e.g., oscilloscope) without requiring bypass of the safety interlock system.
h. All settable values shall be resettable from the front panel.
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i.
All meters and sensors shall be easily accessible for calibration.
j. Transient data necessary for VS-PCS fault diagnostic and performance analysis shall be stored in a
circulating memory and shall be accessible at the VS-PCS front panel. After a fault event data
collected shall be automatically downloaded to permanent storage.
3.2.1.7.4 Fault Conditions
a. The VS-PCS shall enter Mode 4 and set the VS-PCS Fault discrete in the event of any of the
following conditions:
1) AC interface conditions out of tolerance.
2) AC or DC faults within the VS-PCS, unless isolation of the failed module is possible, and then
continued operation at reduced power operation is possible.
3) Reverse phase sequence.
4) VS-PCS overtemperature or loss of cooling, unless overtemperature is limited to one module and
isolation of the affected module is possible, then continued operation at reduced power operation is
possible.
5) Loss of VS-PCS auxiliary power for more than TBD seconds.
6) Opening of VS-PCS cabinet doors or panels.
7) Loss of VS-PCS controller function.
b. Emergency commands from either the magnet, the VS-PCS front panel, or SCADA/RTU shall override any mode of operation.
c. The VS-PCS shall not resume operation after an fault shutdown until the cause of the fault has
been removed and manual reset has been performed. It shall be possible to lock out the manual
reset capability by means of a keylock or padlock on the front panel.
d. When enabled by software selection, the VS-PCS shall automatically attempt one restart after an
internal fault disconnect has occurred. The restart attempt shall occur following a softwareprogrammable delay of TBD to TBD seconds. Additional restarts will require a manual VS-PCS
front panel reset as well as removal of the cause for the disconnect.
e. In the event of disconnect or shutdown, the operation of all isolation switchgear on both the ac
and dc connections to the VS-PCS shall be independent of the VS-PCS controller (i.e., implemented
directly within the ac and dc switchgear). The displays of switchgear status on the VS-PCS front
panel shall also be independent of the VS-PCS controller.
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8 - Appendix – PSCAD Modeling and Simulations
The results below show the machine angles, real / reactive power and the DC current and voltage for a
2000 km and 4000 km between converters at typical HVDC cable parameters and near superconducting
conditions.
2000 km Base Case – typical HVDC cable parameters (Figures 8.1 to 8.2)
Figure 8.1(a)
Figure 8.1(b)
Figure 8.1(c)
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Figure 8.1(d)
Figure 8.1(e)
Figure 8.1(f)
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Figure 8.1(g)
Figure 8.1(h)
4000 km Base Case
Figure 8.2(a)
Figure 8.2(b)
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Figure 8.2(c)
Figure 8.2(d)
Figure 8.2(e)
Figure 8.2(f)
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Figure 8.2(g)
Figure 8.2(h)
To include cases near superconducting conditions and also with three converters.
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9 - References
[1] A superconducting DC transmission system based on VSC transmission technologies
Venkataramanan, G.; Johnson, B.K.; Applied Superconductivity, IEEE Transactions on
Volume 13, Issue 2, Part 2, June 2003 Page(s):1922 - 1925
[2] Analysis of wide area integration of dispersed wind farms using multiple VSC-HVDC links
Gonzalez-Hernandez, S.; Moreno-Goytia, E.; Anaya-Lara, O.; Power Electronics and Motion
Control Conference, 2008. EPE-PEMC 2008. 13th, 1-3 Sept. 2008 Page(s):1784 - 1789
[3] VSC transmission control under faults, Lamont, L.A.; Jovcic, D.; Abbott, K.; Universities
Power Engineering Conference, 2004. UPEC 2004. 39th International, Volume 3, 6-8 Sept.
2004 Page(s):1209 - 1213 vol. 2
[4] Improvement of transient stability in AC system by HVDC Light, Hu Zhaoqing; Mao
Chengxiong; Lu Jiming; Transmission and Distribution Conference and Exhibition: Asia and
Pacific, 2005 IEEE/PES, 2005 Page(s):1 - 5
[5] Performance analysis of a hybrid multi-terminal HVDC system, Fuan, X.F.; Shijie Cheng;
Electrical Machines and Systems, 2005. ICEMS 2005. Proceedings of the Eighth International
Conference on Volume 3, 27-29 Sept. 2005 Page(s):2169 - 2174 Vol. 3
[6] Power control applications on a superconducting LVDC mesh, Johnson, B.K.; Lasseter,
R.H.; Adapta, R.; Power Delivery, IEEE Transactions on Volume 6, Issue 3, July 1991
Page(s):1282 - 1288
[7] Expandable multiterminal DC systems based on voltage droop, Johnson, B.K.; Lasseter,
R.H.; Alvarado, F.L.; Adapa, R.; Power Delivery, IEEE Transactions on, Volume 8, Issue 4,
Oct. 1993 Page(s):1926 - 1932
[8] High-temperature superconducting DC networks, Johnson, B.K.; Lasseter, R.H.; Alvarado,
F.L.; Divan, D.M.; Singh, H.; Chandorkar, M.C.; Adapa, R.; Applied Superconductivity, IEEE
Transactions on, Volume 4, Issue 3, Sept. 1994 Page(s):115 - 120
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