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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 PowerWorld, Inc 1 9 - References ............................................................................................................................................. 49 PowerWorld, Inc 2 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. PowerWorld, Inc 3 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. PowerWorld, Inc 4 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. PowerWorld, Inc 5 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 PowerWorld, Inc 6 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 - PowerWorld, Inc 7 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) PowerWorld, Inc 8 Figure 3.1 – VSC (a) Topology (b) –Controls Figure 3.2 – DC Voltage and Current –Typical HVDC Cable PowerWorld, Inc 9 Figure 3.3 – Voltage and Current – Cable Near Superconducting Conditions PowerWorld, Inc 10 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. PowerWorld, Inc 11 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 PowerWorld, Inc 12 Figure 4.3 – Six-Node Test System Figure 4.4. One-line Six-Node Test System Topology PowerWorld, Inc 13 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% PowerWorld, Inc 14 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 Xd Inertia Resistive Load Basic Generation Characteristics Xd 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) PowerWorld, Inc 15 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 PowerWorld, Inc 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 PowerWorld, Inc 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 0.5-0.6 520 0.150.25 0.070.12 0.2-0.3 0.5-0.6 Turbine Time Constant Gov Time Constant Resistive Load 35% Xd 25% Xd 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 17 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. PowerWorld, Inc 18 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. PowerWorld, Inc 19 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. PowerWorld, Inc 20 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 PowerWorld, Inc 21 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. PowerWorld, Inc 22 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. PowerWorld, Inc 23 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. PowerWorld, Inc Parameter Value TQmeas 0.02 KQ 0.03 TQ 0.20 TVmeas 0.01 KV 0.03 TV 0.20 24 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. PowerWorld, Inc 25 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. PowerWorld, Inc 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 PowerWorld, Inc 27 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 PowerWorld, Inc 28 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. PowerWorld, Inc 29 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. PowerWorld, Inc 30 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. PowerWorld, Inc 31 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). PowerWorld, Inc 32 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 PowerWorld, Inc 33 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 PowerWorld, Inc 34 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 PowerWorld, Inc 35 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. PowerWorld, Inc 36 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) PowerWorld, Inc 37 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. PowerWorld, Inc 38 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. PowerWorld, Inc 39 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. PowerWorld, Inc 40 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. PowerWorld, Inc 41 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. PowerWorld, Inc 42 PowerWorld, Inc 43 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) PowerWorld, Inc 44 Figure 8.1(d) Figure 8.1(e) Figure 8.1(f) PowerWorld, Inc 45 Figure 8.1(g) Figure 8.1(h) 4000 km Base Case Figure 8.2(a) Figure 8.2(b) PowerWorld, Inc 46 Figure 8.2(c) Figure 8.2(d) Figure 8.2(e) Figure 8.2(f) PowerWorld, Inc 47 Figure 8.2(g) Figure 8.2(h) To include cases near superconducting conditions and also with three converters. PowerWorld, Inc 48 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 PowerWorld, Inc 49 PowerWorld, Inc 50