Transmissão em Corrente Contínua em Ultra-Alta Tensão

Transcription

1 Transmissão em Corrente Contínua Panorama Atual e Perspectias Futuras no Brasil Siemens AG 2012 Energy Sector

2 Ultra-High Voltage Transmission Systems 2 Brazilian SC B4 Multiterminal HVDC Systems / VSC Dag Soerangr Siemens AG 2

3 Multiterminal HVDC Systems A multiterminal HVDC system is a system with more than two terminals connected to an HVDC grid. 3 3

4 Existing 3 Terminal HVDC Systems SACOI The QUEBEC - NEW ENGLAND HVDC System 4 4

5 Multiterminal with Line Commutated Conerters The operational flexibility of a multiterminal system with LCC conerters is restricted by the fixed current direction in the conerter. Power reersal is done by changing the polarity of the DC oltage. A polarity change will affect the entire system. Example: Equialent circuit with 3 conerters Ideal Circuit Restrictions on the LCC Circuit -R11 -R

6 LCC Monopole with polarity switching In order to make the power direction of one station independent of the others a polarity switching arrangement is required. Monopolar Arrangement 6 Increased Insulation Leels (same as upper part of the conerter) 6 1

7 LCC Bipole with polarity switching 800 kv -Q11 / -Q12 M M M The Insulation Leels are not affected by this switching arrangement 7 7 1

8 Switching elements for multiterminal systems High Speed High Voltage (HSHV) switch Each station pole group is connected to the DC line ia a High Speed High Voltage (HSHV) switch. Any shutdown sequence, Blocking, De-paralleling or ESOF of a group will mean bringing its DC current to zero and opening the HSHV switch kv HSHV Switch When a conerter group is tripped (ESOF) upon an internal fault the DC current will be brought to zero before the HSHV switch of the faulty group can be opened The "breaking capability" of a mechanical switch depends on I 2 L. 8 8

9 Switching elements for multiterminal systems Low Speed Polarity Reersal Switches Polarity Reersal Switches may be required to enable power for the different operating modes. Polarity Reersal Switches must be designed to proide a dielectric withstand oltage of 1600 kv dc (± 800 kv). 2 x 800 kv Disconnectors 9 9

10 LCC multiterminal with limited flexibility The number of conerters with polarity reersal switching determines the flexibility regarding power directions. Polarity Rerersal Polarity Rerersal

11 Control Concept for LCC point to point Ud (pu) Nom. TAP = DC Line Voltage Drop U d Rectifier Operating Point at l d =1.0 pu 0.8 Operating Point at l d =0.7 pu ld (pu) 11 1

12 Multiterminal with Voltage Sourced Conerters The operational flexibility of a multiterminal system with VSC conerters is not restricted by a fixed current direction in the conerter. Power reersal is done by changing the direction of the current indiidually in each conerter by arying the oltages. Example: Equialent circuit with 3 conerters Ideal Circuit NO Restrictions on the VSC Circuit -R11 -R

13 VSC Multiterminal DC Network Topology One Central Connection Point - HUB A ~ - - ~ D B ~ - - C ~ - Example: A: Strong AC grid B: Islanded Wind farm (may require a break-resistor) C: Islanded Load D: Weak AC grid w. black start requirement 13 13

14 VSC Multiterminal DC Network Topology Connection Points in Conerter Stations 14 14

15 Conerter Topology It seems that different ersions of Multileel Conerters will be the chosen conerter technology. The technologies seem to be basically compatible, but some questions need to be answered: Choice of grounding principle for the DC grid: - Symmetrical high impedance grounding - Effectiely grounded as in a bipole The systems which are currently in planning use the first principle, but larger systems are likely to require a solid grounding of the DC circuit. Choice of standard system oltages Conerter response to DC grid faults - Conerters with full bridge modules can control the DC oltage in order to force the fault current to zero and allow a fast recoery. - Conerters with half bridge modules can only operate within a limited oltage range and hae to be disconnected from the AC system by a circuit breaker or be separated from the faulty DC grid by an extremely fast switching deice with fault current limiting- and interruption capability

16 Conerter Topology This figure shows why a conerter with half bridge modules behaes differently than one with full bridge modules

17 DC Grid Topology The current flow will be determined by relatiely small differences between the conerter oltages. The power flow can be analyzed by assuming the oltage of one conerter to be fixed. The difference of this oltage and the other conerter oltages will be a linear function of their currents. ( (n-1) x (n-1) matrix ) P 1 I 1 P 4 ~ - U 1 -R 1 -R 4 I 4 U 4 - ~ ~ P 2 I 2 - U 2 -R 2 -R 3 I 3 P 3 - U 3 ~ An oerall grid control function is required. This can be realized in a controller hardware with communication to each conerter controller. A limited functionality must be maintained during communication failure

18 24 DC Grid Topology In a meshed grid the power flow may be restricted by the indiidual DC lines or cables. The grid controller must estimate and monitor the line currents

19 Steady State Operation Different control strategies hae been ealuated. It seems that droop control with oltage-power droop, with or without dead band, or oltage-current droop will be the preferred solution. Voltage Current Droop U I U I Voltage - Power Droop I U di du = I = 0 k ( U U0 ) ( I I ) = k k 0 U 0 Uo Io k Limits C&P C&P ~ ~ P 1 I 1 - U 1 P 2 I 2 - U 2 -R 1 Grid Control -R 4 -R 2 -R 3 Uo Po k Limits I 4 U 4 P 4 I 3 P 3 U ~ ~ C&P C&P P U I di du = P = = 0 k ( U U 0 ) ( P P ) P U = 0 k ( k U P ) k U U 0 U 1 U 0 0 It is obious that the oltage-current characteristic of the DC grid seen from a single conerter will be the result of the conerter reference alues of the other conerters and the DC resistances

20 Voltage-power droop with two conerters Example with extreme alues of k Station A: k = i.e. constant oltage the slope reflects 2.5% line resistance Rectifier Station B: k = 0 i.e. constant power Inerter DC Voltage 20 DC Current The green lines are oltage -, current - and power limits 20

21 Choice of Droop Characteristics The choice of droop characteristics and limits will depend on many factors: The number of conerters which shall contribute to the oltage control The limitations of the connected AC grids including dynamic limits - A wind farm or an isolated load will determine the actie power. The conerters connected to such AC grids cannot contribute to the DC oltage control. (k = 0) The DC line or cables ability to withstand temporary oeroltage The DC line resistance alues The expected system response to single conerter run-backs or trips during communication failure or slow communication. The acceptable deiations resulting from oltage measuring errors and uncertainty in DC line resistances. The characteristics of a gien conerter may include a dead band to stabilize the power against small oltage fluctuations, oltage limits with a different droop, i.e. a high alue of k and/or the time staged reduction of limits. Dynamic behaior with and without communication must be studied

22 DC fault clearing DC line fault clearing is a challenge in a DC grid. In a point to point system or a small system with half bridge conerters it may be acceptable to trip the conerters with the AC breakers of all conerters. In a conerter system with high impedance grounding the healthy cable must be discharged by means of a mechanical switch after a single pole fault. In a system with oerhead lines a recoery might be attempted by recharging the entire system. The recoery sequence will be slow. If the recoery is unsuccessful or if only cables are used the faulty section must be disconnected after the trip and a restart of the remaining system may be attempted

23 DC Fault Clearing Larger DC grids would ideally require selectie fault clearing proided by extremely fast protection and line switching deices. In a meshed grid this could minimize the impact on the connected AC grids. A system with full bridge conerters can proide a fast reduction of the DC oltage in order to extinguish the fault current and then allow for a recoery after haing isolated the faulty line by means of fast mechanical switches. AC side Conerter Circuit Breaker Conerter with Fault Current Controlling Capability DC side Conerter Circuit Breaker DC Feeder Circuit Breaker

24 Studies and Verification A large number of studies are required to ensure the reliable operation of the system during steady state conditions and during contingencies. The aailability of realistic and erified models of all parts of the system is essential. This is a challenge in multi endor systems because the endors see the models as intellectual property. Possible solutions are Main studies by owner or third party Standardized models A simulator with RTDS representation of the oerall system and the conerters enhanced with the real controllers with hardware and software proided by the different endors. The complexity of the conerter models is a real challenge

25 Status and Future HVDC Grids are at the Starting Point HVAC is the role model mature technologies based on experience from more than 100 years Grid Codes for system design and operation in place standardized solutions allow competitie supply chains for all equipment innumerous world market for all kinds of equipment HVDC today more than 200 GW installed transmission capacity worldwide tailor made solutions based on dedicated customer requirements nearly all systems are single endor point-to-point connections world wide a small number of projects per year, mostly turn key Multiterminal and HVDC Grids Two LCC three-terminal LCC schemes in operation, one under construction First VSC based schemes under construction or in planning 25 25

26 What HVDC Grids need in the Short Term.. on the Application side common design requirements (DC Grid Codes, specifications).. on the Technology side agreed operating and control principles including load flow control short circuit currents and earthing fault detection and fault clearing common functional requirements for equipment including AC / DC conerters fault clearing and fault isolation deices (e.g. DC circuit breakers or conerters with disconnectors) HVDC Grid Controllers and interfaces to Conerter Station Controllers Current Actiities CIGRÉ SC B4 WGs 56, 57, 58, 59, 60 DKE/CENELEC Study Group "Technical Guidelines for first HVDC Grids" 26 26

27 Thank you for your attention! 27 27