Multi-terminal HVDC operation in a weakly interconnected system: results from Best Paths Demo 3

Transcription

1 Multi-terminal HVDC operation in a weakly interconnected system: results from Best Paths Demo 3 E. Ciapessoni, D. Cirio, A. Iaria, A. Pitto, M. Rapizza RSE HVDC International Workshop Operational experience and technological development for applications worldwide Venice, Italy, March 28-30, 2017

2 BEST PATHS EU FP7 Project DEMO 3: SACOI HVDC 3-terminal interconnection, 300 MW, 200 kv monopolar LCC with sea electrodes Complete rehabilitation foreseen in the Italian grid development plan and included in the ENTSOE TYNDP 2016 Demo 3: Good laboratory for promoting new HVDC technologies! Demo leader TERNA SACOI 1967: Sardinia- Mainland Italy 1987: 50 MW tap station in Corsica Converters TOSHIBA EUROPE System issues RSE TERNA Submarine cables NEXANS Overhead lines and Insulation DE ANGELI RSE TERNA Land cables NEXANS 2

3 SACOI connects 2 countries Different market areas 2 (or 3) electrical AC islands SACOI features Temporary DC faults Unidirectional sea electrodes Aspects of Sardinian system Large RES penetration Another large HVDC link to mainland Italy (2x 500 MW LCC) SACOI as «embedded» HVDC Envisaged objectives of the new SACOI Increase share of RES Support stability Allow restoration

4 LCC Technology options LCC Half Bridge (HB) VSC - Full Bridge (FB) VSC 1 Configurazione HB HB 1 Impact on Transient DC fault V CAP 2 V A V CAP 2 V A Fast power reversal AC fault ride through 1 3 Configurazione FB FB IGBT on V CAP V A 2 4 4

5 LCC Technology options Temporary LCC switch off LCC Half Bridge (HB) VSC - Full Bridge (FB) VSC 1 Configurazione HB HB 1 Impact on Transient DC fault V CAP 2 V A V CAP 2 V A ACCB or DCCB needed Fast power reversal AC fault ride through 1 3 Configurazione FB FB IGBT on V CAP V A 2 4 Blocking capability 5

6 LCC Technology options LCC Half Bridge (HB) VSC - Voltage polarity inversion Switches counter-invert polarity in the 3 rd terminal Full Bridge (FB) VSC 1 Configurazione HB HB 1 Impact on Transient DC fault Fast power reversal V CAP 2 V A V CAP Current inversion 2 V A AC fault ride through 1 3 Configurazione FB FB IGBT on V CAP V A 2 4 Current or voltage inversion 6

7 Technology options LCC Commutation failure LCC Half Bridge (HB) VSC - Full Bridge (FB) VSC 1 Configurazione HB HB 1 Impact on Transient DC fault Fast power reversal V CAP 2 V A V CAP 2 Fault support V A AC fault ride through 1 3 Configurazione FB FB IGBT on V CAP V A 2 4 Fault support 7

8 Unidirectional electrodes: V d loss of DC pole In symmetrical bipolar operation no current flows through unidirectional electrodes. V d - I d P I d - V d V d Bipolar operation Electrodes not involved Loss of one pole (line or converter): depending on the lost pole, switching may be needed to manage direction of the current in the electrodes. V d I d V d Loss of negative pole «Right» direction of the current in the electrode In the worst case (loss of line as in the figure), short interruption is needed V d - I d - V d Loss of positive pole «Wrong» directionof the currentin the electrode allowed only for a limitedtime 8

9 «V DC margin» control strategy Fast Suitable for radial links with one «predominant» interconnected AC system V DC Pmin V DC V DC INV REC INV REC Pmin INV REC V DC refh V DC ref V DC refl Pmax Pmax -Pc Corsica P -Ps Sardinia P Pt=PcPs Mainland P Always: Constant P 3-terminal operation: Constant P 2-terminal operation (no continent): V control V control Power «slack» 9

10 Controls in Sardinia 10

11 Case 1: Power reversal Sardinia from export to import Hypothesis on SACOI3: 2 x 300 MW Powerexport of onesacoi pole [MW] Without fast power reversal Small initial export SAPEI not regulating (e.g. at technical minimum) 280 MW generation loss in Sardinia Frequency With fast power reversal With fast power reversal Without fast power reversal Fast power reversal makes the difference (no load shedding) 11

12 Case 2: Loss of large export by HVDC pole Frequency SAPEI LCC bipolar HVDC 2 x 500 MW Emergency measures to keep stability in case of severe perturbations and low margin scenarios: - High RES - Low load Example: - Loss of 410 MW export - Generation tripping by 180 MW Power export from Sardinia to Corsica via «SARCO» AC cable 12

13 Power export in the safe pole of SAPEI Case 2: Loss of large export by HVDC pole SAPEI LCC bipolar HVDC 2 x 500 MW Emergency measures to keep stability in case of severe perturbations and low margin scenarios: - High RES - Low load Power export in one pole of SACOI Example: - Loss of 410 MW export - Generation tripping by 180 MW 13

14 Case 3: AC fault LCC Commutation failure VSC supports stability -> frequency and angle stability increased Sardinia Per pole 14

15 Restoration scenarios SACOI assumed size: 2 x (300 MW, 100 Mvar), Rating: 2 x 316 MVA 1. VSC as black start source Supply Sardinia from mainland Italy Non black start units F. Santo Codrongianos SAPEI LCC VSC SACOI3 2. VSC as STATCOM Support voltage in conventional restoration Black start units Ottana & Taloro 15 15

16 VSC used as black start source Energise lines Control voltage profile and frequency Supply ballast loads Provide cranking power to non-black start unit Non black start units F. Santo Codrongianos SAPEI LCC Black start VSC SACOI3 G3 unit at F Santo PP ramping up to 110 MW 16

17 V VSC used as black start source Voltage: Soft start control: progressively increase the voltage magnitude from an initial low value to its final value, avoiding overvoltages induced by the energization of no load long lines Reliability in the early stages of the process t Frequency: Primary controller based on a very small permanent droop ( %) and high response speed Frequency deviations at load connection are kept small Reliability and flexibility in the subsequent stages 17 17

18 Powers DIgSILENT VSC P 7 MW 0.40 Mechanical power of F Santo G3 unit Power reversal MW VSC Q Q well inside reactive capability curve [min] PWM Converter/1 DC-CodronPos: Active Power/Terminal AC in MW PWM Converter/1 DC-CodronPos: Reactive Power/Terminal AC in Mvar VSC fast acts and supplies almost the whole ballast load [s] TGOV1ramping: pt TGOV1ramping: o2 in p.u. (base: 0.05 ) [min] TGOV1ramping: pt TGOV1ramping: o2 in p.u. (base: 0.05 ) Mechanical power of ramping up unit only undergoes a slight transient

19 50.04 [-] VSC «Secondary» controller deactivated Frequency profile In bothcases, transientand steady state f s inside 50.04max admissible range [-] VSC «Secondary» controller active Allow involving both pole converters in the black start. Potentially needed in case the size of one converter is not enough (especially in terms of Q capability) [min] regpfcodpos: f [min] regpfcodpos: f 19 19

20 Non black start units F. Santo Codrongianos SAPEI LCC VSC as STATCOM STATCOM VSC SACOI3 Use of VSC as STATCOM to support voltage during conventional restoration path Black start units Ottana & Taloro Units at Taloro PP used as black start units G3 unit at F Santo PP ramping to 110 MW 20 20

21 VSC as STATCOM: voltage profile 1.13 [p.u.] Variation of Vac setpoint of VSC control at Codrongianos: effective contribution to contain voltage Load connections [min] MORCDI1501 SUBNET \MORCDI1501A1 BUS : Voltage, Magnitude NU2CDI1501 SUBNET \NU2CDI1501A1 BUS : Voltage, Magnitude NUOCDI1501 SUBNET \NUOCDI1501A1 BUS : Voltage, Magnitude PT1CTI1501 SUBNET \PT1CTI1501A1 BUS : Voltage, Magnitude 6

22 Conclusions & future work VSC offers opportunities for enhanced stability and flexibility of operation: V control, no commutation failure, FRT, black start, no filters / reactive compensation banks LCC offers good performances in addressing some peculiarities of SACOI: DC fault Unidirectional electrode Topics for future work Coordinated control of HVDC and other components (e.g. RES) for stability enhancement Emergency control of VSC in case of SAPEI LCC commutation failure Fast response of HVDC to mitigate frequency transients Control of HVDC for perturbations involving Corsica-Sardinia AC link 22 22

23 Thank you for your attention! Diego Cirio 23

24 Additional slides 24

25 DC faults LCC can suppress the fault by temporarily switching off the HVDC link HB-VSC needs for AC-CB or DC-CB to clear DC side fault DC-CB allows to achieve protection selectivity FB-VSC needs no CB to clear DC side faults FB topology is capable of reversing the polarity of the line voltage (at least for a short period of time) in order to extinguish and de-ionize the electric arc. A certain number of FB cells is needed (modular multilevel converter MMC) to sustain the DC-side fault current 25

26 Power reversal via voltage polarity inversion or current inversion. LCC can only implement voltage polarity inversion commutation of the operational mode of all HVDC terminals, including the one that is not required to reverse the active power flow e.g. Corsica in case of reversal between Sardinia and mainland need for switches that counter-invert the voltage polarity in the terminal that is not involved, in a stand-by time interval of about 500 ms LCC power reversal cannot be continuous also because of the LCC technical minimum ( 10%) HB-VSC can implement current inversion power reversal between two terminals can be quickly implemented without affecting operating mode of the third one FB-VSC can implement either voltage polarity inversion or current inversion In case of monopolar operation, fast power reversal (within few hundreds of ms) can only be assured by VSC-FB because of the unidirectional electrodes 26

27 AC fault ride through VSC, unlike LCC, is not affected by valve commutation failure, e.g. in case of under-voltages due to severe short circuits in the AC grid. AC Voltage Fault Ride Through capability Possibility to support voltage during fault and preserve some power exchange (ok for angle/frequency stability). U [p.u.] 1 Urec2 Urec1 Ublock Uret 27

28 V DC control in the mainland terminal V DC ref - V DC PI Ku(11/sTu) I dref - V DCref = 1 p.u. Control Vdcby actingon Id I DC U D : directaxis component of V AC V DC Pmin V DC V DC Pmin V DC refh V DC ref INV -Pc Corsica P REC V DC refl INV -Ps Sardinia Pmax REC P INV Mainland Pt=PcPs Pmax REC P 28

29 P- V DC margin control in Sardinia P V DC refl - APR - V DC DC-AVRL Pmax P set-point_controlled V DC refh - V DC DC-AVRH Pmin P Idref Responsetime of 100 ms@ 300 MW ΔP steprequest C V DC V DC Pmin V DC V DC Pmin V DC refh V DC ref INV -Pc Corsica P REC V DC refl INV -Ps Sardinia Pmax REC P INV Mainland Pt=PcPs Pmax REC P 29

30 Controls Based on the AC grids characteristics Sardinia Vdc Vdc Tuscany P -Vdc margin Vac(or Q) Vdc Q SarCo P Vac AC DC Vac P Corsica 30 30

31 Control requirements for black start Dead line energisation Voltage control with «good» lower bound margins (underexcitation) Integral frequency regulation Withstand step load reconnection Restoration criticalities EM transients related to energisation of lines and transformers (in-rush currents), manoeuvring Inadvertent protection intervention Overvoltages Frequency stability 31 31

32 VSC used as black start source Ballast loads generally used to: 1. Keep voltages within range during path energisation 2. Allow ramping of the non black start unit until technical minimum Load connection can occur even in the early ramping stage thanks to the fast frequency control Function 2 not essential as VSC can operate power reversal More efficient and flexible restoration 32 32