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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
BEST PATHS EU FP7 Project DEMO 3: SACOI HVDC
2
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!
SACOI
Converters
TOSHIBA EUROPE
Submarine cables
NEXANS
Land cables
NEXANS
Overhead
lines and
Insulation
DE ANGELI
RSE
TERNA
Demo leader
TERNADemo leader
TERNA
RSE
TERNA
System
issues
1987: 50 MW tap station in Corsica
1967: Sardinia-Mainland Italy
SACOI connects…
• 2 countries
• Different market areas
• 2 (or 3) electrical AC islands
• Large RES penetration
• Another large HVDC link to mainland Italy (2x 500 MW LCC)
• SACOI as «embedded» HVDC
Envisaged objectivesof the new SACOI
• Increase share of RES
• Support stability
• Allow restoration
Aspects of Sardinian system
SACOI features
• Temporary DC faults
• Unidirectional sea electrodes
Technology options
• LCC
• Half Bridge (HB) VSC
• Full Bridge (FB) VSC
Impact on…
• Transient DC fault
• Fast power reversal
• AC fault ride through
4
+
VA
1
2
1
2
+
VA
VCAP VCAP
Configurazione HB HBHB
LCCLCC
+
VA
1
2
3
4
VCAP
Configurazione FB FBFB
IGBT on
++
--
Technology options
• LCC
• Half Bridge (HB) VSC
• Full Bridge (FB) VSC
Impact on…
• Transient DC fault
• Fast power reversal
• AC fault ride through
5
+
VA
1
2
1
2
+
VA
VCAP VCAP
Configurazione HB HBHB
LCCLCC
+
VA
1
2
3
4
VCAP
Configurazione FB FBFB
IGBT on
++
--
Temporary LCC switch off
ACCB or DCCB needed
Blocking capability
Technology options
• LCC
• Half Bridge (HB) VSC
• Full Bridge (FB) VSC
Impact on…
• Transient DC fault
• Fast power reversal
• AC fault ride through
6
+
VA
1
2
1
2
+
VA
VCAP VCAP
Configurazione HB HBHB
LCCLCC
+
VA
1
2
3
4
VCAP
Configurazione FB FBFB
IGBT on
++
--
Voltage polarity inversion
Switches “counter-invert”
polarity in the 3rd terminal
Current inversion
Current or voltage
inversion
Technology options
• LCC
• Half Bridge (HB) VSC
• Full Bridge (FB) VSC
Impact on…
• Transient DC fault
• Fast power reversal
• AC fault ride through
7
+
VA
1
2
1
2
+
VA
VCAP VCAP
Configurazione HB HBHB
LCCLCC
+
VA
1
2
3
4
VCAP
Configurazione FB FBFB
IGBT on
++
--
Commutation failure
Fault support
Fault support
Unidirectional electrodes:loss of DC pole
8
Vd
Vd
Vd
Vd
-
+
Vd Vd
+
Loss of negative pole
«Right» direction of the
current in the electrode
Loss of positive pole
«Wrong» direction of
the current in the
electrode allowed only
for a limited time
Bipolar operation
Vd Vd
-
-
+
-
+
Id
Id
Id
Electrodes notinvolved
Id
PP
In symmetrical bipolar operation no current flows through unidirectional electrodes.
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.
In the worst case (loss of
line as in the figure), short
interruption is needed
RECINV
Corsica
-Pc
VDC
P
RECINV
Mainland
Pmin
Pmax
VDC ref
P
VDC
P
VDC
-Ps
RECINV
Sardinia
Pmin
Pmax
VDC refH
VDC refL
Pt=Pc+Ps
«VDC margin» control strategy
• V control
• Power «slack»
Always:
• Constant P3-terminal operation:
• Constant P
2-terminal operation
(no continent):
• V control 9
• Fast
• Suitable for radial links with one «predominant» interconnected AC system
Controls in Sardinia
10
Case 1: Power reversal – Sardinia from export to import
11
• Small initial export • SAPEI not regulating (e.g. at
technical minimum)• 280 MW generation loss
in Sardinia
Power export of one SACOI pole [MW]Power export of one SACOI pole [MW]
Without fast power reversal
With fast power reversalWith fast power reversal
Without fast power reversal
Fast power reversal makes the
difference (no load shedding)
Hypothesis on SACOI3: 2 x 300 MW
FrequencyFrequency
Case 2: Loss of large export by HVDC pole
12
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
Frequency
13
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 in one pole of SACOI
Power export in the safe pole of SAPEI
Case 2: Loss of large export by HVDC pole
Case 3: AC fault
14
• LCC Commutation failure
• VSC supports stability -> frequency and angle stability increased
Per pole
Sardinia
15
Restoration scenarios
1. VSC as black start source
• Supply Sardinia from mainland Italy
2. VSC as STATCOM
• Support voltage in conventionalrestoration
15
Codrongianos
Ottana &
Taloro
F. Santo
VSC
SACOI3
SAPEI
LCC
Black start
units
Non black
start units
SACOI assumed size:
2 x (300 MW, 100 Mvar),
Rating: 2 x 316 MVA
16
G3 unit at F Santo PP
ramping up to 110 MW
• Energise lines
• Control voltage profile
and frequency
• Supply ballast loads
• Provide cranking
power to non-black
start unit Codrongianos
F. Santo
Black start
VSC
SACOI3
SAPEI
LCC
Non black
start units
VSC used as black start source
17
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
Frequency:
• Primary controller based on a very small permanent droop (0.1-0.5%) and high response speed
17
Reliability in the early stages of the process
Frequency deviations at load connection are kept small
Reliability and flexibility in the subsequent stages
V
t
18
Powers
18
60.0048.0036.0024.0012.000.000 [min]
120.00
80.00
40.00
0.00
-40.00
-80.00
PWM Converter/1 DC-CodronPos: Active Power/Terminal AC in MW
PWM Converter/1 DC-CodronPos: Reactive Power/Terminal AC in Mvar
DIg
SIL
EN
T
Q well inside
reactive capability curve
Powerreversal
60.0048.0036.0024.0012.000.000 [min]
0.50
0.40
0.30
0.20
0.10
0.00
TGOV1ramping: pt
TGOV1ramping: o2 in p.u. (base: 0.05 )
Mechanical power
of F Santo G3 unitVSC P
VSC Q
7 MW
VSC fast acts and supplies almost the
whole ballast loadMechanical power of ramping up unit
only undergoes a slight transient
750.0744.0738.0732.0726.0720.0 [s]
0.085
0.084
0.083
0.082
0.081
0.080
TGOV1ramping: pt
TGOV1ramping: o2 in p.u. (base: 0.05 )
0.7 MW
19
Frequency profile
19
VSC «Secondary»
controller active
60.0048.0036.0024.0012.000.000 [min]
50.04
50.00
49.96
49.92
49.88
49.84
[-]
regPfcodPos: f60.0048.0036.0024.0012.000.000 [min]
50.04
50.00
49.96
49.92
49.88
49.84
[-]
regPfcodPos: f
VSC «Secondary»
controller deactivated
In both cases,
transient and steady
state ∆∆∆∆f’s inside max
admissible range
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)
20
VSC as STATCOM
20
Use of VSC as
STATCOM to support
voltage during
conventional restoration
path
Units at Taloro PP used as
black start units
G3 unit at F Santo PP
ramping to 110 MW
Codrongianos
Ottana &
Taloro
F. Santo
STATCOM
VSC
SACOI3
SAPEI
LCC
Black start
units
Non black
start units
21
60.0048.0036.0024.0012.000.000 [min]
1.13
1.08
1.03
0.98
0.93
0.88
[p.u.]
MORCDI1501________SUBNET__\MORCDI1501A1______BUS_____: Voltage, Magnitude
NU2CDI1501________SUBNET__\NU2CDI1501A1______BUS_____: Voltage, Magnitude
NUOCDI1501________SUBNET__\NUOCDI1501A1______BUS_____: Voltage, Magnitude
PT1CTI1501________SUBNET__\PT1CTI1501A1______BUS_____: Voltage, Magnitude
VSC as STATCOM: voltage profile
21
Variation of Vac setpoint of VSC control at Codrongianos:
effective contribution to contain voltage
Load connections
22
Conclusions & future work
VSC offers opportunities for enhanced stability and flexibility of operation:
• V control, no commutation failure, FRT, black start, no filters / reactivecompensation 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 stabilityenhancement
• 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
Additional slides
24
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 (atleast for a short period of time) in order to extinguish and de-ionize theelectric arc.
• A certain number of FB cells is needed (modular multilevel converter MMC) to sustain theDC-side fault current
25
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
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).
27
U [p.u.]
1
Urec2
Urec1
Ublock
Uret
VDCref= 1 p.u.
UD: direct axis
component of VAC
PIKu(1+1/sTu)
++
-
VDCref
-
+ Idref
VDC
IDC
RECINV
Corsica
-Pc
VDC
P
RECINV
Mainland
Pmin
Pmax
VDC ref
P
VDC
P
VDC
-Ps RECINV
Sardinia
Pmin
Pmax
VDC refH
VDC refL
Pt=Pc+Ps
28
VDC control in the mainland terminal
Control Vdc by acting on Id
APR
DC-AVRH
DC-AVRL+
Pset-point_controlled
P
+
-
VDCrefL
VDC
++
-
VDCrefH
VDCPmin
Pmax
P
VDCC
-
+
RECINV
Corsica
-Pc
VDC
P
RECINV
Mainland
Pmin
Pmax
VDC ref
P
VDC
P
VDC
-Ps RECINV
Sardinia
Pmin
Pmax
VDC refH
VDC refL
Pt=Pc+Ps
Idref
P- VDC margin control in Sardinia
Response time of 100 ms @
300 MW ΔP step request
29
3030
Based on the AC grids characteristics
P - Vdc margin
Vac (or Q)
P
Vac
Vdc
Q
Corsica
Tuscany
PVac
Vdc Vdc
Sardinia
SarCo
AC
DC
Controls
3131
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…
…
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
