Multi-Terminal DC gridsmontefiore.ulg.ac.be/~vct/elec0445/LP_MTDC.pdf · 1\High Voltage Direct...
Transcript of Multi-Terminal DC gridsmontefiore.ulg.ac.be/~vct/elec0445/LP_MTDC.pdf · 1\High Voltage Direct...
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Multi-Terminal DC grids
Lampros [email protected]
May 4th, 2018
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Outline
1 Introduction
2 VSC or LCC for MTDC grids?
3 MTDC grid control
4 MTDC grid power flow
5 Simulation examples
6 MTDC grid code
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Introduction
Up to now, we have seen only point-to-point HVDC links ortwo-terminal DC grids
=≈ = ≈TA TB
A B
Area A Area B
In the following slides we will discuss about multi-terminal DC grids,with more than two terminals
=≈ = ≈TA TB
A B
Area A Area B
=≈TC
C
Area C = ≈TD
D
Area D
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Why build an MTDC grid?
Better asset utilization
Increase security of power transfer
Increased reliability of power transfer
Enhance power trading
Better operating flexibility
and others...
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Current situation: Point-to-point links
Offshore wind farms connectedthrough point-to-point HVDClinks
Wind farms rarely produce theirmaximum power
Therefore, both links areunderutilized
If the upper VSC (or the cable)is tripped, the wind farm poweris lost
Fixing the cable might take verylong!
≈=
=≈
≈=
=≈
≈=
=≈
≈=
=≈
200 MW
100 MW
100 MW
0 MW
P = 200 MW
P = 100 MW
P = 0 MW
P = 100 MW
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Extension to MTDC grid
Better utilization of lower link
In overall, better flexibility
The power can be sharedbetween the two onshoreterminals
Wind farm power can still betransferred onshore, if the upperVSC is lost
Building an additional HVDCline is a considerable investment!
The cost should be compared tothe cost of keeping the WFoffline until the trippedconnection is restored
≈=
=≈
≈=
=≈
≈=
=≈
≈=
=≈
150 MW
100 MW
100 MW
200 MW
50 MW
0 MW
P = 200 MW
P = 100 MW
PN = 200 MW
PN = 100 MW
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HVDC grid suggestions
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What is an MTDC grid?
A DC connection of more than two AC/DC terminals.
Various topologies:
Radial DC grid
No redundancy. If one DC cableis tripped, the DC grid isseparated
DC line flows are controlled
HVDC connection
AC connection
AC/DC terminal
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Full control of DC line flows
Redundancy
Expensive (N lines ⇒ 2Nconverters)
Not a “real” MTDC grid.Actually a set of point-to-pointDC links.
Meshed HVDC grid
Redundancy
DC line flows depend onKirchhoff’ s laws; control is nottrivial
It is considered by some the only“real” MTDC grid
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Full control of DC line flows
Redundancy
Expensive (N lines ⇒ 2Nconverters)
Not a “real” MTDC grid.Actually a set of point-to-pointDC links.
Meshed HVDC grid
Redundancy
DC line flows depend onKirchhoff’ s laws; control is nottrivial
It is considered by some the only“real” MTDC grid
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VSC or LCC for MTDC grids?
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LCC-based MTDC grids
SACOI 3-term. Quebec-New England 5-term.
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SACOI MTDC grid1
The only MTDC grid with long operating experience
Radial MTDC grid
Built in steps. First, the Italy-Sardinia link was built to control thefrequency of the Sardinia system
The Corsica converter was added later
DC current control DC voltage controlMechanical switches
to change polarity
1“High Voltage Direct Current transmission : converters, systems and DC grids,”D. Jovcic and K. Ahmed, Wiley, 2015
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SACOI MTDC grid
Italy station: rectifier controlling the DC current
Sardinia station: main inverter, controlling the DC voltage
Corsica: either rectifier or inverter. Power reversal is achieved by themechanical switches. This means a short interruption of the Corsicastation.
Corsica is also temporarily interrupted, if Italy and Sardinia changedirection.
Corsica station should be able to withstand an AC voltage drop ofaround 20%. This requires a large extinction/ignition angle, whichleads to high reactive power demand and high losses.
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Why VSC?
Independent active and reactive power control - an LCC consumeslarge amounts of reactive power, the VSC can either consume orproduce reactive power depending on the system needs
Smaller filters (if any) - the total harmonic distortion is smaller with aVSC
Black start capability, i.e. energize an AC system after a black out
Suitable for offshore wind farms and islanded grids - the LCC needs astrong AC voltage (large Short Circuit Ratio)
Easy to change direction of power by changing the DC currentdirection (no interruption needed)
Smaller footprint - important for offshore applications where a largerplatform means a big increase in cost
For the above, VSC-based MTDC grids will be described in more detail.
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MTDC grid control
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MTDC grid controlMain objectives
Harvest renewable power and avoid curtailment
Control DC voltages between bounds
Control power flows and power exchange
Avoid overloading of HVDC cables/lines
Ensure stable and secure operation following
loss of an HVDC cable/lineloss of a VSCA fault at the AC side of any of the convertersIn general, the MTDC grid has to withstand the failure of any singlecomponent
Also called N-1 criterion
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Analogy with AC systems
∼
∼
AC network
Mechanical powerproduction
Electrical powerconsumption
' =
' =
'=
HVDC grid
'=
Rectifiers Inverters
What happens when there is an imbalance of powers?
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Analogy with AC systems
Imbalance ⇒ AC frequencydeviation
∼PePm
12Jω
2
governor machine network
Imbalance ⇒ DC voltagedeviation
Pr Pi
12CV 2
rectifiers inverters
DC gridcapacitance
Power balance should be controlled fast and tightlyin an MTDC grid
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DC grid power balance
'=
'=
'=
'=
'=
P1
Pi
P2
Pi+1
PN
∑Nj=1 Pj > 0: The DC capacitors of the VSCs will start storing the
excess energy and the DC voltage level will increase.∑Nj=1 Pj < 0: The DC capacitors of the VSCs will start providing the
additional energy requested by the inverters and discharge.Consequently, the DC voltage level will drop.
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DC Voltage Control
DC Voltage must be tightly controlled:
For the correct control of the VSCs. Too low a DC voltage will causeVSC tripping.
For equipment protection. Too high a DC voltage can destroy theinsulation materials
Already discussed about the DC voltage control in point-to-point links:
One VSC controls its DC voltage to a constant value (Master VSC)
The other VSC controls the power flowing through the link (SlaveVSC)
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DC Voltage ControlMaster-Slave
VDC
VDC
V setDC
+-
Kp
Ki
s +
+
P
P set
+-
Kp
Ki
s +
+
P
Master
Slave
VDC
P
V setDC
VDC
PP set
'=to DC grid to AC grid
'=to DC grid to AC grid
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DC Voltage ControlMaster-Slave
Master-Slave control inMTDC grids
One VSC acts as Master
The rest are Slaves
Any imbalance (change ofWF production, VSCtripping, etc.) is correctedonly by the Master VSC
'=
'=
'=
P1
Pi
P2
Master
Slaves
'=
'=
Pi+1
PN
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DC Voltage ControlMaster-Slave
Master-Slave control inMTDC grids
One VSC acts as Master
The rest are Slaves
Any imbalance (change ofWF production, VSCtripping, etc.) is correctedonly by the Master VSC
'=
'=
'=
P1 + Pi+1
Pi
P2
Master
Slaves
'=
'=
Pi+1
PN
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DC Voltage ControlMaster-Slave
Problems with Master Control:
The Master should be connected to a strong AC area that canwithstand sudden big changes in power injection
What happens if the Master VSC is tripped?
Droop control as an alternative option:
Inspired by control of frequency in AC systems
Allows to share the effort between multiple VSCs
Provides redundancy; if one VSC trips the remaining can keep theMTDC grid operating
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DC Voltage ControlMaster-Slave
Problems with Master Control:
The Master should be connected to a strong AC area that canwithstand sudden big changes in power injection
What happens if the Master VSC is tripped?
Droop control as an alternative option:
Inspired by control of frequency in AC systems
Allows to share the effort between multiple VSCs
Provides redundancy; if one VSC trips the remaining can keep theMTDC grid operating
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DC Voltage Control
Droop control implementation
Each VSC is given a Power and a DC Voltage setpoint, Pset andV set , respectively
Its power follows the following characteristic (positive power forrectifier operation):
P = Pset − Kv
(VDC − V set
DC
)(1)
VDC
P
P set
+-
Kp
Ki
s +
+
P
Droop
VDC
PP set
'=to DC grid to AC grid
VDC
V setDC
-+
+
-
Kv
V setDC
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DC Voltage control
'=
'=
'=
P1
Pi
P2
Slaves
Kv = Kvo
Droop control
'=
'=
Pi+1
PN
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DC Voltage control
'=
'=
'=
P1 −Kv1∆VDC
Slaves
Kv = Kvo
Droop control
'=
'=
Pi+1
PN
P2 −Kv2∆VDC
Pi −Kvi∆VDC
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DC Voltage control
Exercise: Assume the following 5-terminal MTDC grid with three VSCsoperating in droop mode, each with its own droop gain Kv ,i . Following thetripping of the WF find the participation of each VSC and the deviation ofthe DC Voltage. Assume all VSCs operate initially at their setpoints andneglect the DC grid resistances.
'=
'=
'=
P1
P3
P2
Slaves
Kv = Kvo
Droop control
'=
'=
PWF
PPV
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DC Voltage control
Answer: Since we neglect the DC grid resistances, all VSCs have thesame DC Voltage.The total change of the power of the VSCs operating in droop mode willbe equal to the lost WF power (PWF )
3∑j=1
(Pj − Pset
j
)= PWF
From the droop characteristic equation (1):
−3∑
j=1
Kv ,j
(VDC − V set
DC
)= PWF ⇒ VDC − Vset
DC =−PWF∑3
j=1 Kv,j
Substituting the result in (1) for each VSC i yields:
Pi − Pseti = − Kv,i∑3
j=1 Kv,j
PWF
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DC Voltage control
Comments:
The deviation of the DC Voltage has an inversely proportional relationwith the sum of the droop gains
The power sharing between the VSCs depends on the respective ratioKv,i∑3j=1 Kv,j
If all VSCs had the same value Kvo :
P − Pset =−PWF
3, VDC − V set
DC =−PWF
3Kvo
The above are true for an ideal MTDC grid (no losses)!
The droop control seems like the most attractive method to controlthe DC voltage
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DC Voltage controlAlternatives
'=
'=
'=
P1
Pi
P2
Master
Slaves
'=
'=
Pi+1
PN
Back-up Master
Voltage Margin method: same as Master control
a back-up VSC is used if the main Master trips
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DC Voltage controlAlternatives
'=
'=
'=
P1
Pi
P2
Masters
Slaves
'=
'=
Pi+1
PN
Distributed Master: More than one Master VSCs at the same time
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DC Voltage controlAlternatives
'=
'=
'=
P1
Pi
P2
'=
'=
Pi+1
PN
Central unit
Communication-based methods, i.e. a signal is communicated by acentral unit very fast to all VSCs
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Higher control levelsAC frequency control structure
Tertiary control
Secondary control
Primary control
stabilize frequency
cancel frequency offset,restore power transfer
through interconnections
free frequency reserves
seconds minutes minutes to hours
AC system
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Higher control levelsMTDC grid control structure
correct DC voltagedeviations,
adjust power transferin AC/DC converters
satisfy security criteria
Tertiary control
Secondary control
Primary control
milliseconds seconds to minutes minutes
stabilize DC voltages
DC system
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MTDC grid power flow
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MTDC grid power flow
The MTDC grid is not ideal
The DC Voltage is not the same throughout the whole MTDC grid(in contrast to AC frequency which is the same everywhere in an ACgrid). If it were the same there would be no power transfer
We do not know exactly how the power will be shared after adisturbance
For this reason a power flow computation will have to be performed
The power flow computation for MTDC grids is much simpler thanfor AC systems due to the absence of reactive power
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MTDC grid power flow
Power flow through the DC cableconnecting nodes i , j :
Pij = Vi (Vi − Vj) gij
Losses:
Plosij = (Vi − Vj)2 gij = Pij + Pji
DC power of VSC i :
Pi =N∑j 6=i
Pij ⇒ Pi = Vi
N∑j 6=i
(Vi − Vj) gij
Vi Vjgij
' = '=
' =
VN
'=
Vk
gik
giN
Pi
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MTDC grid power flow
Power flow equations:
P1 = V1
N∑j 6=1
(V1 − Vj) g1j
P2 = V2
N∑j 6=2
(V2 − Vj) g2j
...
PN = VN
N∑j 6=N
(VN − Vj) gNj
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MTDC grid power flow
The system can be written in compact form:
P = V ⊗ GV (2)
where ⊗ is the element-by-element multiplication and
P = [P1, . . . ,PN ]T , V = [V1, . . . ,VN ]T
G is the admittance matrix of the DC grid, i.e.
G =
∑N
i=1 g1i −g12 . . . −g1N−g21
∑Ni=1 g2i . . . −g2N
......
. . ....
−gN1 −gN2 . . .∑N
i=1 gNi
Different power flow algorithms are needed for different VSC
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MTDC grid power flow
Power flow calculation for MTDC grid with Master-Slave control
The Master VSC is considered as slack bus (known DC voltage)
The Slave VSCs have constant power (known DC power)
The power flow can be calculated even by using an AC power flowalgorithm and applying the following changes
Type of bus Master (slack) Slave (P or PQ)
Network AC DC AC DC
Known V , δ V , δ = 0 P, Q P , Q = 0Unknown P, Q P V , δ V
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MTDC grid power flow
Power flow calculation for MTDC grid with droop control
The droop characteristic of the VSC should be incorporated in thepower flow equations:
Pset1 − Kv ,1
(V1 − V set
1
)= V1
N∑j 6=1
(V1 − Vj) g1j
Pset2 − Kv ,2
(V2 − V set
2
)= V2
N∑j 6=2
(V2 − Vj) g2j
...
PsetN − Kv ,N
(VN − V set
N
)= VN
N∑j 6=N
(VN − Vj) gNj
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In compact form:P = V ⊗ GV⇒
Pset − diag(KV)(V − Vset
)= V ⊗ GV⇒
Pset + diag(KV)Vset = V ⊗ GV + diag(KV)V
where: diag(KV) =
Kv ,1 0 . . . 0
0 Kv ,2 . . . 0...
.... . .
...0 0 0 Kv ,N
Pset = [Pset
1 , . . . ,PsetN ]T , Vset = [V set
1 , . . . ,V setN ]T
Using an iterative procedure (Newton-Raphson), V can be found ifvectors Pset,Vset are known
Knowing V, P can be found from: P = V ⊗ GV
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Simulation examples
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Test system
WF
= '
= '' =
'=
' =T1 T2
T4T3
T5
∞
∞ ∞
∞Kv = 5 Kv = 5
Kv = 5Kv = 5
6.6 Ω6.6 Ω
3.3 Ω
4.4 Ω
3.3 Ω
5-terminal HVDC grid
Four VSCs in droop control. Kv = 5 for all to share equally the WFpower
We simulate a ramping up of the WF power by 600 MW
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Results
-100
0
100
200
300
400
500
600
0 5 10 15 20 25 30 35 40
Power of T1Power of T2Power of T3Power of T4
Power produced by WF
Power not shared equally due to losses and resistances
The VSCs closer to the WF (T1 and T4) change their power more
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Results
1
1.005
1.01
1.015
1.02
1.025
1.03
1.035
1.04
0 5 10 15 20 25 30 35 40
Voltage at bus DC1Voltage at bus DC2Voltage at bus DC3Voltage at bus DC4Voltage at bus DC5
DC voltage offset due to the droop effect
The voltage deviation is larger near the WF (DC1, DC4 and DC5)
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Results
Comments:
In contrast to frequency control, a simple droop control does notguarantee exact power sharing
Similar to frequency control, the droop control results in asteady-state offset. For security reasons it is necessary to restore theDC voltages close to 1 pu
The correction of the above will be the objective of the secondarycontrol
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Network Code on HVDC Connections
“Establishing a network code on requirements for grid connection ofHVDC systems and DC connected power park modules”
devised by European Network of Transmission System Operators forElectricity (ENTSO-e)
still a draft though
will specify requirements for long distance DC connections, linksbetween different synchronous areas and DC-connected Power ParkModules, such as offshore wind farms, which are becomingincreasingly prominent in the European electricity system
Available in:www.entsoe.eu/major-projects/network-code-development/
high-voltage-direct-current/Pages/default.aspx
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Network Code on HVDC Connections
Indicatively, an HVDC system should:
not disconnect from the network for frequency deviations in apre-specified rangestay connected to the AC grid during an AC fault (called FaultRide-Through) as defined by a voltage vs time characteristicprovide reactive support and/or voltage control to the network
V
time
VSCcan
discon
nect
VSCsho
uldno
t discon
nect
fault fault clearance
1 pu
Vfault
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Network Code on HVDC Connections
Indicatively, an HVDC system should:
provide frequency support
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