Multi-Terminal DC gridsmontefiore.ulg.ac.be/~vct/elec0445/LP_MTDC.pdf · 1\High Voltage Direct Current transmission : ... This requires a large extinction/ignition angle, ... Inspired

$Page 1: Multi-Terminal DC gridsmontefiore.ulg.ac.be/~vct/elec0445/LP_MTDC.pdf · 1\High Voltage Direct Current transmission : ... This requires a large extinction/ignition angle, ... Inspired$
Multi-Terminal DC grids

Lampros [email protected]

May 4th, 2018

Lampros Papangelis [email protected] Multi-Terminal DC grids May 4th, 2018 1 / 53

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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


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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



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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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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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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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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


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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'=

'=

'=

P1

Pi

P2

Masters

Slaves

'=

'=

Pi+1

PN

Distributed Master: More than one Master VSCs at the same time


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'=

'=

'=

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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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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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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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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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

control logicsLampros Papangelis [email protected] Multi-Terminal DC grids May 4th, 2018 41 / 53

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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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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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MTDC grid code


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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


www.entsoe.eu/major-projects/network-code-development/high-voltage-direct-current/Pages/default.aspx

www.entsoe.eu/major-projects/network-code-development/high-voltage-direct-current/Pages/default.aspx

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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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Indicatively, an HVDC system should:

provide frequency support


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Documents

Multi-Terminal DC gridsmontefiore.ulg.ac.be/~vct/elec0445/LP_MTDC.pdf · 1\High Voltage Direct Current transmission : ... This requires a large extinction/ignition angle, ... Inspired