WF Integration Through MTDC-VSC

7/31/2019 WF Integration Through MTDC-VSC

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Abstract —This paper presents a large off shore wind farm

interconnected to the grid using a multiterminal HVDC link. The

200W wind farm consists of 100 individual 2MW turbines

connected using 25 VSC (Voltage Source Converter) converters

to a common DC bus. The transmission system converters enable

variable speed operation and therefore additional converters are

not needed with individual generators, implying savings in

converter costs.

The paper presents PSCAD simulation of the proposed

concept for various changes in wind speeds. The results confirm

the ability to operate at optimum coefficient of performance and

no synchronization problems occur even for severe wind speed

changes. Further tests with faults on AC grid demonstrate

satisfactory recoveries. The proposed concept may enable

integration of large offshore wind farms at considerable

distances, and using optimal number of converters.

Index Terms — Multiterminal HVDC Power Transmission,

Wind farms, Variable speed converter control.

I. I NTRODUCTION

A. Background

ecause of the projected energy shortage and the concerns

about greenhouse emissions, there has been significant

development in renewable energy sources worldwide, in

the past decade. In particular, the UK government aims to

achieve the goal of 20% (up to 40% in Scotland) energyproduction from renewable sources by 2020. It is projected

that the increase in renewable energy share from the present

3% will be largely based on increase in wind energy

generation, which is likely to become the main source of

renewable energy in the UK and in many other countries.

Because of the economy of scale and increasing demand,

future wind farms will have a larger capacity, exceeding a

hundred MW in many cases, implying hundreds of individual

1-5MW units. Considering also the environmental issues, it is

recognized that large size offshore wind farms are the well

placed to accommodate the future increase in the wind energy

generation [1-2]. Presently, there are a number of small-scale

offshore wind farms in Europe, including several in the UK,where the largest is the (160MW ) farm at Horns Rev,

Denmark. Under the “Round two” offshore program, the

British government has recently granted permission for fifteen

more off-shore plants, and many of these are expected to be

rated at 100-500MW . Because of the environmental and social

aspects these wind farms might be located at larger distances,

some approaching 100-150km from the shore [1].

D. Jovcic is with University of Aberdeen, Engineering Department, King’s

college, Aberdeen, AB24 3UE, Scotland. [email protected]

Traditionally, wind energy generation has been

connected to the network grid assuming that its size and

influence are small and therefore the connection requirements

have been less stringent. Typically, wind farms do not

contribute stabilization or regulation of AC grid and in many

cases no detailed transient studies or stability studies are

performed. With the projected power injection in the order of

hundreds of MW, power plants might have significant

influence on the host grid and the interaction issues need to be

carefully investigated. New integration solutions are sought,

taking into consideration the AC system properties including

stabilization, regulation and fault recovery, but also examining

cost effective wind farm topologies, their dynamics, transientsand efficiency.

At present, none of the wind farms, including the large

Horns Rev offshore installation, can contribute to the AC

system control or stability enhancement and they simply

disconnect in case of AC faults. As the power share from wind

farms increase, it is necessary that wind farms should take

more active role in the AC systems regulation and support.

The network operators have raised many issues with wind

power generation, especially for large-scale generation, in

order to enable secure and reliable system operation. The

Danish network operator Eltra, has recently issued a unique

specifications document for wind farms connections to the

transmission grid [3], and comparable documents are inconsultation stages in England, Wales and Scotland. Similarly

as with conventional generators, wind farms are now required

to comply with stringent connection requirements including:

reactive power support, transient recovery, system stability

and voltage/frequency regulation, power quality, whereas

scheduling and reserve availability are also considered. The

conventional wind generation concepts based on doubly fed

induction generators may have difficulties in meeting all the

above interconnection requirements [2].

B. Wind farm interconnection using HVDC

Theoretically, future offshore wind farms at distances

below 60km from the shore can be connected to the grid using

either an AC or DC link whereas at a greater distance only DC

links are applicable [1]. In searching for the adequate wind

energy integration solution, it has been recognized that many

of the above network-connection issues would be eliminated,

and even AC system stability might be enhanced, if the wind

power connection point incorporates a converter system [2].

AC connection link would therefore in many cases require an

additional converter system (like SVC or STATCOM) at the

connection point for reactive power and voltage support. Still,

this shunt converter does not resolve the issues with low

inertia, power control and frequency control/stabilization of

Interconnecting offshore wind farms using

multiterminal VSC-based HVDCD. Jovcic, Member IEEE

B

1-4244-0493-2/06/$20.00 ©2006 IEEE.



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the AC system, which remain significant issue with large wind

farms.

Traditional HVDC controls have in many cases been used

for AC frequency stabilization [6] or AC voltage regulation.

HVDC systems based on VSC converters have more versatile

and faster controls [4],[5], which may be utilized at the wind

farm interconnection point. A VSC converter enables

independent voltage, frequency and power control. A wind

farm interconnected with an HVDC link therefore has the

potential to offer grid control functions similar to a

conventional generator. On the downside, cost of HVDC link

is considerably higher than comparable AC link because of

converter stations.

This paper analyses the option of reducing converter costs

by eliminating primary converter systems associated with

common variable speed wind generators. Such concept has the

potential to offer variable speed operation and all the benefits

of HVDC interconnection without significant escalation in

converter costs.

II. WIND FARM TOPOLOGY

Figure 1 shows the electrical circuit for the considered windfarm. It presents a 200MW off shore wind farm consisting of

100 individual 2MW , 4kV wind generators. It is assumed that

the farm distance from the shore is approximately 100km.

The wind generators resemble the commercially available

2MW units based on permanent magnet synchronous

generators. However, converter systems are not used with

generators, since transmission system converters enable

variable speed operation. This concept implies savings in the

converter costs. The total converter rating in Figure 1 is same

as with conventional fully-fed variable speed wind generators

and Ac interconnection.

The nominal operating frequency of the offshore network is

50Hz (at full power), and the 4-pole generators use gearboxes

(approximately 77 ratio). Note that directly coupled generators

are not suitable in the proposed concept since they would

require operation at very low offshore electrical frequency and

therefore transformers with large cores would be needed.

The offshore electrical network consists of 25 generator groups each connected through a single Voltage Source

Converter (VSC). Each 8MW group includes 4 generators.

There is a single 4kV/90kV transformer per group, which

elevates the generator voltage to the transmission level.

The wind turbines operate at variable speeds in order to

maximize energy capture, reduce stresses and reduce noise.

The generator speed is controlled using the VSC converters,

and all the generators in a group operate at the same speed.

The frequency in a group, and the speed of all generators in

the group, is derived as the average speed considering wind

speeds at individual machines. The inability to operate

individual machines at most optimum speeds is not considered

as great loss in efficiency, since it is expected that the wind

profile will largely be similar on the four closely located

turbines. Note that each group can operate at most suitable

speed, which is independent of speeds for other groups.

The 25 VSC converters are connected in parallel to a

common DC bus, thus operating at the same DC voltage in a

parallel multiterminal HVDC connection. The DC voltage is

maintained at the nominal level (150kV ) by the single VSC

inverter located on-shore.

AC Network on shore SCL=10

+75kV +75kV 0.0135H 0.0135H

0.052H

110kV

110kV

90kV/110kV

Xl=10%

1.85Ω 1.85Ω

4.36Ω

-75kV -75kV

200MW

150kV

100kM DC cable

25 VSC

converters

VSC - Voltage Source Converter

VSC 1

G (2MW)4kV

4kV

4kV/90kV

Xl=12% 8MW

150kV

4

s y n c h r o n o u s g e

n e r a t o r s

∆ Y

VSC 25

G (2MW)4kV

4kV

4kV/90kV

Xl=12% 8MW

4

s y n c h r o n o u s g e n e r a t o

r s

∆

∆

Y

Y

80uF120uF

120uF

34uF

80uF

Figure 1. 200MW off-shore wind farm with parallel multiterminal HVDC connection



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III. WIND FARM MODEL

A. Electrical circuit

A suitable model for the system in Figure 1 is developed on

PSCAD/EMTDC platform [7]. It would be extremely difficult

to model such complex system in detail, and a series of

simplifications is adopted.

The schematic of the PSCAD model is shown in Figure 2.The number of the off-shore converters is reduced to four in

order to save simulation time. The converters are rated 50MW

and they are connected in parallel to represent multiterminal

HVDC operation in the actual system. Only one of the

converters is connected to four machines, to enable studies of

the dynamics within a group.

All the machines models are based on a single 2MW

permanent magnet synchronous machine, which has common

parameters from PSCAD library. The large 50MW generators

are also based on the same 2MW machine model, which uses

PSCAD ability to represent 25 coherent 2MW machines in a

single model.

B. Offshore VSC controller

The adopted principle of a parallel multiterminal HVDC

control is explained with reference to Figure 3. The inverter

station regulates the DC voltage which is common for all

converter stations. Each of the rectifier stations (the offshore

converters) regulates the DC current in its own branch.

The generator speed control can be achieved using the

known principles of flux oriented synchronous machine

control and using position encoder to synchronize the

coordinate frame [8]. However, since a transformer is placed

between machine and converter, it is found more suitable to

use the torque control based on regulation of power transfer

through the transformer. The rotating coordinate frame

position is determined using a PLL, which measures the 4kV

generator voltage. This signal is a good estimate of the rotor

position and therefore the danger of loss of machine

synchronism is avoided.

The control system for each of the offshore converters is

shown in Figure 4. The machine power, and consequently

machine torque, is varied by changing the angle of the VSCconverter voltage M Φ, with respect to the generator terminal

voltage. The power control in VSC transmission is commonly

achieved either using the VSC voltage angle or VSC voltage

D component [9-11]. As shown by the lower control diagram

in Figure 4, the VSC angle control is based on two series

connected controllers. The inner control loop regulates the DC

current (in the concerned DC branch) which improves

performance of the of the outer speed control loop. The inner

DC current loop also prevents overcurrents in the DC system.

The generator speed operating range is approximately

65rad/s<w g <158rad/s, corresponding to the off-shore grid

frequency 20Hz<f s<50Hz, and corresponding to the wind

speed range at optimal c p

5m/s<vw<12m/s. The actual allowed

wind speed range is wider, but the operation beyond these

limits is at lower coefficient of performance.

The reference generator speed w gref is calculated to enable

maximum coefficient of performance as [12]:

2

P g

r

k vw r

r

tsw gref = (1)

Where vw is the wind speed, k ts is the optimal tip speed ratio

(typically k ts=7 ), g r =77 , is the gearbox ratio, P=4 is the

number of generator poles and r r =41m is the turbine radius.

AC Network on shore SCL=10

++75kV 0.0135H 0.0135H

0.052H

110kV

110kV

90kV/110kV

Xl=10%

1.85Ω 1.85Ω

4.36Ω

-75kV -75kV

200MW

150kV

100kM DC cable

VSC - Voltage Source Converter

VSC 1

VSC 2

VSC 3

VSC I

4kV/90kV

Xl=12%

4kV/90kV

Xl=12%

4kV/90kV

Xl=12%

50MW

150kV

50MW

150kV

50MW

150kV

56MW

150kV

∆

∆

∆

Y

Y

Y

VSC 4

G41 (14MW)

G42 (14MW)

G43 (14MW)

G44 (14MW)

G1 (50MW)

G2 (50MW)

G3 (50MW)

4kV

4kV/90kV

Xl=12%

4

s y n c h r o n o u s g e n e r a t o r s

∆

∆

Y

Y

80uF120uF

120uF

34uF

80uF

Figure 2. Simulation model for the test system.



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Idc1 Idc2 Idc3 Idc4

Vdci

Idci

Figure 3. Five terminal HVDC system

ki1*1/s

kp1 Vacmref

Vacm

0.0127Mm

ki2*1/s

kp2Idcref

wgref

wgref

calculation

vwind

Idc

wg

Mφ

ki3*1/s

kp3

Hz n

180

1

=

=

ω

ς

Hz n

10

1

=

=

ω

ς

Figure 4. Controller for a generator side VSC

The VSC converter magnitude control signal ( Mm) is used

to regulate the generator terminal voltage as shown in the

upper control diagram in Figure 4. The terminal voltage is

regulated to be proportional to the generator speed (V/f

control) in order to prevent generator flux saturation and to

enable good gain in the torque control loop [8]. The ratio

between the voltage and speed reference is obtained as:

V n /wn=4kV/314.15rad/s=0.0127kVs/rad .

C. Blade angle controller

The blade angle controller plays the role of a governor in

conventional generators [12]. It is designed to preventgenerator power exceeding rated power, by using either speed

or power feedback. In the proposed wind farm, the blade

angle regulators plays more important role since generators do

not have individual converters for speed regulation. Figure 5

shows the adopted controller which uses generator power

feedback with wind speed feedforward signal.

D. Onshore VSC controller

Figure 6 shows schematic of the controller for the inverter

VSC [10]. The DC voltage is regulated by the exchange of

power on the inverter side using the variations in the converter

AC voltage angle M Φ. The 110kV AC voltage is regulated in a

feedback manner using the converter AC voltage magnitude

Mm, as given by the upper control diagram.

IV. SIMULATION RESULTS

The performance of the above system is tested using

PSCAD/EMTDC simulation for a range of inputs and faults.

Figure 7 shows the simulation results for the changes in

wind speed. Although it is unlikely that the wind speed will

vary significantly across a wind farm, in this simulation

largely different wind speeds are applied at each turbine

group, to

ki6*1/s

kp6

1.812

12

max

Pgref=50MW

Pg

vw

β

β blade anglevw - wind speedPg - generator power

Hz n 10

,1

=

=

ω

ς

Figure 5. Blade angle controller

ki4*1/s

kp4 Vacmref=110kV

Vacm

Mm

ki5*1/s

kp5 Vdcref=150kV

Vdc

Mφ

70

,1

=

=

nω

ς

Hz n

10

,1

=

=

ω

ς

Figure 6. Controller for the grid side VSC

demonstrate the concept of different operating frequencies at

each VSC converter. Figure 7a) shows the wind profile, where

also sharp wind pulses of 3m/s are applied at 9s in order

simulate gusts. Initially, the system is operating at close to

rated power as shown in Figure 7b) and the power reduces as

the wind speed reduces. We observe that around 14s the wind

speed (and power) is above rated and the output power is

regulated by the turbine blade angle controllers. Figure 7c)

shows the generator speeds for each group, which are able to

individually follow the wind speed for the particular group.

Figure 7d) presents the generator terminal voltages that are

regulated to maintain constant V/f ratio. The coefficient of

performance c p for turbine 1 is shown in Figure 7e)

demonstrating that the VSC controller is capable of

maintaining maximum c p (c pmax=0.5) at different wind speeds.

It is seen that the c p is low during transients since the

generator inertia prevents fast changes in rotational speed.

Figure 8 presents the studies of the wind speed changes

within a single group. The four generators in a group operate

at the same frequency and they do not have individual

controllers to regulate speed. Therefore, there is theoretical

possibility that a generator can fall out of step for large torque

inputs. In figure 8a) we observe a conservative scenario of

significant differences in the wind speeds on individual

turbines, including several wind gusts. The generator speed

curve (average speed) in Figure 8b) indicates that the speed is

closely controlled at the reference speed and no danger of loss

of synchronism is present.

Figure 9 shows the simulation of a 0.1s low-impedance

three-phase fault at the inverter AC terminal. During the fault,

the inverter AC voltage and DC voltage are close to zero and

there is no power transfer. However because of the generator



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Pg1

Pg4

Pg3

Pdcinv

wgen1 wgen4

wgen2

wgen3

vw1

vw4

vw3

vw2

Vg4

Vg1 Vg2

Vg3

a)

b)

c)

d)

wind gusts

e)

Figure 7. Wind farm simulation for changes in wind speed. a) wind speeds for

individual turbine groups, b) generated power, c) generator speed, d) generator

terminal voltage, e) Turbine 1 coefficient of performance.

inertia, the generator speed changes very little for the fault

duration and the system recovery is fast.

Figure 10 demonstrates the ability of the wind farm to

regulate the grid voltage for high-impedance faults or changes

in the grid loading. A 1s high-impedance fault is applied that

reduces the onshore voltage by over 10%. It is seen that the

inverter VSC is capable of regulating the AC voltage to the

reference value.

Pg41

Pg42

Pg43

Pg44

Idc4

Idc4ref

vw41

vw42

vw43

vw43

a)

b)

c)

d)

wg4wg4ref

Figure 8. Simulation for changes in wind speed within a group. a) wind speeds

on individual turbines, b) generated power, c) generator speed, d) DC current

for VSC 4.



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Pdcinv

Pg4 Pg1,Pg2,Pg3

Vdcinv Vdcref

Vacinv

Vacref

a)

b)

c)

Figure 9. Simulation of a 0.1s three-phase low-impedance fault at inverter ACterminal, at 5s. a) Generated power, b) DC voltage and c) Inverter AC voltage.

V. CONCLUSIONS

The HVDC interconnection enables location of offshore

wind farms at considerable distances from the shore. A

multiterminal HVDC connection, with an appropriate offshore

circuit design, facilitates the use of transmission converters for

turbine speed control thus avoiding converter systems with

individual turbines. It is concluded that if VSC converters are

employed it is possible to regulate the generator speed and

voltage and therefore simple permanent magnet generators can

be used.

The PSCAD/EMTDC simulation confirms capability of thewind farm to operate each VSC converter at different

frequency enabling optimum speed regulation at each turbine

group. The simulation also indicated that there are no dangers

of loss of synchronism in a single turbine group even if the

wind speed significantly differs at each wind turbine.

The proposed wind farm circuit may enable integration of

large wind farms at considerable distances from the shore, and

also make possible operation at variable speed using minimal

number of converters.

Vacinv

Vacinvref

Vdcinv

Vdcinvref

Pdcinv

a)

b)

c)

Figure 10. Simulation of a 1s three-phase high-impedance fault at inverter AC

terminal, at 5s. a) AC voltage, b) DC voltage and c) DC power.

VI. APPENDIX

TABLE A.1 CONTROLLER PARAMETERS

Notation Value Notation Value

K P1 0.1 1/kV K P4 0.008 1/kV

K I1 1 1/kVs K I4 0.05 1/kVsK P2 7 deg/kA K P5 0.7 deg/kV

K I2 80 deg/kAs K I5 2.5 deg/kVs

K P3 1.2 kAs/rad K P6 0.4 deg/MW

K I3 2.1 kA/rad K I6 0.8 deg/MWs

VII. R EFERENCES

[1] N.M.Kirby, M.J. Luckett, L.Xu, W.Siepmann, “HVDC Transmission for

large off shore wind farms” IEE AC-DC Power Transmission, November

2001, London, Conference publication no 485, pp 162-168.



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[2] L.Holdsworth, N.Jenkins, G.Strbac “Electrical Stability of Large,

Offshore Wind Farms. IEE AC-DC Power Transmission, November

2001, London, Conference publication no 485, pp 156-161.

[3] Eltra, Transmission Systems Planning “Specifications for Connecting

Wind Farm to Transmission Networks” document no 74557, Eltra

Denmark, 2000, http://www.eltra.dk

[4] Kjell Ericsson "Operational Experience of HVDC Light" Seventh

International Conference on AC-DC Power Transmission. IEE. 2001,

pp.205-210. London, UK .

[5] B R Andersen, L Xu, K T G Wong, “Topologies for VSC transmission,”

Seventh International Conference on AC-DC Power Transmission (IEE

Conf. Publ. No.485). IEE. 2001, pp.298-304. London, UK .

[6] C.E.Grund at all. “Dynamic performance characteristics of North

American HVDC Systems for transient and dynamic stability

evaluations” IEEE Transactions on PAS-100, no 7, 1981, pp3356-3364

[7] Manitoba HVDC Research Centre, “PSCAD/EMTDC User Manual,”

Tutorial Manual, 1994.

[8] B.K. Bose “Modern Power Electronics and AC drives” Prentice Hall

2002

[9] B. Ooi, Xiao Wang, “Voltage Angle Lock Loop control of the boost type

PWM converter for HVDC application,” IEEE Trans. on power

electronics, vol 5 no 2, April 1990, Pp 229-235.

[10] D. Jovcic L.A.Lamont, L.Xu: “VSC Transmission model for analytical

studies" Power Engineering Society General Meeting, 2003, IEEE,

Volume: 3, 13-17 July 2003, Pages:1737 - 1742

[11] J L Thomas, S Poullain, A Benchaib, “Analysis of a robust DC-bus

voltage control system for a VSC transmission scheme,” Seventh

International Conference on AC-DC Power Transmission IEE. 2001, pp.119-24. London, UK .

[12] S. Hier, Grid Integration of Wind Energy Conversion Systems, John

Wiley and Sons 1998.

VIII. BIOGRAPHY

Dragan Jovcic (S’97, M’00) obtained a B.Sc. in Control Engineering from

the University of Belgrade, Yugoslavia in 1993 and a Ph.D. degree in

Electrical Engineering from the University of Auckland, New Zealand in

1999.

He is currently a lecturer with the University of Aberdeen, Scotland where

he has been since 2004. He also worked as a lecturer with University of

Ulster, in period 2000-2004 and as a design Engineer in the New Zealand

power industry in period 1999-2000. His research interests lie in the areas of

FACTS, HVDC and control systems.

Documents

WF Integration Through MTDC-VSC