300 MW Variable Speed Drives for Pump-Storage Plant Application ...

May 2004May 2004

300 MW Variable Speed Drives forPump-Storage Plant Application Goldisthal

Aurélie Bocquel

APCG / 4BOC004 (300MW-Goldisthal 13-05-2004).PPT

205.2004 APCG / 4BOC004 (300MW-Goldisthal 13-05-2004).PPT / A.BOCQUEL / Be


Content

� Major benefits of the cyclo-converter driven doubly-fed inductionmachines

� Topology of the pump storage power station� Cyclo-converter topology� Mathematical model and simulation� Power flow at constant torque in turbine and pump operation� P/Q-diagram of the induction machine at different speeds and at the

limits of the cyclo-converter� Control structure of the doubly-fed induction machine with cyclo-

converter� Measurement results (torque steps, synchronisation, speed variation)

� Conclusions


Principle of a pump-storage plant

Maximum dam levelMaximum dam level

Minimum levelMinimum level

Maximum dam levelMaximum dam level

Minimum levelMinimum levelAmount ofAmount of

exchanged waterexchanged water


GoldisthalGoldisthal



Single-line diagram of Goldisthal pump-storage station

380 kV, 50 Hz

IMSM

380 kV, 50 Hz

380 kV, 50 Hz

IM SM

18 kV 18 kV18 kV 18 kV

� 4 turbines with a power of about 300 MVA each� 2 of the electrical machines are designed as converter

driven doubly-fed induction machines� The converter feeding the rotor of the machine is with a

power of 100 MVA one of the biggest cyclo-converters inthe world

� The efficiency of the turbineespecially at partial loads canbe increased by optimising thespeed of operation.

� The converter only needs tosupply about 30 % of the totalpower of the machine;compared to a fully fedsolution the installed powerand the converter losses arereduced.

� The delivered power of theinduction motor can becontrolled with a high dynamicwhich is advantageous for gridstabilisation.

Major benefits of the cyclo-converter driven doubly-fed induction machines


Machine cavern Goldisthal - different phases of the construction



Rotor of Rotor of thethe inductioninduction motormotor underunder constructionconstruction


Machine cavern Goldisthal



AssemblyAssembly of of thethe rotorrotor of ofthethe inductioninduction machinemachine,,statorstator completedcompleted

Machine cavern Goldisthal


� 4 turbines which can work in pumping and generating mode

� 2 turbines equipped with synchronous machines (331 MVA generative power each) :� classical excitement� operation close to their rated power� high efficiency� fixed speed

� 2 turbines connected to doubly-fed induction machines (340.4 MVA generative power each)� dynamic power control� controlled with the cyclo converter� speed range of –10 % to +4 % of rated speed� optimisation of the efficiency varying the speed� operation at partial loads efficient

� 2 start-up converters of 40 MW of rated power each� can start and stop the synchronous and induction machines� each converter can be connected to each of the 4 machines� during start up the cyclo converter excites the induction machine� current source converters� 24 pulse thyristor bridge on the line side� 6 pulse thyristor bridge on the machine side

Topology of the pump storage power station




100 MVA cyclo-converter for the doubly-fed induction machine

3

3

3

3

3

3

3 3

crow bar

18 kV, 50 Hz

neutralpoint

Doubly-fed induction motor (generator)

3 3

3 3

YD

DY

3

3

3

3

3

3

3 3

crow bar

3 3

3 3

YD

DY

3

3

3

3

3

3

3 3

3 3

3 3

YD

DY

Cubicle carrying three independent12 pulse thyristor bridges(width/depth/height in mm: 3400 / 1400 / 2400)

� 1 of these 3 is provided for redundancy (afailing bridge is disconnected via thepyrobreakers)

� Crow bars on the output protect the thyristorsagainst overvoltages and can take overovercurrents generated by the machine

� Each phase of the rotor of the induction machine is fed by two anti-parallel 12 pulse thyristor bridges

� The anti parallel bridges drive circulating current to avoid overvoltages� 3 parallel thyristor bridges carry the rated current

Cyclo-converter topology

crow bar

rated current: 8970 Arated voltage: 3716 V

DC choke (see next slide)


The DC chokesdriving thecirculating currentimprove theprotection of theconverter

Rated current: 6365 A

DC chokes




Schematic model of the induction motor(generator)

i B R RL L i B

� Detailed modelling of the cyclo-converter (72 Thyristors), thecrow bars, the induction machine and the grid

� Simulation together with the drive control� Importance of the simulation

� design and verification of the power components and protection� design and verification of the converter control� calculation of the line distortions

Mathematical model and simulation

Grid

s

usB

s r�s

Lh �r Bj�rB

�r

Cyclo-converterur

B

r

�s Bj�sB

�s B

�r B



Simulation results of athree-phase short circuitin the grid

-10

-5

0

5

10

roto

r vol

tage

s [k

V]

-60

-30

0

25

50

roto

r cur

rent

s [k

A]

-20

-10

0

10

20

cycl

ocon

verte

r cur

rent

s [k

A]

0 0.05 0.1 0.15-50

-25

0

25

50cr

ow b

ar c

urre

nts

[kA]

t [s]

� Due to the short circuit the rotorcurrent rises quickly and reachesthe overcurrent limit of the crowbars.

� The crow bar is triggered andthe pulses of the cyclo-converterare shifted to the end position.

� The fired crow bar greatly reducesthe voltage at the cyclo-converterand takes over the largest part ofthe rotor current.



Power flow at constant torque in turbine and pump operation 1/2

Pr , Ps ,Pmech

Ps

Pmech

Pump operation

nns = nsyn nmax

Pgrid

Pmech

PCYC

Pr

Ps

PD

Pgrid

Pmech

Pr

PCYC

Ps

PD

converter+Transformer

subsynchronous oversynchronous

PNenn

Assumption T = TNom

0nmin


Air gapIM

Air gapIM

nPr = Ps ____ -1

ns

-PNom

Pr , Ps ,Pmech

Pmech

Pgrid

Pr

PCYC

PD

Ps

Air gapIM

Pmech

Pgrid

PCYC

Pr

PD

Ps

Air gapIM


Turbine operation

Pr

PD

Ps

PCYC

Pgrid

2�Pmech = ______ n T

60

oversynchronous subsynchronous

n

Assumption T = TNom

Ps

Pmech

n -nmax 0

Pmech

nPr = Ps ____ -1

ns

-nminns = -nsyn



� The doubly-fed induction machine can operate in a wide speed range relatedto the rating of the cyclo-converter

� Turbine operation: The mechanical power delivered from the turbine isconverted into electrical active power generated in the stator (Ps) throughthe air gap of the machine and partly in the rotor (Pr)

� Oversynchronous turbine operation: Thanks to the cyclo-converter, the activepower in the rotor can be added to the stator power to generate the activegrid power (Pgrid).

� Subsynchronous turbine operation: the cyclo-converter feeds the rotor withpart of the active stator power necessary to generate the mechanical power.

� Induction machine at synchronous speed:� no active power is generated in the rotor� Ps is equal to the mechanical power� excitement by the cyclo-converter with constant current like a synchronous machine.

� The reactive power of the induction machine can be controlled independentof the active power by the cyclo-converter.


Power flow at constant torque in turbine and pump operation 2/2



Simulation of a speed variation at constant torque in pump operation

0 0.2 0.4 0.6 0.8 1 1.2-5

-2.5

0

2.5

5

roto

r vol

tage

s [k

V]0 0.2 0.4 0.6 0.8 1 1.2

-15

-7.5

0

7.5

15

roto

r cur

rent

s [k

A]

0 0.2 0.4 0.6 0.8 1 1.290

95

100

104

mac

hine

spe

ed [%

]

t [s]

� At steady state operation therotor flux amplitude and themagnetising current of thestator are constant.

� With constant torque, therotor current amplitude isconstant and independent ofthe slip of the machine

� The rotor voltage amplitudevaries linearly with theangular velocity of the rotorflux vector related to therotor angle.


P/Q-diagram of the inductionmachine at different speedsand at the limits of the cyclo-converter


-500 -400 -300 -200 -100 0 100 200 300 400 500-400

-300

-200

-100

0

100

200

300

400

500

600

700

P [MW]

Q [

MVA

R]

353 rpm 346 rpm

300 rpm

295 rpm

n = 106 %no n = 104 %

no

n = 88,5 %no

n = 90 %no

stator voltage: 18 kV,grid frequency: 50 Hz,maximum rotor current and maximum rotor voltage

� Advantages of the doubly-fed inductionmachine:� Control of torque and flux with a high

dynamic in all operation modes� Stable operation in all operation modes

within the limits of the converter depictedby the P/Q circlesNote: Operation of synchronousmachines close to those power limits(e.g. with large rotor flux angles) can lead toinstabilities

� The circles are determined by the maximumcurrent of the cyclo-converter

� In the range of capacitive reactive power thepower is limited by the maximum outputvoltage of the converter


� Flux orientedcontrol based ona flux observer(extended Kalman filter)estimating the amplitude andangle of the flux in the machine.

� Torque calculation using theestimated flux.

� Synchronisation control:� control of the flux of the

machine with the cyclo-converter until electrical synchronisationbetween the stator voltage and the grid is reached

� synchronisation below the synchronous speedpossible to reduce the start-up procedure duration

� the line breaker is closing with damped transientcurrents and resulting torque jerks

ASM


Control structure of the doubly-fed induction machine with cyclo-converter

Flux Control

SpeedLimitation

4

36

Fluxand

torqueobserver

76SiCurrentControl

un(L1L2, L2L3)

3

6

�n,p(R,S,T)

i(m)*

i(l)*

�r

�r

�rs

�

�r

2

i(m,l)*

�rs�r

urO(m,l)

control set

is(R,S,T)

T'*

�Rotor�

� 'r*

rs�

un (L1L2, L2L3)

us(R,S,T)

��r

�r*

^

^

^

T^

^ur(R,S,T)

T̂

PLL-Grid�un��un^ ^

2

�r

^

Synchronisation Control

VoltageFeed

Forward

^

��*sync

us(RS,ST,TR)

ir(R,S,T)

Calculation ofCirculating &Rotor Current

irp(R,S,T)

irn(R,S,T)ir(R,S,T)

ik(R,S,T)

TorqueControl

PowerCalculation

Qcos��

3

T*

T

R



Current control structure of the cyclo-converter

IM

irn (R,S,T)

ir*(R,S,T)

calculation ofcirculating &rotor currents

i r (R,S,T)

rotor currentcontrol

i r_circ (R,S,T)*

ir_circ (R,S,T)

PLL andcontrol set

S i

Sni

calculation ofswitching angle

�p(R,S,T)

�n(R,S,T)3

un(L1L2,L2L3)

i rp (R,S,T)

* reference values

circulatingcurrent control

3

u r(R,S,T)*

u r_circ (R,S,T)*

u r0(R,S,T)

336

36

3

3

3

3

3

3

3

3

3

331

� The reference rotorreactive current andtorque generatingcurrent are transformedinto rotor coordinatesand compared to thecalculated rotor currentcomponents ir by therotor current controller

� using voltage feed-forward estimating therotor voltage increasesthe dynamic of thecurrent control

� circulating currentcontroller is onlyactivated if the rotorcurrent is near zero


Simulation of a torque step from zero to the rated torque


Torque steps - simulation and measurement

0.2 0.25 0.3 0.35 0.4 0.45 0.50

5

10

Torq

ue [

MN

m]

0.2 0.25 0.3 0.35 0.4 0.45 0.5-5

0

5

Rot

or v

olta

ges

[kV]

0.2 0.25 0.3 0.35 0.4 0.45 0.5

-10

0

10

Rot

or c

urre

nts

[kA]

0.2 0.25 0.3 0.35 0.4 0.45 0.5

-10

0

10

Brid

ge c

urre

nts

(1 p

h) [k

A]

t [s]

Measurement of a torque step on site(about 40% of rated torque)

2000 kNm/Div

t [20 ms / Div]

This dynamic performances of the drive canbe used to dampen active power oscillationsin the grid.



Connection to the grid at 95 % of the rated speed

� At the end of the starting-up of theinduction machine, the cyclo-converteris synchronising the stator voltage tothe grid

� When electrical synchronisation isreached the line breaker at the statorof the machine is closed.

� The transient currents and resultingtorque jerks after closing the breakerare well dampened and disappear afterabout 500 ms

� Compared to a synchronous machinethis drive can synchronise much fasterto the grid

� the start-up time can be reduced byaccelerating the rotor to only 92 % ofthe synchronous speed andsynchronising to the grid with thecyclo-converter

h:min:s12:59:01,5 12:59:02,0

350

0

-350350

0

-35011000

0

-11000

20000

0

-2000015000

0

stator phase current [A]

rotor phase current [A]

reactive power [MVar]

active power [MW]

torque [kNm] t [250 ms / Div]

Measurement of the drive behaviour while closing theline breaker after synchronisation at almost 95 % ofthe synchronous speed



Speed variation at turbine operation with 300 MW

� Each of the 2 water pipes inGoldisthal is feeding 1 pair ofturbines, consisting of one inductionmachine and one synchronousmachine.

� The induction machine is running inturbine operation at about 300 MW(rated torque: 9000 kNm).

� the synchronous machine is takenout of operation and the waterpressure of the turbine of theinduction machine is changingdrastically

� the delivered power of the cyclo-converter driven induction machine iskept constant to 300 MW bychanging the speed of the turbine.

-15000

h:min:s15:58:40 15:58:45 15:58:50 15:58:55 15:59:00

350

0

-350350

0

-35011000

0

-11000

15000

0

rotor phase current [A]

reactive power [MVar]

active power [MW]

torque [kNm]

t [2,5 s / Div]

Measurement of dynamical speed variations caused bya load rejection of the synchronous machine around thesynchronous point (turbine operation with 300 MW)


Cyclo-converter DrivenInduction Machine

Synchronous Machinewith Standard Excitement

Step response time (P, Q)(zero to rated) 150 ms several seconds

Synchronisation time 10 s 20 s or more depending onmechanical load conditions

Synchronisation transientreactions

damped after500 ms

damped after severalseconds

Average efficiency of thewhole storage unit(turbine + drive)

80 % 70 %


advantages of the cyclo-converter driven induction machinedrive compared to a synchronous machine drive

Conclusion 1 / 2


� A 300 MVA doubly-fed cyclo-converterdriven induction machine drive for thepump-storage station Goldisthal wassuccessfully put in operation.

� The cyclo-converter only needs todrive a third of the total power of themachine at the most. The efficiency ofthe turbine is increased by varying thespeed (–10 % to 4 %).

� The high-dynamic flux oriented control(torque step response time <10 ms)can be employed for a very fast powercontrol for the stabilisation of the grid.

� Application of the drive system couldalso be advantageous for pump-storage stations with much less power(e.g. 50 to 100 MVA).


Shaft between the induction machine and the turbine

Conclusion 2 / 2

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300 MW Variable Speed Drives for Pump-Storage Plant Application ...

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