TGS HVDC Transpower part2HVdc Transmission Tutorial-Transpower December 2010 ©TransGrid Solutions...

HVdc Transmission Tutorial-TranspowerDecember 2010

©TransGrid Solutions Inc., 2009

Innovative Solutions for the Electric Power Industry

HVdc Transmission

Presented by:

TransGrid Solutions Incwww.transgridsolutions.com



AC/DC Conversion Process



Ud

1 3 5

4 6 2

R

ST

Six Pulse Bridge

• valves are turned on (triggered) sequentially, at any moment one of the upper valves and one of the lower valves (from a different phase) are conducting• one valve is triggered every 600



Six Pulse Bridge

Operation at zero delay angle – ideal commutation

V1 V3 V5

V4 V6

R S T

TS R

V2



Six Pulse Bridge


V1 V3 V5

V4 V6

R S T

TS R

V2

RS ST TRUd

RT SRTS

V1 V3 V5V2 V4 V6



Six Pulse Bridge

Operation with delay angle – ideal commutation

Ud

V3 V5V2 V4 V6V1



Instantaneous line-to-line voltage

The limits of integration are (/3 + ) to (2/3 + )

dttEU rmsLLd )sin(23

)cos(23

rmsLLd

EU

Rectifier Equations

)sin(.2 tEU rmsLLRS

Average DC voltage – ideal commutation



Commutation Process

• Due to the inductance in the commutation path there is an overlap during which current in the outgoing valve decays to zero and rise in the incoming valve

• The overlap angle depends on the commutation inductance (Xc), dc current and

Ud

1 3 5

4 6 2

RST



Commutation Process



Commutation process

Udio is the ideal no load direct voltage =

c

dIXEU 3)cos(23

)]cos()[cos(213)cos(

dio

cdiod UIXUU

E23



Inverter Operation



Inverter Operation

• Angle is defined as the angle between the end of current and the start of voltage reversal on the out going valve.

• Gamma should be large enough to allow valve successfully turn off



Inverter Equations + + =

+ =

= +

Inverter Equations

)]cos())([cos(21 diod UU

)]cos()[cos(213)cos(

dio

cdiod UIXUU

c

diodiodIXUUU 3)cos()]cos()[cos(

21



Udr Udi

R

Converter Operation

dc

didrdc R

UUI



Converter Operation

Principle of operation of HVdc system

A

BC

D

F

E

A‘

C‘

D‘

F‘

XC XCR

Normal Operation

Rectifier reduced VoltageOperation

Udi

oU

dio

cos

Udr U

di

Udi

oco

s Udi

o

RectifierInverter



Reactive PowerReactive Power



Reactive Power


=0, =0, =0, Q=0



Reactive Power

Operation at delay angle – ideal commutation

≠0, =0, ≠0, Q>0



Reactive Power

• Non-ideal commutation cause an increase in



Reactive Power

)]cos()[cos(21 diod UU

)cos()cos(23.21.

ddddc IEIUP

dcacacac IIandIEP

6)cos(3

)cos(23 dac IEP

Assuming a lossless converter: Pac=Pdc , therefore:

)cos(23)cos()cos(23.21

dd IEIE

dio

d

UU

)cos()cos(21)cos(



Reactive Power

• The operation of the converter results in a phase angle between the fundamental component of the currents and the phase voltages

• This phase angle in principle is similar to a power factor

• This means that the converter whether a rectifier or an inverter will consume reactive power

• A rule of thumb is that a typical converter at nominal firing angles will consume approximately 60% of its rating in reactive power

• In precise terms the reactive power consumption is a function of the delay angle α , the overlap angle μ and the converter power at that point of operation.



Reactive Power

• The consumption of reactive power by the converter has to be compensated

• Shunt capacitor banks, or a combination of shunt banks and the shunt ac filters are used for this purpose

• As the dc power of the converter is ramped upwards, its consumption of reactive power increases– Shunt elements must be switched on to avoid large reactive

power consumption from the ac system

• Synchronous condensers, SVC’s or STATCOM’s can also be used for reactive power management



Power in pu

Q in MVAR

1 pu

Q filter

Q converter

+Q

-Q

0



Reactive Power Control-Pole 3

• Reactive power control (RPC) is part of the station control

• RPC functions:– Harmonic Performance Control– 220kV voltage control– 110kV voltage control– Reactive Power Control– Interconn. Transformer loadflow between 220<->110kV networks– 220kV overvoltage limitation




• AC filter subbanks are switched by the Harmonic Performance Control based on predefined DC current limits– A further function can limit the converter DC current if there

aren’t enough subbanks available.– Switching of further subbanks is inhibited if the AC voltage is

above a limit (1.09pu)• The 220kV busbar voltage at Haywards is controlled by

means of STATCOMs and SCs 1 to 4– shunt reactors and AC filters will be used if the SCs operate at

their limit or are not available– SCs will be controlled by reactive power. The target is to bring

back the STATCOMs steady state output to zero




• The 110kV busbar voltage at Haywards is controlled by means of tap changers of interconnection transformers T1, T2, T5



Harmonics



Harmonics

Thyristor LCC HVdc converters produce harmonics on the dc and ac side.



Characteristic Harmonics

• Classical theory assumptions:– The three phase supply voltages are displaced by 120o and

consist only of fundmental frequency.– The direct current is constant.– The valves begin conducting at regular time intervals.– The commutation impedance in each phase is the same.



AC Current Harmonics

• For a 6-pulse converter the characteristic harmonics in the output current are of the order 6n+/-1 where n = 1,2,3,4,....

• The harmonics generated are of the order 5,7,11,13,17,19,.... and magnitude of In = 6 Id/n

• Magnitude of harmonics generally increases as increase due to the increased short circuit voltage

• Magnitude of harmonics decrease with increasing



AC Current Harmonics



DC HarmonicsDC Side Voltage Harmonics



DC Harmonics

• The order of the harmonics in a six pulse converter is given by 6n where n = 1,2,3,4,....

• The harmonics generated are of the order 6,12,18,24,...• As increases the harmonics magnitude increase as well.• The higher order harmonics increase faster with .

• Effect of DC side harmonics:– DC current ripple which can cause non-harmonic oscillations in AC

currents in asynchronous systems– Current zero specially at light load– Communication interference

• Can be reduced by increased smoothing reactor size or DC filter improvement



DC Harmonics



Characteristic Harmonics - 12 Pulse converter

If the converter consists of two bridges one with star/star connected transformer and the other with a star/delta transformer, their voltages will be 30 degrees out of phase and so the harmonics will accordingly be out of phase.

Since 30 degrees of main frequency correspond to half cycle of 6th harmonic, therefore the 6th harmonic will be in phase oposition in the two bridges, while for the 12th

harmonic they will be in phase.

Similar effect is also applicable for the ac current harmonics.




Y

Y




The current harmonics will be of the order 12n±1. This means:• 11th and 13th harmonic for n=1• 23rd and 25th harmonic for n=2, etc….The dc voltage harmonics will be of the order 12n. This means:• 12th harmonic for n=1• 24th harmonic for n=2, etc…




Primary side current of the 12-pulse converter

Primary side current of the YY transformer Primary side current of the YD transformer




5th and 7th harmonic cancellation in 12-pulse converter

IY_1

ID_1

IY_5ID_5

30o

5x30o

IY_1

ID_1

IY_7ID_7

30o

7x30o



Non Characteristic Harmonics

Possible Causes:• Firing Error

– Can cause even-numbered harmonics or DC component in AC currents• AC voltage unbalance (negative sequence) or distortion• Direct current modulation from the remote station• Unbalance of converter components (e.g. transformer reactances)

Ways to improve:• Reduced firing angle tolerances• Reduce converter transformer reactance tolerance• Increase smoothing reactor and dc filter effectiveness



Commutation Failure



Inverter Commutation Failures

Commutation failures are the result of the incoming valve failing to take over the current, or re-fire of the outgoing valve. Commutation failures are due to:

• AC system faults & disturbances.

• DC faults or disturbances.

• Equipment failures



6 2

1 3 5

T

S

R

4




Thank You

TGS HVDC Transpower part2HVdc Transmission Tutorial-Transpower December 2010 ©TransGrid Solutions...

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Transcript of TGS HVDC Transpower part2HVdc Transmission Tutorial-Transpower December 2010 ©TransGrid Solutions...