Transcript
Innovative Solutions for the Electric Power Industry
HVdc Transmission
Presented by: TransGrid Solutions Inc www.transgridsolutions.com
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
AC/DC Conversion Process
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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
R
1
3
5 Ud
S T
4 HVdc Transmission Tutorial-Transpower
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2 December 2010 ©TransGrid Solutions Inc., 2009
Six Pulse Bridge V3
V1 R
V5 S
T
S
V2
T
R
V4
V6
Operation at zero delay angle – ideal commutation HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Six Pulse Bridge V1
V1
V3
V5
R
S
T
S
T
R
V2
V4
V6
V2
V3
V4
V5
V6 Ud
TS
RS
RT
ST
SR
TR
Operation at zero delay angle – ideal commutation HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Six Pulse Bridge V1
V2
V3
V4
V5
V6
Ud
Operation with delay angle – ideal commutation HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Rectifier Equations U RS 2 E LL rms . sin(t ) Ud
3 2 E LL rms
Instantaneous line-to-line voltage
sin(t ) dt
The limits of integration are (/3 + ) to (2/3 + )
Ud
3 2 ELL rms
HVdc Transmission Tutorial-Transpower
cos( )
Average DC voltage – ideal commutation
December 2010 ©TransGrid Solutions Inc., 2009
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
R
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3
5 Ud
S T 4
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HVdc Transmission Tutorial-Transpower
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December 2010 ©TransGrid Solutions Inc., 2009
Commutation Process
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Commutation Process
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Commutation process
Ud
3 2E
cos( )
U d U dio cos( )
3IX c
3IX c
1 U dio [cos( ) cos( )] 2
Udio is the ideal no load direct voltage =
HVdc Transmission Tutorial-Transpower
3 2E
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Operation
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Equations ++ = +=
Inverter Equations
=+ U d U dio cos( )
3 IX c
1 U dio [cos( ) cos( )] 2
1 U d U dio [cos( ( )) cos( )] 2
3IX c 1 U d U dio [cos( ) cos( )] U dio cos( ) 2 HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Converter Operation I dc
U dr U di Rdc
R
Udr
HVdc Transmission Tutorial-Transpower
Udi
December 2010 ©TransGrid Solutions Inc., 2009
Converter Operation Rectifier A
F C
A‘
E
D
F‘
C‘
Udio
D‘
XC
Udi
Udio cos
Rectifier reduced Voltage Operation Udr
Udio cos
Normal Operation
R
Udio
B
Inverter
XC
Principle of operation of HVdc system HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Reactive Power
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Reactive Power
=0, =0, =0, Q=0
Operation at zero delay angle – ideal commutation HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Reactive Power
≠0, =0, ≠0, Q>0
Operation at delay angle – ideal commutation HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Reactive Power
•
Non-ideal commutation cause an increase in
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Reactive Power 1 U dio [cos( ) cos( )] 2 1 3 2E Pdc U d .I d . I d cos( ) cos( ) 2
Ud
Pac 3 E I ac cos( ) and
Pac
3 2
I ac
6
I dc
E I d cos( )
Assuming a lossless converter: Pac=Pdc , therefore: 1 3 2E 3 2E . I d cos( ) cos( ) I d cos( ) 2 U 1 cos( ) cos( ) cos( ) d 2 U dio HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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. HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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 HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Q converter
Q in MVAR
Q filter
+Q
0 -Q
1 pu HVdc Transmission Tutorial-Transpower
Power in pu December 2010 ©TransGrid Solutions Inc., 2009
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
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Reactive Power Control-Pole 3 • 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 HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Reactive Power Control-Pole 3 • The 110kV busbar voltage at Haywards is controlled by means of tap changers of interconnection transformers T1, T2, T5
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Harmonics
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December 2010 ©TransGrid Solutions Inc., 2009
Harmonics Thyristor LCC HVdc converters produce harmonics on the dc and ac side.
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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.
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
AC Current Harmonics
HVdc Transmission Tutorial-Transpower
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AC Current Harmonics
HVdc Transmission Tutorial-Transpower
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DC Side Voltage Harmonics DC Harmonics
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
DC Harmonics
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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. HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Characteristic Harmonics - 12 Pulse converter
Y
Y
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Characteristic Harmonics - 12 Pulse converter 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…
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Characteristic Harmonics - 12 Pulse converter
Primary side current of the YY transformer
Primary side current of the YD transformer
Primary side current of the 12-pulse converter HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Characteristic Harmonics - 12 Pulse converter
7x30o IY_5
ID_5
IY_1
ID_7
IY_7
IY_1
30o
30o
ID_1
ID_1
5x30o
5th and 7th harmonic cancellation in 12-pulse converter
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Commutation Failure
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
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
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Commutation Failures
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6
2
3
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S R 1
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Commutation Failures
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Commutation Failures
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2
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S R 1
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Commutation Failures
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2
3
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S R 1
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Commutation Failures
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S R 1
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Inverter Commutation Failures
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
Thank You
HVdc Transmission Tutorial-Transpower
December 2010 ©TransGrid Solutions Inc., 2009
