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HVDC Training Course- Steady State -
DIgSILENT GmbH
HVDC Training Course 2
Introduction
• „HVDC“ - general definition:
High Voltage Direct Current Transmission
Application in Long-Distance and Cable Transmission Systems
• Part of FACTS:Flexible AC Transmission Systems
2
HVDC Training Course 3
AC vs. DC Transmission
HVDC Training Course 4
AC vs. DC Transmission
AC Transmission• Easy, robust and reliable
• Rather cheap• Only applicable to systems with
the same nominal frequency
• Cable capacitance limits the distance of submarine cables (or increases the cost because of additional compensation)
• Contribution to short-circuit currents
• Dynamic/Transient stability limits
DC Transmission• More complex, power electronics,
including controls are required• Expensive technology• Can connect systems of different
nominal frequency/asynchronous systems
• No limitation by cable capacitance
• No contribution to short-circuit current in interconnected systems
• No dynamic or transient stability limits
3
HVDC Training Course 5
AC vs. DC Transmission
HVDC Advantages• Possibility to connect two networks with different frequency or
different power-frequency control strategies.
• Transmitted power can be controlled and can be held constant independent of network situation within power range.
• Control is flexible and different control strategies can be used.
• The control is fast acting, so the transmitted power can be changed rapidly.
• HVDC systems can also be used in parallel to AC lines for stabilizing the network.
HVDC Training Course 6
AC vs. DC Transmission
AC vs. DC Transmission• Break-even-distance with
overhead lines at about 600-800km
• Break-even-distance is much smaller for submarine cables (about 50 km)
• Distance depends on several factors (both for lines and cables) and an analysis is required.
DC transmission can only be justified, if AC-transmission is impossibleor extremely expensive because of additional compensation
4
HVDC Training Course 7
Circuits and Components
HVDC Training Course 8
Diode Turn-On Turn-On & Turn-Off
Thyristor GTO IGBT
Valves/Semiconductor Devices
5
HVDC Training Course 9
Diode Thyristor GTO IGBT
Valves/Semiconductor Devices
Classification of valves into three groups according to their controllability:
Ideal Characteristic:
HVDC Training Course 10
Valve Characteristic Parameters
• Current carrying capability– e.g.: 1000A...4000A (Thyristor, GTO)
• Forward blocking voltage– e.g. 8-10kV (Thyristor)– e.g. 5-8kV (GTO)– e.g. 3-5kV (IGBT)
• dv/dt capability • di/dt capability• Turn-on time and turn-off time• On-resistance (and associated losses)• Switching losses
6
HVDC Training Course 11
HVDC Valves
Thyristor element with Thyristor Control Unit (TCU)
Thyristor Module
HVDC Training Course 12
HVDC Valve Halls
Chandrapur - PadgheHVDC Transmission1500MW, ±500kV800km
New Zealand Inter-Island HVDC Link1240MW, ±300kV600km
7
HVDC Training Course 13
Semiconductor Capabilities
• Capability and usability of valve devices are depending on:
– Rated Voltage
– Rated Current
– Switching Speed
HVDC Training Course 14
Snubber Circuits
• Snubber circuits are used to change the current and voltage waveform of the valve to reduce the electrical stresses on the switching devices to safe levels.
• RC – Snubbers:– Limit the maximum voltage– Limit dv/dt during turn-off or recovery
• LR – Snubbers:– Limit di/dt during turn-on
8
HVDC Training Course 15
Line-Commutated Converter
Vdc
Idc
Vac
Vdc Vac
Idc
HVDC Training Course 16
Self-Commutated Converter
Udc Uac
ACUDC
U
9
HVDC Training Course 17
Self/Line-Commutated Converters
Self-Commutated:• Very good P and Q controllability• Low Harmonic contents (high
switching frequency)• Q can be controlled/provided by
the converters• Independent from the strength of
AC network• High no load losses• New technology (long term testing
required)• Only possible up to 200..330MW
Line-Commutated• Only P controllability, Q resulting• High Harmonic contents, large
filters required• High Q consumption of both
rectifier and inverter• Short-Circuit capacity of network is
important for operation• No load losses can be neglected• Well established, robust technology
• Efficient for high power transfers
HVDC Training Course 18
Self/Line Commutated Converters
Self-Commutated• Modular concept with
standardized sizes possible. • DC circuit is by ‘nature’ a bipolar
technology. Two conductors are required.
• Using turn on/turn off IGBT valves• Very fast and flexible
controllability possible,frequency control possible
• No need of communication between stations
Line-Commutated• always tailor made to suit a specific
application• Can be designed as a monopolar
or bipolar system.• Well established, robust technology• Using turn on GTO valves• Good controllability,
No frequency control
For high power transfers (>200MW), the line commutated converteris still the only possibility
10
HVDC Training Course 19
Applications
Self-Commutated
• HVDC light (<330MW)• FACTS (UPFC, STATCOM)• Variable Speed drives
(machine side)• Doubly-fed induction machines
Line-Commutated
• HVDC (High Power)• Back-to-Back HVDC• Synchronous machine drives• DC-machines
HVDC Training Course 20
Line-Commutated Converter
11
HVDC Training Course 21
Analysis of the Line-Commutated Converter
HVDC Training Course 22
0.015 0.012 0.010 0.007 0.005 0.002 ..
200.00
100.00
0.000
-100.00
-200.00
Rectifier: Phase Voltage/Terminal DC in kVRectifier: Phase Voltage/Terminal DC in kVInverter: Phase Voltage/Terminal DC in kVRectifier: Line-Line Phase Voltage B/Terminal AC in kV
DIg
SILE
NT
DC-Voltage Wave-Forms
α
α
12
HVDC Training Course 23
DC-Voltage
lllllllld UududuVππ
θθπ
θθπ
π
π
π
π
23ˆ3)cos(ˆ3)(3 6
6
6
6
0 ==== ∫∫−−
Diode Rectifier:
Thyristor Rectifier:
ααππ
απαππ
θθπ
θθπ
ααπ
απ
απ
απ
coscos6
sin2ˆ3
6sin
6sinˆ3)cos(ˆ3)(3)(
0
6
6
6
6
dll
lllllld
Vu
ududuV
=⎟⎠⎞
⎜⎝⎛
=⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛ +−−⎟
⎠⎞
⎜⎝⎛ +=== ∫∫
+
+−
+
+−
HVDC Training Course 24
DC-Voltage
lld Uq
qsV ⋅⋅⎟⎟⎠
⎞⎜⎜⎝
⎛⋅=
32sin0
0π
π
n-pulse Bridge:
12-pulse Thyristor Rectifier:
Ideal no-load dc voltage
s0 = sum of valves in seriesq = number of branches in parallel
)cos(232
)cos(32
3sin34)cos(0
απ
αππ
α
⋅⋅⋅
=
⋅⋅⋅⎟⎠⎞
⎜⎝⎛⋅
==
ll
lldd
U
UVV
13
HVDC Training Course 25
AC-Current Wave-Forms
0.030 0.020 0.010 0.000 [s]
0.150
0.100
0.050
0.000
-0.0500
-0.1000
-0.1500
REC 1: Phase Current A/Terminal AC in kA
DIgSILENT Rectifier AC-Current
Date: 2/14/2003
Annex: 1 /7
DIg
SILE
NT
3π
3π
−DI
HVDC Training Course 26
AC-Current Fund. Frequency
DDDAC IIdIIπ
πππ
θθπ
π
π 232
3sin
3sin2cos
22 3
3
=⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛−−⎟
⎠⎞
⎜⎝⎛== ∫
−
RMS-value of fundamental frequency component:
Power Factor:
DDDCACllAC IVPIUP === ϕcos3
απ
ϕπ
cos23cos26
DllDllAC IUIUP ==
αϕ coscos =
14
HVDC Training Course 27
Commutation
HVDC Training Course 28
Commutation
Id
v1(t)
v2(t)
i1(t)
i2(t)
02112 =−+−
dtdiL
dtdiLvv
dIii =+ 21
dtdiLvv 2
12 2=−
15
HVDC Training Course 29
Commutation
( )dtdiLtUvv c
212 2sin2 =+=− αω
( ) ∫∫ =+2
02
0
2sin2it
c diLdttU αω
( )( )αωαω
+−= tL
Uti c coscos22)(2
( )( )αµαω
+−= coscos22
LUI c
d( )llc UU =
HVDC Training Course 30
0.015 0.012 0.010 0.007 0.005 0.002 ..
200.00
100.00
0.000
-100.00
-200.00
Rectifier: Phase Voltage/Terminal DC in kVRectifier: Phase Voltage/Terminal DC in kVInverter: Phase Voltage/Terminal DC in kVRectifier: Line-Line Phase Voltage B/Terminal AC in kV
DIg
SILE
NT
DC-Voltage with Overlap
µ
α µ
α µ
16
HVDC Training Course 31
DC-Voltage with Overlap
ddd VVV ∆−= αcos0
dcdd IZLIV ==∆ ωπ3
αcos0d
V
dI
dV
cZ
( )2
coscos0
µαα ++= dd VV
HVDC Training Course 32
AC-Current with Overlap
0.030 0.026 0.022 0.018 0.014 0.010 [s]
0.150
0.100
0.050
0.000
-0.0500
-0.1000
-0.1500
REC 1: Phase Current A/Terminal AC in kA
Constant x= 0.018 s
-0.000 kA
Constant(1) x= 0.019 s
0.100 kA
DIgSILENT Rectifier AC-Current
Date: 2/14/2003
Annex: 1 /7
DIg
SILE
NT
17
HVDC Training Course 33
AC Current with Overlap
( )2
coscoscos3 0
µααϕ ++= ddACll IVIU
dAC IIπ6
≈Approximation:
( )2
coscoscos µααϕ ++≈
In PowerFactory:Precise expression for AC-current from Fourier analysis used
HVDC Training Course 34
HVDC Configurations
18
HVDC Training Course 35
12-Pulse Configurations
• Monopolar
• Homopolar
• Bipolar
• MTDC (Multi-Terminal HVDC)
- Short-distance connection- Sea cable connection
- Short-distance connection- Sea cable connection
- Long-distance transmission- Sea cable connection
- Long-distance transmissionwith several connections
HVDC Training Course 36
Detailed 12-Pulse Bipolar HVDC System
19
HVDC Training Course 37
12-Pulse Bipolar System in Power Factory
V~ V ~
DIg
SILE
NT
HVDC Training Course 38
HVDC Layout
20
HVDC Training Course 39
HVDC Components
Converter bridges
Converter Transformers: three- or single-phase transformertwo- or three-winding transformernot grounded at valve side
Smoothing reactors: large inductance (<1H)reduces harmonics in DC current and voltageprevent commutation failures and discont. Currentslimit extensive currents at DC short-circuit
Harmonic filters: reduce harmonics at AC and DC sideprovide reactive power for converter operation
Electrodes: use earth or metallic return conductor as neutral
DC Line
HVDC Training Course 40
Large Scale HVDC Projects
1500km2 x 1125MW2 x ±600kVCanadaQuebec-New England
940km2000MW±500kVNelson River 2
890km1854MW±500kVCanada
Nelson River 1
1361km3100MW±500kVUSAPacific Intertie
71km2 x 1000MW2 x ±500kVGB, FranceCross Channel 1+2
1440km2 x 960MW2 x ±270kVMozambiqueCabora-Bassa
800km2 x 3150MW2 x ±600kVBrazilItaipu
890km2 x 3000MW2 x ±500kVChinaThree Gorges
DistancePowerVoltageCountryProject