Ulf Radbrandt, 2011-10-31
System aspects on insulation levels
for HVDC converter stations
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February 3, 2012 | Slide 1
Introduction, Insulation Coordination
Optimization
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February 3, 2012 | Slide 2
Cost
Protection
Equipment
Probability
Type of stresses
Continuous AC - Normal operation
- clean conditions
- contamination
- other
Temporary overvoltage
Rain
Snow & Ice
Birds…
Fires
Vegetation
Abnormal system conditions
System switching operations (lines, loads and equipment)
Lightning overvoltage
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February 3, 2012 | Slide 3
Dry
Switching overvoltage
Lightning flashes to and around lines
Strength and Stress
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February 3, 2012 | Slide 4
Strength and Stress
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February 3, 2012 | Slide 5
Principles of insulation coordination
Primary objective is:
Establish maximum steady state, temporary and transient
overvoltages on equipment
Select
insulation strength and characteristics of equipment and
protective devices
To ensure a safe, economic and reliable installation.
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February 3, 2012 | Slide 6
Insulation coordination Important parameters
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February 3, 2012 | Slide 7
Minimum Short Circuit Capacity,
pre and post fault
X/R
X1/X0
AC fault clearing times
Auto-reclosing? Times?
Maximum system voltage
Arresters in the surrounding a.c. network characteristics
DC and electrode line data
Insulation margins
Voltage profiles - Arrester scheme
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February 3, 2012 | Slide 8
Voltages - Establish operating voltages
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February 3, 2012 | Slide 9
Valve voltages
CCOV=π/3*Udi0absmax
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February 3, 2012 | Slide 10
Surge Arresters – Polymeric type
PEXLIM P arrester
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February 3, 2012 | Slide 11
Surge Arresters - Valve hall arrester
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February 3, 2012 | Slide 12
Surge Arresters
Voltage (p.u.)
Min protection levels in kV (peak)
according IEC60099-4
Region 1
Region 2
Region 3
Protection against lightning overvoltages
2.3
2.0
Protection against switching overvoltages
Rated voltage (Ur)
1.0 x √2
Ires, resistive current
Continuous operating voltage (Uc)
0.8 x √2
Ires
Effect of increased
block temperature on
Ires
Icap
Icap, capacitive current
(no influence from
temperature)
10-5
10-3
102
Current (Ampere)
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February 3, 2012 | Slide 13
103
104
Log scale
Surge Arresters – Protective parameters
Urw = Insulation level of equipment
Up = Protective level of the S.A.
Overvoltage without
surge protection
Urw
Urw
Up
Protective
margin
Overvoltage with
surge arresters
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February 3, 2012 | Slide 14
Insulation Coordination
Exemple of minimum Insulation margins (based on IEC 60071-5):
Valves
AC equipment, line side
20 % for steep front impulses
25% for lightning impulses
15 % for lightning impulses
20 % for switching impulses
15 % for switching impulses
DC equipment
20 % for lightning impulses
15 % for switching impulses
Converter transformer, valve side
20 % for lightning impulses
15 % for switching impulses
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February 3, 2012 | Slide 15
Factors affecting on surge arrester dimensioning
Continuous operating voltage
Climate (ambient temp., rain, sunshine)
Mechanical stresses
Temporary overvoltages, TOV
Transient overvoltages
Protection function
Energy- and current strength
Outer insulation
High outer pollution
Short circuit proof
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February 3, 2012 | Slide 16
Energy dimensioning
Current amplitude
Duration
Time between current pulses
The number of pulses before cooling
Total energy
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February 3, 2012 | Slide 17
Surge Arresters – Difference between AC and DC
AC
•Much higher current through arresters (coordinating
current) for lightning than for switching overvoltages. That
leads to higher BIL than SIL
•The same maximum operating voltage in both ends of a
transmission line
DC
About
the same current through arresters (coordinating
current) for lightning as for switching overvoltages. That
leads to about the same BIL as SIL
Lower
maximum operating voltage in the receiving end of
a transmission line
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February 3, 2012 | Slide 18
Converter Configuration for Xiangjiaba – Shanghai
DC Pole 800kV
DC Filter
DC Neutral
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February 3, 2012 | Slide 19
Electrode
Line
Converter Configuration for North East - Agra
DC Pole 800kV
DC Filter
DC Neutral
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February 3, 2012 | Slide 20
Electrode
Line
Transformer voltage characteristics – Steady State
Transform er voltages Xiangjiaba – Shanghai
1000
1000
900
900
800
800
700
700
600
600
500
500
tfo HV Y
tfo HV D
400
tfo LV Y
tfo LV D
300
u [kV]
u [kV]
Transform er voltages North East - Agra
tfo D
300
200
200
100
100
0
0
-100
-100
-200
tfo Y
400
-200
0
5
10
t [ms ]
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February 3, 2012 | Slide 21
15
20
0
5
10
t [ms ]
15
20
Transformer voltage characteristics – Transient
(kJ)
(kA)
(kV)
Arresters V135 C4 S1
400
300
200
100
0
-100
-200
-300
3.00
2.50
2.00
1.50
1.00
0.50
0.00
-0.50
7.0k
6.0k
5.0k
4.0k
3.0k
2.0k
1.0k
0.0
0.000
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February 3, 2012 | Slide 22
UV1_4S1
UV3_4S1
UV5_4S1
-0.075
-190.975
-190.900
Min -221.920
Max 377.457
IV1_4S1
IV3_4S1
IV5_4S1
-0.000
-0.000
-0.000
Min -0.000
Max 2.654
EV1_4S1
EV3_4S1
EV5_4S1
0.0007k
6.9177k
6.9170k
Min 0.0007k
Max 6.9177k
0.025
0.050
0.075
0.100
0.001
0.099
0.097
Pros and cons with standard insulation levels for HVDC
Pros:
Manufacturers can standardize their product portfolios
Utilities can have the same equipment for several converter stations (same voltage level)
Increased insulation margins for e.g. transformers and bushings because thyristor valves will
be optimized anyway (minimized insulation levels)
Less need for thorough calculations/simulations?
Cons:
Standard levels will mean higher levels (due to e.g. different insulation margins by different
utilities and different minimum short circuit capacity) which will give higher cost for equipment
Higher insulation levels will limit the number of possible test institutes
Higher insulation levels for transformers will lead to increased difficulties for transport
Higher insulation levels for transformers will lead to increased amount for oil
Higher insulation levels for transformer bushings will lead to increased size of valve halls
Correction factors for external insulation might require selection of the next standard level
Higher insulation levels for transformers might lead to higher commutation reactance which will
lead to increased number of thyristors and increased need for reactive power compensation
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February 3, 2012 | Slide 23
Ongoing work within IEC
Extract from IEC TS 60071-5, Insulation co-ordination – Part 5 HVDC:
“Selection of standard withstand voltages for a.c. side equipment only. The
present step is skipped for equipment on d.c. side because there are no
standardized withstand voltage levels for such equipment
For equipment on the d.c. side, specified insulation levels are rounded up to
convenient practical values”
Ongoing work for the revision of IEC 60071-5
The work is going in the direction to even more emphasize the principle with tailor
made insulation coordination and insulation margins instead of standard
insulation levels within the converter
The following line is added in the new draft under Clause 6.1 Essential
differences between a.c. and d.c. systems:
“•
there exist no standard insulation levels in the case of d.c. systems“
The following is added in the new draft under Clause 12 Clearances in air:
“The
clearances in d.c. applications are based on insulation levels of equipment
which are determined to provide the appropriate margin over the protective level
of the arresters rather than on standard equipment levels.“
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February 3, 2012 | Slide 24