on dimensioning lvdc network capacitancies and impact on power losses andrey lana, tero kaipia,...
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ON DIMENSIONING LVDC NETWORK CAPACITANCIES AND
IMPACT ON POWERLOSSES
Andrey Lana, Tero Kaipia, Tuomo Lindh, Pasi Nuutinen, Jarmo Partanen
LUT Energy, LAPPEENRANTA UNIVERSITY OF TECHNOLOGY (LUT)
Lappeenranta, Finland
Andrey Lana – Finland – RIF Session 2 – Paper ID 1099
Frankfurt (Germany), 6-9 June 2011
Presentation outline
Introduction The LVDC network
Dimensioning of the DC capacitors Technical approach
Modelling, analysis and resulting boundaries Power losses Network transient response
Economical approach
Conclusions
Frankfurt (Germany), 6-9 June 2011
Introduction
Frankfurt (Germany), 6-9 June 2011
21/0.56/0.56 kV
20 kVMV network
PCC
AC/DC+750VDC
-750VDC
LVDC network
0.1MVA
Rectifier sideDC capacitors
Inverter sideDC capacitor
Inverter sideDC capacitor
LVDC network
Introduction
What should be taking into account for dimension of DC capacitors? How network power losses are affected by DC capacitor dimensioning? When the dimensioning from harmonic losses is economical profitable?
Frankfurt (Germany), 6-9 June 2011
Voltage ripple
Standard on low-voltage electrical installations [IEC60364] requires that DC voltage ripple (in the systems up to 1500VDC) is in 10% range of rated DC voltage.
The front-end 3 phase six pulse rectifier produces 300Hz voltage ripple.
DC capacitor on customer end
Frankfurt (Germany), 6-9 June 2011
Momentary interruptions
The voltage hold up time during MV supply interruption gives one more guideline for selecting the size of capacitors.
The difference in the energy stored in the system capacitors in the beginning and in the end of the supply interruption is equal to the energy needed for load feed.
Frankfurt (Germany), 6-9 June 2011
Maximum DC capacitance
After voltage sag etc., large amount of recharge current may flow to the DC network.
Maximum amount of recharge current should be restricted below the trip current of protection and current handling capacity of the rectifier.
Start-up control of the system is required or size of capacitors have to be limited (Nuutinen et al., START-UP OF THE LVDC DISTRIBUTION NETWORK”, CIRED 2011)
Frankfurt (Germany), 6-9 June 2011
DC network stability condition is the requirement for customer side DC capacitor size.
derived from applying Liénard–Chipart conditions on system characteristic polynomial
Boundaries for the size of the system capacitors derived from the stability conditions are less then from dc voltage ripple requirements.
The DC network stability condition
Conditions Ripple Stability
Rectifier 30 µF/kW
Inverter 1p 44 µF/kW12 µF/kW
Inverter 3p 15 µF/kW
Frankfurt (Germany), 6-9 June 2011
The DC network resonance
100
101
102
103
104
0
0.5
1
1.5
2
2.5
3
3.5
4
Mag
nitu
de (a
bs)
Bode Diagram
Frequency (Hz)
Cinv
=250uF
Cinv
=500uF
Cinv
=800uF
Cinv
=1600uF
System frequency response. Inverter DC capacitance is varied; Rectifier DC capacitance is fixed at 500µF.
System frequency response. Rectifier capacitance is varied; Inverter capacitance is fixed at 1600µF.
100
101
102
103
104
0
0.5
1
1.5
2
2.5
Magnitude (
abs)
Bode Diagram
Frequency (Hz)
Crec
=20mF;
Crec
=1mF;
Crec
=500uF;
Crec
=250uF
Crec
=20mF & Cinv
=10mF
Frankfurt (Germany), 6-9 June 2011
The DC network resonance
100
101
102
103
104
105
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Ma
gn
itud
e (
ab
s)
Bode Diagram
Frequency (Hz)
Crec
=0uF
Crec
=14uF
Crec
=300uF
Crec
=800uF
System frequency response (from inverter load to rectifier side). Rectifier side DC capacitance is varied. Small size of the rectifier capacitance can cause possible
amplification of the high switching frequency harmonics.
Frankfurt (Germany), 6-9 June 2011
Power Losses in LVDC network
The capacitor placed on the DC terminals of a converter affect directly on the harmonic currents it excites.
The power losses due to current distortion decrease quadratic proportionally to the decrease of harmonic currents
Simulation CASE 1 CASE 2 CASE 3 CASE 4 Capacitors Rated Oversized Undersized Undersized
1.2mF 2.4mF 600µF 1.2mF
720µF 1.4mF 720µF 360µF Power losses in kW
0.485 0.11 1.51 0.54 0.074 0.072 0.067 0.081
1.31 1.31 1.36 1.31 0.31 0.325 0.37 0.355
Total 2.179 1.81 3.3 2.28
Frankfurt (Germany), 6-9 June 2011
Network transient response
Voltage dip @ 1s, t=0.04s
Main : Graphs
0.980 0.990 1.000 1.010 1.020 1.030 1.040 1.050 1.060 ... ... ...
620
820
y (V)
Link voltage at rectifier side + Link voltage at inverter side +
625 650 675 700 725 750 775 800 825
y (V)
Link voltage at rectifier side - Link voltage at inverter side -
-125 -100 -75 -50 -25
0 25 50 75
100
y (A)
Network negative pole current, - Network neutral current Network positive pole current, +
Main : Graphs
0.950 1.000 1.050 1.100 1.150 1.200 1.250 1.300 1.350 1.400 1.450 1.500 1.550 1.600 1.650 ... ... ...
640
660
680
700
720
740
760
y (V)
Link voltage at rectifier side + Link voltage at inverter side +
640
660
680
700
720
740
760
y (V)
Link voltage at rectifier side - Link voltage at inverter side -
-15.0
-10.0
-5.0
0.0
5.0
10.0
15.0 y (
A)
Network negative pole current, - Network neutral current Network positive pole current, +
HSAR from 3p short circuit @ 1s, t=0.5s
Frankfurt (Germany), 6-9 June 2011
Economical approach The increase of capacitance in the DC network to reduce the
losses is justified, if the reduction in the costs of losses is higher than the price of added capacitance.
If the price of losses is 0.05 €/kWh, peak operation time of losses 1000 h, utilisation period 40 a and the interest rate is 5 %, the unit price for power losses over the utilisation period becomes 857.95 €/kW. Thus, the costs of doubling the size of capacitors from the values used in simulation case 1 to the values used in simulation case 2 can be in maximum 323.45 € during the utilisation period regardless of the lifetime of the capacitors.
capacitorlossesh
rtransformeh
networkdc CostpricePP
Frankfurt (Germany), 6-9 June 2011
Conclusions
Voltage ripple (300Hz) due to 6-pulse bridge and due to inverter operation under unbalanced load condition (100 Hz) have to stay in standard range [IEC60364]: 10% of rated DC voltage
Need to ride through possible short MV supply interruptions due to auto-reclosing operations require energy storing in DC capacitors
System stability set requirements on minimum size of capacitors
Capacitor sizing has direct impact on harmonic losses
Resonance situations need to be checked
Charging currents may limit the size of capacitors
System protection have to be considered
Economical profitability of reducing power losses by increasing capacitance size could be estimated
Frankfurt (Germany), 6-9 June 2011
Thank you!
Andrey Lana – Finland – RIF Session 2 – Paper ID 1099