1. ON DIMENSIONING LVDC NETWORK CAPACITANCIES AND IMPACT ON POWER LOSSES
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

2. 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

3. Introduction

4. Introduction LVDC network is based on power electronics.
The front-end rectifier and inverter loads are sources of harmonics in the LVDC system.
Six pulse bridge, 300Hz (6*50Hz)
Inverters, 100Hz (2*50Hz)
DC capacitors are installed on DC terminal
Network front-end rectifier
Customer Inverters
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?

5. 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

6. 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.

7. 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)

8. Frequency-domain model
System transfer function from source to load
System transfer function from load to source
LVDC network resonances and harmonic amplification are examined in frequency domain.
system transfer function from source to load
system transfer function from load to source

9. The DC network stability condition
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.
Conditions
Ripple
Stability
Rectifier
30 µF/kW
Inverter 1p
44 µF/kW
12 µF/kW
Inverter 3p
15 µF/kW

10. The DC network stability condition
Case: A
Case: B
Case: C
A&C: Capacitance is choosen for stability up to maximum power point
A: Normal operational point
B: Close to maximum operational point
C: Capacitance size is half from A case, but above stability condition for normal operation point.

11. The DC network resonance
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.

12. The DC network resonance
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.

13. Time-domain simulation model
Time-domain simulations in electomagnetic transient simulation software PSCAD/EMTDC
Modelling approach is verified against measurments on laboratory prototype
Power losses calculation
Transient response

14. 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

15. Network transient response
HSAR from 3p short 1s, t=0.5s
Voltage 1s, t=0.04s

16. 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 €/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 € during the utilisation period regardless of the lifetime of the capacitors.

17. 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

18. Thank you!
Andrey Lana – Finland – RIF Session 2 – Paper ID 1099