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

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

Introduction

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?

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

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.

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)

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

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

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.

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.

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.

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

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

Network transient response HSAR from 3p short circuit @ 1s, t=0.5s Voltage dip @ 1s, t=0.04s

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.

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

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