Performance of 3-Phase 4-Wire Solid State Transformer under Imbalanced Loads

Transcription

1 1 Performance of 3-Phase 4-Wire Solid State Transformer under Imbalanced Loads Tao Yang, Ronan Meere, Orla Feely and Terence O'Donnell School of Electrical, Electronic and Communications Engineering University College Dublin Dublin, Ireland March 214

2 2 Overview Introduce Solid State Transformers (SST) Advantages and Disadvantages Each Stage of SST Design and Operation How can a SST deal with imbalance? Compare different topologies Conclusions and Future Work

3 3 Solid State Transformer(SST) : Basic Concept Take 5/6 Hz input voltage Use power electronics switches to chop 5/6 Hz into much higher frequency AC (e.g. 1 s khz) Pass this high frequency through a transformer Use power electronics to re-make and regulate the 5 Hz voltage.

4 4 Advantages? Advantages/Disadvantages Reduced size and weight SST is active, controllable device Output and input waveforms can be decoupled Eliminate harmonic distortion Improve voltage regulation Control active and reactive power Could have a DC input/output for connection of DG, energy storage Disadvantages? Efficiency, Reliability Cost

5 Motivation 5

6 6 Motivation of our study Modern Distribution Grid Three Phase Imbalanced Loads Impacts of Imbalanced Loads Cause malfunction Damage to Electric Equipment 1. Improve the disadvantages of SST? 2. In an SST-fed distribution system, Can SST ensure that certain tolerances on phase loads imbalance at its output terminals.(few publications have addressed the issue )

7 7 Imbalance Loads Unevenly distributed single-phase load. Three-phase Imbalance Loads Balanced three-phase load running at a fault condition. Imbalance = Z Max phase Z Min phase Factors of Z Loads 1 Rated phase Imbalance Degree of = Output 2 V a,b,c Max(RMS) V a,b,c Min(RMS) V Rated(Phase RMS) Imbalanced phase loading causes negative sequence and zero sequence to flow in the power system. [1] MIL-STD -74E, Military Standard [2] International Electrotechnical Vocabulary, Group 5, Fundamental Definitions, International Electrotechnical Commission (IEC) Publication 5 (5), 1954.

8 Each Stage of SST Design and Operation 8

9 Voltage/V Voltage, Current, V/A 9 Single Phase Rectifier Stage + V_in - i_l i_l - x i_l* + L PI V_c Single-Phase Full Bridges PWM i_l* + V_DC - x C - V_DC x + Sine PI R Vo_ref Vo_ref Start-Up Vin, Iin V_HVDC Step Load t/s Vin Iin VHVDC

10 Voltage/V Phi/rad 1 Dual Active Bridge(DAB) Stage Iin Phase-shifted Io Vp(V) Phi Vs(V) -Vo' Vin Vo' Vin' -Vo' Vin Lin Cin S1 S3 IL L Vp N:1 Vs S5 S7 Co Vo R IL(A) S4 S2 High Frequency Transformer S8 S6 Iin(A) P o = V in 2 2Lf H N d Φ π (1 Φ π ) d = N V o V in Io(A).3 Phi Pi 2*Pi wt(degree) Phase shift between two bridges in rad Phi Vo_ref Vo(V_LVDC) - + X Phase-shift Control for Single DAB K-factor (P-I) Φref Phase Angle Saturation To Gate Drivers Phase Shift Modulation Start-up Step Load Low DC output voltage of DAB Step Load VLVDC 399 Start-up t/s.6.8

11 Voltage, Current, V/A 11 Single phase DC-AC Inverter Stage VLVDC Sc1 Sc2 Sc3 Kpwm Sc4 Vin L IL + C Io Vo Load 4 2 ILref VLref X PI + X PI + X Kpwm + X 1/sL + X 1/sC + - Vo Voref - + Vin 1/Kpwm - Vo Output voltage and current. IL -Io Vo Vo Io -2-4 Step Load 1 Step Load t/s

12 12 Two Topologies of 3-stage 3-Phase SST 3-phase 3-wire SST(SST1) 3-phase 4-wire SST(SST2) 12.47KV 12.47KV Three Phase Rectifier Dual Active Bridge (DAB) (a) Three Phase Inverter 381V 381V a b c

13 Systems Performances simulations under Imbalanced Loads: 3-phase 3-wire SST VS 3-phase 4-wire SST 13 Input Voltage Vin 3.39*3 kv (amplitude) AC Phase voltage Grid Frequency 5 Hz High Voltage DC reference DAB Frequency Low Voltage DC reference Output Voltage reference 3.8*3 kv 1 khz 4 V 311 V (amplitude) AC Phase voltage Load Conditions Imbalance factors Case1 5% Case2 1% Case3 15%

14 urrent/a Voltage/V Voltage/V Current/A Voltage/V Voltage/V Performance comparison with case 2 loads condition 3-phase 3-wire SST output voltage Ua Ub Uc phase 4-wire SST output voltage Ua Ub Uc Output Current Performance comparison with case 3 loads condition 3-phase 3-wire SST output Voltage Ua Ub 4-4 Uc.2.4 t/s Ia Ib Ic t/s phase 4-wire SST output voltage Ua Ub Uc Output Current Ia Ib Ic t/s

15 Output Voltage Imbalance Degree of SST/% Output Voltage Imbalance Degree of 2 SST under the Different Imbalance Loads 3-phase 4-wire SST 3-phase 3-wire SST I II Imbalance Factors of Loads/ % III Why 3-Phase 4-Wire SST can handle the imbalance loads conditions better Provide neutral wire to handle the neutral current caused by an imbalanced loads; Attain 3 phases fully decoupled and independently controlled.

16 16 Conclusions and Future Work Yes, SST can be an excellent system for ensuring distribution level feeder voltage balance in the face of significant loads imbalance. But Compare both topologies designs to see is there a difference in power efficiency? Is there a design method which can both handle imbalance loads and improve the power efficiency?

17 17 Thank you for listening Questions?