SungPilOe_PhD_Thesis_Presentation
- 1. Microgrid Unbalance Compensator – Mitigating the negative effects of unbalanced microgrid operation Sung Pil Oe, Prof. Mark Sumner, Prof. Mark C. Johnson University of Nottingham, UK
- 2. Content • Background • Motivation • Microgrid Unbalance Compensator • Compensation Strategies • Experimental Results • Conclusion
- 3. Background • Connection of large number of micro-generators and new loads (heat pumps, EV/PHEVs) on the distribution network presents a number of technical challenges to DNOs. • A new approach to the way the traditionally passive distribution system is designed, managed and operated is required. Microgrid: • Small-scale semi-autonomous LV distribution network consisting of loads, micro-generation and energy storage units designed to meet the electrical and heat demands of the energy cell it serves. • Connected to the LV distribution network at the PCC downstream of the distribution substation transformer. • Introduced to facilitate the integration of large numbers of micro- generation, distributed storage and active loads in the LV network. • Allows intelligent coordination of loads, generation and storage to make it appear to the utility as a single controllable entity minimising negative impacts to the wider network.
- 4. Motivation Motivation • Mitigate the negative effects of unbalanced operation propagating to the wider electricity network, increase utilization of existing network assets and reduce losses. • Facilitate the connection of a large number of synchronous generator- based micro-generation units by limiting the technical problems associated with its unbalanced operation Unbalanced Operation VUF exceeds 1.3%
- 5. Microgrid Unbalance Compensator What is a Microgrid Unbalance Compensator (MUC)? Shunt connected three-phase four-leg voltage source converter (VSC) designed to detect the unbalanced three-phase load current and perform unbalanced compensation. Main Components: • Four-leg PWM IGBT- based VSC with LCL filter • Voltage/Current sensors • Advanced control system
- 6. Microgrid Unbalance Compensator Advanced Control System Main Components: • DSOGI-PLL (positive sequence detector) • DC Link Control (PI controller) • Proportional Resonant (PR) current controller + Harmonic Compensator (HC) • Compensation reference current generator (p-q Theory) • Sine-PWM signal generator
- 7. Microgrid Unbalance Compensator PR+HC Current Control Loop
- 8. Unbalance Compensation Strategy • Two main compensation strategies based on the p-q Theory have been identified : 1. “Constant Source Power (CSP)” Strategy • Ensure constant power under unbalanced operating conditions • Aims to provide optimal power flow to the synchronous generator-based micro-generator 2. “Sinusoidal Source Current (SSC)” Strategy • Ensure sinusoidal and balanced source currents even under unbalanced load and unbalanced and distorted source voltage conditions
- 9. Prototype MUC • 15 kW Triphase PM15F42C power module • All controllers implemented in Matlab/Simulink • All data logged through Simulink
- 10. Experimental Setup: CSP Strategy Balanced (Ω) Unbalanced (Ω) Phase A 31.6 15.7 Phase B 31.4 31.4 Phase C 31.2 31.2 Unbalanced microgrid load • Percentage Voltage Unbalance Factor (%VUF) is used as a measure of the 3- phase terminal voltage unbalance %𝑉𝑈𝐹 = 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑉 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑉 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 × 100
- 11. Experimental Results CSP Strategy: • Oscillating power component has been significantly reduced (≈ 87%) reduction). 910𝑊𝑝−𝑝 3900𝑊𝑝−𝑝 1300𝑊𝑝−𝑝 • Neutral current is effectively compensated • %VUF decreases (2.7% 0.2%) Key Result: MUC is able to reduce the oscillating active power component thus reducing the wear and tear of the SG.
- 12. Experimental Setup SSC Strategy Test 1 A programmable AC source is used to generate a three-phase unbalanced and distorted voltage Resistance (Ω) Inductance (mH) Phase A 227.2 15 Phase B 15.7 30 Phase C 11.8 15 <Unbalanced Load>
- 13. Experimental Results Test 1 • After activating the MUC, three-phase currents are balanced and in- phase with the voltage and neutral current is effectively compensated. Before Compensation After Compensation isa 2.32 A 14.07 A isb 19.41 A 13.77 A isc 21.65 A 13.49 A <Compensation Result> Key Result: MUC is able to perform unbalance compensation even under highly unbalanced and distorted operating conditions
- 14. Experimental Setup SSC Strategy Test 2
- 15. Experimental Results Test 2 Phase Before Compensation After Compensation A -8.04 A 4.52 A B 10.16 A 4.13 A C 9.69 A 4.26 A 2.1 kW3.3 kW <Compensation Result> • The total three-phase power imported from the grid is reduced which effectively means that the excess power generated by the single-phase micro-generator is recirculated to the other two phases Test 2 Scenario: • Unbalance caused by a single-phase micro-generator connected to phase A . • The loads on the other two phases are made equal (36Ω + 15mH). • The excess power generated by the micro-generator on phase A (1.6kW) is less than the combined power (≈3.3 kW) required by the loads on phase B & C.
- 16. Experimental Results SSC Strategy Test 3 Test 3 Scenario: The excess power generated by the micro-generator on phase A (3 kW) is now greater than the combined d load demand (≈ 400 W) of the other two phases. Simulates the case of low demand and excess PV generation. Before Compensation After Compensation Phase A -17.87 -4.34 Phase B 2.00 -4.78 Phase C 1.98 -4.73 <Compensation Result> Key Result: MUC allows the surplus power generated within the microgrid to be exported in a balanced manner
- 17. Experimental Setup SSC Strategy Test 4 With Community Energy Storage Prior to activating the battery charger : • Phase A is exporting approximately 3 kW (single-phase generation) • Phase B is importing approximately 340 W (resistive load) • Phase C is importing approximately 340 W (resistive load)
- 18. Experimental Results SSC Strategy Test 4 Test 4 Result : After activating the battery charger to charge 2 kW (without MUC controller enabled): • Phase A is exporting approximately 2.3 kW (3kW → 2.3kW) • Phase B is importing approximately 1 kW (340W → 1 kW) • Phase C is importing approximately 1 kW (340W → 1 kW)
- 19. Experimental Results SSC Strategy Test 4 Continued Test 4 : After activating the MUC controller: • The three-phase power imported from the grid is nearly 0 kW • Power generated from phase A micro-generator is redistributed to the other two phases to meet the demand and the excess power is charged to the battery in a balanced way.
- 20. Power Converter Control Design Consideration • Which feedback signal is used for control? • Cheaper inverters try to cut costs by not having an extra current sensor at the grid-side. • Need to consider the phase shift introduced by the LCL filter capcitor to ensure unity power factor. (Typically sized 5% of rated power) • Possible cause of increasing reactive power at DNO level?
- 21. Conclusion • MUC is able to ensure the microgrid behaves as a “good citizen” from the view point of the distribution network operator (DNO) even under unbalanced operating conditions. • It has been demonstrated that the MUC allows the recirculation of power, thus allowing other customers to benefit from excess generation originating from another phase within the microgrid. Also able to export balanced power back to the grid or charge a battery storage. • MUC will benefit the DNO by reducing losses and increasing utilization of the existing distribution network assets thus deferring unnecessary network reinforcements, especially beneficial for urban areas (smart buildings, smart offices, etc.) • MUC is able to reduce the %VUF at the PCC thus allowing increased connection of micro-generation units. • The MUC will benefit the microgrid owners by maximizing self consumption, protecting synchronous-generator based micro-generators and compensation of reactive power.
Editor's Notes
- #4: Demand side management