Dual DC Buses Nanogrid with Interlink Converter

Transcription

1 1 Dual DC Buses Nanogrid with Interlink Converter SEPOC 2018 Santa Maria RS October 2018 Graduate students and research collaborators of Luiz A.C. Lopes

2 Outline Introduction: From centralized to distributed generation. Types of residential nanogrids: AC or DC? DC bus voltages and number of buses. Control strategy for the DC nanogrids DC bus signaling (DBS) Control strategies for the interlink (LVHV) converter Isolated and nonisolated high gain DC Twostate and tristate modulation strategies Fault detection and clearance in DC nanogrids. 2

3 Introduction (1/2) Conventional power system: Centralized. Load demand increases, then what? Environmental impact: Is renewables the solution? Dealing with the stochastic fluctuating nature of renewables Distributed power generation: DG Issues of protection and power flow control. 3

4 Introduction (2/2) Concept of microgrids and intergrids. Autonomy and coordination of microgrids Microgrids (neighborhoods) and nanogrids (homes) RES for cogeneration and the netzero energy home (NZEH) Impact of electric vehicles on the distribution systems. Linking EVs (picogrids) to residential nanogrids (V2H)? 4

5 Residential AC nanogrid (Boroyevich Optim10) 5

6 Residential nanogrids (1/2) Residential distribution system: Singlephase 3wire (120:240V). Threephase 4wire (127:220V) AC nanogrid: Good choice for a netzero energy home? Sources and storage units are mostly DC. DCAC converters are used now. Modern appliances present an ACDC converter, an unregulated DC bus followed by another DCAC or DCDC converter. 6

7 Residential nanogrids (2/2) What about a DC distribution system? Edison vs. Tesla revisited! Power electronics allow voltage level variations. DCDC converters tend to be more efficient than DCAC. Modern appliances are DC compatible. Remove ACDC converter and PFC elements. No reactive power flow/demand. Conductors can be reduced for the some power transfer. 7

8 Residential DC nanogrid (Boroyevich Optim10) 8

9 Residential AC or DC nanogrid (Riccobono IEEE 16) 9

10 Choice of DC bus voltage Extra Low Voltage (ELV) DC presents a magnitude of less than 120 V and lower risk of electrical shock. 48V DC is a standard telecom voltage level. Supply of kw loads done with >20 A Issue with conductor size for reducing voltage drops Following category is the Low Voltage (LV): <1500V. There is an industry standard at 380 V. Option of dual DC bus system promoted by EMerge Alliance: 24V & 380V. 10

11 EMerge Alliance Dual DC buses: 380 V 24V/100W buses for lighting. 11

12 EMerge Alliance 12

13 380 V DC ecosystem development: Present status and future challenges (INTELEC 2013) 13

14 Advanced LVDC electrical power architecttures and microgrids (Dragicevic,2013) 14

15 The dual DC buses nanogrid 48V for low power loads and 380 V for higher power loads. Generation and storage placed in both buses. Bidirectional interlink DCDC converter. 15

16 Control strategy for the DC Nanogrids Hierarchical control: DC bus signaling (DBS) is used for coordinating the action of interfaces. I o* = f(v o ) or V o* = f(i o ). Variable DC bus voltage Low bandwidth communication link for regulation. 16

17 DC bus signaling and droop control Droop control. Params: Noload voltage and droop slope IDC_x = V NL_x V DC R d_x R d_x = V DC I DC_x I Load = I S I g I b 17

18 DC/DC Interlink Converter Aniel Morais UFU DA B n=

19 Control strategies Usually the DBS is intended for supporting power balancing in one bus. Power injected into one bus will come from the other. Option #1: Averaging DBS efforts I inj_lv = I DBS_LV I DBS_HV = V NL_LV V DC_LV R d_lv V NL_HV V DC_HV R d_hv Option #2: Equalize, by means of a PI controller, the % error between the voltages in the LV and HV bus. Option #3 * : Address the needs of the LV bus, until the HV bus is stressed. I inj_lv = V I NL_LV V inj_lv 0, if V > 390 V DCHV DC_LV I R inj_lv 0, if V DCHV < 370 V d_lv 19

20 Interlink Converter control strategies 20

21 Interlink Converter control strategies 21

22 Interlink Converter control strategies 22

23 Interlink Converter control strategies 23

24 Simulation 24

25 Load 1 Operate from 30 ms to 130 ms Simulation Load 2 Operate from 80 ms to 180 ms 25

26 Simulated Converter 26

27 Averaged droop control Zone I Zone II Zone III Zone IV No Load 0 kw Load kw Loads 1 & kw Load kw No Load 0 kw 30 ms 80 ms 130 ms 180 ms 27

28 Constant voltage ratio Zone I Zone II Zone III Zone IV No Load 0 kw Load kw Loads 1 & kw Load kw No Load 0 kw 30 ms 80 ms 130 ms 180 ms 28

29 DCBus Voltage Zone I Zone II Zone III Zone IV No Load 0 kw Load kw Loads 1 & kw Load kw No Load 0 kw 30 ms 80 ms 130 ms 180 ms 29

30 Alternative control strategy (recall ) The interlink converter is controlled with DBS based (solely) on the voltage level of the LV DC bus (48V). Assuming that the HV DC bus (380 V) is connected to the AC utility grid, a strong system, it can provide/absorb power for balancing the HV DC bus. What if it is not active? The interlink converter is prevented from: injecting power in the HV DC bus, if its voltage is higher than 390 V Drawing power from the HV DC bus, if its voltage is lower than 370 V. 30

31 Loads vary 370 V V HV 390 V Different load variations (time and value) 31

32 Loads vary with power shortage at HV bus 32

33 Loads vary with power surplus at HV bus 33

34 Alternative nonisolated high gain topology: 3switch bidirectional DCDC converter D 1 L T D 2 S 1 S 2 V 1 C 1 S 3 D 3 C 2 V 2 n : 1 I 1 D1 I LT1 I 2 I LT2 D 2 V LT1 V LT2 S 1 I M S 2 L M V 1 S 3 D 3 V 2 n : 1 I 1 D1 I LT1 I LT2 I 2 V LT1 V LT2 S 1 I M S 2 L M V S 1 3 D 3 V 2 D 2 Various possibilities of modulation schemes tristate! 34

35 Static Analysis Conventional (twostate) forward BuckBoost: V S1 V S2 I S1 I M2 I M1 I S2 I S3 I M2 I M1 V 1 nv 2 V 1 nv 2 n ni M1 ni M2 ni M1 ni M2 t 0 t 1 t 2 t 0 t 1 t 2 t 0 t 1 t 2 t 0 t 1 t 2 t 0 t 1 t 2 T S Fig. 6 Forward BuckBoost: Voltage waveforms across switches S 1 and S 2 T S T S T S T S Fig. 7 Forward BuckBoost: Current waveforms in the switches V LM I M I M2 V 1 nv 2 I M1 t 0 t 1 t 2 V2 D1 V n 1 D 1 1 T S t 0 t 1 t 2 T S Fig. 8 Forward BuckBoost: Magnetizing inductance waveforms Fig. 9 Forward BuckBoost: Voltage Conversion Characteristic 35

36 Tristate 3switch bidir. DCDC converter Research question: How can one benefit (and simplify) from this multivariable control scheme? 36

37 Tristate 3switch bidir. DCDC converter 37

38 5switch bidirectional DCDC converter (Power flow direction changed without changing direction of I LM ) S 1 I SD1 1 L 2 S 3 I SD3 V2 D2 V n 1 D 1 2 S 1 I SD1 1 L 2 D 1 I L2 D 3 I 1 I 2 S 3 I SD3 D 1 I L2 D 3 I 1 I 2 S 2 S 4 V 1 I SD2 n L 1 L M I SD4 V 2 D 2 I L1 I LM D 4 S 2 S 4 V 1 I SD2 n L 1 L M I SD4 V 2 D 2 I L1 I LM D 4 I S5 S 5 I S5 S 5 Buckboost forward twostate 38

39 5switch bidirectional DCDC converter (Buckboost reverse twostate) S 1 I SD1 1 L 2 S 3 I SD3 S 1 I SD1 1 L 2 S 3 I SD3 D 1 I L2 D 3 I 1 I 2 D 1 I L2 D 3 I 1 I 2 S 2 S 4 V 1 n V 2 I SD2 D 2 I L1 L 1 I LM L M I SD4 D 4 S 2 S 4 V 1 I SD2 n L 1 L M I SD4 V 2 D 2 I L1 I LM D 4 I S5 S 5 I S5 S 5 Various possibilities of modulation schemes: Tristate, quadstate 39

40 Performance of the twostate current control scheme of the 5switch converter I1_Ref I1_1stOrder_Filter ILm_Ref I(Lm) Time (s) 40

41 Comparison of the twostate current control scheme of the 3 and 5switch converter I1_3 Switches Converter I1_Novel 5 Switches Converter I1_Ref Time (s) 41

42 Fault protection in DC nanogrids Fault detection and selective fault clearance. How to identify where the fault occur and which circuits breakers should open. Established technology for AC distribution systems. What about for DC nanogrids? Issue #1: Very small impedances between branches and nodes. Issue #2: Devices to open high DC currents. Arc extinction Much more expensive, if available, than AC However, the power electronics interfaces provide some sort of control of the current injected into the DC bus 42

43 Fault Current Limitation Traditional (tripping by overcurrent) vs. fault current limitation and interruption with coordination of interfaces and contactors. [3] 43

44 DC Ringbus Microgrid Fault Protection and Identification of Fault Location (2013) 44

45 Configuration of a typical DC Nanogrid 45 45

46 Prospective topology and control logic Normal operation: S1 ON and S3S4 controlled as a classical current controlled class C converter. Fault current detected? Current control through S1 (PWM). iout LV side S1 L S3 HV side V in C in C out V dc il S2 S4 46

47 Control scheme 47

48 Experimental setup 48

49 Experimental setup 49

50 Experimental setup 50

51 Droop, current limit and reduced current 51

52 Additional modulation schemes: Tristate D on D off D on D off D f iout iout S1 S3 R eq S1 S3 R eq L L V in C in C out V in C in C out S2 il S4 V meq S2 il S4 V meq Conventional Boost TriState Boost Conventional has 2 states, D on and D off 3 rd freewheeling state, D f. Here S2 and S4 are On. D on D off D f = 1 Inductor is shortcircuited. No transfer of energy. D off is kept constant to make D on the only control variable. Is there a best sequence of states? 52

53 Switch RMS Current Comparison Forward power flow Reverse power flow 53

54 Thank you for your attention! 54

55 Issues under investigation Control strategy for a single and a dual DC buses DC nanogrid. Hierarchical control with DC bus signaling (DBS) at the primary level. Control scheme for a solar PV converter operating in three modes: Droop, MPPT and current limiting. (AHMAD) Control of a hybrid energy storage system (HESS). Goal is to regulate a DC bus voltage, considering the power and energy density characteristics of batteries and supercapacitors. The attenuation of the voltage distortion caused by a singlephase AC grid interface is also considered. (ZAID) Control strategy for a bidirectional interlink DCDC converter. (ANIELAHMAD). Fault protection. Choice of converter and logic for clearing faulted segments (SAROOSH). 55

56 Zone I No Load 0 kw Zone II Load kw Averaged droop control Zone III Loads 1 & kw Zone IV Load kw No Load 0 kw 30 ms 56

57 No Load 0 kw Load kw Averaged droop control Zone III Loads 1 & kw Zone I Zone II Zone IV Load kw No Load 0 kw 30 ms 80 ms 57

58 No Load 0 kw Load kw Averaged droop control Zone I Zone II Zone III Zone Loads 1 & kw IV Load kw No Load 0 kw 30 ms 80 ms 130 ms 58

59 Averaged droop control Zone I Zone II Zone III Zone IV No Load 0 kw Load kw Loads 1 & kw Load kw No Load 0 kw 30 ms 80 ms 130 ms 180 ms 59

60 Zone I No Load 0 kw Zone II Load kw Constant voltage ratio Zone III Loads 1 & kw Zone IV Load kw No Load 0 kw 30 ms 60

61 No Load 0 kw Load kw Constant voltage ratio Zone III Loads 1 & kw Zone I Zone II Zone IV Load kw No Load 0 kw 30 ms 80 ms 61

62 No Load 0 kw Load kw Constant voltage ratio Zone I Zone II Zone III Zone Loads 1 & kw IV Load kw No Load 0 kw 30 ms 80 ms 130 ms 62

63 Zaid 63

64 Configuration of a Basic DC Nanogrid Hybrid Energy Storage System (HESS) 64 64

65 Conventional HESS control scheme Outer voltage loop Inner current loop Inner current loop 65 65

66 The MPPT Scheme The MPPT scheme is based on linearity (K P ) between the optimization current and the maximum output power of the PV. The maximum power prediction line is used to estimate the maximum power the PV can generate for a measured (instantaneous) current. The solar converter operates at MPPT by comparing this estimated maximum power with the power generated from the PV. 66

67 Case Study 67

68 Simulation and Results MATLAB/Simulink A 5 kw PV array (V MPPT = 232 V and I MPPT = 22 A) was modeled in MATLAB. Solar converter supplying the load in standalone Solar converter and grid interface converter together 68

69 Solar converter supplying the load in standalone 69

70 70

71 71

72 Solar converter and grid interface converter together 72