Power Electronics in PV Systems

Transcription

1 Introduction to Power Electronics in PV Systems EEN 2060 References: EEN4797/5797 Intro to Power Electronics ece.colorado.edu/~ecen5797 Textbook: R.W.Erickson, D.Maksimovic, Fundamentals of Power Electronics, 2 nd ed., Springer 2000,

2 Example: Grid-onnected PV System One possible grid-connected d PV system architecture t I PV D input A output Power V A v t V t 2 sin PV, I PV ac ( ) RMS PV V electronics PV v ac utility array converter grid iac ( t) 2I RMS sint PPV VPV I PV P V I p i ac ac ac RMS ( t ) v i V I cos ac ac RMS RMS RMS 2 t Functions of the power electronics converter Operate PV array at the maximum power point (MPP) under all conditions Generate A output current in phase with the A utility grid voltage Achieve power conversion efficiency close to 00% Pac VRMS I RMS converter P V I PV Provide energy storage to balance the difference between P PV and p ac (t) Desirable features Minimum weight, size, cost High reliability PV PV 2

3 Power electronics converter I PV i ac PV array V PV Power electronics converter Inverter v ac A utility grid One possible realization: I PV i ac PV array V PV D-D converter V D Single-phase D-A inverter v ac A utility grid Energy-storage g capacitor lass objectives: introduction to circuits and control of a D-D converter and a single-phase D-A inverter 3

4 Introduction to electronic power conversion Four types of power electronics converters ontrol is invariably required In the PV system, for example: ontrol input voltage of the D-D input voltage to operate PV at MPP ontrol shape of the D-A output current to follow a sinusoidal reference ontrol current amplitude to balance the input and output power 4

5 High efficiency is essential 5

6 ircuit components for efficient electronic power conversion? 6

7 Ideal switch Power semiconductor devices (e.g. MOSFETs, diodes) operate as near-ideal power switches: When a power switch is ON, the voltage drop across it is relatively small When a power switch is OFF, the switch current is very close to zero 7

8 apacitor p i dv dt ( t) v ( t) i ( t) v i For periodic v (t), i (t): No losses (average capacitor power = 0) P T v ( T ) p ( t) dt v ( t) dv 2 T T 0 v (0) T T v (T ) T 2T 2 v ( T ) v (0) 0 apacitor charge balance (average capacitor current = 0) I 0 i ( t) dt T v (0) dv v ( T ) v (0) 0 8

9 Inductor p v For periodic v (t), i (t): di dt ( t) v ( t) i ( t) No losses (average inductor power = 0) P T i ( T ) p ( t) dt i ( t) di 2 T T 0 i (0) T T i (T ) T 2T v i 2 i ( T ) i (0) 0 Inductor volt-second balance (average inductor voltage = 0) V 0 v ( t) dt T i (0) di i ( T ) i (0) 0 9

10 ircuit components for efficient electronic power conversion Power electronics converters are circuits consisting of semiconductor devices operated as (near-ideal) switches, capacitors and magnetic components (inductors, transformers) 0

11 Boost (step-up) D-D converter switch control Position Position 2 DT s T s = switching period f s = /T s = switching frequency D = switch duty ratio (or duty cycle), 0 D T s

12 Boost converter circuit Power MOSFET and diode operate as near-ideal switches 2

13 Power MOSFETs and diodes haracteristics of several commercial power MOSFETs ow onresistance DT s T s implies low conduction losses Fast switching enables high switching frequencies, e.g. 00 s of khz to MHz haracteristics of several commercial switching power diodes 3

14 Voltage, current and frequency ratings of power semiconductor devices (SR) Voltage rating urrent rating MOSFET: Metal Oxide Semiconductor Field Effect Transistor IGBT: Insulated Gate Bipolar Transistor SR (or Thyristor): Silicon ontrolled Rectifier GTO: Gate Turn Off thyristor 4

15 Boost converter analysis 5

16 Position 6

17 Position 2 7

18 Inductor voltage and capacitor current waveforms D = -D Periodic steady-state operation Inductor volt-second balance: average inductor voltage = 0 apacitor charge balance: average capacitor current = 0 8

19 Inductor volt-second balance 9

20 Boost D voltage conversion ratio M = V out /V g Boost D-D converter steps-up p a D input voltage by a ratio M which is electronically adjustable by changing the switch duty ratio D 20

21 Simulink model Switched-mode Boost D-D converter boost_switching.mdl /00 /Rload iout 00 Vg Vin 0.5 D Duty cycle Vg Iout Boost D-D (switching) Vout I D switch control Boost D-D Vout i Vout switch control Vout Scope Input voltage V g = 00 V Inductance = 200 HH apacitance = 0 F oad resistance R = 00 Switch duty cycle D = 0.5 Output voltage V out = 200 V Input current I g = I = 4 A Power P = 400 W Switching frequency f s = 00 khz Switching period T s = 0 s 2

22 No losses: Averaged (D) model V D out V g V g I g V out I I D g I out Ideal boost D-D converter works as an ideal D transformer with an electronically adjustable step-up ratio out I g I out :n V g V out n M ( D) D 22

23 Modeling of losses osses in switched-mode d power converters: onduction losses, due to voltage drops across inductor winding resistance, and across power semiconductor switches when ON onduction losses depend strongly on the output power Switching losses, due to energy lost during ON/OFF transitions Switching losses are not strongly dependent on output power; a portion of switching loss remains even at zero output power Switching losses are proportional to the switching frequency Other losses, including: osses in magnetic cores Power needed to operate control circuitry 23

24 Switching waveforms and switching losses MOSFET turn-on transition zoom-in v t Drain voltage i t Drain current t v t DT s T s i i d I out I g _ v i t v d _ i t V g v gate v t R v out p t = v t i t DT s T s Switching power loss = Transition energy loss * Switching frequency 24

25 Switching waveforms and switching losses MOSFET turn-off transition zoom-in v t Drain voltage i t Drain current t i t DT s T s i i d I out I g _ v i t v d _ v t V g v gate v t R v out DT s T s Switching power loss = Transition energy loss * Switching frequency 25

26 Averaged (D) model with losses I g R I out D : V g I sw V out Small R models conduction losses due to inductor winding resistance and power switch resistances Small I sw models switching and other load-independent losses Efficiency with losses, when the load current I out is known: 2 R ( Iout I sw) I sw 2 ( D) V I I out out out 26

27 Example: efficiency for various R Assume: Resistive load R = V out /I out I sw = 0 R 2 ( D ) R Note that it is more difficult to achieve high efficiency if a large step-up ratio is required (i.e. if duty-ratio D is close to ) 27

28 Single-phase D-A grid-connected inverter i in i 2 V D 2 i ac v ac Switches in position during DT s, in position 2 during (D)T s Switching frequency f s is much greater than the A line frequency (60 Hz or 50 Hz) By controlling the switch duty ratio D, it is possible to generate a sinusoidal A current i ac ( small switching ripple) in phase with the A line voltage, as long as the input D voltage V D is sufficiently high, i.e. as long as V D is greater than the peak A line voltage 28

29 Position i in i 2 V D 2 v i ac v ac v V D v ac i i ac i in i 29

30 Position 2 i in i 2 V D 2 v i ac v ac v V D v ac i i ac i in i 30

31 Inductor volt-second balance Note that t switching frequency f s >> ac line frequency Over a switching period, v ac (t) const. v V V D D v v ac ac,, 0 t DT s DT s t T s T s V v ( t) dt D( VD vac ) ( D)( VD vac ) (2D ) VD vac 0 T Ts 0 vac M ( D) 2D V D M ( D) V D must tbe greater than the peak of v ac 3

32 ontrol of A line current i in i 2 V D 2 i ac v ac ontrol objectives: i ac = I M sin (t), in phase with A line voltage v ac (t) Amplitude I M (or RMS value) adjustable to control power delivered to the A line p ac v i ac ac ( t) ( t) 2V 2I RMS RMS sin sin t t ( t ) v i V I cos ac ac P V ac RMS RMS RMS I RMS 2 t 32

33 A simple current controller i in i 2 V D 2 i ac v ac sensed switch i control sin(t) _ i ref I Mref i ref i comparator multiplier with hysteresis i ref I Mref sin( t) i < i ref i/2: position i > i ref i/2: position 2 comparator with hysteresis switch control position position 2 i i is always within i/2 of i ref i ref i/2 i/2 current ripple 33

34 Simulink model 200 Vdc Step Switched-Mode D-A Inverter Vdc Iref D-A inverter (switching) vac iac iin switch control D-A Scope dcac_switching.mdl Waveforms v ac (t), i ac (t), i in (t), and switch control over one A line period (/60 s) Input voltage V D = 200 V Inductance = 2 mh A: 20Vrms, 60Hz I Mref = 32 = 4.2 A i =A P ac = 360 W With this simple controller, switching frequency is variable 34

35 Averaged D-A inverter model with losses I in : D R i ac V D I sw v ac ideal transformer Small R models inductor winding resistance and power switch resistances Small I sw models switching and other losses 35

36 D-A inverter efficiency example 200 Vdc 4.5 IRMS Switched-Mode D-A Inverter (averaged model) vac vac Vdc D-A iac iac inverter iin iin (averaged) D Duty Iref pin pin pout pout D-A Scope Simulink model dcac_averaged.mdlaveraged.mdl Input voltage V D = 200 V A: 20Vrms, 60Hz R = I sw = 50 ma s Integrator 60 fac Pin P ac = 0 to 600 W Divide Efficiency s Integrator Inverter efficiency of about 95% is typical At high power levels, conduction losses due to R dominate At low power levels, efficiency drops due to switching and other fixed losses 60 fac Pout % 90% 80% 70% 60% 50% 40% 30% 20% 0% 0% P ac [W] 36