1 EE462L, Spring 2014 Power to Grid

2 There are only two stations, so please use the signup sheet (with one-hour time slots) and be considerate of others who are waiting Check out your inverter at another lab bench before starting (else you won’t finish promptly)

3 The Schematic and Equipment Layout DCGrid Power Important – if using a solar panel pair as your DC source, insert the Solar Interface Circuit. The circuit is made from recycled DBR components. The large electrolytic capacitor supplies the 120Hz ripple current needed by the inverter, thus permitting the panel current to be practically ripple-free. The diodes prevent back-feeding and polarity errors. 10A Shorting Grid Tie Board On/ Off 10Ω + V grid – DC power Pearson coil amp probe + V ac – Inverter Grid Tie Variac Watt meter 120V ac Outlet “The Grid” I grid 40V dc (DBR or solar panel pair) Solar Interface Circuit

4 Some Theory is Needed Equivalent Circuit is mostly resistive unless a large inductor is addedAt 60 Hz, inverter impedance !

5 same as So if the MOSFETs are never switched on and there is an AC source, the H-Bridge behaves exactly like a DBR, where power is moved from AC to DC Thus, the H-Bridge can be either a rectifier or inverter Redrawing shows that the H-Bridge can be a rectifier or inverter !

6 The Electrical Circuit Model InverterGrid I is the phasor current S (complex power) = P + jQ P is the active power Q is the reactive power Impedances Z can be expressed as R + jX

7 The Electrical Circuit Model, cont. For our 60Hz experiment, the circuit resistance R’s are much higher than the inductive reactance X’s For our 60Hz experiment, voltage angle δ is zero because the inverter control signal is a replica of the grid voltage Then, as derived in the lab document, Active powerReactive power Thus, we control the direction and amount of P by adjusting this difference !

8 Effect of real and reactive power from PV inverters

9 The Grid Frequency is Constantly Changing The average frequency is 60Hz The frequency is the same throughout a grid (so, in the US, there are eastern, western, and Texas frequencies) Generator governors hold the grid frequency near 60Hz as the total customer load changes to maintain balance between generation and (load + losses) Time correction is performed when clocks are off by 3 seconds A grid-tie inverter stays perfectly synchronized because it uses the wall outlet voltage as Vcont !

10 Tie Source: NPR Source: Tosaka How many main grids are there in Japan and in the US? US: The same nominal frequency but 3 main grids Japan: Two different nominal frequencies and 3 grids

11 Daily Load Variation Summer dayWinter day

12 0.1Hz 8 minutes Slow Frequency Variation Wednesday, November 7, 2007, 4:15 PM Texas

13 Large Generator Trip Tuesday, November 13, 2007, 4:19 PM 0.16Hz Texas 8 minutes

14 Whoa, Clocks! Intentional Time Correction Taking Place Wednesday, November 14, 2007, 6:00 AM 8 minutes

15 Unusual Wind-Related Event? Sunday, May 13, 2007, 3:11am 8 minutes Intermittent (non-dispatchable) generation sources, such as wind generators or PV modules) may have a severe negative effect on grid’s stability if they are not properly controlled. Proper control may imply having large energy storage integrated in the grid.

Hz 8 minutes Multiple Generator Trips Saturday, August 25, 2007, 3:32 AM California

17 0.2Hz Generator Trip Generator Trip Insufficient Spinning Reserve Onset of Rotating Blackout Monday, April 17, 2006, 4pm (Unusually hot day, and many generators out for maintenance) Texas 8 minutes Voluntary load shedding begins Stage 1 of automatic load shedding (5%) kicks in at 59.7Hz

Traditional architecture (SMA Sunny Boy, PV Powered, Fronius, Xantrex, and others): Grid Tied Inverters “CONNECTION BOX” (NO ELECTRONICS) 18 How can grid tied inverters affect grid stability?

Synchronous generators control Field Excitation Q In large conventional grids, voltage depends on reactive power flow. In synchronous generators, the frequency of the output electrical signal depends on the rotor’s speed. The rotor acceleration depends on the difference between input mechanical power and output electrical power. So frequency can be controlled through the mechanical power. Pmec is increased to increase f Pmec is decreased to decrease f

Voltage and frequency control Operating frequency Higher commanded frequencies No load droop line Higher power output Operator of a generator connected to a large grid After the generator is paralleled to the grid then its output frequency and voltage will remain fixed and equal to the grid’s frequency and voltage, respectively. Output power is controlled by attempting a change in frequency by controlling the prime mover’s torque (the input Pmec). By “commanding” a decrease in frequency, the output power will increase. A similar approach is followed with reactive power control, by controlling field excitation in an attempt to change output voltage.

21 Effect of real and reactive power from PV inverters With grid tied inverters, when PV power increases, the real power provided by the grid is reduced by the reactive power provided by the grid is not significantly changed. Hence, power factor from the grid is reduced.

Traditional architecture (SMA Sunny Boy, PV Powered, Fronius, Xantrex, and others): Fronius PV Powered 22 Grid Tied Inverters

Islanding inverters Alternative to traditional architecture to provide backup and operation without grid connection (SMA Sunny Boy – Sunny Island 5048U). 23

24 Pecan Street Mueller area

Source: Fabian Uriarte Pecan Street Mueller area

Pecan Street

Mueller area. Preliminary results about transformer loading during a winter day (no a/c) including PV and EV contributions Transformer’s cooling (or heating) profiles are affected by PV systems Source: Fabian Uriarte Pecan Street

28 10A Shorting Grid Interface Board On/ Off 10 Ω + V grid – Pearson coil amp probe + V ac – Inverter Grid Tie Variac Watt meter 120V ac Outlet “The Grid” I grid Closing the Grid Tie V ac V grid When these two voltages are the same, you can close the grid tie in two steps: 1. Close the on/off switch 2. Close the 10Ω shorting switch Use same scale on both channels

29 When the grid tie is first closed, there is no power flow Then, lower the grid voltage with the variac, and power will begin to flow from the inverter to the grid The grid voltage will control the inverter voltage because the grid is much “stiffer” than the inverter V grid and I grid (viewed on the low-voltage side of the variac) Save screen snapshot #3 V grid I grid

30 The Inverter Current is Actually Quite Clean (for an Inverter), and Much of the Distortion is a Reflection of the Grid Voltage FFT ofI grid (10dB/division on the y-axis) Save screen snapshot #4 THD  0.20

31 Questions?