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EE462L, Spring 2014 Power to Grid

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• 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)

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The Schematic and Equipment Layout

DC Grid

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 +Vgrid

–

DC power

Pearson coil amp probe

+Vac

–Inverter Grid

TieVariac

Wattmeter

120Vac

Outlet“The Grid”

Igrid

40Vdc

(DBR or solar panel

pair)

Solar Interface Circuit

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Some Theory is Needed

Vdc

Mot

A+ B+

A– –+M

B– –+M

Mot

invZ

Zinv

+ )cos( tVinv

−

Equivalent Circuit

adc

inv mV

V 2

invZ

invZ is mostly resistive unless a large inductor is addedAt 60 Hz, inverter impedance

!

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Vdc

Mot

A+ B+

A– –+M

B– –+M

Vdc

Mot

A+ B+

A– –+M

B– –+M

Vdc

A+ B+

A– –+M

B– –+M

Vac

same as

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 !

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invZ

Zinv

+ )cos( tVinv

−

gridZ

Zinv +

)cos( tVgrid

−

QPSI , , ,S, P, Q

The Electrical Circuit Model

Inverter Grid

• 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

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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,

gridinvtot

gridVV

R

VP 0Q

Active power Reactive power

Thus, we control the direction and amount of P by adjusting

this difference

!

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Effect of real and reactive power from PV inverters

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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

!

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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

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Daily Load Variation

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 3 6 9 12 15 18 21 240.5

0.6

0.7

0.8

0.9

1.0

1.1

0 3 6 9 12 15 18 21 24

Summer day Winter day

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0.1Hz

8 minutes

Slow Frequency Variation

Wednesday, November 7, 2007, 4:15 PM

Texas

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Large Generator Trip

Tuesday, November 13, 2007, 4:19 PM

0.16Hz

Texas

8 minutes

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Whoa, Clocks! Intentional Time Correction Taking Place

Wednesday, November 14, 2007, 6:00 AM

8 minutes

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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.

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0.10Hz

8 minutes

Multiple Generator Trips

Saturday, August 25, 2007, 3:32 AM

California

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0.2HzGenerator

Trip

GeneratorTrip

InsufficientSpinningReserve

Onset of Rotating BlackoutMonday, 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 InvertersGrid 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

2P1P

f

P

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.

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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

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Grid Tied InvertersGrid Tied Inverters

Islanding invertersIslanding inverters

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

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Pecan StreetPecan Street

• Mueller area

Source: Fabian Uriarte

Pecan StreetPecan Street• Mueller area

• Mueller area

Pecan StreetPecan 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 StreetPecan Street

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10A

Shorting

Grid Interface Board

On/Off

10Ω +

Vgrid

–

Pearson coil amp probe

+ Vac

– Inverter

Grid

Tie Varia

c

Watt meter

120Vac

Outlet “The Grid”

Igrid

Closing the Grid Tie

Vac

Vgrid

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

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• 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

Vgrid and Igrid

(viewed on the low-voltage side of the variac)

Save screen snapshot #3

Vgrid

Igrid

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The Inverter Current is Actually Quite Clean (for an Inverter), and Much of the Distortion is a Reflection of

the Grid Voltage

FFT of Igrid

(10dB/division on the y-axis)

Save screen snapshot #4

THD 0.20

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Questions?