1 ee462l, spring 2014 power to grid. 2 there are only two stations, so please use the signup sheet...
TRANSCRIPT
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
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
4
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
!
5
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 !
6
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
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,
gridinvtot
gridVV
R
VP 0Q
Active power Reactive 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
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
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.
16
0.10Hz
8 minutes
Multiple Generator Trips
Saturday, August 25, 2007, 3:32 AM
California
17
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.
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 InvertersGrid Tied Inverters
Islanding invertersIslanding inverters
• Alternative to traditional architecture to provide backup and operation without grid connection (SMA Sunny Boy – Sunny Island 5048U).
23
24
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
28
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
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
Vgrid and Igrid
(viewed on the low-voltage side of the variac)
Save screen snapshot #3
Vgrid
Igrid
30
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
31
Questions?