EE462L, Spring 2014
Power to Grid
1
• 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)
2
The Schematic and Equipment Layout
Power
DC
Grid
Grid Tie Board
DC
power
40Vdc
(DBR or
solar panel
pair)
Shorting
On/
Off
Igrid
10Ω
+
Inverter
Vac
–
Pearson coil
amp probe
10A
+
Vgrid
–
Grid
Tie
Variac
Watt
meter
120Vac
Outlet
“The
Grid”
Solar Interface Circuit
• 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.
3
!
Some Theory is Needed
Vdc
Equivalent Circuit
A+
B+
+
Vinv cos(t   )
−
Mot
A–
B–
–
–
+
M
+
M
At 60 Hz, inverter impedance
Z inv
Zinv
Z inv
Vinv 
Vdc
2
 ma
Mot
Z inv
is mostly resistive unless a large inductor is added
4
Redrawing shows that the H-Bridge
can be a rectifier or inverter
Vdc
A+
!
Vdc
A+
B+
A–
B–
–
–
+
M
+
M
B+
same as
Mot
A–
B–
–
–
+
M
+
M
A+
Mot
B+
Vdc
Vac
A–
B–
–
–
+
M
+
M
• 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
5
The Electrical Circuit Model
Z inv
Zinv
Z grid
Zinv
I , S , P, Q
S, P, Q
+
Vgrid cos(t )
+
Vinv cos(t   )
−
−
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
6
!
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 power
P
V grid
Rtot
Vinv  Vgrid 
Thus, we control the direction
and amount of P by adjusting
this difference
Reactive power
Q0
7
Effect of real and reactive power from PV
inverters
8
!
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
9
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
Tie
Source: NPR
Source: Tosaka
10
Daily Load Variation
Summer day
Winter day
1.1
1.1
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0
3
6
9
12
15
18
21
24
0
3
6
9
12
15
18
21
24
11
Slow Frequency Variation
Wednesday, November 7, 2007, 4:15 PM
Texas
0.1Hz
8 minutes
12
Large Generator Trip
Tuesday, November 13, 2007, 4:19 PM
Texas
0.16Hz
8 minutes
13
Whoa, Clocks! Intentional Time Correction Taking Place
Wednesday, November 14, 2007, 6:00 AM
8 minutes
14
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
15
in the grid.
Multiple Generator Trips
Saturday, August 25, 2007, 3:32 AM
California
0.10Hz
8 minutes
16
Onset of Rotating Blackout
Monday, April 17, 2006, 4pm
(Unusually hot day, and many generators out for maintenance)
Texas
0.2Hz
Insufficient
Spinning
Reserve
Generator
Trip
Generator
Trip
Voluntary load
shedding begins
8 minutes
Stage 1 of automatic load shedding (5%) kicks in at 59.7Hz
17
Grid Tied Inverters
•
Traditional architecture (SMA Sunny Boy, PV Powered, Fronius, Xantrex,
and others):
“CONNECTION BOX” (NO
ELECTRONICS)
• How can grid tied inverters affect grid stability?
18
Synchronous generators control
• 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
Field
Excitation
Q
Voltage and frequency control
• 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.
Higher commanded
frequencies
f
Higher power output
Operating frequency
No load droop line
P1
P2
P
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.
21
Grid Tied Inverters
•
Traditional architecture (SMA Sunny Boy, PV Powered, Fronius, Xantrex,
and others):
PV Powered
Fronius
22
Islanding inverters
• Alternative to traditional architecture to provide backup and operation
without grid connection (SMA Sunny Boy – Sunny Island 5048U).
23
Pecan Street
• Mueller area
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
Closing the Grid Tie
Grid Interface Board
Shorting
On/
Off
Inverter
+
Vac
–
Pearson coil
amp probe
Igrid
10Ω
+
10A
Vgrid
–
Grid
Tie
Variac
Watt
meter
120V ac
Outlet
“The
Grid”
Use same scale on
both channels
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
28
• 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
Igrid
Save screen
snapshot #3
Vgrid and Igrid
(viewed on the low-voltage side of the variac)
29
The Inverter Current is Actually Quite Clean (for an
Inverter), and Much of the Distortion is a Reflection of
the Grid Voltage
THD  0.20
Save screen
snapshot #4
FFT of Igrid
(10dB/division on the y-axis)
30
Questions?
31