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Power Grid Stability:
The Perils and Potential of
Power Electronic Load
Bernie Lesieutre (UW-Madison, LBNL)
Joe Eto (LBNL)
Dmitry Kosterev (BPA)
MidAmerica Regional Microgrid Education and Training Consortium (MARMET)
March 30, 2015, UIUC
2
Grid Operation and Planning
Stability: Concerned with immediate response
to disturbances.
I’m interested in what happens on a fast time scale, seconds.
3
Effect of Loads on Grid Stability
Concern: Smart Energy Efficient Loads, and other
active components, will challenge Grid Stability.
4
Load Composition Impact Study
650
From a grid perspective,
“Contant Power” loads are
bad.
550
500
Basline
Expected Increase in Power Electronics with Constant Power
50% Constant Current / 50% Constant Power
100% Constant Power
450
400
0
10
20
30
Time (sec)
40
50
Transmission Outage
60
Basline
Expected Increase in Power Electronics with Constant Power
50% Constant Current / 50% Constant Power
100% Constant Power
60
59.9
Generator,
Load
Loss
Frequency (Hz)
Voltage (kV)
600
59.8
59.7
59.6
59.5
59.4
0
10
20
30
Time (sec)
40
50
60
5
Grid Perspective: Load Modeling
• The Ancient History of Load Modeling
Exponential Model
“ZIP” Model
6
Grid Perspective: Load Modeling
The ZIP model is inadequate.
August 10. 1996
Next Model
20% Motor
ZIP model
Then, lots of research on load models.
7
Grid Perspective: FIDVR
Meanwhile… Voltage Stability Issue:
Fault Induced Delayed Voltage Recovery
Uncontrolled voltage drop.
Overvoltage.
Weakened system
after recovery.
Hypothesis: Air conditioner
motors stall.
8
Grid Perspective: FIDVR
Voltage Stability:
Fault Induced Delayed Voltage Recovery
Wide variation seen on
distribution feeder.
Laboratory tests show …
Air conditioner motors stall, really easily.
9
Motors Stall.
Stall Voltage vs fault duration,
and point-on-wave varation.
Stall voltage vs. loading.
cycles
10
New (WECC Standard) Load Model
Different motors (large,small),
different mechanical loads. Dynamic.
Includes new AC model
(static performance model)
ZIP Model
Constant Power
Better models for compressor motors and stall does not fix
the FIDVR problem. The models allow simulations to study
the possibility of cascading outages due to FIDVR.
Power electronic loads are modeled as constant power
(in normal voltage range).
11
Load Perspective: Let’s do some work!
From the load perspective, the “ideal”
characteristic is constant power.
• The end-use energy is used to do something.
• Disturbance rejection, from the load
perspective, is to continue normal operating in
the face of variable or uncertain inputs.
Power Electronics allows us to achieve this ideal
while increasing efficiency.
Load Characteristics
• Load characteristics, i.e. power sensitivity to voltage and
frequency, can impact the power system stability
– GOOD: Light dims (consumes less power) when the voltage is
lowered
– BAD: Computer consumes same amount of power when voltage
is lowered
• Trends in load characteristics:
End Use
Old Characteristics
New Characteristics
Fans
Direct-drive motors,
frequency sensitive
Electronically commutated
DC motors – constant power
Lights
Resistive incandescent
LED electronic lighting - varies
Water Heating
Resistive
Heat pump
Cooling pumps
Direct-drive motors,
frequency sensitive
VFD-connected – constant
power
Slide from J. Undrill, “How Electric Vehicle Charges can be friendly with respect to power grid stability ,“ SAE presentation, March 2015.
13
VFD Tests, out of the box
• Test Fan and Pump loads
• Subject to various voltage and frequency
ramps and steps.
• Calculate Active and Reactive Power
14
Fan Test Arrangement
Measurements:
• Input voltage and currents
• VFD DC link voltage
• Motor speed
Compute: Active and Reactive Powers
Calculations: Positive Sequence
Voltage Ramp
P
Q
Positive Sequence,
fundamental frequency
pf ≈0.99 lagging
P≈1500 W Q≈200 VAR
Vfd – current/power drawn
“Rabbit Ears”
The current drawn by
the VFD is NOT
sinusoidal, and has
strong harmonics.
ia
YIKES!
RMS calculations:
P≈ 1500 W Q≈1300 VAR
Pf ≈ 0.75 lagging
We look at fundamental frequency
calculations in this presentation
17
Voltage Steps – Active Power
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Voltage Steps: Reactive Power
Perhaps some variation in reactive power, however the load is nearly unity power factor. The
active power characteristic is dominant.
19
Voltage Ramp: Active Power
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Voltage Ramp: Reactive Power
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Pump Test Arrangement
Measurements:
• Input voltage and currents
• VFD DC link voltage
• Motor speed
Compute: Active and Reactive Powers
22
Voltage Steps: Active Power
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Voltage Ramp: Active Power
24
Constant Power Loads
Many power electronic loads display a constant
power power characteristic.
Is this a problem?
25
System Impact
650
From a grid perspective,
“Contant Power” loads are
bad.
550
500
Basline
Expected Increase in Power Electronics with Constant Power
50% Constant Current / 50% Constant Power
100% Constant Power
450
400
0
10
20
30
Time (sec)
40
50
Transmission Outage
60
Basline
Expected Increase in Power Electronics with Constant Power
50% Constant Current / 50% Constant Power
100% Constant Power
60
59.9
Generator,
Load
Loss
Frequency (Hz)
Voltage (kV)
600
59.8
59.7
59.6
59.5
59.4
0
10
20
30
Time (sec)
40
50
60
26
Power Electronic Loads
Power electronic loads are controllable.
Potentially, we can design controls to help with
grid stability, while delivering energy efficiently.
(Win/Win).
27
What do we want?
Control Objective
Control the power electronic load such that
• Voltage response appear as constant current
• Frequency response D = 10.
28
VFD Simulation with grid helpful control
Rectifier
VFD
PWM Inverter
Input
IM
open loop
volts/hertz
control
speed command
Grid Responsive Control
29
Voltage Ramp Test - Actual
FAN
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Voltage Ramp Test - Controls
FAN
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VFD Controls
In the lab we only had access to a frequency
command for control.
• Confirmed effectiveness control (open loop)
• Feedback control also worked, but some tuning
and further research required.
Future: design more comprehensive control
system to achieve control objectives.
32
Conclusions
Power Electronic loads
• help with energy efficient operation
• allow substantial control
• expected to dominate future load.
• create concern over grid level stability
We need to be aware of and address grid
stability concerns.
33
Recommendations
• For non-critical loads, adopt a constant
current operating characteristic.
• For critical loads, enable a constant current
operating characteristic for short disturbances
while maintaining constant power end use.
(Use energy storage.)