1 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 Voltage (kv) 600 550 500 From a grid perspective, Contant Power loads are bad. 450 Basline Expected Increase in Power Electronics with Constant Power 50% Constant Current / 50% Constant Power 100% Constant Power 400 0 10 20 30 40 50 60 Time (sec) Transmission Outage 60 59.9 Basline Expected Increase in Power Electronics with Constant Power 50% Constant Current / 50% Constant Power 100% Constant Power Generator, Load Loss Frequency (Hz) 59.8 59.7 59.6 59.5 59.4 0 10 20 30 40 50 60 Time (sec)
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 The current drawn by the VFD is NOT sinusoidal, and has strong harmonics. i a Rabbit Ears YIKES! RMS calculations: P 1500 W Q 1300 VAR Pf 0.75 lagging We look at fundamental frequency calculations in this presentation
Voltage Steps Active Power 17
18 Voltage Steps: Reactive Power Perhaps some variation in reactive power, however the load is nearly unity power factor. The active power characteristic is dominant.
Voltage Ramp: Active Power 19
Voltage Ramp: Reactive Power 20
21 Pump Test Arrangement Measurements: Input voltage and currents VFD DC link voltage Motor speed Compute: Active and Reactive Powers
Voltage Steps: Active Power 22
Voltage Ramp: Active Power 23
24 Constant Power Loads Many power electronic loads display a constant power power characteristic. Is this a problem?
25 System Impact 650 Voltage (kv) 600 550 500 From a grid perspective, Contant Power loads are bad. 450 Basline Expected Increase in Power Electronics with Constant Power 50% Constant Current / 50% Constant Power 100% Constant Power 400 0 10 20 30 40 50 60 Time (sec) Transmission Outage 60 59.9 Basline Expected Increase in Power Electronics with Constant Power 50% Constant Current / 50% Constant Power 100% Constant Power Generator, Load Loss Frequency (Hz) 59.8 59.7 59.6 59.5 59.4 0 10 20 30 40 50 60 Time (sec)
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 Input Rectifier VFD PWM Inverter IM open loop volts/hertz control speed command Grid Responsive Control
29 Voltage Ramp Test - Actual FAN
30 Voltage Ramp Test - Controls FAN
31 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.)