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ECE 476 POWER SYSTEM ANALYSIS. Lecture 25 Governors, Harmonics, the Smart Grid Professor Tom Overbye Department of Electrical and Computer Engineering. Announcements. Be reading Chapter 14 Last homework is 12.3, 12.11, 14.15. Needs to be done before the final Project is due today

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ece 476 power system analysis


Lecture 25

Governors, Harmonics, the Smart Grid

Professor Tom Overbye

Department of Electrical andComputer Engineering

  • Be reading Chapter 14
  • Last homework is 12.3, 12.11, 14.15. Needs to be done before the final
  • Project is due today
  • Final exam is as given on the UIUC website. That is, Tuesday Dec 13 from 7 to 10pm here (218 Ceramics).
    • Final is comprehensive, with more emphasis on material since exam 2.
    • Three note sheets allowed (e.g., ones from previous two exams and one new note sheet)
generator governors
Generator Governors
  • The other key generator control system is the governor, which changes the mechanical power into the generator to maintain a desired speed and hence frequency.
  • Historically centrifugal “flyball” governors have been used to regulate the speed of devices such as steam engines
  • The centrifugal force varieswith speed, opening orclosing the throttle valve

Photo source:

isochronous governors
Isochronous Governors
  • Ideally we would like the governor to maintain the frequency at a constant value of 60 Hz (in North America)
  • This can be accomplished using an isochronous governor.
    • A flyball governor is not an isochronous governor since the control action is proportional to the speed error
    • An isochronous governor requires an integration of the speed error
  • Isochronous governors are used on stand alone generators but cannot be used on interconnected generators because of “hunting”
generator hunting
Generator “Hunting”
  • Control system “hunting” is oscillation around an equilibrium point
  • Trying to interconnect multiple isochronous generators will cause hunting because the frequency setpoints are the two generators are never exactly equal
    • One will be accumulating a frequency error trying to speed up the system, whereas the other will be trying to slow it down
    • The generators will NOT share the power load proportionally.
droop control
Droop Control
  • The solution is to use what is known as droop control, in which the desired set point frequency is dependent upon the generator’s output

R is known as the regulation constantor droop; a typicalvalue is 4 or 5%.

governor block diagrams
Governor Block Diagrams
  • The block diagram for a simple stream unit, the TGOV1 model, is shown below. The T1 block models the governor delays, whereas the second block models the turbine response.
restoring frequency to 60 hz
Restoring Frequency to 60 Hz
  • In an interconnected power system the governors to not automatically restore the frequency to 60 Hz
  • Rather this is done via the ACE (area control area calculation). Previously we defined ACE as the difference between the actual real power exports from an area and the scheduled exports. But it has an additional termACE = Pactual - Psched – 10b(freqact - freqsched)
  • b is the balancing authority frequency bias in MW/0.1 Hz with a negative sign. It is about 0.8% of peak load/generation
2600 mw loss frequency recovery
2600 MW Loss Frequency Recovery

Frequency recovers in about ten minutes

power system harmonics
Power System Harmonics
  • So far class has talked about fundamental frequency analysis. Many traditional loads only consume power at the fundamental frequency. However, some loads, mostly electronic-based, tend to draw current in non-linear pulses, which gives rise to harmonics.
    • If current has half-wave-symmetry (values are equal and opposite when separated by T/2) then there are no even harmonics
switched mode power supply current
Switched-Mode Power Supply Current


harmonic current specturm
Harmonic Current Specturm
  • The below figure shows the harmonic current components for an 18-W, electronic-ballast compact fluorescent lamp.

Source: Fig 2.34 of “Renewable and Efficient Electric Power Systems” by Masters

key problems with harmonics
Key Problems with Harmonics
  • A key problem with the third harmonic is neutral current since the fundamental 120 degree phase shift becomes 360 degrees for the third harmonic so the third harmonic values do not cancel (also true for other triplen harmonics)
    • Delta-grounded wye transformers prevent triplen harmonic currents from flowing into the power grid
  • Harmonics cause transformer overheating since core losses are proportional to frequency
  • Harmonic resonance, particularly with shunt capacitors (can be around 5th or 7th harmonic values)
the smart grid
The Smart Grid
  • The term “Smart Grid” dates officially to the 2007 US “Energy Independence and Security Act”, Title 13 (“Smart Grid”)
    • Use of digital information and control techniques
    • Dynamic grid optimization with cyber-security
    • Deployment of distributed resources
    • Customer participation and smart appliances
    • Integration of storage including PHEVs
    • Development of interoperability standards
  • This effort is a continuation of grid advances that have occurred over generations
doe s grid vision
DOE’s Grid Vision

Enable a seamless, cost-effective electricity system, from generation to end use, capable of meeting the clean energy demands and capacity requirements of this century, while allowing consumer participation and electricity use as desired:

  • Significant scale-up of Clean Energy (80% by 2035)
  • Allows 100% customer participation and choice (including distributed generation, demand-side management, electrification of transportation, and energy efficiency)
  • A 100% holistically designed system (including AC-DC hybrid configurations)
  • Global competitiveness and leadership
  • A reliable, secure, and resilient Grid

Slide Source: U.S. Department of Energy, Draft Vision of a Future Electric Grid; 11/15/11 draft. For presentation at National Electricity Forum, February 8–9, 2012

doe s priority needs and focus
DOE’s Priority Needs and Focus




End User

Cleaner generation technologies

Accessing high quality sources of renewable energy and addressing line congestion

Accommodating increase use of EV, PV, DG, and consumer participation

Improved efficiencies in buildings and industry

Integration of renewables: improved operation, planning, etc.

Seamless connection:

two-way power flows and increased data streams

Interface with end users: deployment of AMI, microgrids, etc.

System understanding and control: visualization, communications, computation

System flexibility for stability: storage, demand response, accommodating increased variability

System security: physical security, cyber security, mitigating increased vulnerabilities

Slide Source: U.S. Department of Energy, Draft Vision of a Future Electric Grid; 11/15/11 draft. For presentation at National Electricity Forum, February 8–9, 2012

smart grid drivers
Smart Grid Drivers
  • A key driver for the Smart Grid is to allow for the integration of much more non-controllable electric generation like wind and solar
  • There is a potential need to integrate in a large amount of new load for electric transportation
  • Need is to replace control lost on the generation side with more control on the load side
  • Customers desire appropriate levels of information to make more informed choices
areas of uncertainty
Areas of Uncertainty
  • With the large amount of existing generation, transitioning to large amounts of more renewable sources will take years, and will be driven by fuel prices, such as natural gas
  • Climate change
  • Development of electrified transportation
  • Perceived cyber security concerns
  • Customer acceptance
natural gas prices
Natural Gas Prices

Low natural gas prices tend to result in lower electricity prices.

This makes renewable sources seem more expensive, and reducescustomer’s focus on their electric bills

smart grid and transmission
Smart Grid and Transmission
  • A smarter, more renewable intensive grid would be more complex with both more control (at the load) and potentially less control (at the renewable generation)
    • Overall transmission loading may also increase
  • From a transmission perspective this will result in more challenging operations and planning
  • Much of existing smart grid transmission system investments over the last two years have focused on phasor measurement unit (PMU) installations
smart grid and the distribution system
Smart Grid and the Distribution System
  • Distribution system automation has been making steady advances for many years, a trend that should accelerate with smart grid funding
  • Self-healing is oftenused to refer toautomatic distributionsystem reconfiguration
  • Some EMSs alreadymonitor portions of thedistribution system

S&C IntelliRupter® PulseCloser

demand side management
Demand Side Management
  • The utility industry has a long history of demand side management, datingback into the 1970’s;smart grid technology willnot change the economics
  • Smart grid technologymay have some impacton demand, particularlywith real-time prices
the smart grid and the customer
The Smart Grid and the Customer
  • Initially for some consumers the smart grid may just being able to see how they use electricity; but this requires dollars for smarter meters
  • How quickly and to what extent consumers will accept more load management is still unknown
    • Google retired their PowerMeter service in Sept 2011.
    • A 2011 pilot study by CenterPoint Energy indicated most consumers (71%) adjusted their energy usage when presented with displays showing their consumption
    • There are still conflicting views about how much load reduction, and at what time, is possible
the smart grid and electric cars
The Smart Grid and Electric Cars
  • The real driver for widespread implementation of controllable electric load could well bePHEVs.
  • Recharging PHEVs when their drives return home at 5pm would be a really bad idea, so some type of load control is a must.
  • Quick adoption of PHEVs depends on gas prices, but will take many years at least
  • Investigation into Chevy Volts catching firemay inhibit quicker adoption
the smart grid and cyber security
The Smart Grid and Cyber Security
  • Recently several news stories have raised the question of potential cyber vulnerabilities associated with the smart grid
  • Two way communication withmeters raises the possibility of tampering
    • All security issues will probably never the worked out, but potential risks need to be assessed
  • UIUC, along with three other schools, are working to address these issues through TCIPG