Nonintrusive Appliance Load Monitoring

11/14/13 Nonintrusive Appliance Load Monitoring

www.georgehart.com/research/nalm.html 1/5

Nonintrusive ApplianceLoad Monitoring

Contents:

1. Technical Overview2. Multistate Appliances

3. Development Status4. People to Contact

5. References and Related Links

This page is was created by George W. Hart some 5

years ago, and is now fairly out of date. I am no longer

doing research in this area and can provide no more recent information.

Technical Overview

A Nonintrusive Appliance Load Monitor (NALM) is designed to monitor an electrical circuit that contains anumber of devices (appliances) which switch on and off independently. By a sophisticated analysis of the current

and voltage waveforms of the total load, the NALM estimates the number and nature of the individual loads,

their individual energy consumption, and other relevant statistics such as time-of-day variations. No access to the

individual components is necessary for installing sensors or making measurements. This can provide a very

convenient and effective method of gathering load data compared to traditional means of placing sensors on eachof the individual components of the load. The resulting end-use load data is extremely valuable to consumers,

energy auditors, utilities, public policy makers, and appliance manufacturers, for a broad range of purposes. For

example, a monitor placed outside a home can determine how much energy goes into each of the major

appliances within the home.

In a utility application, a NALM connects with the total load using the standard revenue meter socket interface,as shown in the figure above. This permits very easy installation, removal, and maintenance compared with

traditional intrusive load monitoring techniques that require ``submetering'' and interior wiring. The NALM

monitors the total load, checking for certain ``signatures" which provide information about the activity of the

appliances which constitute the load. For example, if the residence contains a refrigerator which consumes 250

W and 200 VAR, then a step increase of that characteristic size indicates that the refrigerator turned on, and a

decrease of that size indicates the turn-off events. Other appliances have other characteristic signatures. After

determining the exact on and off times from the signature events, any desired statistics, such as energy

consumption vs. time of day or temperature, can be tabulated.

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To appreciate how this works, consider this figure, which plots total (real) power consumption vs. time for a

single-family home over a two-hour period. During this interval, the total load shows activity due to a refrigerator

and a heater. Two different-sized step changes are clearly present, providing characteristic signatures of the

refrigerator and the heater. The refrigerator cycles on and off three times, the heater six times. By measuring the

total load ouside the home, it is not difficult to find these step changes and measure their size. Knowing the time

of each on and off event, the total energy consumption of the refrigerator and the heater are easily determined.By also considering measurements of the total reactive power or harmonic current, along with the real power

shown, changes in the resulting vector function of time would reveal even more information about the particularappliances.

Traditional load research instrumentation involves complex data-gathering hardware but simple software. A

monitoring point at each appliance of interest and wires (or sometimes power-line carrier techniques) connectingeach to a central data-gathering location provide separate data paths, so the software merely has to tabulate the

data arriving over these separate hardware channels. The NALM approach reverses this balance, with simplehardware but complex software for signal processing and analysis. Only a single point in the circuit isinstrumented, but mathematical algorithms must separate the measured load into separate components. In many

load-monitoring applications, this is a very cost-effective tradeoff, which is a major advantage of the NALM.

In order to accurately decompose the aggregate load into its components, a model-based approach fordescribing individual appliances and their combination is used. These models suggest certain signatures which can

be detected in the total load to indicate the activities of the separate components. This leads naturally to practicalarchitectures and algorithms for the NALM. For full details, see the references below. We have implemented

these ideas and carried out a number of initial field tests on residential loads to compare the NALM to traditionalload monitoring techniques employed by electric utilities. Based on these tests, a commercial version of the

NALM is being developed for widespread utility use.

Back to Table of Contents

Multistate Appliances

My recent research has focused on appliances which can be understood as multistate devices, using finite-state

machine (FSM) models. There are three classes of appliance models from the NALM perspective:



ON/OFF (Two-state)

Appliances such as light bulbs or toasters, which are either on or off at any given moment. Early researchfocused on techniques for monitoring these.

MultistateAppliances such as washing machines or dishwashers, with distinct types of ON states, e.g., fill, rinse,

spin, pump, etc. Recent research has extended the methods to apply to the multi-state case.Continuously variable

Appliances like light dimmers and variable-speed hand tools, with a continuous range of ON states. Theseare difficult to monitor nonintrusively, because they do not generate step changes in power.

To learn the FSM control structure of

different multistate appliances, we havedeveloped the portable instrumentationillustrated here. This is a new tool, which

analyzes the behavior of an operatingelectrical load in a novel manner by

automatically describing it with a finite-statemodel of its control structure. A personal-

computer-based system collects samples ofreal and reactive power consumption over time, and automatically learns the control structure of the load in real

time, drawing its finite-state diagram. The system also reports the load's state at each point in time, the total timespent in each state, and total energy consumption for each of the states.

One use of this tool is to provide a database of common appliance FSM structures for the NALM project. This

research also has applications to behavioral analysis, energy monitoring, fault monitoring, fault analysis, andpower quality analysis of many types of electrical loads, controllers, and power sources. Although only tested onresidential loads and consumer appliances so far, the underlying methods should also work on commercial and

industrial loads, e.g., HVAC control systems.

Three-way Lamp

As an example, if a three-way lamp is operated, a plot of powerversus time shows plateaus at the low, medium, and high power

levels. The patern-recognition algorithm in the instrument detectsthese, and constructs the FSM disaram shown, illustrating howthe four states are cyclically connected.

Frost-free Refrigerator with Interior Lamp

A more complex example is this frost-free refrigerator. From the

measured plots of real and reactive power, the six-state FSMshown is generated. The inner three states correspond to the light

being off (the door closed) and the outer three occur when the

light is on. The power plot shows how on/off cycles of the



compressor are followed by a single defrost cycle in which the

motor is off but a heater is on (so there is a large real power, but

no reactive power). The FSM generated captures all thisbehavior

For details of the algorithm, discussion, and many more

examples, see my paper ``Automatic Construction of Finite-State

Load Behavior Models," listed in the references below.


NALM Development Status

The Electric Power Research Institute (EPRI) has sponsoredNALM research since its conception in the early 1980's. EPRI

has chosen Telog Instruments to commercialize the NALM into a

research tool available to electric utilities. A beta-test program of

the commercial version of the NALM is underway, and units are expected to be available to electric utilities in1997. For exact availability information, contact: [email protected]

Telog Instruments, Inc.

830 Canning Parkway Victor, NY 14564-8940

(716) 742-3000


People to Contact

A number of people at a number of organizations are involved in research and development of NALMtechniques.

George W. Hart

Originator and developer of the NALM. Originally at MIT, then at Columbia University, I was briefly atHofstra University. Contact information and additional information regarding my research is available on

my home page.

Lawrence Carmichael

Mr. Carmichael is the project supervisor at the Electric Power Research Institute (EPRI) in charge of theNALM. (415) 855-7982

Mark Malmendier

http://www.epri.com/

mailto:[email protected]

http://www.georgehart.com/index.html

http://www.epri.com/



Mr. Malmendier is the product manager of Telog Instruments, Inc. in Rochester, New York. (716) 742-

3000, email: [email protected]

Leslie Norford and Steven LeebProfs. Norford and Leeb are engaged in research at MIT to explore the possibilities of extending NALM

techniques to transient information in commercial buildings.

Jackie Lemmerhirt and Ralph Abbott

Plexus Research (in Acton, Massachusettes) is involved in coordinating the electric utility community to theNALM development efforts. Ms. Lemmerhirt ([email protected]) is carrying out a project

comparing NALM output with independent instrumentation in a number of test houses. This should

provide solid data on the accuracy of the NALM. Mr. Abbott is president of Plexus.


References

A good introductory tutorial survey is the following article. (The above technical overview is excerpted from it.)

Hart, G.W., ``Nonintrusive Appliance Load Monitoring," Proceedings of the IEEE, December 1992,

pp. 1870-1891.

A complete bibliography of published papers concerning nonintrusive load monitoring is also available.


http://web.mit.edu/


http://www.georgehart.com/research/nalmrefs.html

Nonintrusive Appliance Load Monitoring

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