On-line Method for Identifying Harmonic Sources in Power Systems

Himanshu Jain

Advised by Prof. Robert Cox

University of North Carolina, Charlotte

Problem Description

• Non-linear power electronic loads introduce harmonic currents into power systems

• These currents can have detrimental effects

Problem Description

• Harmful effects include:– Overloading of neutrals– Overheating of transformers– Nuisance tripping of circuit breakers– Over-stressing of power factor correction

capacitors– Unbalanced voltages

• Utilities would like an effective way to charge customers for creating these harmonics

Outline

• Discussion of harmonic sources – Linear vs. non-linear loads– The difficulty of identify harmonic sources

• Proposed measurement approach

• Experimental results

Harmonic Sources

• Non-linear loads are very common– Most common example: Rectifier

• Load current contains many harmonics, typically odd harmonics

Quantifying Harmonics

• Total harmonic distortion

• Each harmonic as a percentage of the total RMS value

• Could customers be charged for their THD?

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Coupling

• Generally, linear loads do not draw harmonics

• Problem: Harmonic currents from non-linear loads produce a distorted voltage waveforms because of line impedances

Example

• Problem: Heater will also distort the PCC voltage because the PCC is distorted– This masks the effects of the rectifier– But, the heater would not have drawn such currents,

however, without the rectifier

Harmonic Number

Current w/ Rectifier in

Parallel (A RMS)

Current w/o Rectifier in

Parallel (A RMS)

1st 6.6 6.7

3rd 0.1 0

5th 0.1 0

7th 0.1 0

Current harmonics for heater

Harmonic Number

Current w/ Heater in Parallel (A RMS)

Current w/o Heater in Parallel (A RMS)

1st 1.1 1.3

3rd 0.9 1.2

5th 0.6 0.9

7th 0.3 0.6

Current harmonics for rectifer

Potential Solutions

• It is difficult to charge customers for harmonics because harmonic voltages in the PCC affect the harmonics that are drawn

• Several methods have been considered:– Power direction method

• Xu et al. 2003 proved that this method is flawed

– Neural network method• Mazumdar et al. 2007 created a NN mapping between

PCC voltage and current – no physical meaning

• Here we present a different approach

Proposed Approach

• One potential method would be to– Detect operation of non-

linear loads– Determine their

harmonic content given “ideal” conditions

• Requires:– Load model– Knowledge of source

impedance

Smart Meters

• As a part of Smart Grid efforts, utilities are deploying Smart meters throughout the country

• Smart meters could perform necessary measurements with software changes

NILM Overview

Transient-Based Identification

• Transient electrical behavior is strongly influenced by the physical task performed by the load

Load Identification

• Templates are matched to power data

Proposed Approach• First step: A Smart Meter with

NILM software must identify the operation of a non-linear load such as a motor drive

• Example templates for VSD:

Impedance Estimation• Line impedance is the major

cause of distortion in the PCC

• Line impedance is small and time-varying

• Typical measurement method: – Inject large currents at the PCC

– Requires additional expensive hardware

• Currents are normally injected very close, but not at, 60Hz

)()( injmSSinjinjm jIRLjjV

If ωinj ≠ ωs

Our Approach• Loads such as motors and power electronics

naturally inject currents into the line that are close to the line frequency.

• Example: 4-pole induction motor current spectrum

• Injected signals are much smaller, so the measurement problem is more difficult, but it is far cheaper and more convenient solution

Our Approach and Results• Traditional frequency domain approaches (FFT) are limited

because of the small signal

• Approach is based on communications theory, i.e. detection of small sinusoidal signals in the presence of noise

• Maximum likelihood estimation scheme is used to estimate the amplitude, frequency, and phase of the “injected” signal

Model Identification

• The problem: When simulating the isolated rectifier with a pure sinusoidal source, how should it be modeled?

• Consideration: Typically, the rectifier is loaded by a constant power load (i.e. a regulator or an inverter driving a motor)

• Our assumption:

– When the converter is isolated in simulation, the power demand on the output side is the same as when the rectifier is connected in the system with the non-ideal source

Model Identification of Non-linear load• Research has shown that a reasonable rectifier model with a

constant power load (i.e. regulator) is the following:

• Question: How do you measure output power??

• Output power can be measured:– In most large power-electronic drives, the DC link voltage is known

– The DC current can be measured from the AC-side current

• The diodes are assumed to be ideal

• The nameplate value for the capacitor is used

Simulation• An ideal source is connected to an isolated rectifier and a simulation is

performed in Simulink• DC current is adjusted until output power matches on-line value

Simulation• FFT of current is used to determine the contribution

of desired harmonics

Experimental Setup• Resistively-loaded rectifier connected in parallel

with 900W heating element• Loads are connected to single-phase test source

through 1.5 Ohms and 0.3mH• Both NILM and a Fluke power meter are used for

measurement

Current Plots Heater Current Total Current

Rectifier Current

Model Validation

• To test the validity of the method, several tests were performed with the rectifier only and the results were compared to the simulation– Test 1: Pure sinusoidal

excitation

– Test 2: Excitation with 10% third harmonic added

Harmonic Number

Simulated Current(A rms)

Measured Current(A rms)

1st 1.3 1.3

3rd 1.2 1.2

5th 1.0 0.9

7th 0.7 0.6

Harmonic Number

Simulated Current(A rms)

Measured Current(A rms)

1st 1.2 1.2

3rd 1.0 1.0

5th 0.6 0.6

7th 0.3 0.3

Test 1

Test 2

Experimental Measurements

• Current drawn by rectifier with and without the heating element turned on:

Harmonic Number

Current w/o Heater

(A rms)

Current w/ Heater(A rms)

1st 1.3 1.1

3rd 1.2 0.9

5th 0.9 0.6

7th 0.6 0.3

Experimental Measurements• Measured and simulated rectifier currents:

Harmonic Number

Simulated Current(A rms)

Measured Current(A rms)

1st 1.3 1.1

3rd 1.2 0.9

5th 1.0 0.6

7th 0.7 0.3

• Simulated current similar to measured current when heater was not connected

• Differences between simulation and measurement are expected – simulated results indicate what should be the case w/o the heater.

• The heater’s harmonics (which are not it’s fault) partially mask the effects of the rectifier

Conclusion and Future Work

• Utilities could potentially bill large customers based on “true” distortion

• The harmonic source detection method has worked well in the lab, field tests may be performed in Fretwell

• This approach could be coupled into Smart Meters

Thank You!

Linear load v/s Non Linear load

Linear Load• Impedance remains constant

throughout its operation

• Load current does not contains harmonics

• Zero neutral current if loads are equally balanced

• May not demand high inrush currents while starting

• Linear load is usually resistive, can have small inductance and capacitance.

• Follows Ohm’s law

Non Linear Load• Impedance changes with applied

voltage

• Load current contains all odd harmonics

• Neutral current could be 2.7 times the line current even if loads are equally balanced

• Essentially very high inrush current (20 time of I Normal) is drawn while starting

• Usually an equipment with Diode and Capacitor and can’t be categorize as leading or lagging load

• Ohm’s law is not applicable