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Power Quality and HarmonicsCauses, Effects and

Remediation Techniques Carol

Gowan Chad Loomis, PE Cornell

University PDC 12/13/2006 Electrical Design

Section

Overview

- What is Power Quality?
- What are some power quality concerns?
- What are Harmonics and what causes them?
- What effects do Poor PQ and Harmonics have on a

building power system and components? - How can Power Quality and Harmonics Issues be

mitigated? - Questions

What is Power Quality?

- Can be defined as
- Any power problem manifested in
- voltage,
- current,
- or frequency deviations
- that results in failure or misoperation of

customer equipment

What are general classes of power quality

problems per IEC?

(Intl Electrotechnical Commission)

- Conducted low-frequency phenomena
- Signal systems (power line carrier)
- Voltage fluctuations (flicker)
- Voltage dips and interruptions
- Voltage Imbalance (unbalance)
- Power frequency variations
- Induced low-frequency voltages
- DC in ac networks
- Harmonics, interharmonics
- Radiated low-frequency phenomena
- Magnetic and Electric Fields

Categories and Characteristics of Power System

Electromagnetic Phenomena (IEC)

Voltage Fluctuations withinCornells Utility

Distribution

- Transients very quick
- Normal cause is lightning strike
- No lights flicker
- Cornell Utilities employs lightning arrestors at

substations and at primary switches located at

each building electric service equipment - End users need to purchase/install TVSS equipment

to further clamp the voltage spike.

Voltage Fluctuations withinCornells Utility

Distribution

- Sags / Swells
- Voltage imbalance lasting from 3-20 cycles
- Typical cause ? NYSEG switching on the incoming

115kV transmission line - Lights flickering are indicative of this fault
- Utilities does not protect for this condition

Voltage Fluctuations withinCornells Utility

Distribution

- Long term voltage fluctuations
- As load increases, voltage drops (and vice versa)
- Cornell Utilities compensates the long-duration

voltage variations through the use of automatic

load tap changers at the Maple Ave. substation - System voltage tolerance limits are set in ANSI

C84.1. The Cornell system voltages are designed

to always operate in the range A limits (108

126V) - refer to IEEE 141-1993

Voltage Fluctuations withinCornells Utility

Distribution

- Harmonic Distortion
- Typically generated within a facility, not a

distribution issue - Utilities does not protect for this condition

CBEMA curve for susceptibility of 120V Computer

Equipment

- Computer Business Equipment Manufacturers

Association - Early 1980s CBEMA designed the curve to point

out ways in which system reliability could be

provided for electronic equipment

ITIC curve for susceptibility of 120V Computer

Equipment

- Information Technology Industry Council

- This derivation was developed to show a curve

that more accurately reflects the performance of

typical single-phase, 60-Hz computers and their

peripherals

Computer Equipment Disturbance Table(Dranetz

-BMI Field handbook for PQ Analysis)

What are NOT proper descriptors of power quality

problems per IEC?

- Blackout
- Blink
- Brownout
- Bump
- Clean Ground
- Clean Power
- Dirty Ground
- Dirty Power

- Glitch
- Outage
- Interruption
- Power Surge
- Raw power
- Spike
- Surge
- Wink

We all need to define the power quality in proper

terms to address the problem. Is it voltage

quality, harmonics, etc? Metering and analysis

is the best way to resolve the issue.

What are the causes of most trouble tickets

issued for computer problems at Cornell?

- Internally (building area specific) induced

problems - Shared neutrals
- Overloaded circuits (breakers tripping)
- Poorly or improperly grounded circuits
- Note there are VERY minimal utility / delivery

issues (we are very fortunate here at Cornell!) - Handful of harmonic issues in the last 20 yrs,

but harmonics are becoming a larger concern with

more sophisticated systems and buildings.

What Are Harmonics?

- A component frequency of a harmonic motion of an

electromagnetic wave that is an integral multiple

of the fundamental frequency - US fundamental frequency is 60 Hertz
- 3rd Harmonic is 3 x 60Hz or 180Hz
- 5th Harmonic is 5 x 60Hz or 300Hz, etc.

What Causes Harmonics?

- Non-Linear Loads
- Current is not proportional to the applied

voltage

Linear vs. Non-linear loads and current waveforms

- Linear loads and current waveforms.
- Pure resistance, inductance, and capacitance are

all linear. - What that means
- If a sine wave voltage of a certain magnitude is

placed across a circuit containing pure

resistance, the current in the circuit follows

Ohm's Law I E R. - So, for a specific value of ohms, the

relationship of volts and amperes is a straight

line. The current will always be a sine wave of

the same frequency. - Linear Loads include Incandescent lighting,

heating loads, and motors

Linear vs. Non-linear loads and current waveforms

- Nonlinear loads and current waveforms.
- Solid state electronics is based on the use of

semiconductors. These materials are totally

different in that their response to voltage is

not a straight line. - What this means
- With a nonlinear load, you cannot easily predict

the relationship between voltage and current

unless you have an exact curve for each device.

With equipment containing many solid-state

devices, such an approach is impossible. - Nonlinear loads are switched on for only part of

the cycle, as in a thyristor-controlled circuit,

or pulsed, as in a controlled-rectifier circuit.

Effect of harmonics on waveform

180 Out of Phase

In Phase

When a waveform is identical from one waveform to

the next, it can be represented as a sum of pure

sine waves in which the frequency of each

sinusoid is an integer multiple of the

fundamental frequency of the distorted wave. The

sum of the sinusoids created by harmonics can be

analyzed using the Fourier series concept

What do harmonics do?

- Harmonics are carried through the system from the

source and can nearly double the amount of

current on the neutral conductor in three phase

four wire distribution systems. - Distorted currents from harmonic-producing loads

also distort the voltage as they pass through the

system impedence. Therefore, a distorted voltage

can be presented to other end users on the

system. - Overall electrical system and power quality is

affected by the introduction of harmonics.

Sources of Harmonics

- Solid State Electronic Devices which contain a

poor power supply - Computers (PCs/CPUs)
- Laser Printers
- Copy Machines
- Solid State UPS Units
- Solid State Devices (Fluorescent lighting

ballasts) - Rectifiers (AC-DC Converters ? VFDs)
- Welding Units
- Arc Furnaces

- Video display terminals
- File Servers
- Battery Chargers

What are the order of typical harmonics generated

by non-linear loads?

Effects of Harmonics

- Distorted Voltage
- Overheated Transformers and Motors
- Increases Hysteresis (magnetization) losses in

steel and iron cores of transformers, motor and

magnetic trip units of circuit breakers

(Equipment inefficiencies and overheating) - Heating of Neutral Conductors
- Skin Effect ? Increased amount of current flowing

on the outside of conductors (overheating) - Low Voltage at End Loads
- High Neutral to Ground Voltages at End Loads

Effects of Harmonics (cont)

- Operation Problems of Relays and Circuit Breakers
- Thermal/Magnetic Trip Circuit Breakers
- Fuses bimetal strips respond to True RMS
- Harmonic currents increase eddy current losses in

the core steel of the metallic strip. - This causes an OVERprotection situation

Increased losses generate additional heat, this

effect the thermal trip of the unit. - Electronic Trip Circuit Breakers
- Magnitude and phase angle(s) of harmonic

current(s) in relationship to the fundamental

current can cause - Overprotection when Peak current sensing True

RMS - Underprotection when Peak current sensing RMS
- Changing power system loads will vary the

magnitude and phase angle, resulting in

inaccurate and unpredictable sensing units and

overload protection

Effects of Harmonics (cont)

- Communication Problems
- If sharing common parallel path, potential for

harmonics to have inductive coupling effect on

unshielded cabling - Current Measurement Problems (distorted waveform)
- Unreliable Operation of Electronic Equipment
- Mis-operation of electronic equipment that

measures frequency or uses the zero crossing

point of a sine wave. - Control of Speed and Voltage Problems on

Emergency Generators (supplying power) - Capacitor Bank Application Problems (heating)
- Computer (PC/CPU) data errors / data loss
- Affects power supplies and sensitive electronics

How can Harmonics be Reduced?

- Isolate harmonic loads on separate circuits (with

or without harmonic filters) - Harmonic mitigating transformers
- Phase shifting (zig-zag) transformers
- Used to cancel out specific harmonics by making

one voltage circuit 180 degrees out-of-phase - Filter capacitor backs

How can Harmonics be Reduced?

- For VFD and UPS specifically
- Line Reactors
- K-Rated / Drive Isolation Transformers
- Harmonic Mitigating / Phase Shifting Transformers
- 12, 18 or 24 pulse Converters
- Passive parallel / series tuned Filters
- Active Filters

How can Harmonics be Reduced?

- PROPER GROUNDING
- Neutral to ground conductor connection at one

location at main panel or transformer secondary - When neutral is connected to ground at multiple

locations, interference can occur with sensitive

electronic devices. - Run power and control conductors in separate

raceways - Sensitive loads should not share neutral and

ground conductors. - Avoid using conduit as the ground return path,

run dedicated ground wire with circuit conductors - Refer to IEEE Std 1100-1992 (Emerald Book)

Computer Equipment Grounding

- TOP ? Radial or daisy chain grounding Although

Code compliant per NEC, small differences in

potential can cause unintended ground loops - BOTTOM ? Best method for grounding is via

dedicated equipment grounding conductors back to

the source

IEEE Defined Harmonic Current Limits

Voltage Limit /HarmonicsEvaluation Procedure

Questions?

- Thank You!
- Cornell Planning, Design Construction
- Carol Gowan 254-1457 cag57_at_cornell.edu
- Chad Loomis 255-8039 cel36_at_cornell.edu

Simplified Distribution System Diagram

1

1

Phase Conductors Heating from Skin

Effect Conduits Heating from Skin Effect and

eddy currents

1

Simplified Distribution System Diagram

2

Circuit breakers False tripping due to high

heat and higher frequencies caused by

harmonics Peak sensing CBs may trip because of

higher peaks

2

Simplified Distribution System Diagram

3

3

Neutral conductors high current from zero

sequence triplen harmonics (3rd 9th) can add as

much as 30 to neutral current even if phases are

balanced. Shared neutrals or reduced size

neutral can be hazardous (no CB to protect from

overcurrent!)

3

Simplified Distribution System Diagram

4

Neutral to ground voltage at receptacle

Excessive neutral current results in high voltage

drops between neutral and ground at the outlet

4

Simplified Distribution System Diagram

5

5

Neutral bus bar overloading and heat from zero

sequence harmonics (3rd and 9th)

5

Simplified Distribution System Diagram

6

6

Panelboard enclosure heating from eddy currents

(induced currents caused by magnetic fluxes), as

well as vibration (buzzing sound).

6

Simplified Distribution System Diagram

7

8

7

8

Transformers heating from eddy currents reduces

the life of a transformer insulation system, and

lowers efficiency.

7

8

Simplified Distribution System Diagram

9

Induction Motor Leads Heating from eddy

currents and negative sequence harmonics (5th,

11th, 17th,etc.)

Note Negative sequence harmonics work AGAINST

a motors natural rotation, making it work harder!

9

Simplified Distribution System Diagram

10

Power factor (kVAR) Capacitors since they are

lowest impedence, resultant high harmonic current

can overheat capacitors and/or blow fuses.

10

Simplified Distribution System Diagram

11

kW and kVAR meter may have reading errors when

harmonic currents are present.

11

Simplified Distribution System Diagram

In addition, emergency generators can have speed

control and voltage supply issues caused by

harmonics, and harmonics can impact

communications cabling and sensitive electronic

equipment.

Typical PQ Evaluation Process

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