Fluke 43B Application Note 2403028

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Application Note

Adjustable speed drives (ASDs)

can be both a source and a victim

of poor power quality.

ASDs as victim loads

Although ASDs are usually

depicted as the culprit in the PQ

scenario, there are ways in

which they can be a victim load

as well.

Capacitor switching transients

High-energy (relatively low-

frequency) transients that are

characteristic of utility capacitor

switching can pass through the

service transformer, feeders, and

converter front-end of the drive

directly to the dc link bus, where

it will often cause a dc link over-

voltage trip. Input diodes could

also be blown out by these tran-

sients.

Voltage distortion

If high-voltage distortion shows

up as excessive flat-topping, it

will prevent dc link capacitors

from charging fully and will

diminish the ride-through capa-

bility of the drive. Thus a voltage

sag which would not normally

affect a drive will cause the drive

to trip on undervoltage.

Improper grounding will

affect the internal control circuits

of the drive, with unpredictable

results.

ASDs as culprit loads

A drive can definitely be a “cul-

prit load” and have a major

impact on system PQ. But before

we talk of problems, let’s put in a

good word for the positive effects

of drives on PQ. First of all, they

offer built-in soft-start capabili-

ties. This means there will be no

From the Fluke Digital Library @ www.fluke.com/library

How adjustable

speed drives affect

power distribution

Line-side measurements on ASDs

Measurement Look for

Voltage waveform •Voltage notching (SCR converters)

•Flat-topping

Harmonic spectrum Harmonic orders and amplitudes, before and after filter

application

Displacement PF For PWM drives, DPF should remain high even at low speeds

(it will typically decrease slightly)

Voltage unbalance Less than drive manufacturer specs, or current overload trips

can result. The drive may have a higher limit for unbalance

than the motor.

Induction

Motor

M

Line

Reactors

inrush current and no voltage sag

effect on the rest of the system.

Secondly, if the drive is of the

PWM type, with a diode con-

verter front-end, the

Displacement Power Factor is

high (commonly > 95 % at rated

load) and more or less constant

throughout the range. This means

that drives can reduce energy

usage and correct for DPF at the

same time. It’s a good thing too,

because drives and PF correction

capacitors don’t mix. Caps are

vulnerable to the higher fre-

quency harmonic currents

generated by drives, since their

impedance decreases as fre-

quency increases.

The type of drive has a major

impact on the PQ symptoms,

because of the different converter

designs (converters or rectifiers

turn ac to dc and are the first

stage of the drive). There are two

major types of converter design.

SCR convertor with

Voltage Source Inverter/

Variable Voltage Inverter

(VSI/VVI) drives

Commonly called six-step drives,

they use SCRs (Silicon -

Controlled Rectifiers) in their

converter front-ends (the follow-

ing discussion applies to CSI,

Current Source Inverter drives,

which also use SCRs). VSI and

CSI drive designs tended to be

applied on larger drives (> 100

HP). SCR converters control the

dc link voltage by switching on

(or “gating”) current flow for a

portion of the applied sine wave

and switching off at the zero-

crossing points. Unlike diodes,

SCRs require control circuits for

gate firing.

Figure 1. Voltage Source Invertor (VSI) ASD.

2 Fluke Corporation How adjustable speed drives affect power distribution

For the SCR converter, there

are three main issues that affect

line-side PQ:

•Commutation notches. SCR

switching or commutation is

such that there are brief

moments when two phases

will both be “ON.” This causes

what is in effect a momentary

short circuit that tends to col-

lapse the line voltage. This

shows up as “notches” on the

voltage waveform. These

notches cause both high V-

THD and transients. The

solution is to place a reactor

coil or isolation transformer in

series with the drive’s front

end to clean up both problems.

•Displacement Power Factor

declines as drive speed

decreases. This is not as seri-

ous a problem as it sounds,

because the power require-

ment of the drive-motor-load

decreases even more.

•Harmonic currents, typically

the 5th and 7th, are generated

by VSI drives.

Diode convertor with

Pulse Width Modulation

(PWM) drives

The other and more common

converter design uses diodes and

is used in the PWM drive. The

diodes require no switching con-

trol circuitry. One of the main

trends in the industry has been

the proliferation of PWM drives,

mainly due to the continued

development of fast-switching,

efficient IGBTs (Insulated Gate

Bipolar Transistors) used in the

inverter section of the drive

(inverters turn dc to ac). For all

practical purposes, PWM drives

are the industry standard.

For the diode converter, the

main PQ issue is harmonics. The

actual harmonic orders being

generated depend on the number

of diodes in the front end. For

three-phase conversion, a mini-

mum set of six diodes is required.

This “six-pulse” converter will

generate 5th and 7th harmonics.

If a 12-pulse converter were

used, the 11th and 13th harmon-

ics will be generated instead of

the 5th and 6th — and, very

importantly, for the same load,

the amplitude of the 11th and

13th would be considerably less

than the 5th and 6th. Therefore,

the THD would be less. The vast

majority of drives, however, are

six-pulse PWM style, which is

one reason we see so much 5th

harmonic on the system.

Harmonics solutions

There are a number of solutions

to mitigating drive-generated

harmonics:

Harmonic trap filters (Figure 5)

These are typically LC networks

connected in parallel at the

source of the harmonics (in other

words, at the drive input). They

are tuned to just below the 5th

harmonic (typically 280 Hz) and

will tend to sink both 5th and

much of the 7th harmonic.

Obviously, they must be sized to

the harmonic-generating load.

Phase-shift transformers

This can be as simple as a delta-

wye transformer feeding one

drive(s) and a delta-delta feeding

another drive(s). There is a 30

degree phase shift effect

between these two configura-

tions, which effectively results in

cancellation of harmonics at the

closest upstream PCC (Point of

Common Coupling). The cancella-

tion effect is optimal when both

loads are more or less equal.

Figure 2. Voltage notching.

Figure 3. Typical ASD harmonic waveform.

Figure 4. Pulse Width Modulated (PWM) ASD.

3 Fluke Corporation How adjustable speed drives affect power distribution

12-pulse converter

If the delta-wye/delta-delta are

packaged together (delta primary,

delta and wye secondary) and

each secondary feeds one of two

paralleled six-pulse converters, a

12-pulse front-end is created

with all the benefits mentioned

above. 18-pulse designs are also

available. Because of the extra

cost, this type of solution tends to

only get used on high HP loads.

Active filters

This relatively new technology is

based on an elegant concept —

using power electronics to solve

the problems created by power

electronics. It senses the instan-

taneous ac sine wave; it then

actively cancels the harmonics it

detects by generating equal and

opposite polarity harmonics, thus

recreating the sine wave.

Commercial packages might pro-

vide voltage regulation as well.

Active PF correction

Another recent solution is for

manufacturers to offer converter

front ends using fast switching

technology that generates a mini-

mum amount of harmonics and

has near unity power factor (both

Total PF and DPF).

There is room for discussion

on which approach to harmonic

mitigation might prove most

effective and economical in a

particular situation. However,

what is often overlooked by the

end-user, and what should be

clear from the information in this

section, is that the total cost of a

drive system should include both

the cost of the drive itself and the

harmonic mitigation (whether

part of the drive or

installed separately).

A comparison of average-responding and true-rms multimeters

Multimeter Reading

Waveform Description Average-sensing DMM True-rms DMM

Sine wave Correct Correct

Square wave (flat-top voltage) 10 % high Correct

Current to single phase diode rectifier 40 % low Correct

Current to 3 phase diode rectifier 5-30 % low Correct

Why True-rms

True-rms test tools are necessary for accurate measurements of

distorted waveforms. For more information, see the Fluke application

note Why True-rms, document number 1260729.

LineLoad

Figure 5. Harmonic trap filter.

4 Fluke Corporation How adjustable speed drives affect power distribution

Is it possible to install “Power

Factor Correction Capacitors” and

have PF get worse? It certainly is,

and a starting place to under-

standing this puzzle lies in the

distinction between Displacement

PF (DPF) and Total Power Factor

(PF). The penalty for not under-

standing the difference can be

blown capacitors and wasted

investment.

Total PF and Displacement PF

are the same in one basic sense:

they are the ratio of Real Power

to Apparent Power, or Watts to

VA. DPF is the classic concept of

power factor. It can be considered

as the power factor at the funda-

mental frequency. Total Power

Factor, abbreviated to Power

Factor (PF), now includes the

effects of fundamental and of

harmonic currents (it is also

referred to as True PF or

Distortion PF, Figure 7). It follows

that with the presence of har-

monics, PF is always lower than

DPF and is also a more accurate

description of total system effi-

ciency than DPF alone.

Strictly speaking, the term

“Power Factor” refers to Total PF,

but in practice can also be used

to refer to DPF. Needless to say,

this introduces some confusion

into discussions of power factor.

You have to be clear which one

you’re talking about.

Displacement power

factor

Lower DPF is caused by motor

loads which introduce the need

for Reactive Power (Volt-Amp

Reactive or VARs). The system

has to have the capacity, meas-

ured in Volt-Amps (VA) to supply

both VARs and Watts. The more

VARs needed, the larger the VA

requirement and the smaller the

DPF. The cost of VARs is

accounted for in a power factor

penalty charge. Utilities often

levy additional charges for DPF

below a certain level; the actual

number varies widely, but typical

numbers are 0.90 to 0.95.

To reduce VARs caused by

motor loads, power factor correc-

tion capacitors are installed.

Upstream system capacity, both

in the plant and at the utility

level, is released and available

for other uses. (Figure 6)

Historically, this has been the

gist of the PF story: a relatively

well-known problem with a rela-

tively straightforward solution.

Harmonics and

capacitors

Harmonics have had a dramatic

impact on our approach to Power

Factor correction. The motor and

capacitor loads described above

are all linear and for all practical

purposes generate no harmonics.

Non-linear loads such as ASDS,

on the other hand, do generate

harmonic currents.

Take a plant which is step-by-

step putting adjustable speed

drives on its motor loads. ASDs

generate significant harmonic

currents (5th and 7th on six-

pulse converter drives). Suddenly

the fuses on existing PF correc-

tion caps start blowing. Since

these are three-phase caps, only

one of the three fuses might

blow. Now you’ve got unbalanced

currents, possibly unbalanced

voltages. The electrician replaces

the fuses. They blow again. He

puts in larger fuses. Now the

fuses survive, but the capacitor

blows. He replaces the capacitor.

Same thing happens. What’s

going on? Harmonics are higher

frequency currents. The higher

the frequency, the lower the

impedance of a cap (XC= 1/2πfC).

The cap acts like a sink for

harmonic currents.

Power system resonance

Hot vibes can result when harmonics and capacitors get together

After: PF = 100 %Before: PF = 42 %

1/6 HP Motor

Active

165 Watts

Reactive

360 VAR

3.3 A

1/6 HP Motor

Active

165 Watts

Reactive

360 VAR

1.4 A

Capacitor

60 µF

Figure 6. Capacitor corrects Displacement Power Factor (DPF).

5 Fluke Corporation How adjustable speed drives affect power distribution

many times greater than the

exciting current. This so-called

“tank circuit” can severely dam-

age equipment, and it will also

cause a drop in power factor.

Perversely, this resonant condi-

tion often appears only when the

system is lightly loaded, because

the damping effect of resistive

loads is removed. In other words,

we have what the audio buffs

call a “high Q” circuit. (Figure 8)

Imagine coming to work on a

Monday and seeing the insulation

on your cables melted off. How

can this happen over a weekend

when there was hardly any load

on the system? Has Ohm’s Law

been overruled? Not quite. Your

power system just spent the

weekend tanked out on the

Harmonics. It was quite a party,

but now comes the clean-up.

Start with harmonics

mitigation

The correct solution path starts

with measuring and mitigating

the harmonics generated by the

drives. Harmonic trap filters

would generally be called for.

These trap filters are installed

locally on the line side of the

drive. Their effect is very much

like the traditional PF correction

cap, in two senses: they reduce

DPF as well as PF, and also they

localize the circulation of the

problem harmonics (generally the

5th). Harmonics mitigation and

traditional DPF correction should

be addressed as one systems

issue. In other words, manage

Total PF, not just DPF.

Power system resonance

In a worst-case scenario, the

inductive reactance (XL) of the

transformer and the capacitive

reactance (XC) of the PF correction

cap form a parallel resonant

circuit: XL= XCat a resonant

frequency which is the same as

or close to a harmonic frequency.

The harmonic current generated

by the load excites the circuit into

oscillation. Currents then circulate

within this circuit which are

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©2004 Fluke Corporation. All rights reserved.

Printed in U.S.A. 10/2004 2403028 A-US-N Rev A

Fluke. Keeping your world

up and running.

A. System Diagram B. Equivalent Circuit

Harmonic

source

XC

XTXT

XS

XS

XC

Figure 8. Resonant circuit when XC = (XT + XS)

kvar

(nonwork producing)

VA

kW

(work producing)

Harmonics

(nonwork

producing)

Figure 7. Total Power Factor increases with harmonics.