Application Note

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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

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 (c

ommonly > 95 % at rated

load) and more or less c

onstant

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/

ariable Voltage Inverter

(VSI/VVI) drives

Commonly called six-step drives,

they use S

Rs (Silicon -

Controlled Rectifiers) in their

onverter front-ends (the follow

ing discussion applies to C

Current Source Inverter drives,

which also use S

Rs). VS

I and

SI drive designs tended to be

applied on larger drives (> 100

P). S

R c

onverters c

ontrol the

dc link voltage by sw

itching on

(or “gating”) current flow for a

portion of the applied sine wa

and sw

itching off at the zero-

crossing points. Unlike diodes,

Rs require c

ontrol c

ircuits for

gate firing

Figure 1. Voltage Source Invertor (VSI) ASD.

Summary of content (5 pages)

PAGE 1
How adjustable speed drives affect power distribution Voltage distortion Adjustable speed drives (ASDs) can be both a source and a victim If high-voltage distortion shows up as excessive flat-topping, it of poor power quality. will prevent dc link capacitors ASDs as victim loads from charging fully and will diminish the ride-through capaAlthough ASDs are usually bility of the drive.
PAGE 2
Diode convertor with Pulse Width Modulation (PWM) drives Figure 2. Voltage notching. 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 collapse the line voltage. This shows up as “notches” on the voltage waveform. These notches cause both high VTHD and transients.
PAGE 3
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.
PAGE 4
Power system resonance Hot vibes can result when harmonics and capacitors get together Is it possible to install “Power Factor Correction Capacitors” and have PF get worse? It certainly is, and a starting place to understanding this puzzle lies in the distinction between Displacement PF (DPF) and Total Power Factor (PF). The penalty for not understanding the difference can be blown capacitors and wasted investment.
PAGE 5
Power system resonance many times greater than the exciting current. This so-called In a worst-case scenario, the “tank circuit” can severely daminductive reactance (XL) of the age equipment, and it will also transformer and the capacitive cause a drop in power factor.