VSD Power Quality

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5 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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VSD Power Quality


The Harmonics Dilemma

Author: Thomas F. Lowery

HVAC Marketing Manager

Presented By: Ed Swan

Hermetic Motor Marketing Manager

Both with Rockwell Automation


Reliance Electric

This presentation was given at the ASHRAE 2000 show in
Dallas


ASHRAE SEMINAR 07

Dealing with Deregulation: What will Happen to Power Quality?

1. Harmonics Issue

2. Cost Effective Mitigation Techniques

3. Future Technology

Harmonics
-

What are they ?



Harmonics is a term used to describe the shape or characteristic of a voltage
or current waveform with respect to the fundamental frequency in an electrical
distribution system

Figure 1 : Fundamental Sine Wave

Figure 2 : 5th Harmonic with

Frequency = 5 times fundamental frequency

+

Figure 3 : Combination of the above two waveforms results in a distorted sine wave

=

Harmonics Distortion
-

the Problem



Energy efficiency is driving the need for more VSD’s. Larger loads such as
chillers are being run on VSD’s . The VSD’s are contributing more non
-
linear
load as a percent of overall electricity consumption.


Current Distortion


Higher current peaks could cause transformer heating or nuisance tripping by
fuses, circuit breakers and other protective devices since they are typically not
rated for harmonically rich waveforms.


Voltage Distortion


A distorted current has higher peak values that cause non
-
sinusoidal voltage
drops across the distribution system The resulting voltage drops add or
subtract from the sinusoidal voltage supplied by the utility. Other utility
customers could get distorted voltage on the downstream side of the power
distribution circuit.


Harmonics Distortion
-

the Problem




Other erroneous electrical problems may occur such as
connected sensitive equipment under or over voltage trips.






Even harmonics cancel each other in a balanced system.
Since harmonics are much smaller than the fundamental,
they have minimal impact on the electrical circuit itself !


Where Does the Power Go?

Similar to
residential energy
consumption,
commercial
buildings use over
40% of electrical
power for HVAC. If
50% of these loads
are on VSD’s, the
non
-
linear load is
roughly 25% of
overall building
electrical usage.

Input rectifier converts AC line voltage to fixed voltage DC.

Most Drives use 6 Pulse Diode Bridge Rectifiers

DC voltage is filtered to reduce ripple caused by rectification. 6
-
Pulse generate
5,7,11,13,17,19,23,25 etc.. …. harmonics

Inverter changes fixed voltage DC to adjustable AC voltage and frequency
which is fed to the motor.

Today’s AC VSD’s are

Pulse Width Modulated

Motor

AC Line

Inverter

Input rectifier


Filter



The IEEE
-
519 Guidelines

IEEE
-
519 is a
recommended guidelines
for
designing electrical systems in buildings, NOT a
mandatory standard.



Harmonics is a system issue more than any particular equipment issue.
IEEE
-
519 sets limits on the voltage and current harmonics distortion at
the point of common coupling (PCC, usually the secondary of the
supply transformer).



The total harmonic distortion at the PCC is dependant on the percent of
harmonics distortion from each non
-
linear device with respect to the
total capacity of the transformer and the relative load of the system.




Consider this...

Line impedance dampens harmonics

Expected harmonic current distortion for VSD’s having various amounts of total
input impedance:










Reactors are by far, the most economical way of reducing harmonic distortion on
a drive system




Consider this too...


Actual harmonic distortion is determined for linear and non
-
linear loads
both on the system. Harmonics distortion depends on the percentage of
non
-
linear loads on the system.











If the load was entirely made up of VSD’s, each having a 5% impedance
line reactor, then the distortion at PCC would be:

35% THID x 100%VSD/100% total load = 35% THID at PCC

Now if the same VSD’s were only 25% of the total load at PCC then:

35% THID x 25%VSD/100% total load = 8.6 % THID at PCC


(Source: Proceedings of the 12th International Power Quality 99 Conference)


Rocks and Ponds…..


Harmonics are the
rocks and the
distribution system is
the pond


Small rocks in big
ponds are not a problem


Large rocks in small
ponds are a concern


Solutions:


Larger Ponds or Smaller
Rocks


Xformer, Reactors, Filters to
dampen the “splash”

Electrical Distribution Effects


Simple HP/KVA Ratios can help
you recognize if Voltage or
Current Harmonics may be a
problem:


HP/KVA >.40
--

Add Additional
Impedance.


KVA/HP < .30
--

Most Meet IEEE
519 without need for additional
equipment

Must Know the system, not just the Drive

To properly project harmonics you
must have the following minimum
electrical system data:



Point of Common Coupling (Where on the
pond are we predicting the splash?)


Upstream Xformer size and % impedance
(How big is the pond?)


Drive HP or running load amps (How big are
the rocks?)


Isolation Xformers, Line Reactors, Bus
Inductors, Filters or Multi
-
Pulse
cancellation devices on the system (splash
guards in the pond)

Voltage THD Vs. Motor HP/Input KVA

0%
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
1.2
0.6
0.4
0.3
0.24
0.2
0.17
0.15
0.13
0.12
0.11
0.1
0.09
0.086
0.08
0.075
0.07
0.067
0.063
0.06
Motor HP/Trans forme r KVA
% of Voltage Distortion
IEEE De d.
IEEE Ge n.
IEEE Spcl.
No AC Choke
5% AC Choke
S12
Current THD Vs. Motor HP/Input KVA

0%
5%
10%
15%
20%
25%
30%
1.2
0.6
0.4
0.3
0.24
0.2
0.17
0.15
0.13
0.12
0.11
0.1
0.09
0.086
0.08
0.075
0.07
0.067
0.063
0.06
Motor HP/Transformer KVA
% Current Distortion
IEEE Gen.
No AC Choke
5% AC Choke
S12
Engineering Specs… Some Points to
Note:


The IEEE
-
519 recommended practice defines “dedicated,” “general,” and “special”
classifications. Hospitals and airports fit into the “special” category while most others
fit in the general classification. Systems with only VSD loads are defined by IEEE as
dedicated and allow higher distortion.



Limits for general installations are 5% Total Harmonic Voltage Distortion (THVD) and
12% Total Harmonic Current Distortion (THID). Limits for special applications are 3%
THVD and 8% THID. Dedicated systems allow 10%THVD and up to 22% THID.



IEEE States that the estimated load current should be an average running current for a
1 year period. If not known 80% of Full Load Amps is a good approximation.



IEEE
-
519 should not be blindly specified. Owners and engineers must be educated on
applying IEEE
-
519. Raising costs for customers without rational clarification of the
guidelines is not the optimum engineering solution
-

its like specifying 100,000 CFM
when only 20,000CFM is required.




Emerging Technologies

Ongoing research on AC to AC Inverter (Matrix
Converter)



Uses 9 Bi
-
directional Transistors (3 per phase) to convert fixed
frequency and voltage to variable frequency and voltage. No
Conversion to DC is required so no capacitors are required to
store energy. Does required a minimum of 5% line
impedance to operate.



Unity Power Factor.



No harmonics produced back on the line.

In Summary...


More VSD’s means more
concern over power system
harmonic effects.


IEEE should be applied at the
system level and may have
impacts as utilities charge
penalties for harmonically rich
waveforms generated on the
grid.


In 3
-
5 years AC to AC
technology will obsolete
harmonic concerns when
applying VSD’s.

Any Questions



?

0%
5%
10%
15%
20%
25%
30%
1.2
0.6
0.4
0.3
0.24
0.2
0.17
0.15
0.13
0.12
0.11
0.1
0.09
0.086
0.08
0.075
0.07
0.067
0.063
0.06
Motor HP/Transformer KVA
% Current Distortion
IEEE Gen.
No AC Choke
5% AC Choke
S12