March 2011, Glass International: "Recent improvements

Eurotherm_00_GI_0909 17/03/2011 14:16 Page 1
Furnace design & operation
Recent improvements in electric furnace
boosting
There seem to be advantages and disadvantages of both
phase angle firing and burst firing modes when it comes to
electrical boosting. René Meuleman* favours burst firing in
multiple zone boosting systems in combination with
predictive load management to avoid flickering, as he
explains for Glass International.
I
also start using less energy-consuming
recipes but again this will have its price.
There are also improved burner
technologies to consider. In fact,
electrical boosting is one of the major
considerations when it comes to
achieving an increase in furnace pull
rate. Imagine waste heat recovery that
feeds electrical furnace boosting...
Boosting considerations
There are several methods of applying
electrical energy to a furnace, such as
multi-tap switched or slide wire
controlled transformers; both of which
have their specific advantages and
disadvantages. Another method makes
use of silicon controlled rectifiers (SCRs)
or what we call thyristors to control
the power.
SCR-controlled boosting systems are
considered to be the latest, solid state
technology, although the principle has
existed for more than 50 years (proposed
by William Shockley in 1955). We have
to understand that the SCR is actually
just a solid state current switching
device. The trick behind those SCRs is
Burst firing involves defining a duty cycle - a window of a specific amount of sine waves - in which we
control single or multiple sine wave packages to control the total amount of power inside that duty cycle.
the complicated and sophisticated
algorithms controlling them, which
became available after the introduction
of ultra-fast micro and digital
signal processors.
With this technology, we are now
capable of controlling SCRs so that they
can adapt to all kind of situations and
run these applications at the highest
efficiency and best power factors
possible. That is why we do not intend
to fire SCRs in phase angle firing mode
anymore but instead run them in burst
firing mode where the specific
application allows it.
Phase angle and burst firing
Firstly, let’s look at both phase angle and
burst firing modes to understand what
they mean and what advantages and
disadvantages they have. Phase angle
firing is the oldest method of controlling
electrical power with SCRs since this
firing method could be handled by
analogue circuitry. It offers a very
smooth control but unfortunately, it
continued »
Eurotherm Invensys recently installed a ninezone furnace boosting system that runs in burst
firing mode.
Glass International March 2011
www.glass-international.com
n recent years we have witnessed
increased interest in glass furnace
electrical boosting systems. Is this
becoming part of new business strategy
in glass manufacturing?
Instead of aiming to increase furnace
lifetime an alternative business case may
be to produce more tons per square
metre. This increased pull rate may wear
out a furnace more quickly.
Our objective is therefore not to have
a furnace lifetime of +12 years, but to
accept a lifetime of +6 years while
achieving double the throughput. In
fact, this carries some further advantages
such as a faster return of investment and
the opportunity to innovate more often.
Such a concept faces the constraints of
what the infrastructure, such as
refractory material, is capable of
handling when it comes to more
extreme high temperatures; one of these
being of course the maximum allowable
crown temperature.
This is where electrical boosting comes
into the equation. Of course, we can
increase the amount of cullet, but for
that cullet has to be available. We could
19
Eurotherm_00_GI_0909 17/03/2011 14:16 Page 2
Furnace design & operation
Thyristor 1
Gate
Gate
Thyristor 2
To be able to control an alternating current, two SCRs need to be in anti-parallel positions.
The drawings above show in more detail how phase angle controls the power and the effect the firing
angle has on the power factor. Running a phase angle fired system below 80% of the set-point may already
result in an unacceptable low power factor.
www.glass-international.com
This drawing illustrates the difference
between phase angle firing and burst
firing at same equal power levels. Since
the sharp waveforms of phase angle
firing course the harmonics and
reduced power factor it is obvious that
burst firing, due to controlling only full
sine waves, will not have those
drawbacks.
20
generates a lot of harmonics and
normally runs at unacceptable power
factors if not running above at least 70%
of set-point.
An alternative method of controlling
power with SCRs is known as burst
firing. For this we define a duty cycle - a
window of a specific amount of sine
waves - in which we control single or
multiple sine wave packages to control
the total amount of power inside that
duty cycle. The major advantage is that
such a burst firing system runs at very
good power factors and minimises
harmonics.
However,
a
major
disadvantage with high power loads or
multiple power loads is that burst firing
may cause ‘flickering’.
Overcoming disadvantages
With EPower we have a solution for the
poor power factor and harmonics
generation of a phase angle fired system
called ‘load tap changing’. We also have
a solution for the flickering disadvantage
of multiple burst fired systems, which is
called ‘predictive load management’.
Even a combination of both load tap
changing and burst firing is possible and
Glass International March 2011
will normally give the best result.
However, for that solution we would
need to have multiple SCRs on multiple
tapped transformers, thus introducing
additional costs on both the power
control system and on the transformers.
Nevertheless, an acceptable return on
investment can normally be achieved
and should be always the subject of a
power system’s layout consideration.
Usually, high power values are
running in glass furnace boosting
systems, therefore power factor,
harmonics and flickering need to be
considered. With simple phase angle
firing, we avoid flickering issues.
However, with this method it is
common to run into some constraints,
such as losing most of our control
freedom or running the system with a
bad
power
factor
introducing
harmonics. That is why InvensysEurotherm introduced burst firing in
multiple zone boosting systems in
combination with predictive load
management to avoid flickering. The
result of such a system is optimum
power factor, minimum harmonics and,
due to predictive load management - the
distribution of sine wave packages over
the total duty cycle - no flickering.
Recently, we installed a nine-zone
furnace boosting system that runs in
burst firing mode. This installation has
now been running successfully for
several months and demonstrating the
benefits detailed above.
One of the biggest advantages of
running a solid state EPower system is
that it needs no maintenance and is not
subject to wear. Even constantly
controlling power fluctuations or active
glass temperature control will not harm
the system. In fact, it provides an
additional controlled parameter to your
glassmaking process, which will become
important as soon as you consider the
use of advanced process control
methods.
Fundamentals
For a better understanding we need to
know that a single SCR will behave as a
diode, the only difference being that it
starts conducting only if the gate is
triggered by a pulse. To be able to
control an alternating current, two SCRs
need to be in anti-parallel positions: One
SCR triggering for the positive sine wave
and the other triggering for the negative
part of that sine wave.
To operate such a system, we need to
start applying trigger signals to both
SCRs using the sine wave zero crossing as
a reference. For each SCR, there is a
firing angle of 1800 and the longer we
wait to trigger the SCR(s) the less time it
will be in conduction, and consequently
less power is applied to the load.
Controlling the firing angles of both
SCRs also controls the applied power.
Summary
There are many different considerations
to be made during the design of a
furnace boosting system. Initial costs,
return on investment, freedom of
control and mean time before failure are
just a few. Using burst firing modes in
combination with predictive load
management can provide smooth,
constant control, a good power factor
and minimum harmonics at an
achievable price level. Finally, it is
maintenance-free. *René Meuleman, Global Glass
Industry Technical Leader, Invensys
Operations, The Netherlands.
Email: [email protected]
Website: www.eurotherm.co.uk/
industries/glass