Simple on-off switching was found to be too brute a force
for electrode boilers over 100kW at Collins Walker,
manufacturer of electric steam and hot water boilers. As a
result, thyristor manufacturer CD Automation has helped
the company to switch from contactors to thyristor control

An electrode boiler conducts an
electric current through water
between a pair of electrodes and is
more robust than an immersion type. One of
the reasons for this is that it does not use
heating elements and associated switching
needed to control them. It is more tolerant of
current and voltage fluctuations making it
popular in Africa and the Middle East where
power supplies are not always as reliable as
those in Europe and the US.

Since the company was set up in 1972,
Collins Walker’s immersion boilers have
used relay contactors to control power
switching to the elements. Because of their
smaller rating, this can amount to a bank of
up to 60 contactors on larger boilers. With
their associated wiring, contactors can be a
source of faults, and require regular
maintenance.

Although having multiple contactors gives
some degree of control, by staging the
switching, the boiler and the electrical supply
can still be subject to power surges. With
larger boilers rated at thousands of kilowatts,
even Collins Walker’s power station
customers are concerned about the huge
draw when boilers are switched on. The
heating elements themselves suffer thermal
shock each time the power is switched, and
this leads to element failures.

Contactor burnout
CD automation replaced all 60 contactors
with a single thyristor, which cut down
wiring significantly. Contactors have a finite
life and start burning out after prolonged
use, giving rise to maintenance issues. A
solid state thyristor is contactless so there is
no arcing.

CD Automation’s thyristor controllers are
designed for ease of installation and Collins
Walker was able to configure and set them
up in house without the need for engineering
consultancy.

The current handled by the boiler is 850A
at 415V, and for loads above 700A, CD
Automation offers its Multidrive thyristor
controller. Multidrive is a fully digital
thyristor power controller available in 21
current ratings from 35A to 2.7kA. The front
panel keypad and display allows total
flexibility for the user with full configuration
of the process and digital inputs types and of
firing and control modes to suit the
application.

Real time values such as voltage, current
and power can be read from the display.

Internal quick-blow fuses provide complete
protection for the thyristor. Additional
features such as current limit, profiling,
heater break alarm and RS485
communications come as standard.

The most important advantage of thyristor
control is the ability to gradually apply power
to the elements in the boiler, virtually
eliminating the problem of thermal shock,
and extending the life of the heating
elements.

Chris Goddard of Collins Walker says:
“CD Automation’s units are simple to set up,
controllable, and cost effective. In addition,
the team at CD Automation are very
knowledgeable and helpful. Whenever you
ring them, they are always there to answer
the phone.

“During the installation of the first CD
Automation thyristor controller, our team at
Collins asked lots of questions, and were
sometimes on the phone with CD
Automation for over an hour. CD
Automation did not spare any effort in
ensuring we were able to successfully install
the first thyristor controller ourselves, so I
am quite impressed with them.”

Gradual power switching
Collins Walker now uses CD Automation’s
Multidrive thyristor controllers on all but the
smallest boilers – those less than 100kW
which only have one of two contactors. The
controllers are programmed to bring in the
power gradually, for example starting at 10%
for a few seconds before moving on to 20%
and so on.

The power is also reduced gradually
instead of being abruptly switched off,
smoothing the whole process of switching the
power on and off. This gradual power process
is similar to soft starting on electric motors
and has been found to be particularly useful
at one of Collin’s Walker customers – Jersey
Power Station – where the need is to heat
water by gradually switching in heater banks.

The thyristor is attached to a
programmable logic controller (PLC) forming
part of a proportional integral derivative
(PID) loop. This is a specialised feedback
process which helps to anticipate the lag in the
system, so the on or off switching occurs just
before it is needed. This prevents thermal
overshoot, where the elements are still
supplying heat after the set temperature has
been reached and the power switched off,
making the boiler more energy efficient.