Thyristor power

London – Simple on-off switching was found to be too brute a force for electrode boilers over 100kW at Collins Walker Ltd, a manufacturer of electric steam and hot water boilers.

An electrode boiler conducts an electric current through water between a pair of electrodes. An electrode boiler 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 places like Africa and the Middle East where power supplies are not 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

The company, therefore, engaged CD Automation to replace 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 over time. A solid state thyristor is contactless so there is no arcing of the type that accelerates contactor wear.

Collins Walker was able to configure and set the thyristor controllers up in-house without the need for expensive 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 the 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.

During Collins Walker’s specification stage, there were two main contenders offering units with broadly similar benefits for the boilers. One company produces very sophisticated thyristor controllers. However, from Collins Walker’s perspective, there was too much functionality built in to the units. Much of it was of no practical use to the company, and meant the units were complicated to set up.

In fact, this process was so complex an engineer from the supplier was needed to set up each thyristor during production of the boiler. This would have complicated production scheduling for Collins Walker, and could have resulted in delays if the engineer needed to come out again.

Collins Walker now uses CD Automation’s Multidrive thyristor controllers on all but the smallest boilers - those less than 100 kW 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 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.

According toi CD Automation, this is only possible with thyristor control as a PID Loop cannot be implemented on a boiler using contactors. A PID temperature controller, it noted, can take advantage of burst firing, which is a time-proportioned on-off method.