Custom Controls & Power Supplies:
Relative Merits of Solid State Power Supplies
for the Refractory/Glass Industry
Background There have been many technological advances that have dramatically effected industrial controls and processes from instrumentation to total supervisory control. And yet, the electric melting process in the glass industry appears to still rely on conventional equipment utilizing tap changing equipment on the primary of a furnace transformer and the skill of the operator to deal with the variables in the melting process.
Opportunities with Solid State Devices No one can argue the advances made in AC/DC drives using solid state controls. There are equal opportunities in similar technology for power supplies that can replace expensive and inefficient conventional transformers with conventional tap changing equipment. The solid state power supplies are adaptable for retrofits as well as new installations.
So why isn't the industry jumping at this opportunity? Not sure, but if I had to guess, the systems that they have been using for many years have worked and just maybe no one has convinced this industry that solid state controls will meet the harsh requirements.
What Does it Consist Of? The neat part about the solid state tap changing approach is that you don't have to change distribution and you can utilize the same real estate. As a matter of fact, it may take up much less space. The building blocks are very much the same as the conventional system consisting of:
Transformer equipment having the same primary voltage and the secondary would have a series parallel scheme to swing the voltage 2 to 1 in the operating range of the melter.
Solid state switching equipment in the secondary side of the transformer provides the secondary voltage to the electrodes. No tap changing required.
Regulating system that will measure and feed back to the power supply to regulate voltage, current, KW, or impedance.
Instrumentation/control package to provide metering such as electrode voltage, current, KW, and the required annunciation.
Typical power levels for solid state switching range from 1.5 MVA to 5 MVA but are not limited to this level.
How Does It Operate? The transformer is constructed in such a manner as to have two secondary windings each being identical to the other to provide a 2:1 voltage range. For the sake of discussion, let each transformer have 150 V secondary, so in parallel, the voltage is 150 V and in series it would be 300 V. By means of solid state switching between series and parallel, and SCR network within the secondary of the transformer which is gated at a conducting angle to a predetermined voltage output allowing a smooth and continuous voltage over a wide range. By simply dialing in the power level via a potentiometer, which is networked with a regulating system, a power or current level can be obtained to provide an adjustment throughout the range for which it is designed.
So What's The Catch? There is none! This scheme has been utilized for many years in plasma melting applications at power levels of 2500 KVA and higher. There may be a reluctance in accepting a method that has not been used, with some of the issues being:
Establish proof that it will work in our industry.
What about harmonics and power factor degradation?
Will this survive the harsh environment that exists in our industry?
Sounds complex and my operations people will resist and not understand.
Well, Is This Solid State Approach Real or What! We would be remiss if we simply made a blanket statement that all glass applications are excellent candidates. Like any application problem to a process, we need to have some parameters and possibly a field trip to evaluate the relative merits of a solid state approach. Once the information is qualified, these various concerns can be addressed and a proposal can be prepared.
Let's Summarize The technology for solid state power supplies has long been established and the opportunities for the glass industry have not taken full advantage of this technique. Ironically, the cost of a new or retrofit installation in the 2500-5000 KVA range or more is very cost effective over the conventional power transformer with tap changing equipment and affords the following advantages:
Initial costs are similar to conventional methods in many cases
Provides field proven reliable control
Energy transfer using solid state can yield 8% to 10% more efficiency
Simple to operate and lends itself to microprocessor based control for future considerations
Takes up less space and due to its efficiency, lends itself to indoor installation
This approach has been field proven
Why Homewood? Homewood, through its affiliates, has been in the process controls, heating systems and melting applications for a broad section of industries for over 20 years. We have extensive systems experience, fabrication capability and dedication to selecting and applying an array of devices to optimize the design and assure reliability. We also offer complete documentation and system start-up. Our reputation is based on our ability to work closely with our clients and our primary goal is to exceed their expectations.
There have been many technological advances that have dramatically effected industrial controls and processes from instrumentation to total supervisory control. And yet, the electric melting process in the glass industry appears to still rely on conventional equipment utilizing tap changing equipment on the primary of a furnace transformer and the skill of the operator to deal with the variables in the melting process. 