High efficiency bushing control systems for glass fiber production

However, the nature of phase angle control is electrically noisy, causing distortion of the AC waveform (known as ‘harmonics’) in the transformer and cables of the system. The harmonic distortion causes extra voltage and current to be present in the cables, which not only wastes energy through excess heat loss but also negatively affects the measurements (for example, power factor) made by electricity suppliers in order to charge for energy.

To achieve an acceptable power factor close to 1, the thyristor controller needs to supply as clean a waveform as possible with the least amount of harmonics. In this case, a power demand setpoint somewhere above 80% of range. However, in an average bushing application, at least another 10% of control either way is required in case the bushing temperature needs to be adjusted quickly after an incident.

Energy efficiency

Experienced supplier of power and control technologies Eurotherm by Schneider Electric has developed a high efficiency solution for bushing power control. The Eurotherm approach considers each part of the system from the process power supply to the ‘ears’ of the bushing and up to the information management system level. Starting at the power controller, rather than using phase angle firing mode, which ‘chops’ the waveform causing harmonics and negatively affecting the power factor, the Eurotherm EPower advanced power controller is used in automatic load tap changer control mode.

Process efficiency

Sophisticated control strategies, able to precisely control the temperature of the glass, aid the manufacture of consistently uniform fiber strands with high tensile strength, while a well-designed HMI enables operators of varying skill levels to run the process without affecting the quality. Typically, a Eurotherm T2750 Programmable Automation Controller (PAC) system provides the bushing temperature control strategy, utilizing the AVEVA System Platform (formerly Wonderware) for supervisory control and data acquisition (SCADA) to give sophisticated bushing position benchmarking and performance information for key performance indicators.

To minimize unscheduled downtime, the dual redundant feature of the T2750 PAC (a secondary processor that takes over control in the case of a problem in the primary processor), can offer high availability of the system, as well as redundant inputs and output modules if required.

Productive trial

In a 24 bushing position control system upgrade for a leading glass fiber manufacturer, the Eurotherm bushing power supply solution achieved a 9.4% power saving. During an initial trial, two of the bushing positions were modernized to assess the possible improvement. The system was run in full operation for several months to compare the performance and efficiency of the two bushing positions fitted with the Eurotherm load tap changer solution, against the other 22 positions. Four bushings were controlled by each control panel, which contained four EPower controllers and a local panel PC HMI. Several of these bushing control cabinets were attached to a single mains power distribution cabinet to create an integrated and flexible solution suitable for almost every glass fiber forehearth configuration.

The trial bushings employing the cleaner power waveform supplied by the EPower controller with load tap changer, in combination with low inductance busbars and transformer, showed a dramatic reduction of harmonics. This led to a 50% improvement in power factor and reductions of 40% in line current; 51% in kVA (apparent power), and more than 9% in real power (kW) compared to the regular phase angle controlled position. The result in cost savings was significant.

Bushing positions with the automatic load tap changing system installed consumed only 9.6kW against the normal bushing positions’ consumption of 10.6kW. The result correlated with the load tap changer-controlled position achieving a 9.4% total real power saving, which in this case equated to a saving of €21,000 per year (based on 0.1 euro/kWh, continually running 24 bushings). The ROI was approximately 18 months.

Additional benefits

In a traditional system, vibrations can cause weakness in the glass fiber strands and can also eventually cause damage to system components including the bushing itself. There were noticeably fewer vibrations in the improved system due to the reduction of harmonics. This improved the yield by reducing fiber breakage and enabled the system components to naturally last longer, reducing the time and cost of maintenance. The EPower load tap changer option also saved time for the engineer by automatically switching power to the transformer taps. This was normally a manual task that interrupted the process during changeover. After the successful completion of the trial phase, all 24 positions were modernized to the Eurotherm solution, and the system was designed with future scalability built-in if required.

With an improved power factor, capacitor banks are no longer necessary which helps with CapEx and ROI and further reduces maintenance cost. On a new installation, with the line current typically reduced by 40% per position and kVA less than half, compared to a traditional SCR controlled system, significant capital cost savings can also be achieved by reducing the size of all components, from the main distribution transformer to circuit breakers and cables.

Conclusion

On refurbishment projects, the Eurotherm bushing control solution with automatic load tap changer design helps to enhance each part of the system to achieve improved energy efficiency and glass fiber quality by significantly improving the power factor and reducing total harmonic distortion (THD) all with an acceptable return on investment. The energy savings over the lifetime of the system (typically 15 years) are substantial. Energy assessments can be carried out to calculate expected ROI and the solution can be fitted in modernization steps to suit budgetary requirements. Even larger capital savings can be achieved on new installations through the appropriate design of the complete system. For glass producers ready to transition to more efficient technology, Eurotherm can help.

About the authors: 

Mikael Le Guern is Business Leader for Global Glass and Amber Watkin is Application Marketing Engineer at Eurotherm.

The full version of this article appeared in the May/June 2021 issue of Glass Worldwide alongside a broad cross-selection of editorial that assists with all areas of production and processing.