Power Control and Energy management Solutions

Electrical heating

The debate over increasing energy cost has caused suppliers in the glass production and glass processing industry to look at control refinements for heating systems.

Reducing energy costs remains a key area of focus and successful companies are finding ways to improve their competitiveness by concentrating investment in this area of their business. This paper aims to highlight further savings, which can be made by paying careful attention to the way electrical energy is used and distributed around thermal processing

equipment.

Energy monitoring

Since energy use in glass production and glass processing is a major cost factor, there is a need to record and store energy usage data. With the advent of simple communicating power metering equipment it is a natural extension of the control system to embed plant energy usage in to the stored records. Having access to energy data in real time and historic

format allows users to evaluate the following:

• Instantaneous overall power demand

• Instantaneous power demand for individual processes

• Energy usage against pull

• Energy usage against plant utilisation

• Plant priority for load shedding

   

Using wireless technology as a cost effective way to acquire and distribute energy information over Ethernet allows the data to be shared in real time around groups of internal management and engineering clients. Experience has shown that where users have access to energy data it has always been possible to define areas of savings.

Electrical energy switching methods

Except for the most complex heater loads i.e. those element materials which have resistance change with temperature or complex transformer coupled loads, it is recommended that simple whole cycle switching methods are employed to control electrical energy with thyristors.

The continuing use of Phase Angle (Cycle Chopping) for simple heaters including modern Silicon Carbide causes disadvantages to users through poor power factor, harmonic disturbance on the supply and RF interference around the installation.

Figure 1 shows typical harmonic disturbance associated with phase angle firing for single and 3 phase loads. It can be seen from the diagrams that when switching the sine wave at 90 degrees a high proportion of odd harmonic current is reflected into the supply.

Poor power factor associated with phase angle firing is the principal concern for energy cost. Since most electrical installations are designed to operate at around 50% output power at the nominal operating setpoint, the supply mains cycle will be chopped at the worst case of 90 degrees when operating in PA mode. Under these conditions the resultant power factor could be as low as 0.72 instead of above the desired level of 0.95. Dependent on the metering type and the supply impedance, this could have a very detrimental effect on the billing value - adding 7-10% cost with no benefit to the process.

Two simple solutions can reduce or overcome the disadvantages associated with phase angle thyristor control:

1. For installations where phase angle control cannot be avoided, using an electronic supply tap changer will automatically keep the power factor and supply disturbance to a minimum.

Figure 2 shows typical response from a .4- tap change control system. Notice how the power factor is above 0.9 for most tappings at the critical 50% demand level compared to the fundamental curve which shows 0.72 for 50% demand.

2. For non complex heater loads it is possible to use any of the whole cycle firing modes, including single cycle and advanced single cycle switching methods, to satisfy the watts density loading and thermal mass characteristics of most common heaters - whilst eliminating the poor power factor and harmonic problems.

Advanced single cycle can be particularly effective for shortwave infra-red loads or for loads where it is desirable to minimise the effect of long bursts of power on the elements.

For element materials, which have a positive resistance/temperature coefficient, it is also possible to use intelligent thyristors to switch from phase angle firing to whole cycle firing when the element resistance increases to allows full mains volts to be impressed across the load.

Information is available from Eurotherm® on the benefits of alternative thyristor switching methods for particular heater materials.

Load sharing and load shedding techniques

Where a glass processing shop has a large installed base of electrically heated thermal processing equipment, it is often desirable to sequence the firing of individual loads to  inimise the supply fluctuation. By using intelligent thyristor firing methods it is possible to limit the power surge and instantaneous supply loading associated with any installation through a selectable combination of firing patterns. In this mode none of the zones are switched on simultaneously and individual load power demands are synchronised to give a very even loading on the factory supply.

The following figures show a zone sequencing pattern and the overall effects on the supply by evening out the load on the plant.

The benefits enable clients to operate higher  nstalled equipment base from the existing supply.

A further benefit can be obtained from this solution by setting a threshold on the smoothed power level to trap excursions of energy use through the site maximum demand point. Setting alarms on the threshold level can trigger prioritised load shedding and thus avoid

costly excess-tariff penalties.

The same principle would be applicable to tinbath roof heating and Lehr heating.

Out of hours furnace setpoint control Using the intelligence of modern control systems, it is possible to automate out-of-hours setpoint control for thermal processing equipment. In the example shown below, the control systems understand the dynamics of the furnace and can recognise the power required to maintain the standby setpoint and the power required to achieve the operating setpoint.

The controller has a user screen which allows the operator to enter the required duty setpoint and time for the furnace to be back at temperature. The controller uses the furnace tuning information and an internal real time clock to ensure the furnace is back at the

duty setpoint as required for work.

Benefits enable much more consistent and repeatable energy savings through the use of out-of-hours furnace turn down.

Eurotherm supply control systems are specially designed to incorporate their unique energy saving routines.