Power Limiting on Multi Zone Furnaces using Thyristors

Purpose 1: To prevent Power Bursts

Purpose 2: To limit the Power usage when there is insufficient supply

On multi-loop furnaces the installed power can be rather high compared to the available supply. In cases when the supply is limited or the power is supplied by e.g. Emergence or a Load Power Transformer, methods should be available to control the amount of supply used.

Also, when the supplier is monitoring, the demand and penalties/fines are raised when the usage exceeds certain limits or is peaking.

When the furnace is turned on all control loops will want to start firing and a sudden power burst can occur running up to or over 100% of power. The sudden draw of current will cause a supply voltage to drop (noticeable by the dimming of lights), fuses can be blown or supply transformers damaged. A generator cannot handle the increase of this demand  instantaneously and needs some time to run to that power. It is even possible for the generator to stall. These situations occur when the power lines are not designed for this amount of power. It is also possible that there is a general lack of available power. Another consideration is the investment required for this higher power demand.

When the furnace is at temperature the control loops need less power. A well designed furnace will run around 50% of its installed power.

Frequently, due to construction limits or other restraints, the furnace will run at less than 30% of its power.

To control the instantaneous maximum power usage is not easy. Most methods apply on an averaging basis over time. With Analogue/Communication controlled thyristors the input signal can simply be limited, but this has other impacts depending on the firing mode:

1. Analogue fired thyristors modulate their firing over time. It is a matter of coinciding firing since thyristors are unsynchronized units and they all can fire at the same moment. Each thyristor adds its demand to the total on a Sine Wave Level (Superposing). When the firing mode is burst firing the thyristor can syncopate; noticeable due to oscillations in the current by either light or sound.

2. When the firing mode is Phase Angle the effects are noticeable by humming of transformers but this has a drastic/catastrophic affect on the wave form.

3. With Logic driven thyristors one has to disable units or even complete  roups over time. Doing this Manually or Hardware wise has a very course effect on the heating up.

Controlling the maximum power:

1. Calculate the installed power by summing the individual power of each loop.

2. Continuously calculate the power usage by summing the actual output times the loop power. This figure is an average for analogue fired thyristors and actual for logic fired thyristor.

3. Set the maximum output of each loop to the maximum allowed percentage. This is not an absolute limitation but gives feed back to the loops that they are limited in output and can prevent integral windup and output bursts if power limitation is no longer needed.

4. An individual loop basis checks if that loop wants to fire the thyristor and that the loop power added to the power usage does not exceed the maximum allowed power to enable the firing. If it does exceed the limitation, disable the firing. Doing exactly this creates a variation in firing all thyristors.

5. By applying a positive ramp rate (Ramp Up) on the maximum  allowed power, a gradual demand increase can be simply achieved e.g. at startup.

6. Depending on the mechanism used, the above can lead to situations that certain loops are never allowed to fire at all.

More variations have to be brought in:

Incremental: Allows only one loop to switch each cycle.

Sequential: Follows a strict order in switching.

Rotational: The first loop disabled is the first one to be enabled again.

Time wise: Firing allowed for a certain duration.

Priority: First firing high priority loops then lesser priorities.

Deviation: The lowest deviation is the first to be turned off.

Priority changing: Change priority when needed, e.g. on a time base.

The above technique requires powerful mathematical capabilities within the control system.

The Eurotherm EPower thyristors have been developed to incorporate power limiting calculations - Predictive Load Management. These units can be arranged in a networked group to communicate with each other and manage the multizone plant load requirements.

Setting the Maximum Power limitation mode will enable the group to behave as a single unit. They will configure themselves as to how and which thyristor is allowed to fire depending on parameters set in the instruments.