Varmeteknikk AS
Phone +47 23 37 55 00 August 2020 V2
12
Power control
On systems with varying current loads, the electrical subscription can be utilized to the maximum
by using power regulation. The total power is measured in the building's main switchboard and by
means of the boiler's regulator, the power that is available below the subscription limit is supplied
at all times. Necessary equipment is 3 pcs. measuring transformers for placement in the building's
main board and 3 pcs. secondary transformers for connection to the controller.
Example:
The building's maximum load is 200 kW, which at 400 V corresponds to 289 A. Suitable current
transformers for the system, 300/5. The factor for this will then be: 300: 5 = 60. The system is
desired to be regulated at a maximum of 289 A, which gives 4.8 A. (289: 60 = 4.8).
Settings must be made in the service menu. By holding the menu button inside min. 5 sec. you will
have access to this. With the «-» button the current limit is set to 4.8 and the new value is saved by
pressing the «E» button.
Current
0.0(4.8)A
Calculation of appropriate current margin
When the power limitation has limited the boiler to a certain power, you get to a point where a step
will constantly switch on and off. To prevent the boiler from switching on and off the power stage, a
certain "power margin" must be available before a new stage enters. This margin should be at
least the current corresponding to the difference between two arbitrary power levels on the boiler.
If the power levels are different, the calculation should be performed according to the largest power
level.
The value is calculated according to the following formula:
P x Is______
U x
√3 x Ip
P = The boiler's largest power change in W between two stages
U = Main voltage at the system
√3 = 1.732
Ip = Max. primary current on the plant's current transformers
Is = The secondary current on the system's current transformer (normally 5A)
Example:
The boiler is 140 kW with a 7-stage regulator.
Largest power change 20 kW, current transformers on
200/5 A and main voltage 400 V.
P x Is
= 20000 x 5
= 100000
≈ 0.48
U x 1.732 x Ip
400 x 1.732 x 300 207840
To avoid ending up too close to the limit for self-oscillation in the system, you should set the power
margin approx. 0.1 higher than the calculated value. In the example above, this entails
a setting of 0.58.
Cooke Industries - Phone: +64 9 579 2185 Email: [email protected] Web: www.cookeindustries.co.nz
