PowerController
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Use
The power controller model controls the power output of different power generators. Power control of co-generation units is possible, if heat output is a process by-product or if the heating medium is cooled at a corresponding cooling tower.
The PowerController covers the following applications:
Basic: The PowerController provides the generator with a switch-on and switch-off signal as well as the flow rate of the circulation pumps, i.e. the generator is either on at full power or completely off. The flow rate of the circulation pumps is controlled using a flow setpoint temperature.
Modulation: The output of the generator is additionally modulated, i.e. the generator is not only switched on and off, but can also only call up part of its maximum output during operation.
Cascade: This assumes that the generator has several generator stages or that there is a cascade of generators. These are used one after the other depending on the power requirement.
Parameters and Connectors
Basic
The controller requires the setpoint reference power production (▶ReferencePowerProduction) and the actual value (▶Pel) of the power prodcution. If the ▶ReferencePowerProduction value rise above the SwitchOnPower, the generator is switched on (▶UNITon). In addition, the flow temperature (▶TFlow) and the return temperature (▶TReturn) are required at the generator in order to switch off the generator (emergency shutdown) if limit values (limit parameters TFlowMax and TReturnMax) are exceeded.
In addition, a specification for the desired flow temperature (▶TFlowRef) is required to determine the volume flow from the generator to the heat sink (volume flow of the circulation pumps - ▶qvRef). This is regulated in the limits qvMax and qvMin using the difference between the actual flow temperature and the set flow temperature as well as the temperature limits deltaTFlowRefMax and deltaTFlowRefMin. In addition, a Boolean signal is provided via the output ▶CPon to control the pumps.
Delays (tDelayUnit and tDelayCP) are set between switching the circulation pumps and the generator on and off in order to prevent excessively high temperatures in the generator and the resulting switch-off. In addition, minimum switch-on and switch-off times (tUnitminOn, tUnitminOff) of the generator are defined to prevent excessive clocking of the generator.
The ▶Enable input can be used for the targeted switch-off of the generator by a higher-level control system, seasonal releases or similar.
Modulation
The ConstantPowerOutput parameter allows the generator not only to be switched on and off, but also to only call up part of its maximum output during operation. The technologically defined minimum modulation is mapped by the parameter RelModMin. The power is adjusted with the information about the ElectricalPowerMax and the maximum power difference between ▶Pel and ▶ReferencePowerProduction defined with the parameter PDiffCtrl. The result is provided to the generator via the available connector (▶RelativeModulation) for connection.
If the maximum current output is unknown or cannot be specified for another reason, it is also possible to specify the maximum heat output of the generator (HeatPowerMax) at using the parameter ParameterInputHeat. In this case, the current ▶Power2Ratio of the generator must also be connected to the controller. The same calculation is then carried out at.
In addition, a time constant tMod is applied to the modulation, which reduces the rate of change and ensures numerical stability.
Cascade
The nStages parameter can be used to control several stages of the power generator or a cascade of generators. The ▶ActiveUnits connector is used for this purpose. For the calculation, it is assumed that the next stage is switched on or off if necessary when the time tUnitStateChange has elapsed.
Model Background
Volume Flow Control
The volume flow of the circulation pump is defined by a linear approach as shown in the following figure. The decisive factor is the temperature difference between ▶TFlow and ▶TFlowRef. At the value deltaTFlowRefMax (in the image 3 K) the maximum volume flow qvMax is assumed to reduce the temperature stroke inside the generator. At the temperature difference deltaTFlowRefMin (in the image -10 K) the minimum volume flow qvMin is used to higher the temperature stroke. Linear interpolation takes place between the extreme values in order to achieve the desired set flow temperature ▶TFlowRef.
Calculation of Modulation
The modulation of the generator is determined using the following formula. In the case of a multi-stage generator, all stages have the same modulation.
Switch on/off
The switching on and off of the generator, the stages and the auxiliary heating is controlled via a hysteresis with the specified upper and lower limits of the temperature difference between setpoint and actual temperature.
Pre- and Post-run Time of Pumps
The logic of the time delay can be seen in the following figure. The UNIT (▶UNITon) is only switched on after the circulation pump is switched on and the time tDelayUnit (3 seconds in the figure) has elapsed. The logic for tDelayCP is analogue. This results in the ▶UNITon signal being switched on at 4 s after the circulation pump is switched on at time step 1 s. The circulation pump is then switched off 4 s (tDelayCP) after the unit is switched off.
Switching on additional stages
An additional stage is switched on if the following condition is met.
An additional stage is switched off if the following condition is met.