ConstantPowerLoad

Constant Power Load

The ConstantPowerLoad represents the performance of a generic constant power load (CPL). Both on and off-design, the CPL draws a constant specified power with losses according to a fixed efficiency. Supports DC, 1-phase, and 3-phase.

Usage

• The input design parameters that must be specified by the modeler when creating an instance of this ConstantPowerLoad component are mass specific power (SpecificPower) and design efficiency (eff). This component will consider draw whatever current is needed to deliver the power it is commanded to produce (Pdemand), with losses according to its design efficiency. ONDESIGN, it calculates its mass (Mass) by dividing its ONDESIGN input power by its specific power.

• This component supports DC and AC (single or multiphase) power. At instantiation, this component's power type is unspecified. NPSS Power System Components need to have a power type in order to correctly run power calculations. For this component, the power type is typically set by calling the findSourcesAndPropagate() method at the top level assembly (see ElectricPort).

• This component transforms power, from electrical power at its input port, to useful output (load) power. Because this component represents a transformation of power, it includes a node. Being a component including a node, its voltage is an indepedent variable (or 2 variables, real and imaginary if its connection to the rest of the power system dictates that it is an AC component). It also features a dependent that ensures that the electrical power at the input port, is sufficient to produce the output power demanded, plus the losses in this component (EP_I.S.r * eff == Pdemand). Lastly, if the component is an AC component, it also features a dependent that ensures the power factor at the input of this load is equal to a a specified value.

• As a component that contains a node and has its voltage known at the beginning of the solverSequence[], it uses its prePass() method to call the electrical port update function to pass this voltage information to electrical components that it is connected to. Note that in the current implementation of the NPSS Power System Library, these components containing nodes do not know what currents are going through their ports at the beginning of an iteration. Because of this, they must be connected to power transmission components (cables and breakers), and these transmission components must be run before node-bearing transformation components like this one, as the transmission components will calculate and populate these current values. Note that this design is intended to be analogous to a common approach taken in fluid networks within NPSS rocket models.

• This component, like other power system components in the NPSS Power System Library, can optionally include thermal models. An optional thermal model is enabled by setting switchThermPort to TRUE, and plugging an EThermalMass subelement into the S_eThermMass socket. Doing these will add a temperature state (existing within EThermalMass) and a thermal port to the model. The thermal port is intended to connect this component to a second component that represents the mechanism by which heat is extracted from this component. This second component could represent a heat exchanger, cold plate, or just model heat transfer from the first component to the surrounding environment. For more information see EThermalMass.

• Note, there is no sizing logic included in this component, other than the mass calculations described above.