Constant power loads/sources
Description of the constant power loads/sources components in Schematic Editor
There are two types of constant power loads/sources components: single phase and three phase. The components consist of sine current sources (first harmonic only) which are controlled in a way to achieve a given power flow. Active and reactive power references are set from HIL SCADA. Positive active power references refer to the load mode, while negative power references refer to the source mode. Constant power load/source components operate in a frequency range from 15Hz to 100Hz. Outside that range the sources will turn off.
Tab: General
The Current limit defines the RMS value at which the output current will saturate.
- When the SCADA inputs method is selected, active and reactive power setpoints are provided via built-in SCADA inputs (Pref and Qref).
- When the Signal inputs option is selected, active and reactive power setpoints are provided through signal input ports. In this mode, the constant power load/source internal PLL status and measured frequency are provided as signal outputs. If Pin to System CPU is checked, the Output execution rate property defines the output execution rate. If Pin to System CPU is unchecked, the output execution rate is the same as Slow execution rate.
If Pin to System CPU is checked, signal processing code in the component will be mapped to the System CPU. In this case, execution rates for signal processing code inside the component are defined in the Signal processing settings. If Pin to System CPU is unchecked, signal processing code in the component will be mapped to the CPU according to standard CPU partitioning algorithms. More about this algorithm can be found in the Signal processing settings. In this case, it is possible to specify Slow and Fast execution rates.
Tab: V-F droop
Both voltage and frequency droops can be defined through a "csv" file (Comma Separated Value). Using the preview button on the left, the defined droop functions can be seen. Voltage droop is defined as a Reactive power modulation index that depends on the effective value of an applied voltage. Frequency droop is defined as an Active power modulation index that depends on the frequency of an applied voltage. Voltage and frequency droop functions are optional, and may not be used if not needed.
Tab: Snubber
Since Constant power loads/sources are modeled as current sources it is a good practice to add large resistances in parallel in order to avoid degeneration in the circuit. An example where these snubber resistances are required is when the contactor is directly connected to the power source. Without snubber resistances, the contactor switches cannot be opened as there must be a path for the current from the current source(s).
Single Phase Constant Power Load
The symbol for the Single Phase Constant Power Load in Schematic Editor is shown in Table 1. Parameters, including the ability to convert the power load into a power source, are described in detail above.
component | component dialog window | component parameters |
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Single Phase Constant Power Load |
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The functional diagram of the Single Phase Constant Power Load is shown in Figure 4. It consists of a controlled current source, meters, an optional snubber resistor, and current control logic. The inputs for the control logic are: reference for active power Pref, processed through the frequency droop look up table; reference for reactive power Qref, processed through the voltage droop look up table; input voltage Va; current value Ia. Droop look up tables are optional and can be enabled if needed.
Internal analog measurements to the Single Phase Constant Power Load
Analog output variable name | Description |
---|---|
name_ia | Current of the Single Phase Constant Power Load |
name_va | Voltage of the Single Phase Constant Power Load |
Internal digital probes to the Single Phase Constant Power Load
Digital output variable name | Description |
---|---|
name_active | Signalizes that the power source is working properly |
Three Phase Constant Power Load
The symbol for the Three Phase Constant Power Load is shown in Table 2. Parameters, including the ability to convert the power load into a power source, are described in detail above.
component | component dialog window | component parameters |
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Three Phase Constant Power Load |
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The functional diagram of a Three Phase Constant Power Load is shown in Figure 5. It consists of two controlled current sources, meters, optional snubber resistors, and current control logic. Inputs to the control logic are: reference for active power Pref, processed through the frequency droop look up table; reference for reactive power Qref, processed through the voltage droop look up table; input line-to-line voltage Vab; current value Ia. Droop look up tables are optional and can be enabled if needed.
As can be seen in Figure 5, only one of the three currents is measured and only one of the three line-to-line voltages is measured. In other words, it is assumed that the grid is balanced. The Three Phase Constant Power Load will not work as expected if connected to an unbalanced grid.
Internal analog measurements to the Three Phase Constant Power Load
Analog output variable name | Description |
---|---|
name_ia | Current of phase A of the Three Phase Constant Power Load |
name_ib | Current of phase B of the Three Phase Constant Power Load |
name_ic | Current of phase C of the Three Phase Constant Power Load |
name_vab | Voltage between phase A and B of the Three Phase Constant Power Load |
Internal digital probes of the Three Phase Constant Power Load
Digital output variable name | Description |
---|---|
name_active | Signalizes that the power source is working properly |