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.

There are two control methods avaliable: SCADA inputs and Signal inputs.
  • 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.

Figure 1. General tab in the Constant power loads/sources Properties window

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.

Figure 2. V-F droop tab in the constant power load/source Properties window

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).

Figure 3. Snubber tab in the constant power load/source Properties window

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.

Table 1. Single Phase Constant Power Load
component component dialog window component parameters

Single Phase Constant Power Load

  • Control method
  • Output execution rate
  • Pin to System CPU
  • Slow execution rate
  • Fast execution rate
  • Current limit
  • Enable voltage droop checkbox
  • Voltage droop look up table csv file
  • Enable frequency droop checkbox
  • Frequency droop look up table csv file
  • Snubber resistance

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.

Figure 4. Single Phase Constant Power Load functional diagram

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.

Table 2. Three Phase Constant Power Load
component component dialog window component parameters

Three Phase Constant Power Load

  • Control method
  • Output execution rate
  • Pin to System CPU
  • Slow execution rate
  • Fast execution rate
  • Current limit
  • Enable voltage droop checkbox
  • Voltage droop look up table csv file
  • Enable frequency droop checkbox
  • Frequency droop look up table csv file
  • Snubber resistance

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.

Figure 5. Three Phase Constant Power Load functional diagram

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