Description of the Single-phase Variable Load component in Schematic
Editor.
The Single-phase Variable Load component, shown in Table 1, is a Schematic Editor
library block from the Loads section, of the Microgrid
library. This component is based on variable impedance, built using variable passive components. It supports
distorted voltage on its terminals, meaning it will consume the demanded power when
distortions appear. There are two options for providing the references:
Internal: Through SCADA inputs which are located inside of the component
External: By using signal processing to calculate the references and provide those as
signal processing inputs. Enabled by checking the corresponding property in the Tab: "Inputs/Outputs".
Table 1. Single-phase Variable Load component in the Schematic Editor core library
The control algorithm is based on feed-forward control. Grid voltage is measured and, based
on that voltage and the reference power input, the required impedance value is calculated
and the appropriate contactors are controlled. A single-phase PLL is used in order to
synchronize with the grid and extract the exact value of voltage amplitude and frequency.
Synchronization with the grid is a necessary condition for the load to connect to the grid,
meaning that there might be some delay between the enabling command and the load connection.
When compared to some form of closed loop control, such as the one used in VBR Variable Load, this control approach provides several advantages:
It's more robust: Its operation is mostly independent of the grid dynamics and it is
less sensitive to grid events.
It has no overshoot: Since it's based on a calculation, rather than a closed loop
system, it does not exhibit overshoot and bandwidth issues.
Disadvantages of this control approach:
Steady-state error: Since there is no closed-loop feedback, it is not ensured that
steady-state error in power measurements is equal to zero. However, calculations are
designed in such a way that the steady-state error should remain reasonably small even
under distorted voltage conditions.
Electrical solver resources utilization: Since the component is comprised of several
variable elements, it requires a significant amount of electrical solver resources.
Necessity for snubber elements: In order to ensure the numerical stability of the
circuit, snubber elements are added. Active power consumption of the snubber circuit
represents the minimum active power that can be consumed by the load.
The component consists of an electical part that consists of series RC
connection in parallel with an inductance, as shown in Figure 1. Figure 1. Single-phase core schematic block diagram
Component dialogue box and parameters
The Single-phase Variable Load component dialogue box consists of three tabs for specifying
parameters of the component.
Tab: "General"
In this component tab, general parameters of the Single-phase Variable Load can be
specified.Figure 2. Single-phase Variable Load "General" parameters.
List of harmonic order values that are expected to be present in the grid
voltage.
Execution rate
execution_rate
Execution rate of the internal signal processing. [s]
Tab: "Advanced snubber options"
In this component tab, additional parameters that are related to the Load's snubber can be
specified. Figure 3. Single-phase Variable Load "Advanced snubber options" parameters.
When the external control mode is enabled, the appropriate signal
processing ports appear on the left side of the component. Available inputs are given in
Table 5.
Available outputs
are given in Table 6. These
measurements are available through probes inside of the component, but can also be available
as an output vector with 6 elements.
Table 5. Single-phase Variable Load user interface inputs
Number
Input
Description
Signal range
Default value
0
Enable
A digital input that enables the load. The load is enabled when the input value
is high.
0 or 1
0
1
Pref
An analog input that sets the active power consumption reference. [pu]
0 to 1
0.5
2
Qref
An analog input that sets the reactive power consumption reference. [pu]
0 to 1
0.5
3
ROC
An analog input that sets the rate of change of the power reference ramping.
[pu/s]
>= 0.001
5
Table 6. Single_phase Variable load user interface outputs
Number
Output
Description
0
Enable_fb
A digital output describing the load's enable/disable state. The load is
enabled if the output is high.
1
MCB_status
A digital output reporting the information about the state of the main circuit
breaker (contactor). The contactor is closed if the value is high.
2
Pref_fb_kW
An analog output with the value of the applied active power reference in the
load. [kW]
3
Qref_fb_kVAr
An analog output with the value of the applied reactive power reference in the
load. [kVAr]
4
Pmeas_kW
An analog output reporting the value of the active power output of the load.
[kW]
5
Qmeas_kVAr
An analog output reporting the value of the reactive power output of the load.
[kVAr]
6
V
Voltage measurement.
7
I
Current measurement.
A dedicated Library
Widget for controlling the Single-phase Variable Load component is available.
