Dynamic Load

R2026b

Dynamic load for DC or AC supply

Libraries:
Simscape / Electrical / Passive

Description

The Dynamic Load block implements a dynamic load for a DC or AC supply.

If you set the Load type parameter to DC:

The consumed power of the block is equal to the P port as long as it is greater than the value of the Minimum consumed power parameter and the voltage from the DC supply is equal to or greater than the value specified for the Minimum supply voltage parameter.
When the voltage from the DC supply drops below the Minimum supply voltage, the load behavior changes and the block acts as a resistive load. If the supply voltage becomes negative, the block acts as an open circuit conductance.

To ensure smooth transitions between these behaviors, the block uses a third-order polynomial spline with continuous derivatives. You can specify the width of this transition using the Transition voltage width parameter.

If you set the Load type parameter to AC, this table shows how the block calculates the consumed active and reactive power based on the voltage from the supply, V.

Range of V Values	Consumed Active and Reactive Power
$V > V_{\min}$	$\begin{array}{l} P_{a c t u a l} = P_{i n} \\ Q_{a c t u a l} = Q_{i n} \end{array}$
$V < V_{\min}$	The block models a load with an impedance equal to: $\begin{array}{l} R = V_{\min}^{2} \frac{P_{i n}}{P_{i n}^{2} + Q_{i n}^{2}} \\ X = V_{\min}^{2} \frac{Q_{i n}}{P_{i n}^{2} + Q_{i n}^{2}} \\ Z = \sqrt{R^{2} + X^{2}} \end{array}$

In these equations:

V_min is the value of the Minimum supply voltage (RMS) parameter.
P_in is the input active power of the Dynamic Load block, determined by external input port P.
P_actual is the consumed active power of the Dynamic Load block.
Q_in is the input reactive power of the Dynamic Load block, determined by external input port Q.
Q_actual is the consumed reactive power of the Dynamic Load block.

The Dynamic Load block consumes or generates active power depending on the polarity of input active power P_in. If P_in is greater than 0, the block consumes active power. If P_in is less than 0, the block generates active power.

Faults

To model a fault in the Dynamic Load block, in the Faults section, click Add fault next to the fault that you want to model. For more information about fault modeling, see Fault Behavior Modeling and Fault Triggering.

The Dynamic Load block allows you to model an electrical fault as an open circuit. The block can trigger fault events at a specific time.

Load-Flow Analysis

If the block is in a network that is compatible with the frequency-time simulation mode, you can perform a load-flow analysis on the network. A load-flow analysis provides steady-state values that you can use to initialize a machine.

For more information, see Perform a Load-Flow Analysis Using Simscape Electrical and Frequency and Time Simulation Mode.

Assumptions and Limitations

The Dynamic Load block reports thermal dissipation using DQ-averaged quantities but the energy accounting framework computes port power instantaneously. The block uses a harmonic estimator that integrates the supply voltage over one full cycle of the rated frequency to extract the voltage amplitude. This integration window fills after approximately the reciprocal of the rated frequency plus the voltage time constant. When you set the Load type parameter to AC and get energy information for this block using the getEnergyInfo function, the energy accounting results are inaccurate during this initial transient period. Once the estimator converges, the ports and external energy align on a cycle-averaged basis and the results are accurate at steady state. To reduce the duration of the inaccuracy, set the Rated frequency parameter to a larger value. When you set the Load type parameter to DC, energy accounting is accurate at all times.

Ports

Input

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P — Real power, W
physical signal

Physical signal port associated with real power, specified as a scalar.

Q — Reactive power, V*A
physical signal

Physical signal port associated with reactive power, specified as a scalar.

Conserving

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+ — Positive terminal
electrical

Electrical conserving port associated with the dynamic load positive terminal.

- — Negative terminal
electrical

Electrical conserving port associated with the dynamic load negative terminal.

Parameters

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Main

Load type — Load type
`DC` (default) | `AC`

Type of load. Select DC to use this load with a DC supply or AC to use this load with an AC supply.

Minimum supply voltage — Minimum DC supply voltage
`20` `V` (default) | scalar

Minimum voltage the DC supply needs to give the block to act as a constant power load.

Dependencies

To enable this parameter, set Load Type to DC.

Minimum consumed power — Minimum consumed power
`1e-3` `W` (default) | scalar

Minimum amount of power consumed by the block.

Dependencies

To enable this parameter, set Load Type to DC.

Reverse conductance — Reverse conductance
`1e-6` `S` (default) | scalar

Value of the reverse conductance when the supply voltage becomes negative.

Dependencies

To enable this parameter, set Load Type to DC.

Transition voltage width — Transition voltage width
`1e-3` `V` (default) | scalar

Transition voltage width. This value is used to ensure smooth transitions between the block's behaviors when the supply voltage drops below the Minimum supply voltage or becomes negative.

Dependencies

To enable this parameter, set Load Type to DC.