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Implement three-phase dynamic load with active power and reactive power as function of voltage or controlled from external input

Simscape / Electrical / Specialized Power Systems / Fundamental Blocks / Elements

The Three-Phase Dynamic Load block implements a three-phase, three-wire dynamic load whose active power P and reactive power Q vary as function of positive-sequence voltage. Negative- and zero-sequence currents are not simulated. The three load currents are therefore balanced, even under unbalanced load voltage conditions.

The load impedance is kept constant if the terminal voltage V of the load is lower than a specified value Vmin. When the terminal voltage is greater than the Vmin value, the active power P and reactive power Q of the load vary as follows:

$$\begin{array}{c}P(s)={P}_{0}{\left(\frac{V}{{V}_{0}}\right)}^{{n}_{p}}\frac{1+{T}_{p1}s}{1+{T}_{p2}s}\\ Q(s)={Q}_{0}{\left(\frac{V}{{V}_{0}}\right)}^{{n}_{q}}\frac{1+{T}_{q1}s}{1+{T}_{q2}s},\end{array}$$

where

*V*_{0}is the initial positive sequence voltage.*P*_{0}and Q_{o}are the initial active and reactive powers at the initial voltage V_{o}.*V*is the positive-sequence voltage.*n*and_{p}*n*are exponents (usually between 1 and 3) controlling the nature of the load._{q}*T*_{p1}and*T*_{p2}are time constants controlling the dynamics of the active power*P*.*T*_{q1}and*T*_{q2}are time constants controlling the dynamics of the reactive power*Q*.

For a constant current load, for example, you set *n _{p}* to
1 and

**Nominal L-L voltage and frequency**Specifies the nominal phase-to-phase voltage, in volts RMS, and nominal frequency, in hertz, of the load. Default is

`[ 500e3 60 ]`

.**Active and reactive power at initial voltage**Specifies the initial active power, Po, in watts, and initial reactive power, Qo, in vars, at the initial voltage, Vo. Default is

`[50e6 25e6]`

.When you use the Machine Initialization tool of the powergui block to initialize the dynamic load and start simulation in steady state, these parameters are automatically updated according to the P and Q points specified for the load.

When you use the Load Flow tool of the powergui block to initialize the dynamic load, these parameters represent the P and Q reference powers used by the load flow.

**Initial positive-sequence voltage Vo**Specifies the magnitude and phase of the initial positive-sequence voltage of the load. Default is

`[0.994 -11.8]`

.When you use the Load Flow tool or the Machine Initialization tool of the powergui block to initialize the dynamic load and start simulation in steady state, this parameter is automatically updated using the values computed by the load flow.

**External control of PQ**If selected, the active power and reactive power of the load are defined by an external Simulink

^{®}vector of two signals. By default, the check box is cleared.**[np nq]**Specifies the

**np**and**nq**parameters that define the nature of the load. Default is`[1.3 2]`

.**Time constants [Tp1 Tp2 Tq1 Tq2]**Specifies the time constants controlling the dynamics of the active power and the reactive power. Default is

`[0 0 0 0]`

.**Minimum voltage Vmin**Specifies the minimum voltage at which the load dynamics commences. The load impedance is constant below this value. Default is

`0.7`

.**Filtering time constant**Specifies the time constant, in seconds, of the current filter of the Three-Phase Dynamic Load block. Default is

`1e-4`

.

If **External control of PQ **is
selected, a Simulink input, labeled `PQ`

, appears.
This input is used to control the active and reactive powers of the
load from a vector of two signals [P, Q].

The `m`

output is a vector containing the following
three signals:

positive-sequence voltage (pu)

active power P (W)

reactive power Q (vars)

The `power_dynamicload`

model
uses a Three-Phase Dynamic Load block connected on a 500 kV, 60 Hz
power network.