Documentation

Impedance Measurement

Measure impedance of circuit as function of frequency

Library

Measurements

Description

The Impedance Measurement block measures the impedance between two nodes of a linear circuit as a function of the frequency. It consists of a current source Iz, connected between inputs one and two of the Impedance Measurement block, and a voltage measurement Vz, connected across the terminals of the current source. The network impedance is calculated as the transfer function H(s) from the current input to the voltage output of the state-space model.

H(s)=Vz(s)Iz(s).

The impedance (magnitude and phase) as function of frequency is displayed by using the Impedance vs Frequency Measurement tool of the Powergui block.

The measurement takes into account the initial states of the Breaker and Ideal Switch blocks. It also allows impedance measurements with Distributed Parameter Line blocks in your circuit.

The source impedances are taken into account in the impedance calculations. By definition, the impedance of voltage sources is 0 (voltage source blocks are considered as short-circuit) and the impedance of current sources is infinite (current source blocks are considered as open-circuit).

In some applications, you may need to manually delete current or voltage source blocks in order to ignore their impact on impedance calculations.

Dialog Box and Parameter

Multiplication factor

If you use the Impedance Measurement block in a three-phase circuit, you can use the Multiplication factor parameter to rescale the measured impedance. For example, measuring the impedance between two phases of a three-phase circuit gives two times the positive-sequence impedance. Therefore you must apply a multiplication factor of 1/2 to the impedance in order to obtain the correct positive-sequence impedance value.

Similarly, to measure the zero-sequence impedance of a balanced three-phase circuit, you can connect the Impedance Measurement block between ground or neutral and the three phases connected together.

In that case, you are measuring one third of the zero-sequence impedance and you must apply a multiplication factor of 3 to obtain the correct zero-sequence value.

Limitations

The only nonlinear blocks that are taken into account during the impedance measurement are the Breaker, Three-Phase Breaker, Three-Phase Fault, Ideal Switch, and Distributed Parameter Line blocks. All other nonlinear blocks, such as machines and power electronic devices, are not considered, and they are disconnected during the measurement.

If you plan to connect the Impedance Measurement block in series with an inductance, a current source, or any nonlinear element, you must add a large resistor across the terminals of the block, because the Impedance Measurement block is simulated as a current source block.

See Also

power_zmeter

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