An accurate impedance measurement method for a power system transmission line is disclosed for improving various protection functions, i.e., distance protection and/or fault location estimation. The method is less sensitive to harmonics and other transient problems introduced to power systems by series capacitance and the like, and is easily incorporated into existing protective relays. In the method, a number (n) of current and voltage samples (I.sub.k, V.sub.k) representative of values of current and voltage waveforms are measured, respectively, at successive instants of time on a conductor in a power system. The number n is an integer greater than I and the index k takes on values of 1 to n. Resistance (R) and inductance (L) values are computed in accordance with an equation in which R and L are related to sums of differences in values of successive current and voltage samples. A prescribed power system function is then performed based on the computed R and L values.
The method described here makes it possible to determine the impedance of a line (1) by measuring the voltage (u) applied across the line and the time derivative of the current (i) flowing through the line. The measured values of the differentiated current are not integrated in this case, but rather are substituted directly, together with the measured voltage values, into an equation system from which the values of the inductance (L) and the resistance (R) of the line (1) can be estimated. In this way, integration of the values of the differentiated current is obviated.
A method for characterizing a load on a data line includes the steps of: (A) Applying at least three successive voltages to the data line. Each respective odd-numbered successive voltage of the at least three successive voltages has substantially a first voltage value displaced a first voltage interval from a reference voltage value. Each respective even-numbered successive voltage of the at least three successive voltages has substantially a second voltage value displaced a second voltage interval from the reference voltage value. (B) Measuring a respective current value on the data line while each of the at least three successive voltages is applied to the data line. (C) Comparing the respective current values for selected successive voltages of the at least three successive voltages to determine whether a hysteric impedance change occurs when voltage on the data line is varied.
A technique for determining inductive and resistive components of power line impedance. A measurement circuit switches a burden or drain resistor between power line conductors to cause a droop or sag in a voltage waveform. The voltage waveform is sampled prior to inclusion of the resistor in the circuit, as well as after to identify the droop. The short circuit between the power lines is then removed by opening the circuit and a capacitor in the test circuitry causes a resonant ring due to the inductive component of the power line impedance. Based upon the period or frequency of the resonant ring, and upon the voltage measurements with and without the resistor in the circuit, the inductive and resistive components of power line impedance can be computed.
A method and apparatus is disclosed for determining the power line parameters of a system. Specifically, there is provided a method comprising perturbing a voltage waveform through a first connection, measuring a characteristic of the perturbation through a second connection, and calculating a line impedance based on the characteristic of the perturbation.
