Instantaneous Active Currents Research Articles

Digital differential protection technique of transmission line using instantaneous active current: theory, simulation and experiment

Current differential protection is widely used as the main protection in UHV/EHV (Ultra/Extra High Voltage) transmission lines with advantages of simple principle, high reliability and inherent phase-selection capacity. However, as the voltage level and line length increase, the performance of the current differential protection is severely restricted by distributed capacity current. Aimed at this problem, a rapid extraction method of single-phase active current is proposed in this paper. The method first uses single PLL (phase-locked loop) to track the phase of the corresponding fundamental voltage. Then the current signal is transformed from the abc static coordinate system to the pq rotating coordinate through transformation matrix. By filtering out the high-frequency AC signal on p axis with low-pass filter, the active current component remaining being in the same direction with voltage phase can be obtained. On this basis, a novel active current differential protection method is proposed, with the advantages of high sensitivity and strong capacity of enduring transition resistance without requirement for the acquisition of line impedance parameters. Finally, the proposed method is tested by a series of dynamic simulations through PSCAD and experiment. The simulation results show that the method has merits of effectiveness, robustness and rapidity.

On Some Misinterpretations of the Instantaneous Reactive Power<tex>$phbox-q$</tex>Theory

The main features of the instantaneous reactive power (IRP) p-q theory, considered as a power theory of three-phase systems, are analyzed in this paper using the theory of the currents' physical components (CPC). This analysis shows that the p and q powers are not associated with separate power phenomena, but with multiple phenomena. Moreover, the results of the IRP p-q theory contradict some common interpretations of power phenomena in three-phase circuits. Namely, according to the IRP p-q Theory the instantaneous reactive current can occur even if a load has zero reactive power, Q. Similarly, the instantaneous active current can occur even if a load has zero active power, P. Moreover, these two currents in circuits with a sinusoidal supply voltage can be nonsinusoidal even if there is no source of current distortion in the load. The analysis shows that a pair of values of instantaneous active and reactive p and q powers does not enable us to draw any conclusion with respect to the power properties of three-phase unbalanced loads even in a sinusoidal situation. Thus, the instantaneous reactive power p-q theory does not identify power properties of such loads instantaneously. This conclusion may have an importance for control algorithms of active power filters. The paper reveals the relationship between the p and q powers and the active, reactive and unbalanced powers, P, Q, and D and specifies the required energy storage capability of active power filters operated under sinusoidal unbalanced conditions.

Variation power, variation currents: Physical background and compensation rules

AbstractDerived from instantaneous power and average power the instantaneous power currents as well as the instantaneous active currents are clearly defined under steady state conditions. As a consequence instantaneous active power is also defined without any ambiguity. It is shown that if there is any difference between instantaneous power and instantaneous active power the apparent power becomes larger than the absolute value of the active power. Furthermore the rules are derived whether a power oscillation should be compensated or not and what energy storage capability of the compensator is then necessary.