The present invention relates generally to an impedance monitoring station, and more particularly to a hybrid passive agent system impedance monitoring station and method.
System protection and personal safety is directly related to system impedance which specifies, by Ohm's law, the maximum current that can flow in a circuit. Despite the fact that system impedance is the most important parameter in determining arc fault current that will occur when a system is short circuited, electrical one-line diagrams are often not kept up-to-date with actual system wiring, making it difficult to determine system impedance.
Prior art methods of determining system impedance typically involves either solving only directly or interpretively the power frequency impedance. As a result, prior art methods encompass only limited information.
For example, one method used to measure system impedance requires the test load to be considerably large so that it creates a significant voltage drop between measurement points. The driving point impedance is determined in the time domain using the voltage drop between source and load, the associated current, and the current derivative. Two voltage measurements are required, but the method is unable to determine the change in voltage phase angle. This procedure is also subject to errors caused by capacitor banks in the system such that the resistance is over-estimated.
In another example, the procedure requires either passive or active harmonic current injection for frequency bands around the fundamental. This procedure is disclosed in U.S. Pat. No. 5,587,662. From the corresponding harmonic voltages induced, one can solve for the harmonic impedance in the frequency domain. The disadvantage of this procedure is that the solution can not be made for the driving point impedance due to the dominance of existing line voltage. The fundamental impedance must therefore be interpolated from the trend of harmonic impedances. The interpolation may be linear; however, it does not always have the same curve fit. This can occur in the situation that capacitor banks and resonance exist in the system. Resonance at low frequencies will cause the system inductance to be under-estimated.
Another procedure, disclosed in U.S. Pat. No. 7,164,275, inserts a capacitor into the system and uses the ringing frequency to determine the system inductance. In the presence of a capacitor bank, the known test capacitor will be in parallel with the bank. The system inductance will then be subject to erroneous results. The system inductance is then used to determine the system resistance looking at the magnitude of voltage drop caused by a resistive load. When the system is contaminated with voltage harmonics, the use of magnitude voltage drop will further increase error.
Additional methods, such as those disclosed in U.S. Pat. Nos. 6,713,998, 6,801,044, and 5,631,569, measure the system impedance in the time domain. These methods require making multiple voltage measurements along the energized line in question. The driving point impedance is determined with the voltage drop, the associated current, and the current derivative. It is problematic that the waveforms do not align with the appropriate time stamp or phase angle when two voltage measurements are taken at different locations. Along this length of line, it is also possible for other radial loads. The voltage drop would therefore not be associated with the current measured at either end of the line in question.