The present invention relates to phase angle meters and in particular, to a portable phase angle meter instrument useful in measuring the phase angle between the voltage and the current in a utility power distribution system.
Phase angle measurements generally require installation of current and potential transformers at various locations in the power distribution system. Such installations are time consuming, expensive and frequently disturb the operation of the power distribution system. Presently, there is no portable phase angle measuring instrument for making direct phase angle measurements at distribution system voltages. Consequently, substantial uncompensated phase shifts between the voltage and current occur in distribution feeders causing substantial inefficiencies in the distribution network.
In order to increase the efficiency of such electrical power distribution networks and to decrease the apparent power losses due to phase unbalance and shift, particularly in feeder lines which have no in place instrumentation it is desired to have a portable instrument which may be hooked across two power distribution feeder lines and generate a current signal which in phase with the current in one of the conductors and a voltage signal which is substantially in phase with the voltage between the conductors.
The present invention provides an instrument, embodied in a portable "hot stick" for measuring phase angle on energized distribution feeders without interfering with their operation. Complete safety to the operation is provided even though the hot stick instrument is connected directly to conductors having a voltage up to about 35,000 volts carrying and a current up to about 800 amps.
The present portable instrument may be used on single phase, or 3-phase star connected power distribution systems where the phase angle between voltage and current may be read phase to neutral, without further correction, or in delta connected distribution systems where an appropriate correction factor of 30.degree. is either added or subtracted from the phase angle measurement.
The invention also provides a reverse phase detection means to indicate that the portable hot stick instrument is attached to the power distribution conductors in a reverse position with respect to the direction of the feeding bus.
The elements of the phase angle meter instrument, in accordance with the invention, include a current probe assembly, a potential probe assembly and an electronic circuitry enclosure assembly attached to the current probe assembly.
More specifically, the current probe assembly generally comprises an insulated pole with a sensing head attached to its upper end and an electronic circuitry enclosure attached at substantially mid-pole. A high voltage capacitor having a capacitance of about 0.001 microfarads is positioned inside the current probe pole.
The sensor head has two basis elements. First, a conductive plate by which the sensor head makes direct electrical contact with a reference conductor, and secondly, a current sensing device including the associated circuitry to generate a signal which is either related to or in phase with the current in the reference conductor. This signal will hereinafter be referred to as the "current signal".
In one embodiment, the current sensing device comprises a Hall generator with associated circuitry necessary to generate the current signal.
In an alternative embodiment, the current sensing device may be a transformer type device which is enclosable about the reference conductor. In this embodiment a first current signal is derived from the secondary windings of the transformer. This first current signal may be transformed again to produce a second current signal, which is then digitized as hereinafter explained.
Regardless of the type of current sensing device utilized, the resultant current signal has a fixed phase relationship with the current in the reference conductor. The current signal is coupled to the circuitry in the enclosure assembly which is attached to the current probe housing.
The potential probe assembly comprises an insulated pole with a conductive hook on one end for being hooked over a first conductor. A high voltage capacitor having a capacitance of about 0.001 microfarads is positioned in the interior of the potential probe pole and is coupled to the current probe capacitor via a high voltage cable. The conductor hook, the potential probe capacitor, the current probe capacitor, a filter circuit in the enclosure assembly, and the plate of the current probe sensing head are coupled in series. Appropriate processing circuitry is coupled across the filter circuit to generate a signal which has a fixed phase relationship with the voltage between the first conductor and the reference conductor. This signal will hereinafter be referred to as the "voltage signal".
Circuitry is next provided to digitize the current and voltage signals. The digitized current and voltage signals vary between a zero voltage level and a normalized non-zero voltage level, hereinafter referred to as being a (logical) zero and a (logical) one, respectively.
The digitized voltage signal makes the transition from zero to one upon the occurrence of a positive going zero crossing of the voltage signal and makes the transition from one to zero upon the occurrence of a negative going zero crossing of the voltage signal.
The digitized current signal similarly varies between zero and one making a zero to one transition upon the occurrence of a positive going zero crossing of the current signal and a one to zero transition upon the occurrence of a negative going zero crossing of the current signal.
The digitized current and voltage signals are next coupled to a microcomputer which includes combining circuitry, counting circuitry and a microprocessor. The digitized voltage signal (E) and the digitized current signal (I) are combined in an exclusive OR gate to generate a signal which is at one whenever the logic levels of the digitized voltage signal and the digitized current signal are not the same (i.e. E'=I), thus indicating either a lead or a lag condition. This signal is then coupled to logic circuitry which generates a lead signal or a lag signal depending on whether the current is leading or lagging the voltage signal. The lead or lag signal are next combined with the E'=I signal and a clock signal in a NAND gate to generate a pulse train, in which clock pulses occur only during the time that the digitized current signal and the digitized voltage signal are not at the same logic levels and the current either leads or lags the voltage.
The number of pulses which occur during lead conditions over a specified period of time, preferably in terms of 36 cycles of the digitized voltage signal, are accumulated in a first register designated the lead register. Similarly, the number of pulses which occur during lag conditions over the same specified period of time are accumulated in a second register designated the lag register. Concurrently, the total number of pulses which occur during the specified period of time are accumulated in a register designated the window register.
At the end of the specified period of time, the accumulated count in the three registers are combined by a microprocessor according to a programmed algorithm to compute the phase angle. The phase angle may then be corrected by adding or subtracting appropriate angles and constants. The microprocessor provides a value of phase angle which may be displayed on a display or may compute the power factor directly by taking the cosine of the phase angle and thereafter display the value of the power factor.