Insulation power factor alarm monitor

A method for operating apparatus in conjunction with the aid of a digital computer to monitor the insulation quality of a plurality of pieces of high voltage ac electrical equipment while said equipment is operating in its intended environment including being connected to its normal high voltage source which method comprises:

• (A) providing said computer with a data base accumulated and utilized in later mentioned steps (B) et seq. for said plurality of pieces of high voltage ac electrical equipment including at least:
• (1) analog-to-digital conversions of capacitance tap adaptor and reference voltages for power factor and capacitance computations, which voltages are functions of electrical parameters of the interface network including ADC's operatively associated with said computer, one or more reference voltage sources, and each of the plurality of capacitance tap adapter means coupled with each capacitance tap associated with each of said pieces of equipment being monitored;
• (2) analog-to-digital conversions of the analog output of one or more temperature transducers for use in making temperature corrections to power factor computations;
• and
• (3) correction factors for use with the analog output of each of said capacitance tap adapter means to allow said computer to make amplitude, phase, and temperature corrections to power factor and/or capacitance values computed from said analog outputs;
• (B) initiating a power on or reset routine to initialize the instruction program operatively associated with said computer;
• (C) initiating the startup routine of said instruction program by initializing peripheral ports for printers, terminals, alarms, and analog sampling control interrupters operatively associated with said computer;
• (D) clearing to zero all accumulator memory utilized for summations and to clear all status and control flags;
• (E) computing correction factors for subsequent use of power factor and capacitance determinations;
• (F) sequencing to a wait-for-event routine to determine status of check time instructions wherein:
• (1) if check time instructions are no, proceed to keyboard entry instructions for manually entered operator entries including system parameters wherein:
• (a) if keyboard entry instructions are yes, update variables, and/or list variables, test command tables, and/or print records, and return same to said check time instruction of wait-for-event routine (F) supra;
• or
• (b) if keyboard entry instructions are no, return to said check time instruction of wait-for-event routine (F) supra;
• (2) if check time instructions are yes, initiate action instruction set including:
• (a) begin measure subroutine with setup for data accumulation, clear memories, initialize counter and pointers, and setup arrays for data accumulations;
• (b) begin analog-to-digital conversion and data accumulation;
• (c) repetitively performing intermediate accumulation of power factors and capacitance values for each of said pieces of equipment being monitored by repetitively performing computations, including numerical integrations to calculate, from a series of reference voltage and capacitance tap current measurements summed for 16 cycles at 32 sample times throughout each cycle, both the power factor and capacitance characteristics of each of said pieces of equipment according to the following formulas, which formulas include correction for the 90 degree delay caused by the capacitive shunt in the capacitance tap adaptors:
• ##EQU5## where:
• D is a correction factor, determined during calibration, to compensate for phase shift in the analog circuits;
• X and Y are the sample sums of the voltages delivered to the computer from the capacitance tap adaptor and the referenced source, respectively;
• T.sub.i is the most recently computed estimated internal temperature in degrees Fahrenheit;
• T.sub.i(m) is the estimated internal temperature being computed;
• T.sub.i(m-1) is the estimated internal temperature computed one measurement period previously;
• T.sub.a(m) is the most recently measured ambient temperature in degrees Fahrenheit;
• A is 1/K;
• K is the time constant expressed in units of the measurement period (usually minutes;
• B is 1-A;
• H.sub.0, H.sub.1, H.sub.2, H.sub.3 . . . H.sub.8 are the coefficients of the best fit eighth order power series, computed by the method of least squares, to describe the power factor temperature characteristics according to the standard instrument transformer correction curve;
• C.sub.s is the total shunt capacitance, including the capacitance tap, the capacitance tap adaptor, and the cable, in picofarads (pf);
• R.sub.p is the potential transformer (pt) ratio;
• G.sub.p is the gain of the voltage reference interface amplifier;
• and
• G.sub.c is the gain of the capacitance tap interface amplifier;
• wherein all summations in the above formulas are carried out from n=1 to n=32, the number of samples per cycle;
• and
• (G) continuing said repetitively performed computations, until all samples for a predetermined measurement interval set forth in said check time instruction of wait-for-event routine (F) supra are obtained;
• (H) determine the final results of, for each piece of equipment being monitored, the percent power factor, the temperature corrected percent power factor, and the capacitance;
• (I) determine good data from (H) supra and reject bad data;
• (J) sum said good data from (I) supra to accumulators for monthly and daily averaging;
• (K) determine events outside predetermined limits of alarm conditions;
• (L) determine if data should be printed, and if so print data and averages as required;
• and
• (M) return to said check time instruction of said wait-for-event routine (F) supra.

A continuous monitoring system for simultaneously determining, in situ, the insulation characteristics of a plurality of separate pieces of high voltage alternating current electrical equipment including instrument transformers, high voltage bushings of power circuit breakers and transformers, and other like equipment, each of said pieces of electrical equipment being provided with a capacitance tap, said system comprising:

• (A) separate capacitance tap adapter means operatively associated with each of said capacitance taps;
• (B) separate impedor means operatively associated with each of said capacitance tap adapter means and having an electrical impedance so low that current in said impedor means is substantially determined by the impedance of the insulation of said pieces of equipment being monitored;
• said impedor means comprising at least two shunt capacitors of capacitance value selected such that added together they provide a shunt to ground that, with normal insulation current flowing therethrough, develops a voltage ranging between about 10 to about 140 volts ac RMS;
• (C) separate connector means permanently associated with each said impedor means for permitting coupling of a later mentioned external circuit to said impedor means without interruption of current through said insulation;
• (D) voltage reference source means operatively associated with high voltage alternating current bus means common with each group of said pieces of equipment being monitored and coupled therewith for providing a predetermined fraction of each bus voltage associated with each such voltage reference source means and in substantial phase relationship therewith for coupling with later mentioned external circuit means;
• and
• (E) external circuit means adapted to be coupled with said voltage reference source means and each of said impedor means, and responsive to signals from said voltage reference source means and to signals applied to said impedor means from said pieces of electrical equipment, the insulation characteristics thereof being monitored, said external circuit means comprising, for each said piece of electric equipment being monitored, either in total or on a time-share basis:
• (1) voltage interface amplifier means predisposed for providing compatibility of the voltage supplied by said capacitor tap adapter means to said external circuit means and characterized by its substantial elimination of all 60 Hz interference from extraneous high current conducters juxtaposed said separate connector means, said voltage interface amplifier means comprising transient supressor means, low-pass filter means and differential amplifier means and outputting substantially true square waveforms of the voltage signals from said voltage reference source means and said separate capacitance tap adapter means with transitions at 0.degree. and 180.degree. of said voltage signals;
• (2) noninverting buffer means operatively associated with said voltage interference amplifier means for providing waveforms therefrom in a form fully compatible with later mentioned circuitry and comprising first gate means having a positive output when the waveforms outputted from step (E)(1) supra are instantaneously of opposite polarity, thus having a zero output when said waveforms are exactly in phase and have the same symetry, and said first gates predisposed such that when said waveforms are but slightly out of phase with one another the output therefrom comprises a string of relatively short pulses with the length and position of said pulses being in proportional relationship with the time between the zero crossing of said two waveforms;
• (3) monostable multivibrator means including timing network means for outputting pulses substantially longer than said short pulses in said string of relatively short pulses in step (E)(2) supra for determining which of two said waves first crosses zero, said zero and said zero crossings relative to the midpoint of the power supply voltage;
• (4) second gate means operatively associated with the output from step (E)(3) supra for outputting either a positive going, or a negative going pulse indicative of which of said waveforms is first reaching said zero crossing;
• (5) current summing amplifier means operatively associated with the output from step (E)(4) supra and provided with inverting means such that said negative going pulses drives the output therefrom positive and said positive going pulse drives the output therefrom negative thereby providing circuit means for effecting both positive and negative power factor determinations and predetermined alarm conditions by outputting predetermined signals to any of several possible output conditioning means.

A continuous monitoring system for simultaneously determining, in situ, the insulation characteristics of a plurality of separate pieces of high voltage alternating current electrical equipment including instrument transformers, high voltage bushings of power circuit breakers and transformers, and other like equipment, each of said pieces of electrical equipment being provided with a capacitance tap, said system comprising:

• (A) separate capacitance tap adapter means operatively associated with each of said capacitance taps;
• (B) separate impedor means operatively associated with each of said capacitance tap adapter means and having an electrical impedance so low that current in said impedor means is substantially determined by the impedance of the insulation of said pieces of equipment being monitored;
• said impedor means comprising at least two shunt capacitors of capacitance value selected such that added together they provide a shunt to ground that, with normal insulation current flowing therethrough, develops a voltage ranging between about 10 to about 140 volts ac RMS;
• (C) separate connector means permanently associated with each said impedor means for permitting coupling of later mentioned first external circuit means to said impedor means without interruption of current through said insulation;
• (D) voltage reference source means operatively associated with high voltage alternating current bus means common with each group of said pieces of equipment being monitored and coupled therewith for providing a predetermined fraction of each bus voltage associated with each such voltage reference source means and in substantial phase relationship therewith for coupling with later mentioned first external circuit means;
• (E) first external circuit means adapted to be coupled with said voltage reference source means and each of said impedor means and responsive to signals from said voltage reference source means and to signals applied to said impedor means from said pieces of electrical equipment, the insulation characteristics thereof being monitored, said first external circuit means comprising voltage interface attenuator/amplifer means predisposed for providing, through later mentioned ADC means, compatibility between later mentioned CPU means and the voltage supplied by said capacitance tap adapter means, said voltage interface attenuator/amplifer means characterized by its substantial elimination of all 60 Hz interference from extraneous high current conducters juxtaposed said connector means operatively associated with said first external circuit means, said voltage interface attenuator/amplifier means comprising transient suppressor means, low-pass filter means, and differential amplifier means;
• (F) ambient temperature probe means operatively associated with said pieces of equipment being monitored and coupled through later mentioned ADC means to later mentioned CPU means;
• (G) microcomputer means operatively associated with said first external circuit means and said ambient temperature probe means, said microcomputer means comprising ADC means, CPU means, parallel interface means, battery backed random access memory means, realtime clock means, and power supply means;
• and
• (H) second external circuit means comprising synchronized frequency multiplier means operatively associated with said ADC means and adapted to be coupled therewith, provide timing pulses therefore, and therethrough determine the number and timing of voltage samples taken each cycle of each input wave of the signals applied by said bus means to said impedor means.