VOLTAGE SENSING, MEASUREMENT & PHASING
Hugh C. Ross
Sensing of energized lines, accurate voltage measurement, and phasing by portable isolated instrumentation has proved practical and reliable on modern underground systems. Specifically designed solid state HV voltmeters and accompanying test and verification devices have proved useful and reliable in conjunction with the capacitance voltage test point now found on many devices. Standardization on practical values of capacitance and surface leakage on voltage test points is still a consideration to achieve maximum usefulness.
Underground systems with shielded cables and connectors provided few points for direct hot line measurement. This presents considerable difficulty in determining the presence of voltage on an energized line and even more difficulty in accurate voltage measurement and phasing with a portable voltmeter. Considerable help has had been provided by the voltage test point, a capacitance type voltage tap now being used in several types of cable connectors, elbows, splices, and equipment. This type of tap is essentially a capacitive voltage divider which makes use of the cable's, connector's, or bushing's high voltage insulation between the HV conductor and a conductive, isolated, plate or band to create a useful HV capacitor under the outer shield (see Figure 1).
In many of the cable terminations now being used, the HV capacitance of the voltage test point between the HV conductor and the isolated tap plate is in the order of 1.6 to 1.8 picofarad (C1 of Fig. 1). This value is obtained by choosing the proper size of a pickup plate or band, its distance from the HV conductor, and the arrangement of shielding in the molded connector or equipment.
Accompanying this HV capacitance is low voltage capacitance in the order of 6 to 8 picofarads to ground (to shield) (C2 of Fig.1), thus creating a combination of two capacitances giving a voltage divider effect. These values give a basic average ratio of (7+1.7) / (1.7) or (8.7) / (1.7) which reduces to approximately (5.1) / (1). Larger values of the LV capacitance could also be used as a standard. For a 10 to 1 ratio approximately 15.75PF would be necessary, or it can be even larger. The low voltage end capacitance is not critical if the HV end is standard. Voltmeter impedances can be low enough to "swamp out" a 6 to even a 50PF capacitance, the voltmeter itself then establishing its own ratio independent of the LV end capacitance.
The voltmeter impedance must be kept very high, to more easily match the variations in the HV end and the overall ratio is then mostly dependent on the capacitance ratio, then this LV end capacitance also becomes critical in establishing the ratio.
In the design of the capacitances of the voltage test point, shielding positions as well as sizes of conductors, and thickness, dielectric constant and operating temperature of the dielectric must be considered. Capacitance variation, with variation in applied voltage level, can also be a consideration.
In order to obtain the desired HV end capacitances, the size of the pickup plate, which is generally at a required distance from the conductor due to required insulation level, is the major variable available. This plate can be enlarged to a band around the entire conductor insulation, since the size of the HV conductor plays a critical part in determining the capacitance.
In the plug-in cable connectors, if internal grading shields are not shaped properly to shield the tap plate from the cable conductor, variations in size of cable conductor and dielectric constant of cable insulation will appreciably change the HV end capacitance to the tap plate.
One utility found it could even determine the size of a conductor by the voltage reading. The larger the cable conductor, the higher the voltage reading with the same style of elbow connector and a fixed line voltage. Some brands show considerably more variations due to conductor size variation since the plate is not as remote from the incoming cable.
A major design consideration of a voltage test point also is the external insulation path to ground. Some present designs have extremely short paths for an area that should maintain over 1,000 megohms leakage resistance if a very high input impedance voltmeter is used. A long skirted path is desirable. On one of the widely used elbows, the voltage output of the test point unloaded is 1/5 of the 7.2kV line to ground voltage, or approximately 1.4kV for a 12.5kV system. This, of course, is so limited in current output (5 to 10 microamps) by the low source capacitance that it in itself is not dangerous, but it can cause leakage paths to form on the short insulation path if cleanliness is not constantly observed.
Some brands and models of taps have somewhat different capacitances and therefore somewhat different ratios, some varying as much as 2 or 3 times these amounts. This creates considerable difficulty in designing a universal voltmeter to use these taps and an effort is being made to standardize on at least the most important, the HV end capacitance in the order of 1.75 picofarad. The low voltage end capacitance is less important; however, it has been suggested that it be standardized in the value of 7PF to give a basic minimum ration of 5 to 1, which then can be padded externally to give any higher ratios desired.
A much higher HV end capacitance would be considerably better, perhaps in the order of 3PF or even 10 or 15PF, since the very low value of 1.75PF presents considerable difficulty for accurate voltage measurement with an economical device. The higher capacitance would also reduce the effect of moisture or dirt which could cause low or even zero readings if the test point were not cleaned properly. However, it appears that increasing this HV capacitance involves changing present production molds and adds considerable development expense to the connectors. Thus it may be economically unfeasible for an established item to be modified to meet the desired value.
Sensing Energized Lines
Using this capacitance voltage tap, or voltage test point as has been suggested for a standard name, for merely indicating an energized line is relatively easily done with a moderately priced high impedance sensing and indicating device. Using it for accurately measuring actual voltages line to ground is more difficult because of meter loading and external interference. Further, using it for line to line voltage measurement and phasing becomes even more difficult with added external capacitance problems. However, with proper matching impedance, input voltages, and shielding in an objectively designed high impedance voltmeter, all of these measurements can be made generally within better than 10%, usually better than 5%, depending primarily on the accuracy of the voltage test point capacitances themselves.
At this time there are different capacitances in some manufacturers’ models. For accurate measurement the voltmeter then must have different input matching and ratios for voltage test points on some of the different brands and types of connectors and equipment.
Portable HV Voltmeter
A solid state portable voltmeter and associated accessories, called the Hi-Z® HV Voltmeter (see Figure 2), has been developed for hot stick use by Ross Engineering Corporation to match the voltage test point on the Elastimold 15KV elbow. It also matches the Burndy 15KV load break elbow test point, the ITT Blackburn elbow test point and others, without need for changing voltmeter probes, but will also match the brands and models with different capacitances in their test points by changing either the voltmeter's removable probe or a selector switch. Experience with production units in the field for the past three years has shown this method is reliable for portable voltage and phasing measurement.
With such an extremely high impedance source (1.75 picofarad is approximately 1,500 megohms 7PF is approximately 370 megohms at 60Hz) extreme care must be taken in shielding the higher impedance (100-1000 megohm) pickup probes and voltage test points, particularly if exposed HV taps or lines are near the test point. External pickup is not nearly as critical when using the 1 megohm to 85 megohm probes which can be used with specific brands of connectors or equipment.
For line to line phasing by use of test points, capacitance loading must be carefully balanced to prevent pickup or unequal changes in capacitance ratios, which could cause serious errors in line to line or phasing differential readings.
The voltmeter is provided with both line to ground (Figure 3) and line to line and phasing probes (Figure 4) which are shielded and balanced for accurate measurement by voltage test points for lines from a few volts to 25kV, as a line to ground, a line to line, or a phase differential voltage.
This has increased its versatility considerably, since it also can detect the presence of AC voltage, even at the surface of many semi-conductive shielded concentric neutral cables, thus indicating voltage existing on the internal conductor when there is no voltage test point available. Due to the variations in semi-conductive shielding resistance in various brands of cables, not all cables will indicate properly. If voltage presence is indicated, the cable is definitely energized. However, for some types of semi-conductive shielded cables with very low or very high surface resistance, or for metallic shielded cables, insufficient indication may result.
Since most voltage test points have such high impedence, they can be subject to leakage contamination if care is not taken to keep the tap clean and dry while making a test. Usually a protective grounding cap is provided and silicone grease is liberally applied to the external insulating portion of the tap. Leakage resistance in the order of 1,000 megohms (in some types 100 megohms) or less can seriously effect the voltage test point calibration. Leakage resistance under 20 megohms on a voltage test point can indicate practically zero reading when using the higher impedance voltmeters.
A voltmeter supplied with a combination AC voltmeter tester-verifier and DC leakage current-megohmmeter, provides both 400V AC for testing the voltmeter before and after a measurement, and 500V DC for measuring the leakage resistance of the voltage test point or any other HV insulation in the range of 20 megohms to 2,000 megohms. This leakage or contamination testing unit for verifying voltage test points is designed for portable hot stick use. While the standard unit is rated for 600V maximum, some models can safely contact an energized HV line directly. They can also measure insulation leakage resistance at 500V DC to 2000 megohms, or 0-15kV DC or 0-36kV DC for higher voltage insulation to 30,000,000 megohms, even with the presence of up to 15,000V AC, if the 85 megohm or higher HV probe is used as a limiting resistance.
With the increased use of capacitive voltage test points, sensing of energized lines and accurate voltage measurements and phasing has been shown to be practical and desirable for underground systems.
Test point capacitance values still leave much to be desired for uniformity and greater reliability. One suggested method of standardizing on only a ratio of HV to LV capacitance is considerably less desirable than standardizing on a fixed value of HV end capacitance and also a ratio, although a ratio is less critical.
The HV end capacitance should be established at as high a value as practical to minimize contamination and external shielding problems.
Voltmeters with accessories have shown it is feasible and reliable to safely and accurately measure voltages and phase on underground systems with the capacitive voltage test point. Sufficient varieties of hotstick designed measuring and testing devices now exist to obtain most HV conductor information by use of portable isolated devices.
1. Proposed IEEE - NEMA Joint Standard for Separable Insulated Connectors January, 1970
Sponsored by The Task Force on Switching and Overcurrent Protection, Underground Distribution Subcommittee, IEEE Transmission and Distribution Committee and The Primary Connector Working Group, Technical Committee, Electrical Connector Section, NEMA
Mr. Hugh C. Ross is President and Chief Engineer for Ross Engineering Corporation.
Specifications are for reference only and are subject to change.
Contact Ross Engineering Corp. for current information.