Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-11-25
2001-05-01
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S551000
Reexamination Certificate
active
06225813
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a method and apparatus for inspecting and evaluating water cooled stator windings for possible penetration of water into the groundwall insulation and more specifically to a method for measuring the capacitance of the insulation on a stator winding (bar) in a motor or generator with the field “in situ” (i.e., without disassembling/removing the magnetic field producing components of the motor/generator).
BACKGROUND
In large industrial or utility motors and generators, the stator windings, also known as armature windings, are inspected from time to time to confirm the integrity of the insulation. Each stator winding includes a conductive bar(s) wrapped in layers of insulation. The insulation confines the current in the bars to the bars, prevents arcing of current between windings, and shields the bars against stray objects that could electrically short the bars and to protect people and equipment. In view of the high current levels that flow through industrial generators and motors, the insulation on stator bars must provide an effective and complete barrier surrounding the bars. If the insulating properties of the insulation degrades because it becomes damp or for other reasons, then voltage arcs may jump from the stator bars through degraded regions of the insulation to cause electrical shorts that can harm people and damage equipment.
The insulation on stator bar windings are inspected from time to time to determine whether the insulation has degraded and, if so, to what extent. Stator bars on older water-cooled generators are especially susceptible to water leaks and must undergo regular periodic inspection and testing. One test which has proven very reliable in identifying stator bars with deteriorated groundwall insulation is a capacitance “mapping” of the stator bars. Since the dielectric constant of an insulator provides a measure of its insulating properties, the insulation of an stator bar winding can be inspected by determining the dielectric constant of the insulation. The dielectric constant of the insulator can be calculated by measuring the capacitance of the insulation on the stator bars. The dielectric constant indicates such conditions as the amount of dampness in the insulator. A damp insulator may indicate a leak in the water passages in a water cooled stator. A damp insulator may be water damaged and not functioning as an effective insulator.
All insulating materials have a dielectric constant, which is a measure of the amount of energy the insulating material stores when a voltage is applied across the material. The approximate dielectric constant for air is 1.0 and 80 for water. However, for Micapal™, a common insulation material for stator windings, it is approximately 4 (for undamaged Micapal™). Where there is a mixture of air, water and insulation, the measured capacitance will be a different composite number. Because of the large difference in the dielectric constants for Micapal™ (and other winding insulators) and water, the dielectric constant changes relatively dramatically when an insulator for a winding becomes damp. Accordingly, measuring the dielectric constant of an insulator provides an effective means for detecting water logged insulation on stator windings.
The dielectric constant for a stator insulator can be calculated using capacitance values measured across the insulator. Capacitance and the dielectric constant are related as described in the following equation:
C=kDA/t
Where: D=dielectric constant of the insulation
A=area of the probe electrode
t=thickness of the insulation
Because the area (A) of a probe electrode and thickness (t) of the insulator are know quantities and the capacitance (C) of the insulator is a measured quantity, the dielectric constant (D) can be relatively easily calculated with the above equation. A meter is used to measure the capacitance across the insulation between the electrode and the stator bar conductor. Each stator bar in the winding is measured at both ends of the core and statistical analysis is used to identify those bars with higher than normal expected capacitance. A relatively higher capacitance is a good indication of moisture present in the insulation.
To obtain accurate capacitance measurements of the insulation on stator bars, the capacitance measurement probe must be precisely inserted into the proper position between adjacent stator bars and the electrode placed firmly against the insulated surface of the particular stator bar measured. In this manner, a “mapping” of insulation capacitance for a particular motor or generator could be compiled and used to identify stator bars with damaged or faulty insulation. Consequently, a special capacitance probe having an inflatable bladder was developed for improving the accuracy and ease with which such measurements are made. The special probe developed is the subject of commonly assigned U.S. Pat. No. 5,546,008 issued Aug. 13, 1996, to Sminchak et al., entitled “INFLATABLE CAPACITANCE MEASURING DEVICE”, which is incorporated by reference herein. Unfortunately, capacitance mapping using such a probe required disassembly of the generator and removal of the field producing component (i.e., the rotor) so that the stator bars were accessible to a service technician for inserting and properly positioning the probe at the desired positions between adjacent stator bars. Since removal of the field from a large motor or power generator is an expensive and time consuming process, capacitance mapping was usually a last resort maintenance or inspection procedure.
DISCLOSURE OF THE INVENTION
To satisfy the need to reduce the time and expense of performing stator bar inspection and capacitance mapping for generators and motors, the current invention presents an improved inspection method and capacitance mapping kit developed to allow measurement of the capacitance of the insulation wrap on stator bars with the field “in situ” (i.e., still in place) such that less disassembly of the electromagnetic generator or motor is required. The kit includes several diagnostic tools including a capacitance probe tool having an improved capacitance probe head structure (which uses a version of the inflatable electrode bladder described in above mentioned patent) and a versatile probe head support arm for accurately positioning the probe head in between stator bars from a remote location—thus, precluding the need to remove the field producing component of the motor or generator. In addition, the kit includes one or more pressure regulator (step-down) valves, a small remote “portable” pressure regulator actuator unit (which may be conveniently worn on the belt of a user), a capacitance meter, as well as various lengths of appropriate electric cables and air hoses as appropriate. The capacitance probe tool consists of a narrow elongated pole (support arm) having a telescoping main section and a bendable, rotatable articulated tip section at one end. The improved capacitance measuring probe head structure is attached to the support arm at the end of the articulated tip section which remains bent at a particular desired configuration until manually reconfigured. The improved probe head includes a bladder support plate which acts as a spacer and is adapted to accept one or more additional spacer plates for changing the overall width of the probe head structure. The probe head support arm also supports an electric cable from the probe electrode to a remote electric meter and an air hose to a remote fluid source for inflating/deflating the probe bladder. The arm may also support a mount for a miniature TV camera, a telescoping mirror, and/or an electric lamp for illumination.
In accordance with the method of the present invention, the probe head may be accurately positioned by an inspection technician between stator bars at the requisite measurement sites which were—previous to the advent of this invention—inaccessible without removal of the field (rotor). To accomplish this, the capacitance prob
General Electric Co.
Metjahic Safet
Nguyen Vincent Q.
Nixon & Vanderhye
LandOfFree
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