In the area of electrical power, a bushing is an insulated device that allows an electrical conductor to pass safely through an (usually) earthed conducting barrier such as the wall of a transformer, circuit breaker or reactor. A condenser bushing is a bushing in which metallic or nonmetallic conducting layers are arranged within the insulating material for the purpose of controlling the distribution of the electric field of the bushing, both axially and radially by capacitive grading. In many cases, bushings may be used with very high voltages such that a failure in the bushing may lead to arcing and a catastrophic failure of the attached device. Accordingly, testing and monitoring of the bushing is very advantageous.
The current solutions for on-line transformer monitoring are passive systems that can only provide bushing capacitance and power factor values for the inner condenser section of energized bushings. The capacitance and power factor values for both the transformer and for the outer condenser section of an energized bushing cannot be measured. The embodiments described herein are a significant improvement in the art that address, or at least ameliorate, some of the shortcomings of the current art.
FIG. 1 illustrates a current design of a bushing monitoring system comprising a condenser-type bushing 10, a passive sensor 20, and a monitor processor 30. High voltage condenser-type bushings 10 may be designed in a wide variety of constructions, and the description herein is not intended to describe all of the possible types of designs. However, the majority of high-voltage condenser-type bushings 10 currently manufactured are constructed with an inner capacitive condenser section 12 referred to as C1, and, an outer capacitive condenser section 14, referred to as C2. These bushing sections C1 and C2 are wound radially around the bushing's central conductor 11. The inner C1 section 12 comprises the bulk of the total condenser 13. The outer C2 section 14 of the condenser 13 normally contains only 5% to 10% of the total number of layers in the complete condenser 13. This type of bushing 10 is normally equipped with an external tap connection 16, which is coupled to a foil layer between the inner C1 section 12 and the outer C2 section 14. The tap 16 may be referred to by several different names such as a test tap, capacitance tap, voltage tap, power factor tap or other designations. For the purposes of this document, we will refer to the tap 16 as a test tap.
The inner C1 section 12 of the bushing condenser 13 provides a capacitive structure, which is intended to provide a consistent voltage stress gradient from the energized inner conductor 11 to the outer layer of the inner C1 section 12. The outer layer of the inner C1 section 12 is normally connected to an earth ground 18 when the bushing 10 is in-service. Grounding of the inner C1 section 12 is normally accomplished by attaching a cover onto the test tap 16. The cover electrically connects the test tap 16 to the grounded metal flange of the bushing.
In many condenser designs, the outer layer of the outer C2 section 14 is also grounded. In such embodiments, the outer layer of the outer C2 section 14 of the bushing 10 may be comprised of a conductive foil which is connected to the grounded metal flange of the bushing 10. When this type of bushing 10 is in-service and the test tap cover is installed, the C2 condenser section 14 is either directly or effectively earth grounded on the inner and outer layers. However, in some bushings, normally in bushings designed for lower voltages, the outer layer may be ungrounded and separated from the grounded metal flange of the bushing 10 by an insulating fluid or dielectric.
Typically, the C1 and C2 bushing condenser sections 12 and 14 have capacitance values in the pico-farad range which result in high impedances and low leakage currents through the condenser 13. Typically, the C1 and C2 condenser sections 12 and 14 also have low power factor values that are normally in the 0.2% to 0.5% range as a result of the low power losses in the condenser 13.
During the normal manufacturing process, the C1 and C2 bushing sections 12 and 14 are tested for capacitance and power factor. This ensures the bushings sections 12 and 14 are manufactured without defects. In addition, field tests can be performed after the bushings are installed to detect deterioration or damage to the condenser 10. If the condenser 10 deteriorates sufficiently, the bushing 13 can fail catastrophically, which can cause consequential equipment damage or unsafe conditions for workers in the area. Often, off-line tests and on-line monitoring are performed with the goal to detect such deterioration before a failure occurs.
Bushings are commonly used with transformers or reactors. The transformer or reactor windings have self and mutual capacitances that represent the quality of the insulation for each winding to the grounded parts of the transformer, or between the different windings inside the transformer. Each winding has a capacitance to ground that is a function of the physical dimensions and clearances of the windings as well as the condition of the insulation. For the high voltage (“HV”) winding, this has traditionally been referred to as CH. For the low voltage (“LV”) winding, this has traditionally been referred to as CL. And the capacitance between the HV and LV windings has traditionally been referred to as CHL. Similar designations can be made for three-winding transformers with HV, LV and tertiary voltage (TV) windings to provide CH, CL, CT, CHL, CHT and CLT values.
Off-line tests, when the equipment is not in-service, have been performed on transformers, reactors and bushings for several years. In order to test a condenser-type bushing's inner C1 section 12, the bushing's test tap cover is removed and test voltages are applied to the center conductor 11 and the resulting current magnitude and relative phase angle are measured from the test tap 16. In order to test the outer C2 section 14, the test tap 16 is energized and the resulting current from the grounded metal flange of the bushing is measured for amplitude and relative phase angle. Off-line tests on higher voltage bushings are normally performed at significantly reduced voltage levels since it is not practical to produce the rated bushing voltages with portable field test equipment. Off-line transformer tests are performed at the same reduced test voltage levels with the voltage applied to the top terminals of the bushings and the resulting current magnitude and phase angle shift are measured to calculate capacitance and power factor values for each winding.
On-line bushing monitoring, when the equipment is energized and in-service, has been performed for a shorter time. The current art connects a passive sensor 20 to the test tap 16 and measures the resulting test tap current magnitude and phase angle when the bushing 10 is energized by the power system. As may be seen in FIG. 1, the typical passive sensor 20 included a shunt resistor or capacitor 22 (labelled “A”) and a voltage arrestor 24 (labelled “B”). This method only measures the power frequency current 26 (labelled “IP”) through the inner C1 section 12 of the condenser 13 so only the C1 capacitance and power factor can be determined. Neither the outer C2 section 14 of the condenser 13 nor the transformer winding capacitances can be monitored with the current method.
Additionally, transformers with wye winding connections may have neutral bushings that are connected to earth ground either directly or through an impedance, to stabilize the three-phase voltages and limit the phase-ground voltages to safe levels. These neutral bushings can be tested with off-line testing equipment, but neither the inner C1 section 12 nor the outer C2 section 14 of the bushing 10 can be monitored on-line with the current art since the center conductor 11 is effectively grounded and no voltage nor current are produced at the test tap 16.