Loadbreaking and the Bazooms Curve
Carlo B. DeLuca, PE, MBA
This course will
discuss power connectors capable of manual live connection and disconnection
of high voltage, high amperage circuits. This connector system offers a versatile
and economical application to convenient load switching operations in confined
areas. Some applications will be discussed.
Originally, the objective was to connect and disconnect 15 kV primary distribution circuits with switching voltages at 8.3 kV; however, the voltages were later increased to include 25 and 35 kV. Connector manufacturers not only had to be thoroughly concerned with long term connection at amperages of at least 200 amperes, but also had to study the switching performance and details of the connector itself. The connector is designed to be manually connected and disconnected at the end of a hot stick (an insulated live line pole) by a lineman.
Our discussion analyzes the performance of the switching system based on data received from power testing. It will describe some constraints that had to be applied to performance and standards of performance for the connector system. Basically, the requirements were to be able to switch at the high voltage and high currents for a prescribed amount of switching and also be able to safely connect (fault close) a high current (10,000 amperes RMS) in the event of a fault. Even though the connector was not required to be used again, this operation had to be done safely by a lineman. As we continue through the discussion, it will also be realized that there are other constraints that the user (primary electric utilities) would require from this connecting system. The analysis of the fault close operation was a key in evaluating the connector system inasmuch as most of the early failures of the system occurred during fault close. A unique system is described to analyze real time occurrences at the point that the connector connects fault close at the higher voltages, and also to determine the most critical area. In addition, equipment is described to be able to produce such a connection with a good sense of predictability in order to limit the amount of samples being tested.
The body of the course is outlined as follows:
1. Loadbreak connector
A cutaway of a loadbreak connector system is presented in Fig. 1. Critical elements are described and pointed out using this illustration.
2. Standards of
operation and performance.
Requirements for standards are described including the voltages and currents to be used in performance. Fig. 2 depicts a cutaway of a bushing and elbow at a position where contacts are separated in order to give the reader an understanding of the operations within the bushing itself.
3. Problem analysis
and the Bazooms Curve.
Fig. 3 has shown how it was developed to analyze the movement of the elbow/bushing assembly during fault close. In addition, it illustrates the critical areas of fault close in relation to the velocity and position of the elbow and bushing at contact separation.
Fig. 4 is an analysis curve developed from Fig. 3, illustrating contact separation on load disconnection. It describes the voltage and current phase lag relating to the position and velocity of connector separation upon disconnection. Through its illustration, it presents a logical explanation of another problem encountered.
4. Definition of
problems and solution.
A review of the problems, problem symptoms and the graphical analysis is shown to disclose not only some of the basic causes of problems, but also suggests resolutions of these problems. Two proven approaches are shown as a resolution and a third is also suggested.
Present and known applications are enumerated. Additional applications are suggested. Sample product catalogue pages are provided.
This course includes
a multiple choice quiz at the end, which
is designed to enhance the understanding of the course materials.
which is designed to enhance the understanding of the course materials.
This course will assist the students' learning by:
This course is intended for engineers, architects and contractors engaged in construction power distribution of large facilities, particularly over long distances. It is also helpful for construction requiring many power switching points and, of course, for utility power engineers and technicians in becoming more familiar with loadbreaking. It can also be of value to engineers and designers in demonstrating approaches to performance analysis of products using simple graphical analysis. The reader should have a familiarity with AC circuits and power factor.
Benefit to Attendees
The paper enables engineers, architects and contractors to become more aware of a new and economical approach to power distribution. Although the systems have been used in power distribution with power utilities, many engineers, architects and contractors may not be totally familiar with the application of this versatile connector system. In addition, it will benefit the reader by providing another viewpoint on analysis of design and performance in integrating switching performances using velocity, contact position and voltage/current wave forms.
During the 1960's,
underground power distribution was becoming more popular, replacing overhead
distribution in many areas. During that time, the First Lady of our nation,
Lady Bird Johnson, was on a very strong, influential program to beautify America.
Part of her program was to replace overhead power distribution and install underground
wiring to eliminate unsightly poles and wires along the roadways. Obviously
this was limited because of the terrain in many parts of our country but nevertheless
the idea became quite popular. In reality, the major influence was the desire
on the part of utilities to provide more reliable electrical power. This need
was particularly recognized with the tremendous growth of power usage and storm
Secondary distribution (600 v and less) was being accomplished; however, the need for primary underground distribution was more difficult. Due to the voltage involved, primary voltage conductors were of a special nature. Safety codes and local requirements required the cables to be externally shielded at ground potential so that any static electricity or build up of electromotive force on the outside of the cable would not be a factor. This was particularly true for cables that could be directly buried but also was applied to cables placed in proximity to each other in ducts. This type of construction made conductors susceptible to failures until better insulation structures were developed. While insulation systems were vastly improved, however, it was still difficult and time consuming to detect failure locations of conductors, particularly when they were directly buried. Distribution circuits were then devised to allow disconnection of the faulted circuits from the remainder of the distribution network, thus allowing power to the rest of the users while locating the fault on the isolated faulted circuit. Many distribution circuits provided a radial distribution configuration to achieve this. This requires switching at the transformers to bypass the faulted circuit. For each transformer therefore, switches were needed at voltage ratings from 15 kV to 35 kV with loads of 200 amperes RMS. It was also realized that the connection could be accidentally made to a faulted circuit. Thus, the requirement was imposed to safely connect a faulted circuit with currents as high as 10,000 amperes.
Connector manufacturers realized a particularly attractive market for connectors that could not only connect the distribution conductor to the transformers, but also be able to be easily connected and disconnected under load. This would reduce and in some cases eliminate the need for costly switchgear. Product development progressed and early developments produced patents to such a connector system in the 1970's. The product consisted of a male connector called an elbow to be connected to a female connector called the bushing. The bushing is mounted and electrically connected to the transformer. The elbow can be manually removed from the bushing to disconnect the transformer, and also provide junctioning to the rest of the distribution net. Not only were the bushings provided in a configuration to be mounted within the transformer, but also provided as busses in order to bypass transformers or other parts of the circuit.
Initial field use revealed the use for additional performance requirements. Originally, the bushings were vented to the inside chambers of the transformer. This allowed gasses to contaminate transformer oil. Also, fault close tests were originally performed on samples that did not experience load switching. When unvented, seasoned connectors were tested, many failed to safely connect a fault. This prompted further investigation and greater development with more complex designs.
Course Content The
course content is in a PDF file (96 KB) Loadbreaking
and the Bazooms Curve. You need to open or download this document to
study this course.
The course content is in a PDF file (96 KB) Loadbreaking and the Bazooms Curve. You need to open or download this document to study this course.
problems can be approached with the same basic procedures. First, identify the
problem. Then define the problem. Determine the performance that the solution
requires. Invent the solutions and test for performance and re-invent until
the solutions meet the requirements. Just as graphical analysis solves complex
mathematical problems, it can also solve less complex engineering challenges.
When instrumenting devices to be observed, the environment of testing and performances
must be carefully and cautiously approached. We used the "bazooms"
curve to achieve solutions and particularly enjoyed naming it.
The development of loadbreak connectors followed this pattern directly and provided a valuable contribution to power distribution for underground and construction programs.
Shurter and DeLuca, May 1974.
Patent #3,917,374; Murdock, November 1975.
Patent #3,982,812; Boliver, September 1976.
Patent #4,068,912; Stanger et al, January 1978.
Patent #5,266,041; DeLuca, November 1993.
Basic Electrical Engineering, McGraw Hill, Second Edition, 1957.
IEEE 592-1977, IEEE Standard for exposed semi-conducting shield on pre-molded high voltage cable joints and separable insulated connectors.
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.