EDS - Electric Distribution Systems
Who Should Attend?
It is imperative that managers not send people to this class who do not have formal electrical training in the form of an engineering degree, an electronics degree, completion of an apprenticeship program, or completion of PTS's Electric Utility System Operation class.
The EDS class is targeted to those employees and their support staff who have direct responsibility for the construction, operation and maintenance of an electric utility's distribution system. This includes line workers, substation workers, recent electrical engineering graduates, and apprentices in all of the electrical trades. It is a continuation of PTS's 3-day Electric Utility System Operation (EUSO) class.
What Does the EDS Class Cover?
The first day of the EDS class starts with a discussion regarding distribution system reliability based around the distribution substation. Significant emphasis is placed on the substation transformer. A complete description is provided of a transformer sealing system used to prevent water from getting inside, and the sealing systems used in bushings. Preventative maintenance testing of the transformer includes turns ratio testing, dissolved gas analysis, power factor testing, hi-pot testing and partial discharge testing.
Safety aspects inside a distribution system include thoroughly discussing both step-potential and touch-potential.
EDS covers the various systems that a customer might request including single-phase 120/240, and 3-phase systems that include 120/240 delta, 120/240 open delta, 208Y/120, and 480Y/277. It covers the principles behind selecting a service including the advantages, disadvantages and limitations of each of the services. In the open delta system, the wild leg (also called the power leg, the bastard leg, the stinger, and the high leg) is described including the danger it poses to customer equipment. The reliability aspects regarding transformers connections include open neutrals that result high voltages that destroy electronics, and stray voltages associated with bad connections and with bad grounding.
The details of both overhead and underground distribution systems are covered. Particular emphasis is placed on the necessity of shield cables above 5 kV. Various types of shielding are described including concentric neutrals. The design concept behind an open-loop underground distribution system is described showing how it can be used to quickly and easily isolate faults and then restore power to customers. The role of fault-current indicators is emphasized in this process.
The design philosophy behind ground fault circuit interrupters is described and how they can cause customers numerous headaches until they realize how they operate, and how they are connected.
Faults are explained. Students see how fault current can be many times more than normal full load current. The role of transformer impedance in determining fault current is covered in the area of fault current studies. Equipment fault current bracing is covered.
The reliability aspects on the utility side of the meter include open neutrals and resulting high voltages that destroy electronics, stray voltages, and lightning and switching surges. It covers sizing conductors for voltage drop. The basics of fault current studies and coordinating protective devices are included.
The second day of the EDS class covers transformer polarity, vector analysis, and the role of transformer polarity in determining the phase-shift within a transformer. The cause, characteristics and prevention of ferroresonance is also included in this part of the EDS class.
Sizing conductors for load and for voltage drop is discussed. The reliability aspects associated with the line equipment, including reclosers, sectionalizers, cutouts, series reactors, and current-limiting fuses, are discussed in their application in performing a coordination study.
Switching surges caused when disconnecting underground cables or when disconnecting capicitors are covered along with solutions to the problem. Capacitors for voltgae control and the setting of a voltage regulator is covered.
Emphasis is placed on the harmonics caused by high-speed electronic switching in equipment such as computers, fax and copy machines, dimmers, electronic ballasts in lighting fixtures and variable speed drives. Identifying harmonics and resolving the problems harmonics cause are covered.
The class ends with substation communication systems that include fiber optics, microwave, carrier current using coupling capacitors and wave traps, cell phones and dedicated telephone lines are covered. Transfer trip systems are also included in this area.