Electrical power, produced at the power plant, is usually produced as three phase AC voltage. It is transmitted in the form of three phase voltage over long distance power transmission lines. Electrical power is normally distributed either in the form of single phase or three phase alternating current (AC) voltage.
SINGLE PHASE SYSTEMS
Electrical power systems are capable of supplying both three phase and
single phase loads from the same power lines
Single phase is supplied either by a two wire system or a three wire system
THREE PHASE SYSTEMS
AC voltages that are transmitted to the distribution substations at
high voltages, which must be stepped down by three phase transformers,
to meet the necessary load requirements.
THREE PHASE TRANSFORMER CONNECTIONS
There are five ways in which the primary and secondary windiness of three phase transformers may be connected.
TYPES OF THREE PHASE SYSTEMS
Three phase power distribution systems are classified according to the number of phases and number of wires required. There are three classifications.
GROUNDING OF DISTRIBUTION SYSTEMS
A ground is a reference point of zero volt potential, which is usually
a connection to earth ground. Grounding in electrical power distribution
system is very important. Distribution systems must have grounds. If a
grounded conductor is opened, the ground is no longer functional and can
present severe safety problems to the public and cause abnormal system
Distribution systems must be grounded at substations, and at the end of the power lines, before the power is delivered to the load. Grounding provides points for transformer neutral connections for equipment grounds. At substations, all external metal parts must be grounded, including the transformers, circuit breakers and even fencing around the substation. The actual ground connection is made by connecting the item (using wire colour coded green) to a grounding electrode (metal rod usually copper), which is then buried in the ground.
There are two types of grounding
(1) System grounding
(2) Equipment grounding.
System grounding is the grounding of a current carrying conductor (the
neutral). The wye system has an obvious advantage over the delta system,
since one side of each phase is connected to ground. The common terminal
of the wye system, when connected to a ground, become the neutral conductor
of the four wire system. In the delta system, since there is no common
neutral is not grounded. This introduces the problem of a ground
fault. System grounding conductors are insulated with white or grey material
for easy identification.
As the name suggest, this is grounding of equipment. The conductor that
is used for this purpose is either bare wire or a green insulated wire.
The NEC describes conditions that require fixed electrical equipment to
be grounded. All fixed electrical equipment located in industrial plants
or commercial buildings should be grounded
GROUND FAULT PROTECTION
Ground fault interrupters (GFI) are used extensively in industrial,
commercial, and residential power distribution systems. It is required
by the NEC that all 12 volt, single phase, 15- or 20 ampere receptacle
outlets that are installed outdoors or in bathrooms have ground fault interrupters
connected to them. These devices are also called ground fault circuit interrupters
(GFCls). These devices are designed to eliminate electrical shock hazard
resulting from individuals coming in contact with a hot AC line. The circuit
interrupter is designed to sense any change in circuit conditions. The
operating speed of the GFI is so fast that the shock hazard to individuals
is greatly reduced, since only a minute current opens the circuit. To understand
the need for a ground fault circuit interrupter certain basic facts must
first be understood. One important fact is that a person's body resistance
varies with the amount of moisture present on the skin, the muscular structure
of the body, and the voltage to which the body is subjected. The resistance
of the body (hand to hand) is lower for higher voltages. This is
because higher voltages are capable of "breaking down" the outer layers
of the skin.
GROUND FAULT PROTECTION FOR THE HOME
Ground fault interruptions for homes are of three types:
(1) circuit breaker,
(2) receptacle and
(3) plug-in types.
Ground fault protection devices are constructed according, to standards
developed by the Underwriters' Laboratories.
The GFI circuit breakers combine ground fault protection and circuit interruption in the same overcurrent and short-circuit protective equipment as does a standard circuit breaker. A GFI circuit breaker fits the same space required by a standard circuit breaker. It provides the same branch circuit wiring protection as the standard circuit breaker, as well as ground fault protection. The GFI sensing system continuously monitors the current balance in the live conductor and the neutral conductor. If the current in the neutral wire becomes less than the current in the hot wire a ground fault develops. When this occurs, the sensor sends a signal to the solid state circuitry, which activates a trip mechanism. This action opens the live wire, causing the circuit breaker to interrupt the circuit. GFI receptacles are available for 120V, 208V or 240 volt AC systems. The GFI receptacles come in 15 and 20 A designs. The 15A unit has a receptacle configuration for use with 15A plugs only. The 20A device has a receptacle configuration for use with either 15 or 20 A plugs. These GFI receptacles have connections for hot, neutral, and ground wires. All GFI receptacles have a two pole tripping mechanism, which breaks both the hot and the neutral load connections when a fault occurs. The plug-in GFI receptacles provide protection by plugging into a standard wall receptacle.
GROUND FAULT PROTECTION FOR POWER DISTRIBUTION EQUIPMENT
Ground faults can destroy electrical equipment. Some ground faults produce an arcing effect from relatively low currents that is not large enough to trip conventional protective devices. Arcs can severely burn electrical equipment. High current faults are quickly detected by conventional over current devices. Low current faults must be detected by GFIS. Ground faults that cause an arcing effect in the equipment are probably the most frequent faults. They result from damaged or deteriorated insulation, dirt, moisture, or improper connections. They usually occur between one hot conductor and the grounded equipment enclosure, conduit, or metal housing. The impedance of the conductor and the ground-return path depends on many factors. As a result, the fault current value cannot be predicted.
Ground current relaying is one method used to protect equipment from
ground faults. Current flows through a load or fault along the hot
and neutral conductors and returns to the source on these conductors and
to some extent, along the ground path. The normal ground path current
is very small. If a ground fault occurs, the ground current will
increase and current will flow through the fault and return via the ground
path. As a result, the current returning on the hot and neutral conductors
is less than the amount going out. The difference is an indication
of the amount of current in the ground path. A relay, which senses
this difference in currents, can act as a ground fault protective device.
GROUND FAULT PROTECTION FOR ELECTRIC MOTORS
Motor protective systems offer protection in the 5- to 100 ampere range.
This type of ground fault protective system offers a protection against ground faults in both the single phase and the three phase systems.