Note: Descriptions are shown in the official language in which they were submitted.
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GROUND FAULT PROTECTION CIRCUIT FOR A
MULTIPLE SOURCE SYSTEM
F1TLD OF ~ '
This invention relates generally to ground fault protection circuits for
electrical distribution equipment. and more particularly, to a ground fault
protection
circuit for switchboards and switchgear having multiple sources and grounds.
Ground fault protection ("GFP") circuits are commonly used for providing
automatic circuit interruption upon detection of undesired short circuit
currents
which flow as a result of a ground fault condition in electrical power
distribution
systems. Such GFP circuits ordinarily include means for Quickly sensing and
individually isolating any faults occurring in a respective branch circuit of
the power
distribution systems and utilize selective coordination to instantly respond
and
intemtpt power only to the system area where a fault occurs thereby preventing
unnecessary loss of power to other areas. One example of such a GFP circuit is
described in U.S. Patent No. 3,259,802 entitled "Ground Fault Responsive
Protective
System for Electric Power Distribution Apparatus." Protective circuits of this
type
utilize zone protective interlocking to be capable of selectively clearing a
fault in a
specific system area without interrupt interrupting power to other sections of
the
system. In such GFP circuits. a restraining signal is transmitted from a
downstream
circuit breaker (remote for the source) to an upstream circuit breaker (closer
to the
source) instructing that circuit breaker not to trip, permitting the
downstream circuit
breaker to trip and isolate the fault. Thus, when a ground fault occurs on a
feeder
circuit, the main circuit breaker is prevented from interrupting while
allowing a circuit
breaker on the feeder circuit to interrupt. The main electric power bus will
consequently remain in service while the faulted feeder circuit is interrupted
and the
fault is isolated.
GFP circuits of this type are quite effective in single source electrical
distribution systems where only one circuit breaker is required to trip and
clear the
fault. However, as modern power distribution systems become increasingly
complex
and use multiple power sources and current paths. such systems require added
circuit
breakers for adequate circuit protection. More complex ground fault protection
circuits are consequently required.
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Other types of GFP circuits using secondary circuits have been designed to
accommodate multiple sources; circuits of this type are disclosed in U. S.
Patent Nos.
3,949,272, 4,068,275 and 4,110,808. Such circuits utilize a secondary circuit
for
routing control or tripping currents to actuate ground fault relays and cause
designated circuit breakers to trip thereby interrupting power only to the
portion of
the primary circuit which has a ground fault. However, a disadvantage to these
protective circuits is that time-current trip coordination or zone selective
interlocking
schemes are required to isolate a ground fault.
Another drawback with these protective circuits is that auxiliary control
contacts must be used to control the circuit breakers. In particular, a
disadvantage
with the protective circuit shown in U.S. Patent No. 4, I 10,808 is the
difficulty in
coordinating the specific circuit breaker to be tripped. This causes more
circuit
breakers to trip during a ground fault than is required, thereby causing more
loads to
lose power than is necessary. Additionally, circuits of this type do not allow
all of the
circuit breakers to be closed during normal operations. Circuits of the type
shown in
U.S. Patent No. 4,068,275 have the disadvantage of being expensive because
they
require more equipment, such as current transformers, to isolate and interrupt
the
power in the section of the system where the ground fault occurs. A drawback
of
circuits of the type shown in U.S. Patent No. 3,949,272 is the inability to
utilize the
advance capabilities of some of the modern day circuit breakers.
Accordingly, there is a distinct need to provide an improved ground fault
protection circuit which overcomes the limitations of the prior art and is
capable of
effectively protecting electrical power distribution systems having multiple
sources,
loads and grounds while, at the same, providing sufficient zone selectivity to
prevent
needless loss of power.
The present invention provides an improved ground fault protection system
for protecting an electrical distribution system having multiple sources and
multiple
grounds. In accordance with a preferred embodiment, an electrical power
distribution circuit having a primary electrical circuit for distributing
electrical power
from a plurality of sources to a plurality of loads is provided with a ground
fault
protection circuit magnetically coupled to the primary circuit. The primary
electrical
circuit includes breakers for interrupting power from flowing in the buses.
The
ground fault protection circuit includes current sensors, tripping functions
or ground
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fault relays associated with each circuit breaker and a novel use of an
auxiliary
transformer. When a ground fault occurs, the ground fault protection circuit
is capable
of sensing and determining the specific bus in which a ground fault condition
exists. It
then selectively sends a trip current generated by the associated current
sensor only to
the proper tripping functions for tripping the appropriate circuit breakers in
the
affected bus in order to isolate the ground fault; this avoids unnecessary
tripping of
circuit breakers in bus portions which are not affected by the ground fault.
The
auxiliary transformer uniquely sends the trip current to portions of the
ground fault
protection circuits that would not have been connected, or accessible, in the
ground
fault protection circuits of the prior art. The use of the auxiliary
transformer i)
eliminates the need for auxiliary contacts, ii) allows all of the circuit
breakers in the
primary circuit to be originally closed and iii) eliminates the need for time-
current trip
coordination and zone selective interlocking schemes for preventing
unnecessary
tripping of circuit breakers in bus portions which are not affected by the
ground fault.
In accordance with one aspect of the invention there is provided a ground
fault protection circuit for an electrical power distribution system having i)
a plurality
of polyphase power sources, each including a plurality of phase conductors and
a
neutral conductor, ii) a polyphase main bus connected to each one of the power
sources and including a plurality of phase conductors and a neutral conductor,
iii) a
main circuit breaker electrically connected in each of the main buses for
interrupting
power flowing therethrough, iv) a polyphase tie bus connected between each of
the
main buses and including a plurality of phase conductors and a neutral
conductor, v) a
tie circuit breaker electrically connected in each of the tie buses for
interrupting power
flowing therethrough the ground fault protection circuit comprising a current
sensor
associated with each one of the main and tie buses having a pair of output
terminals
and located adjacent to each one of the main and tie buses for generating a
trip current
through the output terminals that varies directly with the vector sum of the
currents
flowing through the phase conductors and the neutral conductor as the location
of the
current sensor; a trip function associated with each one of the main and tie
circuit
breakers, wherein current flowing through the trip function causes its
associated
circuit breaker to trip, and an auxiliary transformer for sending the trip
current to a
portion of the ground fault protection circuit so that the appropriate
tripping function
receives the trip current and causes its associated circuit breaker to trip
when a ground
fault occurs in the power distribution system.
In accordance with a further aspect of the invention there is provided a
ground
fault protection circuit magnetically coupled to an electrical power
distribution system
having i) a plurality of polyphase power sources, each including a plurality
of phase
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conductors and a neutral conductor, ii) a plurality of polyphase main buses,
each
including a plurality of phase conductors and a neutral conductor, iii) a
plurality of
main circuit breakers, wherein each one of the plurality of power sources and
main
buses having one of the plurality of main circuit breakers electrically
connected
between the phase conductors thereof for interrupting power flowing
therethrough, iv)
a plurality of polyphase tie buses, each having a plurality of phase
conductors and a
neutral conductor, wherein each one of the plurality of tie buses connecting
one of the
plurality of main buses to another one of the plurality of main buses, v) a
plurality of
the circuit breakers, wherein, each one of the plurality of tie buses having
one of the
plurality of tie circuit breaker connected therein for interrupting power
flowing
therethrough, the ground fault protection circuit comprising a plurality of
current
sensors, each having a pair of output terminals, for magnetically coupling the
ground
fault protection circuit to the plurality of main and tie buses, each one of
the plurality
of main and tie buses having one of the plurality of current sensors located
adjacent
thereto for generating a trip current through the output terminals that varies
directly
with the vector sum of currents flowing through the phase conductors and the
neutral
conductor at the location of the current sensor; a plurality of trip
functions, each
associated with one of the plurality of main and tie circuit breakers, wherein
current
flowing through a particular one of the plurality of trip functions causes its
associated
circuit breaker to trip; and an auxiliary transformer having a primary coil
and a
secondary coil for dividing the ground fault protection circuit into a first
portion and a
second portion, wherein the auxiliary transformer isolating the first portion
from the
second portion; wherein the plurality of current sensors, the plurality of
trip functions
and the auxiliary transformer being so arranged that in response to a ground
fault in
the power distribution system, the trip current is routed through the tripping
functions
that are operable to produce tripping of each of the plurality of circuit
breaker that are
required to be tripped to isolate the ground fault; wherein one of the
plurality of trip
functions is connected in parallel to the secondary winding and is associated
with one
of the plurality of tie circuit breaker, wherein the trip current only flows
through the
secondary winding when the one of the plurality of tie circuit breakers is
required to
trip in order to isolate the ground fault.
In accordance with a further aspect of the invention there is provided an
electrical power distribution system for transmitting current from a first
power source,
a second power source and a third power source to a plurality of loads, the
electrical
power distribution system comprising an electrical power distribution circuit
comprising a first main bus and a first main circuit breaker electrically
connected
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between the first power source and the first main bus; a second main bus and a
second
main circuit breaker electrically connected between the second power source
and the
second main bus; a third main bus and a third main circuit breaker
electrically
connected between the third power source and the third main bus; a first tie
bus
connected between the first main bus and the second main bus; a second tie bus
connected between the second main bus and the third main bus; a first tie
circuit
breaker electrically connected in the first tie bus for interrupting current
flowing in the
first tie bus; a second tie circuit breaker electrically connected in the
second tie bus
for interrupting current flowing in the second tie bus; and a ground fault
protection
circuit magnetically coupled to the electrical power distribution circuit
having a trip
current flowing therethrough only in response to a ground fault condition in
the
electrical power distribution circuit, the ground fault protection circuit
comprising a
first current sensor having first and second output terminals and located
adjacent the
first main circuit breaker for generating the trip current through the output
terminals
that varies directly with the vector sum of the currents flowing through the
first main
bus; a second current sensor having first and second output terminals and
located
adjacent the second main circuit breaker for generating the trip current
through the
output terminals that varies directly with the vector sum of the currents
flowing
through the second main bus; a third current sensor having first and second
output
terminals and located adjacent the third main circuit breaker for generating
the trip
current through the output terminals that varies directly with the vector sum
of the
currents flowing through the third main bus; a fourth current sensor having
first and
second output terminals and located adjacent the first tie circuit breaker for
generating
the trip current through the output terminals that varies directly with the
vector sum of
the currents flowing through the first tie bus; a fifth current sensor having
first and
second output terminals and located adjacent the second tie circuit breaker
for
generating the trip current through the output terminals that varies directly
with the
vector sum of the currents flowing through the second tie bus; a first trip
function
associated with the first main circuit breaker wherein current flowing through
the first
trip function causes the first main circuit breaker to trip; a second trip
function
associated with the second main circuit breaker wherein current flowing
through the
second trip function causes the second main circuit breaker to trip; a third
trip
function associated with the third main circuit breaker wherein current
flowing
through the third trip function causes the third main circuit breaker to trip;
a fourth
trip fiznction associated with the first tie circuit breaker wherein current
flowing
through the fourth trip function causes the first tie circuit breaker to trip;
a fifth trip
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function associated with the second tie circuit breaker wherein current
flowing
through the fifth trip function causes the second tie circuit breaker to trip
and an
auxiliary transformer, having a primary coil and secondary coil, wherein the
fifth trip function is connected in parallel to the secondary coil; wherein i)
the first
output terminal of the first current sensor is coupled to one side of the
fourth trip
function, the first output terminal of the second current sensor, the first
output
terminal of the third current sensor, one side of the fifth trip function and
one side of
the secondary coil of the auxiliary transformer, ii) the second output
terminal of the
first current sensor is coupled to the first output terminal of the fourth
current sensor
and one side of the first trip function, iii) the second output terminal of
the fourth
current sensor is coupled to the first output terminal of the fifth current
sensor, the
second output terminal of second current sensor and one side of the primary
coil of
the auxiliary transformer; iv) the other side of the primary coil of the
auxiliary
transformer is coupled to one side of the second trip function; v) the other
side of the
second trip function is coupled to the other side of the first trip function
and the other
side of the fourth trip function; vi) the second output terminal of the fifth
current
sensor is coupled to one side of the third trip function and the second output
terminal
of the third current sensor and vii) the other side of the third trip function
is coupled to
the other side of the fifth trip function and to the other side of the
secondary coil of
the auxiliary transformer.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will be apparent from the
following detailed description and the accompanying drawings in which:
FIG. 1 is a schematic representation of a representative power distribution
system having a ground fault protection circuit according to a preferred
embodiment
of the present invention; and
FIGS 2A-2D are schematic representations of the power distribution system
shown in FIG. 1, each representing an individual step in analyzing the
circuit.
While the invention is susceptible to various modifications and alternative
forms, a specific embodiment thereof has been shown by way of example in the
drawings and will be described in detail. It should be understood, however,
that is not
intended to limit the invention to the particular form described, but on the
contrary,
the invention is to cover all modifications, equivalents, and alternatives
failing within
the spirit and scope of the invention as defined by the appended claims.
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DETAI1 ED DEC 1PTION OF T E P EFE RFD EMBODIMENT
For a better understanding of the present invention together with other and
further advantages, and capabilities thereof, reference is made to the
following
disclosure and appended claims in connection with the above-described
drawings.
FIG. 1 shows a one-line schematic diagram of a three-phase, four-wire
electrical power distribution system 10 in accordance with the preferred
embodiment.
The system 10 includes a primary circuit and a secondary or ground fault
protection
circuit in accordance with the preferred embodiment of the present invention
for
protecting the primary circuit against currents caused by ground fault
conditions that
may occur in the system 10. The primary circuit consists of three main buses
12, 14
and 16 and two tie buses 18 and 20 interconnecting the main buses. Power
sources,
such as three-phase grounded-neutral transformers 22, 24, 26, supply power to
loads
28, 30, 32 which are respectively connected to the main buses 12, 14 and 16
through
main switches or circuit breakers M1, M2 and M3, respectively, when the main
circuit breakers are closed. Opening of the main circuit breaker M1
disconnects the
source 22 from its associated bus 12, opening the main circuit M2 disconnects
the
source 24 from its associated bus I4 and opening the main circuit breaker M3
disconnects the source 26 from its associated bus 16. The tie bus 18 contains
a tie
switch or circuit breaker T 1 that, when closed, connects the main buses 12
and 14
together and, when open, opens the tie bus 18 to disconnect the main buses
from
each other. The tie bus 10 contains a tie switch or circuit breaker T2 that,
when
closed; connects the main buses 14 and 16 together and, when open, opens the
tie
bus 20 to disconnect the main buses from each other. The main buses 12, 14 and
16
and the tie buses 18 and 20 consists of three phase conductors respectively
designated as 12a, 14a 16a, 18a and 20a and a neutral conductor respectively
designated as 12N, 14N, 16N, 18N, and 20N.
The sources 22, 24 and 26 are shown as each comprising a transformer
secondary having three phase windings connected in wye configuration with
their
neutral points solidly grounded at 22a, 24a and 26a, respectively. The neutral
point
of source 22 is connected to the neutral conductor 12N, while the neutral
point of
source 24 is connected to the neutral conductor 14N and the neutral point of
source
26 is connected to the neutral conductor 16N.
The ground fault protection circuit in accordance to the preferred
embodiment of the present invention is magnetically coupled to the primary
circuit
and is provided to protect the system 10 from ground faults that may occur in
the
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system. For example, if a ground fault occurs on the main bus 12, it is
usually
necessary to open the main circuit breaker M I . and the tie circuit breaker T
I to isolate
the fault from the rest of the system. Under such circumstances, the remaining
circuit
breakers M2, M3, and T2 should remain in the position they were in prior to
the
occurrence of the ground fault to permit uninterrupted power from the sources
24
and 26 to continue over the sound buses 14, 16 and 20.
Similarly, if a ground fault should occur on the main bus 14, the main circuit
breaker M2 and the tie circuit breakers TI and T2 should be opened to isolate
the
fault from the remaining buses, while the main circuit breakers M 1 and M3
should
remain closed to maintain power from the sources 22 and 26, respectively, to
the
main buses 12 and 16, respectively.
The ground fault tripping function of the main circuit breakers M1, M2 and
M3 and the tie circuit breakers T I and T2 are controlled by trip functions or
ground
fault relays represented as GFM 1, GFM2, GFM3, GFT I and GFT2, respectively.
Effective energization of these ground fault relays (i.e., energization by
current
flowing therethrough above a predetermined level) causes the associated
circuit
breaker to open, if it is closed. For example, if current in excess of a
predetermined
level flows through the ground fault relay GFM 1, then main circuit breaker M
I will
open.
In accordance with the principles of the present invention, effective
energization of the ground fault relays GFM 1, GFM2, GFM3, GFT 1 and GFTZ is
controlled by current flowing through the ground fault protection circuit. The
ground fault protection circuit includes the ground fault relays, current
sensors 34,
36, 38, 40 and 42 and an auxiliary transformer AT. The current sensors are
responsive to the vector sum of the currents flowing through the primary
conductors
at the location of the individual sensor. The auxiliary transformer AT
preferably has
a transformation ratio of 1:1 or 5:5.
Each of the current sensors consist of four current transformers (not shown)
for sensing the current flowing in its associated phase and neutral buses.
These
current transformers develop current signals representative of the currents
flowing in
the phase and neutral buses for separate application to the ground fault
protection
circuit. The four current transformers having primary windings coupled to the
three
phases and neutral and secondary windings connected in parallel, and the
parallel
combination has output terminals which are coupled in series with the ground
fault
protection circuit. The current sensors develop a current through their
terminals that
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is substantially proportional to the vector sum of the currents flowing
through theta
three primary conductors and the neutral conductor at the location of the
current
sensor. So long as this vector sum is zero, the current sensor develops no
effective
secondary current through its terminals, but this secondary current increases
as this
vector sum increases. Although the preferred embodiment is shown utilizing
current
sensors having four current transformers, it should be understood that current
sensors
utilizing any number of current transformers may be utilized so long as the
same
principles of current sensing is followed. That is, the current sensor
produces no
effective secondary current through its terminals when the vector sum is zero
and
produces a secondary current when the vector sum is not zero.
The polarities of the current sensors 34, 36, 38, 40 and 42 and the auxiliary
transformer AT are indicated by the square black dots (polarity mark) adjacent
the
windings. More specifically, when primary current enters a given primary
winding
through the black dot adjacent this primary winding, secondary current leaves
the
associated secondary winding through the black dot adjacent the secondary
winding.
When the direction of the primary current is reversed, the direction of the
secondary
current is correspondingly reversed.
The current sensor 34 is located in the region of the main circuit breaker Ml
and having four current transformers (not shown) having primary windings
coupled
to the main bus 12 adjacent to the main circuit breaker M 1. The secondary
windings
are connected in parallel, and the parallel combination having terminals 34a
and 34b
coupled to nodes 44 and 46, respectively, in the ground fault protection
circuit. The
current sensor 34 develops a current through the terminals 34a and 34b,
however,
this secondary current increases as this vector sum increases.
For sensing the vector sum of the current through the main bus 14, the
current sensor 36 is provided in the region of the main circuit breaker M2.
The
current sensor 36 comprises four current transformers (not shown) having
primary
windings coupled to the main bus 14 adjacent to the main circuit breaker M2.
These
secondary windings are connected in parallel, and the parallel combination
having
terminals 36a and 36b coupled to nodes 44 and 47, respectively, in the ground
fault
protection circuit. The current sensor 36 develops a current through the
terminals
36a and 36b that is substantially proportional to the vector sum of the
currents
through the main bus 14 at the location of the current sensor. So long as the
vector
sum is zero, current sensor 36 does not develop any effective secondary
current
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through the terminals 36a and 36b; however, this secondary current increases
as this
vector sum increases.
For sensing the vector sum of the current through the main bus 16, the
current sensor 38 is provided in the region ofthe main circuit breaker M3. The
current sensor 38 comprises four current transformers (not shown) having
primary
windings coupled to the main bus 16 adjacent to the main circuit breaker M3.
These
secondary windings are connected in parallel, and the parallel combination
having
terminals 38a and 38b coupled to nodes 48 and 50, respectively, in the ground
fault
protection circuit. The current sensor 38 develops a current through the
terminals
38a and 38b that is substantially proportional to the vector sum of the
currents
through the main bus 16 at the location of the current sensor. So long as the
vector
sum is zero, current sensor 38 does not develop any effective secondary
current
through the terminals 38a and 38b, however, this secondary current increases
as this
vector sum increases.
For sensing the vector sum of the current through the tie bus 18, the current
sensor 40 is provided in the region of the tie circuit breaker T1. The current
sensor
40 comprises four current transformers (not shown) having primary windings
coupled
to the tie bus i 8 adjacent to the tie circuit breaker T1. These secondary
windings are
connected in parallel, and the parallel combination having terminals 40a and
40b
coupled to nodes 46 and 52, respectively, in the ground fault protection
circuit. The
current sensor 40 develops a current through the terminals 40a and 40b that is
substantially proportional to the vector sum of the currents through the tie
bus 18 at
the location of the current sensor. So long as the vector sum is zero, current
sensor
40 does not develop any effective secondary current through the terminals 40a
and
40b; however this secondary current increases as the vector sum increases.
For sensing the vector sum of the current through the tie bus Z0, the current
sensor 42 is provided to the region of the tie circuit breaker T2. The current
sensor
42 comprises four current transformers (not shown) having primary windings
coupled
to the tie bus 20 adjacent to the tie circuit breaker T2. These secondary
windings are
connected in parallel, and the parallel combination having terminals 42a and
42b
coupled to nodes 52 and 50, respectively, in the ground fault protection
circuit. The
current sensor 42 develops a current through the terminals 42a and 42b that is
substantially proportional to the vector sum of the currents through the tie
bus 20 at
the location of the current sensor. So long as the vector sum is zero, current
sensor
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42 does not develop any effective secondary current through the terminals 42a
and
42b; however, this secondary current increases as this vector sum increases.
The ground fault relay GFT 1 has one side coupled to the terminal 34a of the
current sensor 34 at the node 44 and its other side coupled between one side
of the
ground fault relays GFM 1 and GFM2 at a node 54. The other side of the ground
fault relay GFM1 is coupled to the terminal 34b of the current sensor 34 and
the
terminal 40a of the current sensor 40 at the node 46. The other side of the
ground
fault relay GFMZ is coupled to one end of the primary winding of the auxiliary
transformer AT. The other end of the primary winding of the auxiliary
transformer
AT is coupled to the terminal 40b of the current sensor 40, the terminal 42a
of the
current sensor 42 and the nodes 47 and 52.
One end of the secondary winding of the auxiliary transformer AT is coupled
to the terminal 34a of the current sensor 34, the terminal 38a of the current
sensor
38, one end of the ground fault relay GFT2, a node 60 and the nodes 44 and 48.
The
other end of the secondary winding of the auxiliary transformer AT is coupled
to the
other end of the ground fault relay GFT2, one end of the ground fault relay
GFM3
and a node 62. The other end of the ground fault relay GFM3 is coupled to
terminal
38b of the ground fault sensor 38 and the terminal 42b of the ground fault
relay 42 at
the node 50.
The operation of the ground fault protection circuit under various conditions
will now be described. In analyzing where the currents will flow in the ground
fault
protection circuit, it must be remembered that a) for a significant current to
flow in a
given transformer secondary winding, there must be a corresponding current
flow in
its primary winding and (b) Kirchofl's first law must be satisfied at each
junction
point or node (i.e., the algebraic sum of all instantaneous currents at each
such node
must equal zero).
In analyzing the ground fault protection circuit, certain assumptions must be
made, for example an assumption must be made as to what circuit breakers are
closed
or open and where the current is flowing over the main and tie buses. In the
figure,
the direction the current is flowing is represented by arrows and the
magnitude is
represented in a per unit basis and indicated adjacent to the arrows. FIG. 2A
shows
that we assume that all of the circuit breakers are closed and that 1 unit of
current is
flowing from the source 22 on the main bus phase conductors 12a, 0.5 unit of
current
is flowing from the source 24 on the main bus phase conductors 14a and 0.5
unit of
current is flowing from the source 26 on the main bus phase conductors 16a.
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Additionally, we assume that a ground fault condition, designated as GF, is
located
on the main bus 14 and consists of 2 units of current. We have a total of 2
units of
current going into the system I 0 from the sources and 2 units of current
leaving the
system in the form of a ground fault. The summation of the currents flowing
into and
out of the system 10 is zero.
According to Kirchoff's first law, if current is flowing from the sources,
then
current must return to the sources and whatever current returns to the source
must
equal that which is going out. In electrical power distribution systems, there
a are
many paths over which the current may flow as it is returning back to the
source. For
example, the current may flow in the sheet metal of the enclosure or the
neutral
conductor. In the system shown, it is important to consider those currents
which may
flow through the neutrals.
FIG. ZB shows the assumption we make in analyzing the system 10 of how
the ground fault current may re-enter the system and return to the sources.
Let us
assume that 0. 5 unit of current flows into the grounded neutral point 22a of
the
source 22, 1.5 units of current flow into the grounded neutral point 24a of
the source
24 and zero units current flow into the grounded neutral point 26a of the
source 26.
Thus we have 2 units of current flowing back into the system 10, which
balances the
2 units of current flowing out of the system through the ground fault GF. The
current going into each source must equal the current going out. If the
current
entering a particular grounding point is not in balance with the current going
into the
source, then the excess or deficiency must come from the neutral conductor.
Therefore, the currents must flow in the neutral conductors from the neutral
conductor. Therefore, the currents must flow in the neutral conductors 12N,
14N,
16N, 18N and 20N as shown to balance all of the currents returning to the
sources.
For example, 1 unit of current must flow into the neutral conductor 14N and
then
split at a node 56 so that 0.5 unit of current flows in the neutral conductors
18N and
20N.
FIG. 2C shows the currents flowing through the main conductors 12a, 14a,
16a, 18a and 20a at each of the current sensors and indicates the currents
leaving the
current sensors. For example, 1 unit of current is flowing into the polarity
mark of
the current sensor 34 through the phase current conductors 12a and 0.5 unit of
current is flowing away from the polarity mark of the current sensor 34
through the
neutral conductor 12N. The sum of these two currents is 0.5 unit of current
flowing
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into the polarity mark thereby causing 0.5 unit of current to flow from the
secondary
side of the current sensor 34 at the terminal 34a.
At the current sensor 36, 0.5 unit of current is flowing into the polarity
mark
through the phase conductors 14a and 1 unit of current is flowing into the
polarity
mark through neutral conductor 14N. Therefore, 1.5 units of current are
flowing
from the terminal 36a of the current sensor 36.
At the current sensor 38, 0.5 unit of current is flowing into the polarity
mark
through the phase conductors 16a and 0.5 unit of current is flowing away from
the
polarity mark through neutral conductor 16N. Therefore, no current is flowing
from
the secondary side of the current sensor 38.
At the current sensor 40, 1 unit of current is flowing into the polarity mark
through the phase conductors 18a and 0.5 unit of current is flowing away from
the
polarity mark through neutral conductor 18N. Therefore, 0.5 unit of current is
flowing from the secondary side of the current sensor 40 at terminal 40a.
At the current sensor 42, 0. S unit of current is flowing from the polarity
mark
through the phase conductors 20a and 0.5 unit of current is flowing into the
polarity
mark through neutral conductor 20N. Therefore, no current is flowing from the
secondary side of the current sensor 42.
FIG 2D shows the per unit current representation flowing in the ground fault
protection circuit. From FIG. 2C, we see that 0.5 unit of current enters the
node 46
from the terminal 40a and that 0.5 unit of current leaves the node 46 towards
the
terminal 34b. Because the current entering and leaving the node 46 is
balanced, there
can not be any current flowing through the ground fault relay GFM1 into the
node
46, as shown in FIG. 2D. Therefore, the ground fault relay GFM1 does not cause
the
main circuit breaker M1 to trip. Additionally, the current entering the node
54 from
the ground fault relay GFM 1 is zero.
Looking at the node 52 in FIG. 2C, we see that 0.5 unit of current must leave
and enter the terminal 40b and that no current enters the node from the
terminal 42a.
Therefore, 0.5 unit of current must enter the node 52 from the node 47 as
shown in
FIG. 2D. Because 1.5 units of current flow into the terminal 36b of the
current
sensor 36, as shown in FIG. 2C, then 2 units of current must flow from the
node 54,
through the ground fault relay GFM2, through the primary or bottom coil of the
auxiliary transformer AT and into the node 47. This 2 units of current is then
divided
into 1.5 units leaving the node 47 toward the terminal 36b of the current
sensor 36
and 0.5 unit leaving the node toward the node 52 as shown in FIG. 2D.
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As shown in FiG. 2D, because there are 2 units of current flowing through
the bottom coil of the auxiliary transformer AT, then 2 units of current must
flow
through the secondary of top coil of the auxiliary transformer AT and into the
node
60. Because 2 units of current must enter the top coil of the auxiliary
transformer
AT to match the 2 units of current leaving the top coil. the 2 units of
current must
flow from the node 60, through the ground fault relay GFT2, into the node 62
and
out of the node 62 towards the top coil of the auxiliary transformer AT. The 2
units
of current flowing through the ground fault relay GFT2 causes the tie circuit
breaker
TZ to trip. Because the current leaving the node 60 towards the node 62
matches the
current entering the node 60 from the auxiliary transformer AT, then the
current
entering the node 60 from the node 48 must equal zero. FIG. 2D shows the
current
entering the node 60 from the terminal 38a of the current sensor 38 is zero;
therefore,
according to Kirchoff's first law, the current entering the node 48 from the
node 44
must also be zero.
FIG. 2D shows that 0.5 unit of current enters the node 44 from the terminal
34a and 1.5 units of current enter the node from the terminal 36a. Because
there is
no current between the node 44 and the node 48, the 1.5 units of current
combines
with the 0.5 unit of current to create 2 units of current flowing from the
node 44 and
through the ground fault relay GFT 1 to the node 54 thereby causing the tie
circuit
breaker T 1 to trip.
Because the current entering the node SO from the terminal 38b and the
terminal 52b is zero, then the current leaving the node 50 is also zero.
Therefore, the
current flowing through the ground fault relay GFM3 is zero and the main
circuit
breaker M3 is not caused to trip.
The ground fault protection has now been fully analyzed to assure that all
laws of circuit analysis have been satisfied. The resulting current path in
the ground
fault protection circuit establishes a current flowing through the ground
fault relays
GFT l, GFMZ and GFT2. This current flowing through these ground fault relays
causes their respective circuit breakers T1, M2 and T2, respectively, to trip
thereby
isolating the main bus 14 from the other buses of the system 10. It should be
noted
that no current flows through the ground fault relays GFM 1 and GFM3 thereby
ensuring that main circuit breakers M 1 and M3 do not trip, thereby allowing
the main
bus 12 and the main bus 16 to deliver power.
In other types of ground fault protection circuits, the main circuit breakers
M1 and M3 may have received trip signals, however, restrained from tripping by
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selective coordination or restraint signals sent from other circuit breakers.
The
advantage of the ground fault protection circuit of the present invention is
that it is
self selecting by the architecture of the ground fault protection circuit and
use of the
auxiliary current transformer AT. In other words, by using the ground fault
protection circuit of the present invention there is no need for selective
coordination
or restraint signals from other circuit breakers in the system. Additionally,
the
present invention provides the advantage of not requiring the use of auxiliary
control
contacts and will function properly independent of the status of the circuit
breaker
contacts.
It is also important to note that the current entering the node 62 from the
ground fault relay GFM3 remains at zero under any assumption so long as the
ground
fault GF is located on the main bus 14. This is important when considering the
function and activity of the auxiliary current transformer AT in relation to
the
operation of the ground fault relay GFT2. There is a natural tendency to
consider
that the current from the auxiliary current transformer AT may also flow to
the
ground fault relay GFM3 and/or other places in the right hand portion of the
ground
fault protection circuit. By the same token, if current is being directed to
the ground
fault relay GFT2 by means of the ground fault relay GFM3, there is a tendency
to be
concerned about the current that might be transferred through the auxiliary
transformer AT through the ground fault relay GFM2. However, a degree of
isolation is provided by the auxiliary transformer AT to make connections from
two
differing parts of ground fault protection circuit possible.
The foregoing description is not limited to the specific embodiment herein
described, but rather by the scope of the claims which are appended hereto.
For
example, although the preferred embodiment has been described with reference
to an
electrical power distribution system having three sources and three loads, the
design
may be easily adapted to an electrical power distribution system having more
than
three sources and three loads. In these systems, more than one auxiliary
transformer
will be required for distributing trip current to the proper circuit breaker
tripping
function.
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