Electricity: electrical systems and devices – Safety and protection of systems and devices – Feeder protection in distribution networks
Reexamination Certificate
1999-12-09
2002-12-17
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Feeder protection in distribution networks
C361S062000, C361S064000, C361S066000
Reexamination Certificate
active
06496342
ABSTRACT:
BACKGROUND OF THE INVENTION
Electric power is typically conveyed from electric power generators to users via a network of transmission and distribution circuits. Electric power is commonly generated as three-phase alternating current (AC) at a frequency of 50 Hz or 60 Hz. Each phase requires a current-carrying wire, and has voltage and current nominally lagging or leading any other phase by 120 degrees. Power is generated, for example, at 4 kV voltage, stepped up to 128 kV or 750 kV for transmission over long distances, and then stepped down in stages to 4 kV or 33 kV for distribution to various neighborhoods. Voltage may further be reduced, by pole-mounted or pad mounted transformers, for delivery at 120V and 240V to residential and commercial users within these neighborhoods.
Large voltage transformations typically take place at transmission or distribution substations. Functions of the substations may include voltage transformation, regulation and control, power-factor (e.g. capacitor-bank) and load balancing, monitoring, and protection of hardware. Proper monitoring and protection is extremely important in preventing damage to equipment, reducing hazards and minimizing the number of users who may have to be disconnected from an electric distribution network due to damage or equipment failures. Conventional protection systems include fuses and relays, each having predetermined response times and zones of control to minimize propagation of failures.
Fuses are located throughout the electric distribution network and disconnect circuits experiencing excessive current flow due to equipment failure, storm damage, etc. Each fuse is selected with a predetermined response time to accommodate the need of the circuit being protected. The fuse does not blow under normal operation or a momentary over-current condition, but is designed to blow under a true, sustained fault situation. Relays are similarly used to detect faults and initiate disconnection of faulted circuits; relays, however, are typically more complex than a fuse. A relay typically includes a voltage and/or current sensor and a set of electrical contacts driven by the sensor. Electrical contacts in the relay may be connected to circuit breakers which, in turn, physically disconnect faulted lines or circuits when the breaker is opened (“tripped”). Modern relays may use solid state switches in place of electrical contacts.
There are many types of relays, each having different characteristics. Depending on its electrical and physical characteristics, the response time of the relay may be varied. For example, a relay may be made to trip a breaker in one or two cycles of the 50 Hz-60 Hz frequency when it detects excessive current. This type of relay is known as an instantaneous over-current (OC) relay (Type
50
). Another type of relay is the time-over-current (TOC) relay (Type
51
). The TOC relay may include an adjustable delay so that it may respond quickly to large over-current conditions, but more slowly to small over-current conditions. Predetermined response curves (TOC Curves) are usually provided by the manufacturer of the relay to aid in the selection and adjustment of a Type
51
relay.
Small current overloads in a local circuit may be tolerated if the magnitude and duration of the overload are not expected to damage the distribution network. For example, temporary overloads lasting a few hours may be acceptable in order to maintain service to users during a hot summer day when peak load periods are expected. Concurrent with maintaining service during expected overload conditions, the relay must still effectively protect the network in the event of a true, sustained circuit fault. Some networks may use a Type
50
relay in parallel with a Type
51
relay to provide better response to large fault currents, while not over-reacting to small, temporary overloads.
Relays are installed at various locations in a network. With respect to electromechanical relays, each protective function for one phase of a circuit generally requires a separate relay. Providing separate relays for each function per phase is expensive, because of space requirements and installation/wiring costs for so many relays. More recent designs provide for microprocessor-based relays. Microprocessor-based relays are able to combine protective functions for all three phases into one unit. Furthermore, microprocessor-based relays may be remotely reset and adjusted to provide responses that vary depending on the nature of the electrical load during the year. For example, TOC curves and operating points may be changed in anticipation of changes in loads.
A conventional relay, either electromechanical or microprocessor, only senses current or voltage on a circuit to which the relay is connected and only disconnects a breaker for that circuit. The relay does not communicate its measurements with any other relay during the period in which it senses a fault and trips the breaker. Some relays may communicate status information, but do not share measurements to make protective decisions. Using TOC curves, for example, is one of the important ways to limit the relay's “zone of control”. For example, relays protecting the spokes or “feeders” of a radially distributed power network may be adjusted to respond faster (and at lower trip currents) than relays protecting the hub of the network. In this manner, a faulted feeder may be disconnected before disconnecting the hub and, consequently, all the remaining feeders.
FIG. 1
illustrates a conventional electric utility network protected by relays. As shown, a 128 kV transmission line feeds the primary windings of each of two power transformers
30
and
31
by way of circuit breakers
18
and
24
, respectively. The secondary of each of the two power transformers
30
and
31
feeds electric power at 13 kV to feeder bus
1
and feeder bus
2
, respectively. Feeder bus
1
transmits electric power at 13 kV by way of two circuit breakers
44
and
52
to two feeders, feeder #
1
and feeder #
2
. Similarly, feeder bus
2
transmits electric power by way of two circuit breakers
47
and
55
to two feeders, feeder #
3
and feeder #
4
. Feeders #
1
-#
4
provide power, for example, to a neighborhood, factory or shopping center. Tie-breaker
40
provides an alternative path of electric power in the event that power transformer
30
or power transformer
31
is taken out of service. It will be appreciated that each power line shown in
FIG. 1
represents three power lines corresponding to the three phases of electric power. Depending on the network, there may actually be three times the number of breakers and relays shown in FIG.
1
.
Also shown in
FIG. 1
are current transformers
14
,
16
,
32
,
34
,
41
,
42
,
50
and
51
feeding current to various relays. The current transformers each provide an output current in proportion to the current flowing through each line. For example, the current flowing through a line may be 1200 amperes, whereas the corresponding current transformer may provide an output of 5 amperes. As shown, current transformer
14
provides current to OC relay
20
, TOC relay
21
and differential relay
22
. Differential relay
22
, which also senses current from current transformer
32
, reacts to an imbalance between current flowing into and out of power transformer
30
. OC relay
20
, TOC relay
21
and differential relay
22
control breaker
18
, and each may individually trip the breaker if a predetermined condition occurs. Similarly, OC relay
26
, TOC relay
27
and differential relay
28
control breaker
24
. OC relay
45
and TOC relay
46
control breaker
44
. OC relay
48
and TOC relay
49
control breaker
47
. OC relay
53
and TOC relay
54
control breaker
50
. OC relay
56
and TOC relay
57
control breaker
55
.
Also shown in
FIG. 1
are under-voltage relays
37
and
39
connected to potential transformers
36
and
38
. Under-voltage relays
37
and
39
are shown connected to the secondary of each potential transformer to protect against tran
Butt James Allen
Horvath Vincent Victor
Bitronics Inc.
Jackson Stephen W.
RatnerPrestia
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