Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2000-05-31
2002-10-01
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S078000, C700S292000
Reexamination Certificate
active
06459557
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to relays for interrupting power on power conductors upon occurrence of a fault condition, such as a phase loss, ground fault, overload, or undercurrent condition. More particularly, the present invention relates to a relay that can be selectively configured to operate in either a single-phase mode or a multi-phase mode and which determines and reports diagnostic parameters associated with current flow through the power conductors. The relay is further configured to provide protection for fault conditions based on the determined diagnostic parameters, regardless of the selected mode of operation.
2. Description of the Related Art
Single-phase and multi-phase (e.g., three-phase) power systems typically include an overload relay for interrupting power in the power conductors when a fault condition occurs, such as a ground fault, phase loss, overcurrent, or undercurrent condition. A variety of types of overload relays are available, ranging from simple big-metal or eutectic overload relays to more complex, solid-state relays which may include some intelligence and/or reporting capabilities. Big-metal and eutectic overload relays include heater elements in each phase which open when an excessive current flowing through the heater elements causes the element to exceed a specific temperature. Solid-state relays, on the other hand, include electronic devices for monitoring phase current and for determining, based on the monitored current, whether a fault condition has occurred. Thus, solid-state relays typically can be configured to provide protection for ground fault, undercurrent and phase loss conditions, in addition to overcurrent conditions.
To provide such protection, however, the electronic devices included in a solid-state relay require power for proper operation. Such power may be provided externally from a separate source, or, the relay may be self-powered, meaning the power for the electronic devices is derived from an internal source, such as the relay's current transformers which are monitoring the current in each phase. The solid-state relay may also be configured to include reporting capabilities. For example, such a relay may communicate diagnostic information, such as an average current in the power conductors or a percentage current imbalance between the conductors.
Both big-metal/eutectic relays and solid-state relays are available in a single-phase configuration and a three-phase configuration. A typical overload relay configuration for a three-phase application is illustrated in
FIG. 1
, and a typical overload relay configuration for a single-phase application is illustrated in FIG.
2
.
FIG. 1
illustrates the conventional use of an overload relay
16
in a three-phase application. In
FIG. 1
, three-phase power conductors
10
a,
10
b
and
10
c
are connected to a motor
11
through short-circuit protection devices
12
a,
12
b,
and
12
c
(e.g., circuit breakers, fuses, etc.), a contactor
14
(including contact pairs
14
a/a
′,
14
b/b
′, and
14
c/c
′), and an overload relay
16
(including relay paths
16
a,
16
b,
and
16
c
), as shown. Relay “paths”
16
a,
16
b,
and
16
c
may be the heater elements of a big-metal or eutectic relay which are in series with the power conductors and open to interrupt current flow through the power conductors upon occurrence of an overcurrent condition. Alternatively, paths
16
a-c
may simply be pass-through conductors through which the phase currents flow through relay
16
and on which phase currents are monitored. In such a device, relay
16
interrupts current flow upon detection of a fault condition by generating a trip signal which, in turn, causes an interruption in current flow through the power conductors. For example, such a trip signal may be used to de-energize the coil in a contactor (such as the coil of contactor
14
), which results in opening of contactor pairs (e.g., pairs
14
a/a
′,
14
b/b
′, and
14
c/c
′) connected in series with the power conductors. The designations “a”, “b”, and “c” are used herein to identify elements associated with phase “a”, phase “b”, and phase “c” of the single-phase or multi-phase system.
In
FIG. 2
, overload relay
16
is configured for use in a single-phase application in which current is conducted through power conductors
10
a
and
10
b
(i.e., phase “a” current and phase “b” current). As shown, the components have been wired such that motor
11
is connected only to overload relay paths
16
a
and
16
c.
The phase “c” load current provided to motor
11
is routed through overload relay path
16
b
and contactor pair
14
b/b
′, and then through overload relay path
16
c,
contactor pair
14
c/c
′, and short circuit protection device
12
b
(i.e., the phase “b” components are connected in series with the phase “c” components).
Proper operation of the overload relay
16
requires that the phase “b” current must be routed through both the phase “b” components and the phase “c” components, even though such a configuration results in extra wiring costs (as well as other drawbacks which will be explained below). For example, if overload relay
16
is a big-metal or eutectic overload relay, load current must be routed through all three heater elements to ensure accurate overload trip protection. Otherwise, special calibration or adjustments must be performed such that the big-metal or eutectic relay will operate properly. If relay
16
is a self-powered solid-state overload relay, current may need to flow through the current transformer in all three phases such that the current transformers can provide sufficient energy to power the relay's electronics. Further, if a self-powered or externally-powered overload relay is to provide phase loss protection, current must be routed through all three conductors to prevent an improper phase loss indication. That is, an apparent current imbalance would be indicated if phase loss protection is enabled and current is not routed through one of the three phases. Still further, a solid-state overload relay with a reporting feature will inaccurately calculate and report average current and current imbalance if current is not routed through each of the phase “a”, phase “b”, and phase “c” conductors as shown in FIG.
2
.
Although the configuration illustrated in
FIG. 2
resolves many of the problems that arise when using an overload relay in a single-phase application, problems still remain. In particular, a solid-state overload relay used in a single-phase system configured in accordance with
FIG. 2
cannot provide ground fault protection. Three-phase solid-state relays typically detect the occurrence of a ground fault in a three-phase system by monitoring or determining the vector sum of the currents in each phase. Normal operation is indicated when the phase currents substantially cancel, and a ground fault is indicated if the vector sum of the phase currents exceeds a predetermined threshold value. If, however, such an overload relay is used in a single-phase application and configured as shown in
FIG. 2
, the vector sum of the phase currents would be equivalent to the magnitude of the single-phase current (i.e., the vector currents through phase “a” and phase “b” would cancel such that the resulting vector sum would be the current through phase “c”), resulting in inaccurate determination of the vector sum and improper indication of a ground fault condition.
To avoid the loss of ground fault protection when using a solid-state overload relay in a single-phase application, the system can be configured as shown in FIG.
3
. In
FIG. 3
, the load current for conductor
12
a
(i.e., phase “a”) is routed to the motor through the phase “a” components (i.e., short circuit protection device
12
a,
contactor pair
14
a/a
′, and relay path
16
a
). Similarly, the load current for conductor
10
b
(i.e., phase “b”) is routed to the motor through the phase “b” components (i.e., short ci
Haensgen Steven T.
Jansen Ronald N.
Gerasimow Alexander M.
Huynh Kim
Rockwell Automation Technologies Inc.
Walbrun William R.
Yoder Patrick S.
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