Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2001-10-03
2003-12-30
Sherry, Michael (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S063000, C361S065000, C361S076000, C361S078000
Reexamination Certificate
active
06671151
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to network protector relays used to control circuit breakers and, more particularly, to such network protector relays for circuit breakers connecting feeders to low-voltage secondary power distribution networks. The invention also relates to a method of controlling a circuit breaker employing two trip characteristics.
2. Background Information
Low-voltage secondary power distribution networks consist of interlaced loops or grids supplied by two or more sources of power, in order that the loss of any one source will not result in an interruption of power. Such networks provide the highest possible level of reliability with conventional power distribution and are, normally, used to supply high-density load areas, such as a section of a city, a large building or an industrial site. Each power source is a medium voltage feeder supplying the network and consists of a switch, a transformer and a network protector. The network protector includes a circuit breaker and a control relay. The control relay senses the transformer voltages, the network voltages and the line currents, and executes algorithms to initiate breaker tripping or reclosing action. Trip determination is based on detecting reverse power flow, that is, power flow from the network to the primary feeder.
Examples of network protector relays are disclosed in U.S. Pat. Nos. 3,947,728; 5,822,165; and 5,844,781.
Traditionally, network protector relays were electromechanical devices, which tripped the circuit breaker open upon detection of power flow in the reverse direction. The electromechanical network protector relays are being replaced. One type of electronic network protector relay mimics the action of the electromechanical relay by calculating power flow.
Another type of electronic network protector relay uses sequence voltages and currents to determine the direction of current flow for making tripping decisions. Sequence analysis, upon which such relays are based, generates three vector sets to represent a three-phase voltage or current: (1) a positive sequence vector, (2) a negative sequence vector, and (3) a zero sequence vector. U.S. Pat. No. 3,947,728 discloses a sequence based network protector relay, which uses the positive sequence current and positive sequence voltage vectors to make trip decisions.
More recently, digital sequence based network protector relays have been utilized which periodically sample (e.g., 8, 16, 32 times per cycle) the current and voltages.
FIG. 1
illustrates a secondary power distribution network system
1
, which includes a low-voltage grid
3
servicing various loads
5
. The secondary network bus or grid
3
is energized by multiple sources in the form of feeders
7
a,
7
b,
7
c,
7
d.
Feeders
7
a
and
7
b
are supplied directly from substations
9
a
and
9
b
, respectively. Each of the feeders
7
a
-
7
d
respectively includes a feeder bus
11
a
-
11
d,
a switch
13
a
-
13
d,
a feeder transformer
15
a
-
15
d,
and a network protector
17
a
-
17
d.
The secondary network system
1
and its components are three-phase wye or delta connected, although
FIG. 1
shows these as a single line for clarity. Each of the network protectors
17
a
-
17
d
includes network protector circuit breakers
19
a
-
19
d
and network protector control relays
21
a
-
21
d,
respectively.
As disclosed in U.S. Pat. No. 5,822,165, which is incorporated by reference herein, the control relays
21
a
-
21
d
each include a microcontroller-based circuit (not shown) which monitors the network phase to neutral voltages Vn (e.g., Van, Vbn, Vcn), the transformer phase to neutral voltages Vt (e.g., Vat, Vbt, Vct), and the feeder currents I (e.g., Ia, Ib, Ic).
Typically, control relays include a communication module for communication with a remote station over a communication network (or “communication subsystem” in order to avoid confusion with the secondary network bus
3
). For example, one or more MPCV control relays, which are marketed by Cutler-Hammer of Pittsburgh, Pa., may be connected to the communication subsystem (e.g., without limitation, INCOM physical communication layer, and PowerNet or IMPACC Series III communication software, as marketed by Cutler-Hammer) to allow remote access to protector measurement data of interest. In turn, the control relays perform breaker trip and reclose functions.
Advances in solid-state technology continue to improve the functionality of network protector relays.
U.S. Pat. No. 5,822,165 discloses a network protector relay, for example, whereby the flexibility of more powerful processing resources relative to first generation solid-state relays and the even older electromechanical designs allow for providing a more robust and safe low-voltage power distribution network.
The primary responsibility of a network protector relay is to recognize and react to backfeed conditions (i.e., power leaving a low-voltage network grid). In the event that the amount of backfeeding power meets the programmed setpoints of the relay, then the relay trips the network protector circuit breaker and, thus, isolates the feeder circuit. The other major responsibility of the relay is, of course, deciding when the transformer voltage conditions are within programmed parameters relative to the network bus voltages, in order to command a reclosure of the network breaker.
Referring to
FIG. 2
, a phasor diagram
31
shows a traditional network relay trip characteristic. The network voltage phasor reference is shown as vector V
N
33
at 0°. A normal, lagging network load current vector is included for reference as vector I
LOAD
35
. A network relay should trip the protector circuit breaker on backfeed conditions that will occur when the feeder circuit is faulted or when the feeder circuit is opened. I
SC
is shown representing a feeder fault backfeed vector
37
, lagging the 180° reference
39
due to the dominating network transformer leakage inductance and feeder cable inductance combination. For the case of an open feeder, the dominating term is typically the transformer secondary winding magnetizing inductance, as indicated by current vector I
M
41
.
FIG. 2
shows the network protector relay tripping characteristic region
43
(shown in cross-hatch in
FIG. 2
) with a +5° counterclockwise tilt
45
, the purpose of which is to trip on backfeeding currents that may be highly leading the 180° reference
39
due to system cable charging currents indicated by I
C
, which is represented by vector
46
. A corresponding non-trip region
47
is shown above the trip region
43
. The threshold line of the trip region
43
is sloped 5 degrees to compensate for phase shift (e.g., in the network transformers; in the current transformers which measure the currents). This avoids unnecessary tripping in response to temporary reverse current conditions which could be caused for instance by a regenerative load on the network
3
.
The described vectors
33
,
35
,
37
,
41
,
46
are not drawn to a particular scale, but are simply graphical representations of various system conditions. RT represents the reverse trip setpoint
48
of the network protector relay. In this regard, the circuit breaker
103
has a rated value of current. Typically, the reverse trip setpoint RT
48
is about 0.2% of the rated value of current and is associated with the positive sequence current vector I
SC
37
. The intention is that at every network protector relay location, each relay has all possible system current backfeed conditions fall within the trip region
43
of FIG.
2
.
There is room for improvement in network protector relays.
SUMMARY OF THE INVENTION
These needs and others are satisfied by the invention, which is directed to the trip functionality of a network protector relay, which employs two different trip characteristics by adding a “Gull-Wing” trip region to the existing traditional trip region.
As one aspect of the invention, a network protector relay for controlling a circuit breaker connected between a polyphase feeder bus and a po
Kenny Thomas Joseph
Moffat, Jr. John Robert
Smith David Robert
Eaton Corporation
Laxton Gary L.
Moran Martin J.
Sherry Michael
LandOfFree
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