Parallel-feeder directional overcurrent protection

Details Parallel-feeder directional overcurrent protection Parallel-feeder directional overcurrent protection

1999-06-16

2001-10-23

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Feeder protection in distribution networks

C361S080000

Reexamination Certificate

active

06307723

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an arrangement for countering an undesired operation of directional overcurrent protection relays situated in parallel feeders of each phase of a multiphase AC power system, in particular—but not exclusively—a three-phase power system.
2. Description of the Related Art
Overcurrent protection systems are known in which (see
FIG. 1
) a multiphase (normally 3-phase) power source
10
feeds a correspondingly multiphase load
11
by way of respective source and load busbars
12
,
13
and an arrangement of first feeders
20
and parallel-connected second feeders
30
linking these busbars. Three sets of relays
21
/
31
,
22
/
32
and
23
/
33
are connected in the feeders
20
/
30
. Each feeder
20
and
30
has a non-directional overcurrent protection relay
21
and
31
, respectively, on the load side of the source busbars
12
and a directional overcurrent relay
22
and
32
, together with a non-directional overcurrent relay
23
and
33
, respectively, on the source side of the load busbars
13
. These latter two sets of relays may be realized in each case as a single set of devices having two different characteristics (see later). As an alternative arrangement (FIG.
2
), the feeders
20
/
30
may be connected to the secondaries
24
of respective transformers
25
/
35
, the primaries
26
/
36
being fed from respective sets of non-directional relays
21
/
31
.
To explain the mode of operation of overcurrent relays in general, reference will be made to
FIG. 3
, in which a three-phase feeder is shown in simplified form as a single line containing, in this case, three sections each having a non-directional relay
40
/
41
/
42
and an associated current transformer
50
/
51
/
52
and circuit-breaking device
60
/
61
/
62
. The relays are arranged to operate at decreasing overcurrent settings from AC source
10
to fault location F and to have similarly decreasing response times, which in a typical system might involve a difference of around 0.4 s from one relay to the next. When a fault F occurs as shown, it is required that only the circuit breaker
62
in the section concerned be tripped, the remaining breakers continuing to provide a closed path to maintain supply at substations A, B and C which may be feeding loads
63
as shown. Thus, due to the lower fault current levels associated with the location of this fault (the impedance of the feeder is greater) and the above-mentioned so-called grading between the relays in terms of current and time-response, only relay
42
will operate in response to the fault current, leading to tripping of the circuit breaker
62
. In like manner, were a fault to develop in the middle section, the fault current would be greater and would serve to operate the relay
41
, and thereby trip the circuit breaker
61
, and so on.
The relays may have any of three response characteristics, namely instantaneous-time (this is assumed not to be the case in FIG.
3
), dependent-time and independent-time. The later two characteristics arc shown in
FIG. 4
, which is a graph of time “t” against current “I”. The curve
70
represents a dependent-time characteristic and curve
71
an independent-time characteristic. Curve
70
provides a response time which varies according to the level of fault current seen by the relay, response being slower at lower current values than at higher current values. For curve
71
, the response time is invariant when fault-current exceeds a certain value I
1
.
Directionality of a relay is achieved by the use of a reference quantity, normally system voltage, against which current is compared, the relative phase between these two quantities determining the “direction” of the current. The relay then only responds if that direction is the one to which the relay has been configured to respond. Thus, a directional relay has two inputs: voltage and current. The reference voltage is often referred to as the “polarizing voltage”.
Referring now to FIGS.
5
(
a
) and (
b
), these represent a simplification of
FIGS. 1 and 2
, respectively, inasmuch as the three-phase system is represented as a single line diagram containing two parallel-connected feeders and the associated directional and non-directional relays. If it is assumed that no fault exists on the AC system and that the system is working at full capacity, the feeders
20
/
30
will each pass half the total rated load current I
L
. However, the relays will be designed to have a minimum safe overcurrent setting in excess of the maximum load current I
L
on the assumption that one of the feeders might be out of service. In the event a fault develops on one of the feeders, that current setting may be exceeded for one or more relays, causing that or those relays to register a fault current level (i.e., to “pick up”) and, following a time delay, to trip its associated circuit breaker (i.e., to “time out”). Of course, the “time out” only occurs if the fault is not cleared before expiry of the relay's time delay.
In the case of
FIG. 5
, it is assumed that a fault has developed on one feeder at the location F
1
shown. The fault may be either phase-to-phase or phase-to-earth. In the absence of the fault, power flow was from source to load and therefore the directional relays
80
,
81
were nominally insensitive to the normal load current which consequently flowed. However, with the appearance of the fault at F
1
, current flow changes so that, if the fault current is represented as I
F1
, a proportion of that current (e.g., I
F1
/2) will flow into the fault branch via non-directional relay
82
and the remainder (I
F1
/2) via non-directional relay
83
and directional relay
91
. The actual value of fault current in each branch is dependent on fault position and will not necessarily split equally between the branches. Under these circumstances current flow in the relay
81
is in the right direction for it to operate. It will in fact operate provided that firstly, the fault current exceeds its fault-current threshold, and secondly, its set response time is shorter than the duration of the fault. Similar considerations hold for the transformer feeder arrangement of FIG.
5
(
b
).
FIGS.
6
(
a
) and
6
(
b
) show two possible relay co-ordination diagrams for the various relays shown in FIG.
5
. In the first case, the relays are configured as dependent-time relays having the characteristic shown as curve
70
in
FIG. 4
, but with the non-directional relays
84
,
85
and
86
graded in terms of response time for a fault current I
F2
. The current-threshold settings of relays
83
and
85
are identical; (these settings are, incidentally, stored in non-volatile memory in the relay). It is the fact that relays
83
and
85
are set the same which causes the initial problem (which is resolved by the use of directional overcurrent relays), i.e., that for a fault at F
1
, relay
85
could operate prior to relay
83
, effectively isolating the load, since relay
82
would also operate. The directional relays
80
,
81
are arranged to operate (trip their associated circuit breaker) at a lower current level, as it is commonly perceived that their setting is not restricted by the value of load current. The situation is similar in FIG.
6
(
b
), but with all the relay characteristics being of the independent-time type.
As mentioned briefly earlier, it is possible to combine the functions of relays
81
and
83
in one unit, and this may be either a microprocessor-based or numerical relay having two separate sets of overcurrent protection elements.
It is known that the use of a directional relay in each branch can serve as an effective method of isolating all feeder faults with minimal system disruption. Further to this, it has also been recognized as desirable to set the current threshold of the directional relays below normal rated load current, since this can assist with the co-ordination (grading) of the various relays present in the system, in the manner illustrated in FIG.
3
. However, there has to date b

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