07-02-2013, 12:23 PM
Advances in Breaker-Failure Protection
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Abstract
In this paper we first review backup protection
concepts and analyze the impact of breaker-failure protection on
power system stability. We show the importance of having differ-
ent breaker-failure operating times for different types of faults.
We then introduce breaker-failure schemes and also a scheme
for breaker-flashover protection. We discuss the effect of fault-
detector reset time and describe fast-reset instantaneous over-
current elements. Next we discuss alternatives for initiating
breaker-failure operation in protection schemes where different
relays trip the same breaker for faults in different protection
zones. We also review breaker-failure tripping practices.
Later we discuss the advantages of integrating bus and
breaker-failure protection in the same digital relay for substa-
tions with complex bus arrangements. We finally present an eco-
nomical solution for breaker-failure protection in distribution
substations.
INTRODUCTION
For many years, protection engineers have applied local
breaker-failure protection to high-voltage (HV) and extra-
high-voltage (EHV) systems with electromechanical relays
and solid-state relays. These breaker-failure schemes were
designed within the limits of the prevailing technology, such
as inaccurate timers with overtravel and slow-reset overcur-
rent elements. The development of microprocessor-based re-
laying, digital input filtering, and low-burden CT secondary
circuits allows easier breaker-failure protection application
with measurable gains in performance. This is the reason for
revisiting breaker-failure protection in this paper.
BACKUP PROTECTION CONCEPTS
Given the importance of power system protection and con-
sidering that a protection system may fail to operate, it is common to have two protection systems: primary and backup protection. This is particularly important for short-circuit pro-
tection, because the short circuit is the most frequent type of system failure. Therefore, the probability for the primary short-circuit protection to fail is higher than that of the protec-
tion against other abnormal operating conditions.
Primary protection is the first line of defense. Primary short-circuit protection operation should be as fast as possible, preferably instantaneous, for stability and power quality rea-
sons, and to prevent equipment damage.
IMPACT OF BREAKER-FAILURE
PROTECTION ON POWER SYSTEM STABILITY
Breaker-failure tripping times can have a significant impact on system stability. A breaker-failure operation may result in additional line or equipment outages, as well as the increased duration of the fault. For the most part, the critical clearing time is determined from stability studies where a three-phase fault is applied at key locations on the system. A three-phase fault is considered the worst-case condition as power cannot be transferred through the faulted part of the power system and the result is a maximum change in the nearby machine angles. Three-phase faults do not happen very often, and es-
tablishing a breaker-failure delay using these criteria usually results in short delays with little margin.
The following examples show that different fault types re-
sult in different critical clearing times. It may be beneficial to increase the breaker-failure delay using other fault types to improve margin and the security of the scheme recognizing that three-phase faults are unlikely to occur.
The following waveforms and results are test cases from a system model [2] developed on a real-time digital simulator manufactured by RTDS Technologies. The system shown in the Appendix is used for these tests.
FAST-RESET OVERCURRENT FAULT DETECTORS
A factor influencing breaker-failure protection is the pres-
ence of subsidence current. Fig. 17 shows decaying current following interruption of an offset current signal. The subsi-
dence current resulting from energy trapped in a CT magnetiz-
ing branch after fault clearance can delay the resetting of overcurrent elements. This delay can result in breaker-failure relay overcurrent elements still being picked up although pri-
mary current has stopped flowing.
BREAKER-FAILURE TRIPPING
It is a common practice to use a breaker-failure trip to initi-
ate a lockout relay for breaker tripping. This may be the 86B
bus lockout relay with appropriate breaker-failure targeting or
a dedicated 86BF lockout relay. The electromechanical lock-
out relay impairs scheme reliability, because it is a possible
point of common-mode failure. Furthermore, the lockout relay
adds approximately one cycle to breaker-failure clearing time.
Modern microprocessor-based relays have many output
contacts available. In some relays there are high-speed con-
tacts [7] [8] [9]. We may use relay contacts to directly trip the
backup breakers. Another alternative is to apply the high-
speed contacts in parallel with 86BF contacts for faster
breaker-failure clearing time (see Fig. 23). This reduces total
breaker-failure clearing time in about one cycle. In integrated
systems with fiber-optic communications between the IEDs
and the switchyard equipment, it is possible to use protection
logic processors to distribute backup breaker tripping signals.
Fully redundant schemes provide high reliability [10].
BREAKER-FAILURE TRIPPING
It is a common practice to use a breaker-failure trip to initi-
ate a lockout relay for breaker tripping. This may be the 86B
bus lockout relay with appropriate breaker-failure targeting or
a dedicated 86BF lockout relay. The electromechanical lock-
out relay impairs scheme reliability, because it is a possible
point of common-mode failure. Furthermore, the lockout relay
adds approximately one cycle to breaker-failure clearing time.
Modern microprocessor-based relays have many output
contacts available. In some relays there are high-speed con-
tacts [7] [8] [9]. We may use relay contacts to directly trip the
backup breakers. Another alternative is to apply the high-
speed contacts in parallel with 86BF contacts for faster
breaker-failure clearing time (see Fig. 23). This reduces total
breaker-failure clearing time in about one cycle. In integrated
systems with fiber-optic communications between the IEDs
and the switchyard equipment, it is possible to use protection
logic processors to distribute backup breaker tripping signals.
Fully redundant schemes provide high reliability [10].