For those who operate power transmission and distribution networks, the overriding requirements are safety and minimising network downtime, writes Stan Thompson – former product manager at Megger.
Nevertheless, faults will inevitably occur, so any device that will help to minimise the impact of these faults will achieve ready acceptance. Auto-reclosers are just such a device.
Auto-reclosers are most commonly found in overhead cable networks and they are useful because, in such networks, only around 7% of faults are permanent. In fact, the vast majority of faults – around 80% – are transient, caused by events like lightning strikes and arcing, and will disappear in less than a second.
The remaining 13% of faults are semi-permanent and are typically caused by animals or branches bridging the power lines. Even these faults, however, usually burn away and clear in a relatively short time.
With this information about fault duration in mind, it becomes clear that a lot of unnecessary downtime will result if the protection scheme simply isolates the section of a network where a fault occurs and then waits for someone to go out into the field, check the fault status and reset the breaker.
A much more effective strategy is to isolate the affected part of the network and then, after a short delay, re-energise it to determine whether the fault has cleared. If it has, the network can operate normally but if it hasn’t, the affected area can once again be isolated.
This isolation followed by re-energisation process is exactly what an auto-recloser does. In principle at least, it’s a simple device. Essentially, it comprises a circuit breaker with associated protection relays, a mechanism that will allow it to be closed automatically after a trip, and a controller that provides the auto-reclose functionality.
The ANSI standard device number for the controller is 79. Typically the complete assembly is installed at the top of one of the poles or towers used to support the power lines.
The operating principle of an auto-recloser is equally simple: when a fault is detected, the breaker trips. The controller then waits for a predetermined time before reclosing the breaker.
If all is well, that’s the end of the process, but if the fault persists the breaker trips again and, after a further delay, the controller recloses it for a second time and checks again for the continuing presence of the fault. If the fault is still present after this second reclose, the breaker trips yet again.
Clearly, this cycle of closing and tripping could, in theory, continue indefinitely but this would be pointless since a fault that persists for more than a few seconds is likely to be permanent. And, of course, continually de-energising and re-energising a line would create a safety hazard as well as potentially degrading network stability and risking further damage to assets.
For these reasons, auto-reclose controllers are designed to lock out after a certain number of trip/reclose cycles – usually two, three or four cycles, depending on the application.
The use of auto-reclosers brings many benefits for network operators not the least of which is that it minimises the power interruption time resulting from faults. This helps the networks operators to improve their performance in relation to the SAIDI (system average interruption duration index), SAIFI (system average interruption frequency index) and MAIFI (momentary average interruption frequency index) standards.
In turn, this means reduced loss of revenue. Note that while the SAIDI, SAIFI and MAIFI are US standards, comparable network performance standards apply in Europe and in most other parts of the world.
Other benefits of using auto-reclosers include enhanced system stability and reduced manpower requirements, as it is not necessary to visit the fault location to reset the breaker. Improved customer satisfaction, as power interruptions are shorter. In addition, using auto-reclosers makes it easier to operate substations unattended.
With their benefits and principle of operation now firmly established, let us now move on to look at auto-reclosers in a little more detail. Figure 1 shows a very simple arrangement incorporating an auto-recloser, and illustrates the sequence of operations with a permanent fault.
In this instance, the circuit breaker (52) is assumed to be fitted with basic protection in the form an instantaneous overcurrent relay (50), but the same arrangement works equally well with other forms of protection.
Figure 2 shows the timing scheme for this arrangement, and illustrates some of the key parameters related to the setting and operation of auto-reclosers. Definitions of these parameters are:
• Operating time:
o of the relay – time from inception of fault to trip coil energising
o of the circuit breaker – time from trip coil energising to fault arc quenching
• Dead time:
o of the circuit breaker – time from fault arc quenching to circuit breaker contacts remaking
o of the auto-reclose controller – time from re-close scheme initiation to close coil energising
• Reclaim time:
o of the auto-reclose controller – time from close coil energising to arc quenching of the next trip cycle
• Lock out:
o of the circuit breaker and the auto-reclose controller – a feature that limits the number of trip/reclose cycles that the system will attempt
Auto-reclosers are most commonly available in three-pole and single pole versions. With the three-pole versions, all three phases are disconnected under fault conditions, regardless of the fault type. This allows generators on the system to drift out of synchronisation, which means that synchronisation checks must be made before the reclose operation takes place.
In contrast, with single pole types, only the faulted phase is disconnected. This means that the effect on the load is minimised and synchronisation of the generators is maintained. However, phase-selective relays must be used to control the tripping and the recloser requires individual closing/tripping mechanisms for each phase. As a result, the single-phase option is more complex and more expensive. It also has the additional disadvantage that capacitive coupling with the phases unaffected by the fault leads to longer arcing times.
The choice of recloser type depends on the application and it is worth noting that the principal aim of using reclosers often differs between medium-voltage distribution applications and high-voltage transmission applications. In the first case, the main emphasis is likely to be on ensuring the continuity of power supply; so three-phase reclosers are often chosen. In the second case, however, the emphasis shifts to ensuring stability and synchronisation, so single-phase reclosers may be more appropriate.
In distribution applications, reclosers are usually configured for either “fuse save” or “fuse blow” operation. With the fuse save scheme, the protection system is set for fast operation on the first recloser shot (or the first two shots, depending on the application), so if the fault clears quickly the fuse providing back-up protection for the circuit doesn’t blow. After the first one or two shots, the protection switches to a slow operating curve to ensure that the fuse DOES blow, as the fault is considered to be permanent, and the fuse must isolate the faulty circuit so that power can be restored to the rest of the system.
This type of operation has the advantage that it protects the expensive fuse in the case of transient faults, but the downside is that there is likely to be more disruption of the supply to consumers connected to the feeder served by the recloser.
With fuse blow operation, the protection system associated with the recloser uses only the slow operating curve, so any fault – transient or permanent – causes the fuse to blow in the fault area. The benefit is that the power outage only affects the faulty circuit and larger customers connected to the feeder see no disruption. The disadvantages are that the costly fuse must be replaced after every fault, and that consumers fed by the circuit the fuse was protecting will be off supply for as long as it takes to go to the site and fit the replacement.
To illustrate the types of settings found on a typical recloser, Figure 3 shows the details for a type SEL-315R unit that has been set to lock out after three shots. All time intervals are specified in terms of the number of cycles of the supply – that is, on a 60 Hz system, a setting of 60 is equivalent to one second.
In these settings, 79OI1 is the open interval before the first reclose attempt, and is set to 120 cycles, or two seconds. 79OI2 is the open interval before the second reclose attempt and is also set to 120 cycles. 79OI3, the open interval before the third reclose attempt, is however, set to 300 cycles or five seconds. 79OI4, the open interval before the fourth reclose attempt, is set to zero, which tells the relay that no fourth attempt should be made – that is, it should lock out after three attempts.
79RSD is the time after a fault-free reclose before the relay is reset and sees further faults as separate events. Similarly, 79RSLD is the reset time after a lockout, which is the time the breaker must remain closed when it has been reset after a lockout before the functioning of the auto-reclose relay is fully reset. 79CSLD … (this isn’t explained in the presentation – we need either to delete this reference or add an explanation.)
Megger’s innovative ERTS test simulator
Auto-reclosers can be tested using standard protection relay test sets such as those in Megger’s SMRT range. A complication arises, however, when it comes to making the connections between the relay test set and the auto-recloser system. Connections to the auto-recloser are invariably in the form of multi-way cables and connectors, with 14-pin, 19-pin and 32-pin variants commonly being used. It is, of course, possible to make up ad-hoc cables for testing specific types of reclosers, but this is a tedious and time-consuming process which is particularly undesirable when an engineer is struggling to return the recloser to service as quickly as possible.
In order to address this challenge Megger has developed the new ERTS electronic recloser test simulator, which is shown in Figure 4. Designed to sit between the auto-recloser and an SMRT relay test set, this novel instrument will accepts standardised 14-pin, 19-pin and 32-pin cables, and will allow the signals they carry to be quickly and easily patched through to the inputs and outputs of the SMRT test set.
This arrangement is quick and easy to use, and it means the extensive facilities of the SMRT test set can be used to evaluate the performance of the auto-recloser system. A test module specifically for testing auto-reclosers is, in fact, already included in the library of the AVTS software used with the SMRT.
Aside from ease of use, an important benefit of the ERTS and SMRT combination is that, unlike other options for recloser testing, it fully supports both single-phase and three-phase reclosers, and it allows the operation of the auto-reclose controller and the recloser itself to be checked and verified.
Reclosers play an important role in enhancing the reliability and stability of power networks and, because the success rate of reclosers in dealing with faults is high – around 80% of faults are cleared at the first shot – they make a big contribution to improved revenue flow for network operators and utilities. And now, with the introduction of Megger’s innovative ERTS test simulator, testing reclosers is easier, faster and more convenient than ever.
About the company
Megger designs and manufactures portable electrical test equipment. Megger products help you install, improve efficiency, reduce cost and extend the life of your or your customers’ electrical assets.