An Alternative Approach to Improving SAIDI and SAIFI Indicators Philippe Deschamps - Schneider Electric Industries - France Jean-Christophe Orsini - Schneider Electric Industries - France Kaare Seest Rasmussen - DONG Energy - Denmark
Summary Abstract...p 1 Introduction...p 2 DONG Energy's network...p 3 The concept of a SACSe substation...p 4 SACSe control unit architecture...p 7 F200C: Possible further enhancement...p 8 Conclusion...p 9
Abstract Under the pressure of regulating authorities, utilities are being challenged to improve their SAIDI and SAIFI indicators. Usually, to reach this goal, more intelligence is put inside DMS, increasing at the same time the amount of data to be processed and the importance of communication in the network. An alternative way, discussed in this paper, consists of using local automation, allowing faster reactions, and therefore a drastic reduction of the number of customers affected by permanent faults, without requiring heavy communication infrastructure. In addition, this paper introduces a further enhancement allowing a faster power restoration for the remaining affected customers. Paper 0635 1
Introduction Challenged by the Danish authorities, DONG Energy has for a long time experimented with several methods to optimize network management and particularly increase power availability. An important step was reached when it was decided to reduce the maximum time to reenergize a faulty location to less than a minute to avoid the outage to be counted as a long one, that is degrading the SAIDI and SAIFI indexes. Paper 0635 2
DONG Energy's network DONG Energy is a major DNO in Denmark, however this experience involves only one part of its network. FLA04 FLA10-622 3383 4033 FLA10-6202 FLA10-3722 FLA10-7007 FLA10-3369 2017 823 3304 FLA03-4554 FLA03-4553 4956 3320 2365 FLA03-3972 FLA03-3971 FLA03-4524 5199 2366 FLA03-4914 FLA09-3024 FLA09-5562 2928 2367 3568 2726 FLA03-3970 4915 4916 FLA15-998 104 Fig.1 - Typical feeder (neighbouring feeders in white) The 10 kv grid in DONG Energy North, is a 100 per cent underground radial network, with a high degree of access to neighbouring feeders for backup, as shown in Fig. 1. The grid is mainly urban and consists of approximately 7000 substations on 600 feeders. Paper 0635 3
The concept of a SACSe substation First of all, let s clarify the management of phase to-phase and phase-to-earth faults in the DONG Energy network: phase-to-phase faults: these cause the tripping of a breaker and therefore an outage which explains how SACSe limits the number of customers affected phase-to-earth faults: the network is Petersen coil grounded, allowing temporary operation with single earth fault. However, SACSe provides an earth-fault localization information because a second earth fault would cause a tripping. SACSe means "Sectionalizing And Changeover System enhanced". The concept of SACSe described below is the latest evolution of the network, which has already seen significant improvement over the last 10 years. A first step was taken by deploying circuit breakers in the network [1]. The goal is to put in one strategic MV/LV substation, a line circuit breaker intended to split the feeder into 2 segments in order to minimize the number of customers affected by a fault, if it happens on the downstream side. Time selectivity is used to prevent primary substation CB from tripping simultaneously. In some cases, there could be up to two line CBs on the feeder: in such a case they would use the same setting and trip simultaneously. The second concept is a step further. In case of a phase-to-phase fault upstream in the same substation, it allows the faulty segment to be automatically isolated and the healthy ones to be automatically resupplied. This process is based on local automation, allowing a fast reaction, and not needing any human intervention or communication infrastructure. The solution was developed in collaboration with Schneider Electric, which offers a wide range of products for network automation. SACSe - Main principle First of all, in a given feeder, one or two substations are selected to support the automation. They are chosen because of their location: they divide the feeder into two or three segments, each supplying roughly the same number of customers. They are also chosen because of the ease of upgrading them. The principle of layout in the network of these substations is described in the ref [2]. Second, a backup line coming from the neighbour feeder is connected to these strategic MV/LV substations to allow a backup supply if required (Fig. 2). Incoming/outgoing/backup MV/LV transformer Fig. 3 - Typical make-up of a switchgear in a SACSe MV/LV substation HV/MV sub Feeder 1 Feeder 2 Feeder n Fig. 2 - Principle of a SACSe substation layout in DONG Energy s network These substations are equipped with a fourfunction switchgear (Fig. 3): 1 fuse switch or circuit breaker for the transformer protection. It can be forgotten since it has no role in the concept. 2 circuit breakers and 1 switch distributed among the incoming, the outgoing, and the backup lines. There are two circuit breakers among the three to be sure that there will be at least one in the circuit in all network configuration cases. Backup line MV/LV sub SACSe MV/LV sub Paper 0635 4
If a fault occurs downstream, the breaker opens instantaneously, isolating the downstream segment(s). The status of the switchgear is then sent to the SCADA. If a fault occurs upstream, the switchgear acts as an auto changeover (ACO): the circuit breaker will not trip, but the lack of voltage releases the auto changeover process, ordering the incoming to open and the backup to close, which supplies the healthy downstream segment. This is the general principle. Two cases are shown in Fig. 4. Since the breaker in the HV/ MV substation is delayed and those in the SACSe are not, the breaker in the HV/MV sub will only trip when the fault is in the first segment (Fig. 4 - case 1). Trip Open + backup Case 1 Case 2 Fig. 4 - ACO - principle of fault restoration Trip Open + backup In a particular case (2 SACSe on the same feeder), a fault downstream in the second SACSe results in both SACSe circuit breakers tripping. In order to resupply the segment between the two SACSe circuit breakers, the downstream one will act as a Reverse Auto Changeover (RACO): based on the CB tripping information, it will keep the incoming closed, and order the backup to close, therefore resupplying the upstream segment (Fig. 5). Trip Trip + reserve backup In all cases, the time required to reconfigure is less than 20 seconds, saving automatically up to 2/3 of the customers normally affected by such a fault. In addition, several logic processes are embedded in the automation in order to avoid any improper action in certain cases: once the ACO/RACO has operated, it is blocked until there is some human interaction from the control center to prevent further action in a nonstandard network configuration a logic process is in action to avoid any ACO and/or RACO action if the network is subject to a global outage, from the transport for instance. This event is detected by monitoring the voltage presence on the backup line. If an absence of voltage is observed on both incoming and backup lines, then no action is taken in case of a frequency drop, a recovery process exists in the HV/MV substation and it, it reacts by switching off some feeders. So in this case, the ACO/RACO is inhibited to allow the load-shedding process to be prioritized. Fig. 5 - ACO - principle of fault restoration Paper 0635 5
Infrastructure Basically, the system does not require any means of communication to react properly and fulfil its purpose. In fact this is even an advantage in terms of security and reliability. But of course, it is linked with the SCADA to report the outage on the faulty segment, in order to restore the power. In addition, it reports its status to the operating staff, as Danish regulations require the operator to know the real status of all switches and breakers in the network all times. By these means, other information available in the MV/LV substation can also be transmitted. In this case, and thanks to good coverage of the area by GSM infrastructure, GPRS has been chosen to transfer data in a very economical way. Earth fault As explained above, SACSe also integrates directional fault passage indicators (FPIs). These FPIs allow the staff to locate the fault in the field more accurately and faster, and therefore prevent a tripping due to a second fault. All these features are embedded in a single box provided by Schneider Electric, which is described below. Paper 0635 6
SACSe control unit architecture Functional description The unit integrates the following functions (Fig. 6): phase-to-phase fault protection ensured by two protection relays, for the incoming/outgoing and backup breakers phase-to-earth fault detection ensured by two directional fault passage indicators on the same functions voltage presence on incoming, outgoing, and backup cables automation ensuring the ACO/RACO logic communication with the SCADA on the status of the substation power supply with batteries to supply the unit under all conditions. In addition, LV and MV measurement features have been included in the unit to provide a much better knowledge of the substation and the network load. Phase I protection I Eart detection U Fig. 6 - General architecture of the SACSe system U I ACO RACO logic Com Power supply Design SACSe is designed as an industrial solution integrating existing or slightly modified products from the Schneider Electric Easergy offer: relays from the SEPAM range FPI for earth protection is based on the Flair range automation is based on the Easergy T200 which includes peripheral functions such as backup/main power supply and modem. All these products were designed specifically for the network automation environment. As a consequence, they comply with the specificities of this application: harsh environment withstand (temperature, EMC, electrical insulation -10 kv on mains, dust, humidity, etc.), allowing the high level of robustness required by such an application, openness in terms of communication protocols, and a ready-to-use product to minimize the settings on-site during the installation. All these products are integrated in a single box (Fig. 7) in a way to make it easy to integrate into DONG Energy's existing installed base. No assembling is needed on-site, just the mounting of one box, and the connection of the sensors and the commands in the switchgear. In terms of size, the system is made to be mounted directly on the wall of the substation. The dimensions allow it to be used even in a compact substation with reduced volume available. Interfaces are designed to make installation simple and minimize the risk of mistakes by using prefabricated cables and connectors to link the unit with sensors. The box is equipped with a transparent window on the front door, allowing all indicators of the status of the system to be read locally. If needed, Fig. 7 - SACSe control unit operating staff have access to the local operation by opening the front door. Paper 0635 7
F200C: Possible further enhancement Thanks to the unit presented above, the SAIDI index is drastically reduced, but ultimate quality requires some additional instrumentation of the network: In case of a phase fault, only half (if 1 SACSe) or one third (of 2 SACSe) of the feeder customers are affected by an outage more than one minute. However, the half or third of customers in the faulty segment still need to get power back as soon as possible. In case of an earth fault, the SACSe control unit can indicate whether the fault is upstream or downstream the SACSe substation. But a more accurate localization is required in order to allow faulty cable isolation without power interruption for any customer. Accurate localization would ideally mean knowing which cable between the two substations is faulty. Schneider Electric has recently launched a new communicating fault passage indicator, the Flair 200C, combining in a single cost-effective product the phase-to-phase and phase-to-earth fault detection and the GPRS communication functions. This would suit the above purpose particularly well. It should be noted that this new Flair 200C embeds a new algorithm for detection of phase-to-earth faults on compensated networks not requiring voltage sensing. This allows the commissioning of such devices to be drastically simplified, especially for retrofit in existing substations, and it reduces the global cost. Of course, for obvious cost and complexity reasons, more than two SACSe substations on a feeder are not possible. Increasing the number of SACSe substations is therefore not a way to achieve ultimate quality. Additionally, the Flair 200C can optionally provide measurements, further increasing the knowledge of network load. This means a device allowing accurate localization of both phase faults and earth faults is needed in the other substations, both upstream and downstream from the SACSe. Paper 0635 8
Conclusion Increasing pressure on SAIDI/SAIFI improvement usually requires more performance from DMS, and at the same time the management of more and more information. A complementary solution involves the use of local automation able to perform simple actions quickly. By this means, when a fault occurs, a rough but fast reconfiguration is done while the operation is simplified. The implementation of this requires a dedicated device to prevent complex commissioning from spoiling the solution, and therefore the Return On Investment. References [1] Peter Vinter, Henrik Vikelgaard, 2005, "A novel approach to distribution grid automation at NESA A/S", CIRED Conference 18th international conference on electricity distribution. Session 3. [2] Kåre Seest Rasmussen, 2009, "A real case of self healing distribution network", CIRED Conference 20 th international conference on electricity distribution. Session 3. Paper 0635 9
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