Switchgear Evolved: Hybrid Switchgear in the Modern Distribution Network
Why anyone in their right mind would use a Recloser as a Sectionaliser?
Modern distribution network switchgear is nothing short of an engineering marvel. Born to operate in the most demanding conditions under 30 years of electrical abuse, blizzards, dust-storms and everything in between, Reclosers have been the go-to solution for most distribution network engineers since their invention eons ago. Whilst the fundamental concept of reclosing has varied little over the years, the evolution of their controllers has made even the most advanced substation relay a little nervous for competition. Recloser Controllers are essentially industrial computers, which is usually an amalgamation of the SCADA Remote Terminal Unit (RTU), the driving control of the attached recloser and the protection relays. With a functionality checklist starting to resemble a dictionary, some developments have come to light that a recloser can be operated as a sectionaliser. This concept is considered theoretically possible in any recloser controller, but it is bewildering in terms of its practical network application. Why would anyone in their right mind take a piece of equipment as valuable as a recloser and dumb it down to act like a sectionaliser? In the modern distribution world, this very application is gaining significant traction and it is definitely time to explore the rationale behind this deployment. Maybe a Hybrid switch, a device which can act as a recloser, sectionaliser and load break switch on demand is the ideal solution to a glaring challenge.
Firstly, it is appropriate to explore the classic role of the network device hierarchy. At the lower end of the pecking order, in both cost and complexity, is the Load Break Switch. Typically designed to open under normal load conditions at worst, the LBS is a cheap solution to the age old problem of how to disconnect portions of network. Over time, this technology has developed more and more sophistication, with the addition of sensors and controllers. By adding some CTs and a controller, the LBS can warn operators if it shouldn’t be opened when high currents are flowing, but perhaps more importantly this LBS can share a little more information to a centralised control center via SCADA.
In the context of a Smart Grid, information is paramount to making intelligent switching decisions. The “intelligent” LBS, one with some CTs and a communications capability, can be deployed as what has been termed a Fault Passage Indicator. Suddenly a device which has essentially no grading requirements due to its lack of protective interruption capabilities can report a datapoint to a central automation SCADA system to inform that “Fault Current Passed here”. With this information, operators can work to pinpoint fault locations and even re-switch the network to isolate the faulted location. A slightly smarter LBS might very well be worth the investment when considering time and money saved on any future interruptions.
The next switchgear step up is widely known as the Sectionaliser. These distribution network items are essentially the same as the aforementioned Fault Passage Indicator, with the exception that now the switching operation is automated into the switch. Instead of waiting on a remote SCADA operators command, the sectionaliser counts the operations of the upstream recloser as it watches the power supply turn on and off again through the reclose sequence. Since the sectionaliser doesn’t have the capability to break fault current, it can however open when the upstream device has broken the fault current. Programming a sectionaliser only requires setting how many trips of an upstream device it must see within a given window before it trips to segment a section of line.
The beauty of this solution was two-fold. In the days of yore, a recloser was a sizeable investment for a utility to make in comparison to a sectionaliser. Today, this is no longer the case (if you still think it is, maybe an email to email@example.com will change your mind), but nonetheless sectionalisers had another advantage. Sectionalisers are immune to grading requirements.
As a sectionaliser does not interrupt fault current, it has increased resilience to malgrades on fault operations. Traditionally, with a substation circuit breaker set to 1 second clearing time for a fault and a conservative grading margin of 250ms between devices, a maximum of four reclosers could be deployed on any given feeder. Many applications, particularly tee-offs from mainlines and longer feeders could certainly use more reclosers than the four on offer. Whilst today, advanced reclosers like the NOJA Power OSM® series with an RC10 control could easily manage a grading margin of 150ms, sometimes the utility needs more. This is where sectionalisers were deployed.
By further segmenting the lines utilities have greater capability to restore power and isolate faults. Whilst this operation between reclosers and sectionalisers works well for radial feeders, the challenge arises when distributed generation and alternative energy has upset the apple cart. All the old radial feeders of years ago suddenly don’t act quite as “radially” as they used to, where distributed generation and renewable energy can offer multiple fault flow paths. This is compounded by grading requirements on ring feeds, as a loss of a certain supply path may result multiple feeders being tied together. When a four recloser long feeder becomes an eight recloser long feeder, grading margins are certainly going to become a little tighter. This is the driving force for hybrid switchgear.
When current is flowing under conventional load conditions with an open tie point, grading is all well dealt with. The challenge arises when a supply source is lost, and a tie point is closed (Figure 2). The issue is associated with meeting grading requirements under a change of current flow direction. A modern recloser controller which can identify different current directions through sequence component torque angle application can selectively act as a sectionaliser or recloser depending on the network configuration automatically. For example, if Substation 1 is lost, R1 has opened, but R2 may struggle to maintain grading for a fault downstream of it in relation to a supply from Substation 2. To address this challenge, R2 can be instructed to act as a sectionaliser under this current flow path to ensure that grading is met. During both conventional and reverse case operation (loss of Sub 2 and closed tie point), the current flow is in the opposite direction and this device now becomes the recloser 2nd closest to the substation. In this operation, the switch should behave as a recloser as grading is far less challenging at this point and a second midline recloser is certainly expected to interrupt downstream faults. With exceptional versatility, utilities are now armed with the capability to maintain grading even under compromised supply conditions. This level of performance ensures that the network remains adaptive, regardless of the configuration, improving performance and reliability.
Practically speaking, a recloser could also fulfill the role of a fault passage indicating Load Break Switch. Oftentimes during network switching operations, utility operators disable protection functions on reclosers as intermittent switching transients could cause spurious tripping and inadvertent supply interruptions. Nonetheless, it would be wasteful to disable protection in these devices and essentially blind the operators from any unintentional overloads which the switching operations might cause. The solution deployed in the NOJA Power RC10 is what is referred to as “Alarm Mode”, wherein disabling protection whilst in this mode removes all protection functions, but SCADA alarms are still generated for overloads. It is a simple application to ensure that operators are provided with the important information they require for their applications, but simultaneously allow them to continue to work as they always have, disabling protection functionality when they are unsure about network response parameters.
“Reclosers have evolved from both a cost and functionality perspective to allow them to be deployed as the sole overhead protection and control device on modern distribution networks eliminating the need for sectionalisers and remote controlled load break switches all together,” says Neil O’Sullivan, Group Managing Director of NOJA Power. “By adopting this strategy utilities are also best preparing themselves for the automated smart grid their customers will demand in the future.”
This hybrid capability opens up a world of opportunities for utilities, as utilities can replace aging sectionalisers with modern reclosers, and remotely update settings to convert them to reclosers at a later date once feeder study and development has been completed. Better yet, this level of versatility has meant for some utilities that they order only NOJA Power RC10’s to meet their entire suite of distribution network switching requirements. This has many cost saving advantages, with a common device to meet all distribution network applications. There is minimal warehousing cost, minimal training costs, minimal spares stock, and linesman only need to know how to install one piece of switchgear.
This level of evolution is what a utility has come to expect from their distribution network system assets. All the aforementioned capability are available as standard in the NOJA Power OSM® Recloser with RC10 Controller, once again demonstrating why the NOJA Power Recloser is clearly the most advanced recloser product on the market today. With this unrelenting investment in R&D designed to solve the modern utility networks requirements, NOJA Power provides an amalgamated solution to meet your network protection, switching and sectionalising demands.