Improving Reliability for Critical Customers

2/6/2018

By Eero Saarijärvi

RNA (Reliability Network Analysis) enables quantifying various reliability indices (e.g., customer interruption cost, SAIDI, SAIFI) for a given distribution network area. The evaluation is based on the componentwise failure impact and failure probabilities applied over the whole network. Network components in this context are, e.g., lines, disconnectors and distribution substations. Any component that can fail can potentially cause supply interruptions for the customers in the network; however, the likelihood and severity of these contributions strongly depend on where and which type of component is in question. For example, a failure on a trunk line of a radial feeder near to a feeding substation will likely contribute more customer interruptions than a failure of a line at some peripheral part of the feeder. This all, of course, is affected by the topological details of the network in question; whether there are adequate reserve connections available, how fast the fault can be located and isolated using the switches with different operating times (manual, remote of recolser), standby generators, etc.

Let’s think about this through an example case. Say we have a current network that is mainly overhead lines and among the customers we have one that is especially critical - a hospital, as illustrated below. As we can see, the hospital has two alternative supply directions; given the line from the North can supply the required load during contingency. However, there seems to be quite a few other things we could do to increase the electricity supply reliability for this critical customer.

Probably the cheapest thing to do would be to reconfigure the open points so that the long radial overhead line in the North-East part of the area would be removed from the feeder supplying the hospital. With this, the failures on that radial peripheral wouldn’t show on the hospital feeder (except for some voltage dips possibly). In this case, we could attach the long radial to the feeder on the left (with green topology highlighting). Of course, prior to such changes we need to check the line ampacities, voltage constraints, relaying, etc.

Next, we might do something for the remaining parts of the feeder that is supplying the hospital. There seems to be a rather long overhead line connection to the feeding substation (marked with the rectangle in the South). Perhaps we could allocate more maintenance resources in this area; to make sure the feeder corridors are safe from risky vegetation - let’s put some LiDAR bearing drones in the air! Alternatively, we could think of placing the supplying line underground.

Finally, we could make sure we have fast enough switching to be able to quickly take our reserve connections in use; maybe upgrading some of the manual disconnectors to remote ones and maybe even a recloser on the normal downstream side of the hospital. This would help to eliminate possible reclosings contributed from the downstream line sections. And, of course; the hospital itself has to check that the standby generators are always in a good shape and that the most critical instruments are backed up with UPS (uninterruptible power supply) to help overcome, e.g., voltage dips originating from the subtransmission side.

As we see, there are many alternative means to consider while aiming towards more reliable electricity supply of critical network customers - what would you propose? Perhaps RNA could reach its helping hand to get a good grip of this all.

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