Underground Secondary Networks

Secondary networks are commonly used to serve downtown business and mixed residential/commercial districts in an underground urban setting. They are typically made up of ‘square blocks’ requiring highly reliable service using underground facilities from just inside the substation all the way to the meter. The Figure shown to the left is an IEEE diagram that shows a typical secondary grid configuration, with 3 to 6 primary feeders feeding multiple transformers (typically 500-2500 kVA), that in turn feed a true network, (i.e., an interconnected system with loop flow). The principal secondary cables are called mains, from which services can be tapped. A network protector attached to each transformer prevents the network from experiencing reverse flow in the event of a primary fault (i.e., the network cannot feed the fault on the primary feeder even after the primary circuit breaker opens).

Generally Reliable, But Not Without Challenges

Secondary networks are generally highly reliable because a single failure on a primary feeder typically does not cause an outage on the network. Similarly, network mains provide redundancy to the system. So, if one main fails, the rest can serve the load without difficulty.

There are, however, challenges associated with these secondary networks:

• They are generally quite old, having been in service since the early years of the power industry. Many use Paper Insulated Lead Cable (PILC) often laid in old conduits and spliced via a method known as a ‘lead wipe’. Very few technicians are still trained to perform this properly properly.
• They function as a ‘black box’ in the manner in which they deliver electricity (unpredictable loop flows respond dynamically and automatically to load variations), and their overall health and condition (the feeders are underground and the overall system can function quite well even if significant portions of the network are not working properly). Secondary networks can often serve customers even when many of the mains have failed (advantage of the redundant nature of secondary networks). In fact, failed components are usually discovered when performing an inspection unrelated to system performance as customer outages are few and far between.
• However, when secondary networks do fail, they tend to fail catastrophically. Such failures may cause a widespread outage of a city’s central business district or densely populated urban areas. Moreover, service restoration time can be extraordinarily long and the consequences devastating.

By contrast, radial systems, though more prone to failure, are typically configured to have fewer numbers of customers affected, and will generally experience quicker restoration times.

Secondary networks have relatively high reliability but because of the three vulnerabilities listed above, most electric utilities have begun to shift load off of their secondary networks by taking advantage of building renovations and other construction to convert customers to a radial system.

Underground Secondary Networks Failures can be Catastrophic

The following incidents are examples of recent catastrophic failures of secondary networks in the U.S.:

• Commonwealth Edison, Chicago (August, 1999) – Failure of some cables in the secondary network resulted in a loss of service to the South Loop, including the Mercantile Exchange. Commonwealth Edison subsequently invested nearly $1 billion to address this and other challenges to their downtown system, including six additional substations
• Consolidated Edison, New York (July, 1999 and July, 2006) – Failure of the Washington Heights network in 1999 spurred inquiries at the city, state and even federal level, as the neighborhood north of Harlem lost power on one of the hottest days of the year. Consolidated Edison increased its capital budget by $200 million dollars to improve 57 secondary networks in an attempt to avoid recurrence. Seven years later, its largest secondary network (over-400MW Long Island City network in Northwest Queens) experienced blackouts in multiple areas, burning up mains trying to remain in operation in the face of failure to 10 network primary feeders.  After three separate inquiries that led to over 400 proposed remedial actions, litigation is ongoing and the costs to permanently fix the system are still under review.
• Potomac Edison Power Company, Washington DC (March 2000 through August 2001) – The Georgetown network, located in a historic district, experienced a number of incidents involving smoking manholes, explosions, launched covers, and extended blackouts (3-days). Three independent studies provided numerous recommendations, the primary one being a $60 million project to completely rework the heart of the network along Wisconsin and M streets.