Distributed Generation Safety for Lineworkers
Distributed generation (DG) is also referred to as on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy, distributed energy and district energy. Its definition varies slightly from source to source, but for lineworkers, DG is anything that generates power, is connected to the grid and is not part of the normal generating system of whomever we are working for at the time.
A personal generator is the most common type of DG you will come into contact with in the field. These types of generators can range from small, portable, gas-powered units that customers use to run a few appliances during an outage, to large diesel or natural gas units that automatically start during an outage and supply power to an entire home or business. One of the primary safety concerns for lineworkers during storm work is backfeed on lines due to customer generators.
DG in the form of wind and solar generating facilities is becoming much more common around the country. It can appear as a single windmill or a few solar panels supplementing a residence or small business. The true usefulness of this type of power, however, is when it is derived from large-scale generating facilities that can include hundreds of windmills or acres of solar panels, which are also called “photovoltaic” or “PV.” These facilities often are not owned by the local utility, but are connected to their grid. And if there is a power source connected to the grid you are working on, you need to understand how it is going to interact with the system and what you need to do to isolate it and protect yourself from it.
DG System Components and Hazard Considerations
Without going into too much detail, it is important to understand the basic components of a DG system. There is a generation source (windmill(s) or solar panel(s)); a DC disconnect (power is generated as direct current in a PV system and can be either on wind farms); an inverter and transformer combination to change the current to AC and boost/reduce it to system voltage; an AC disconnect; and a meter. There can sometimes be an additional transformer between the AC disconnect and the meter depending upon the needs of the facility. And these components will be the same on a residential setup as they are in a generating facility, just on a much smaller scale. The inverter and disconnects for a residence can often literally fit on half a sheet of plywood. For large facilities, there can also be a set of disconnects or a gang switch on the pole where it connects to the utility grid. System voltage and configuration will determine whether this is feasible or even desirable.
A lineworker’s first instinct when dealing with a hazard of this nature is – or should be – to isolate it entirely. Just go to the source, find the disconnects, open them, tag them out until power is restored, and then reverse the process. Unfortunately, the location of these DG facilities, or even the exact number, is often not formally recorded by the utility. There could be hundreds or even thousands of them on a large system, and the cost and logistical difficulty of tracking them all on system maps, or of finding and opening them all during an outage, is staggering. The question then becomes, if you can’t isolate them from the system, how do you protect yourself from the hazard? There are three primary considerations.
1. External Disconnect Switches
These are the AC disconnect switches that were mentioned earlier. Even small residential systems are required to have them, and if you are aware that there is a facility – residential or commercial – and can find the disconnect, you can use it to isolate the hazard. The primary governing document for installations of this type is UL1741, and it does require the external disconnect switch. But where it has to be, and whether or not lineworkers can access it, is not spelled out.
The UL requirement for inverters on DG systems mandates that they have sensing capabilities to determine when the grid experiences an outage and that they react to the outage by isolating themselves from the grid within very few cycles. These inverters have been shown to be virtually 100 percent reliable. There has never been a known injury to a lineworker from backfeed onto a line from a DG facility associated with an inverter failure.
3. Standard Safety Precautions
Whether you are working under a planned outage or during a storm, there are basic safety procedures for isolating and protecting yourself in these situations. Test and ground every time, but be aware that generators may not trip when only the primary is grounded due to the voltage passing through the transformer impedance. You will have to ground or isolate the secondary to ensure generators cannot backfeed the line. Use nearby switches or other sectionalizing devices to clear yourself from the rest of the system. Pull meters or services if necessary. Wear your PPE. There is not now, nor will there ever be, any substitute for protecting yourself and your co-workers by using these basic protective measures during an outage.
DG systems have become a fact of life on our utility systems, so it is important that you educate yourself about how they function and be prepared when you encounter them in the field. They tend to be intrinsically safe to work around, but there is never an excuse for failing to follow basic safety procedures during an outage. Be safe!
About the Author: Mike Caro, CUSP, is senior transmission safety specialist at PowerSecure. He has 17 years of experience as a lineman and more than eight years of experience as a safety professional. Caro is a board member of the Utility Safety & Ops Leadership Network and serves on the national leadership committee for the Utilities Practice Specialty of ASSE.