Purchasing, sizing, maintaining and testing batteries is key to power reliability in a substation.
The heart of a substation is the battery bank. If this were to fail, an electric utility could expose all feeders associated with the station to a condition where they could not ever trip in a fault. Not only that, but any backup devices, such as the main breaker on the low-voltage side or the high-voltage side protection of the power transformer, would all be inoperative, leaving the transmission grid protection as the only possible backup.
In many cases, however, the transmission grid cannot perform this function because a fault on the low-voltage side of a Delta Wye transformer, especially a phase-to-ground fault, will convert to a phase-to-phase fault on the high side. This would be particularly true if the fault was out a few miles from the station. This could then cause such catastrophic consequences as burning wire down across town and eventually destroying the substation transformer. After the smoke clears, much of the substation could be heavily damaged and the power transformer could be in flames. This is not to mention the hazard it would cause to the public.
Here are some dos and don’ts about proper substation battery maintenance that could save you and your utility time and money and give you peace of mind.
When trying to figure out what types of batteries you need, don’t skimp on your purchase. Instead, carefully think about what kinds of batteries you need to buy and install at your substation.
When trying to size batteries, you must first determine your maximum load. Since substation devices normally trip in milliseconds and the 1-minute rate of the size of batteries far exceeds what we need, this is usually not the deciding rating.
One thing to pay attention to, however, would be in the case where you have bus clearing schemes that will operate many devices simultaneously. This is especially important if circuit switchers are involved, which can have quite a large draw. After you have accounted for this, the next step is to determine how long you want to operate the station in an outage condition.
One way to do this is to plan for a minimum of 24 hours. This means that in a large systemwide event where resources are spread thin, you can monitor the station on supervisory control and data acquisition (SCADA) for 24 hours, close and open devices if needed, and then in the event power will not be coming back on in the area, get out to the station and either shut down the DC or set up a portable generator.
To size a battery for this part, consider your entire continuous load. This would include SCADA, lights, computers, pumps and control relays. Once you have this current, you can calculate the amp-hour capacity you need to buy. Lead-acid batteries are normally rated at the 8-hour rate and NiCad batteries are normally rated at a 5-hour rate. Also, as far as lead-acid batteries go, a battery has reached its end of life when it only has 80% capacity left. As such, you need to figure on 80% of purchased capacity if you intend to run these all the way to the end of life. The slower you discharge the battery, the more capacity it has in amp-hours. Therefore, if the battery has a published 8-hour rate of 100 AH and there is not a longer rate given, the 24-hour rate will contain at least as much energy as the 8-hour rate. Eighty percent of 100 AH is 80 AH. Divide this by 24 giving 3.33 A. This would be the draw you could have for 24 hours.
The battery is considered discharged at 1.75 V per cell or for a 48-V bank is 42 V. The above battery would hit this 42 V in 24 hours with 3.33 A draw at end of life.
Always check published data because the manufacturer may have some discharge rates for longer times than 8 hours.
Battery inspections should be performed monthly. These must consist of checking the electrolyte level and checking the float voltage of each cell and the bank float voltage. Don’t add distilled water until they get at least halfway down in the level operating zone. After distilled water is added, equalize it for at least 48 hours or until all cells are gassing vigorously to mix the newly added water with the electrolyte. If the voltage between cells gets too far off (per the manufacturers recommendations), you can equalize the batteries for at least 48 hours to try to bring them back in line. You can also take a specific gravity reading of a pilot cell, which will tell you the state of charge. This is normally okay if you have the float and equalize voltages set correctly.
Check every month that the battery bank is not grounded at either end. It should float with both ends aboveground. This is an easy test with a voltmeter and some chargers will give an alarm when this occurs.
Testing Battery Banks
You should perform a resistance test on the cells every year and perform a capacity test every five years. There are provisions to continue operating batteries once they fall short of stated capacity as long as you load test annually.
It’s often not advisable to follow this course of action, however, because the batteries are so critical to the overall reliability of the substation. If you’ve gotten 20 to 25 years out of them and they are getting down towards 80% to 90% capacity, think about replacement. The station batteries is not a good place to get the last little bit of use out of.
It’s critical that the batteries be monitored 24/7. You cannot chance them going dead due to a charger failure. If you don’t have SCADA at the station, you can put in cell phone monitoring similar to what you would have on a house as a monitored security system. This way, at least you can be notified if the charger fails and you can get ahead of a problem. Other than that, just keep them clean and keep the connections clean and greased.
Connecting the Loads to the Batteries
It’s important to spend a little time engineering these connections. To start with, DC is much harder to interrupt than AC because you never cross a current zero. The breakers in the DC panel must be rated for DC. The rule of thumb for fault current of lead-acid batteries can be about 12 times the 1-minute rate. It’s even higher for NiCad. Check with the manufacturer to get the available fault current and make sure the DC breakers have adequate AIC.
Don’t use a main breaker in your DC panel. Instead, order it with main lugs only. If you consider that the batteries are going to provide energy for your power circuit breakers to trip off faults, and if a power circuit breaker fails, an upstream power circuit breaker will back it up, but only if the upstream power circuit breaker has DC. If you use a main breaker on the DC panel, and for some reason that main breaker trips, you have no battery at the station and you have no back up. Look at what you have to lose and what you have to gain by using a main breaker in the DC panel, and it’s a no brainer.
Also, you should run conductors from the battery to the panel direct with no splices, no disconnects, but rather just extra flexible, CU, THHN, 600-V insulated wire. You should also consider using a single two-pole DC breaker per control panel. Other than that single breaker, you can slug any other protective devices in this DC path including the DC fuses in the power circuit breaker. That way, there is no risk of a tired fuse letting go and causing it not to trip when needed. As a last way to confirm that one DC breaker is not tripped, you can monitor it with SCADA. You can also use SCADA to monitor the trip coil circuit. With this plan, hopefully you will never experience a situation where you can’t trip when called on.
Gary Wright (email@example.com) consults for McMinnville Water & Light and Forest Grove Light & Power in Oregon. He started out in the power industry in 1977, and first worked at Cowlitz County PUD in substation design and distribution system planning.
Editor’s note: For more information on selecting the right type of battery for your substation application, please visit our website at www.tdworld.com/electric-utility-operations.