By George Brendahl, Marketing Manager, Nuclear and Utility, EnerSys
The reliability of the electrical grid is directly dependent on switchgear, which basically consists of disconnect switches, fuses, circuit breakers and batteries used to manage electrical equipment.
One of the main functions of these devices and the back-up DC power that batteries provide is to interrupt shorts and overloads on the electrical grid while continuing to provide service to customers.
The electric utility switchgear application is typically characterized by a complex duty cycle that has changing loads throughout the discharge period. The duty cycle typically has a high, short duration load in the beginning of the discharge for applications such as breaker tripping, followed by one or more constant loads for applications such as control circuits, communications and monitoring circuits, or for lights and alarms.
If the end-of-the-duty cycle is met before the interruption is cleared, there is an additional high, short duration discharge. That occurs as circuit breakers function to disconnect the load to avoid low-voltage damage to both the loads and the batteries.
Factors to Consider When Choosing a Battery
Switchgear applications typically use flooded wet cells, although other technologies are considered and used at some sites. Here are a few factors to consider when choosing the proper battery for switchgear use:
· One-Minute Rate: The first minute of the duty cycle — usually characterized by a peak current attributed to critical breaker tripping operations — often determines the size or capacity of the battery required.
· Duty Cycle versus Ampere-hour (Ah) Rating: Selecting a battery dependent on Ah alone is generally not recommended (unless replacing batteries with original models). Battery attributes such as plate size and thickness, amount of active material and concentration of acid help regulate how much current is provided for a given time signature. Ah rating is typically provided at the eight-hour rate down to an end voltage of 1.75 volts per cell (VPC). One-minute rates can vary among manufacturers and differently designed battery models even though the eight-hour rate is the same. It is critical to review the sizing calculations before selecting a battery based on Ah rating alone.
· Design/Operational Life: It is important to understand the difference between the warranty’s term and design life. For example, European and North American batteries are specified on different specific gravity and operating temperature ranges. This needs to be taken into account when comparing the two, as higher temperature and specific gravity will increase the grid corrosion rate and, subsequently, decrease life expectancy.
· Shelf Life: Batteries have a limited storage life, and often are not installed immediately. If kept in storage, the battery must be charged periodically to counteract the self- discharging that occurs. Length of storage and frequency of charge is dependent on environmental considerations. Be sure to follow the manufacturer’s manual to ensure proper battery function once the battery is installed.
· Hydrogen Evolution and Ventilation: All batteries produce gases that can concentrate into combustible levels under both normal and abnormal functioning. Different battery chemistries vent varying amounts of gas, and room ventilation must be addressed, especially for wet cells. It is extremely important to provide proper ventilation to exhaust the gases produced. Many substations were constructed years ago with specific battery chemistry in mind. Existing ventilation must be able to support the new chemistry.
· Temperature and Capacity: The relationship of output capacity to temperature is an inverse function. Battery ratings are based partly on a given temperature, which is optimal around 77 F (25 C) for IEEE standards or 68 F (20 C) for DIN standards. If the operating temperature is lower than this, capacity is reduced. If operating temperature is higher, the resulting capacity is increased. It is always important to size the switchgear battery for the lowest operating temperature.
· Life Cycle Cost: Lower specific gravity and additional lead weight can lessen positive grid corrosion, which can yield a longer operational life. Conversely, frequent cycling and high temperatures can reduce operational life. These factors are important and should be considered in battery selection.
The battery is an integral and extremely important part of the switchgear application. Choosing the properly sized battery for the site requires assessing many factors, including the proper technology for the intended use. Each technology -- whether flooded flat plate, rod plate, nickel cadmium, tubular or Thin Plate Pure Lead (TPPL) -- has its own advantages and disadvantages.
Ultimately, the user selects what is needed. A full-service battery manufacturer that offers several different technologies can help provide insight into the many factors involved in battery selection for specific applications.
EnerSys, the global leader in stored energy solutions for industrial applications, manufactures and distributes reserve power and motive power batteries, battery chargers, power equipment, battery accessories and outdoor equipment enclosure solutions to customers worldwide. Motive power batteries and chargers are used in electric forklift trucks and other commercial electric powered vehicles. Reserve power batteries are used in the telecommunication and utility industries, uninterruptible power supplies, and numerous applications requiring stored energy solutions including medical, aerospace and defense systems. Outdoor equipment enclosure products are utilized in the telecommunication, cable, utility, transportation industries and by government and defense customers. The company also provides aftermarket and customer support services to its customers from over 100 countries through its sales and manufacturing locations around the world.