DC (Battery) Control Power Equipment
The following is offered as a general guide for the selection and application of DC battery equipment.
The duty cycle imposed on the battery will depend on the DC system design and the load requirements. The duty cycle showing the battery loads in amperes and the lengths of time for which they must be supported will determine the sizing of the battery. Loads may be classified as continuous or noncontinuous. Continuous loads are classified as steady-state loads and noncontinuous loads lasting 1 min or less are known as momentary loads or short-time loads. Continuous loads are energized throughout the duty cycle and are normally supplied by the battery charger. Examples of typical continuous loads are
• Indicating lights
• Continuously energized coils and operating motors
• Inverters and annunciator loads
Examples of typical noncontinuous loads are
• Circuit breaker operations
• Motor operated valves
• Inrush currents of motors or other devices
• Field flashing of generators or synchronous motors
To size a control power source, each type of load must be known. For batteries, the steady-state and short-time loads must be converted to a common rate base. Also, the long-time loads must have specified time periods, since the battery is not a continuous source when the charger is off. After all the loads have been totaled, the next higher size of control power source should be selected. The reader is urged to consult IEEE standard 485-1997 (P485/D5, May 2008), “IEEE recommended practice for sizing large lead storage batteries for generating stations and substations” when selecting a battery for control power or other usage.
The capacity of the battery is usually expressed in ampere-hours, that is, the product of discharge current and time in hours. The basic rate is normally expressed for 8 h; however, many other rates are used to express battery capacity. In switchgear application, the short-time rates, such as 1 min rate per cell, is frequently used to express the terminal voltage drop early in the discharge period. The manufacturer’s data are usually given for cells at 25°C (77°F), and when the battery is at a lower temperature than the stated tem- perature, the battery rating must be reduced. The voltage drop due to discharge current specified in terms of 1 min rate per cell for a nickel–cadmium battery is 1.14 V, and 1.75 V for the lead–acid battery, at 25°C.
To convert the 1 min rate loads to the equivalent ampere-hours rate, the battery or switchgear manufacturer should be consulted. For sizing the capacity of a battery for switchgear, the worst case should be assumed. The worst case occurs when the battery has carried steady-state load for 8 h and then is sub- jected to maximum load involving 1 min rate. However, it is not uncommon for some installations to require batteries to be sized to carry the anticipated control and tripping loads for up to 24 h. For indoor locations, the battery temperature is assumed to be 25°C, and for outdoor application, −10°C.
Types of Batteries
For switchgear applications, two types of batteries are used: lead–acid batteries (flooded cells and valve regulated) and nickel–cadmium batteries. Lead–acid batteries are made in several types:
• Pasted plate with lead–antimony grids:
This is a basic lead–acid battery, similar to the common automobile battery. However, for switchgear control work thicker plates and lower gravity of acid pro-vide longer life. It is also suitable for long-time float charging. The expected life of this battery is from 6 to 14 years depending upon the
plate thickness. It is also the lowest-cost battery.
This is basically pasted-plate construction, with antimony replaced by calcium for additional grid strength. It has an expected life of about 25 years. Because of pure lead electrochemical characteristics, it requires slightly different charging voltages.
• Tubular positive:
This is also known as an iron-clad battery. These batteries are suitable for large stations and long-time load applications.
This is a long-life battery, with expected life of 20–25 years.
In this battery, the positive plate is formed form pure lead. Its short-term rates are somewhat higher and ampere-hours slightly less as compared to pasted-plate types. This is the most expensive lead–acid battery.
• Valve-regulated lead–acid (VRLA):
This is also known as maintenance-free battery. The cells of this battery are sealed with the exception of a valve that opens to atmospheric pressure by a preselected amount.
This battery provides means for recombination of internally generated oxygen and the suppression of hydrogen to limit water usage. The cells of this battery are sealed from the environment unless internal pressure operates the release valve.
• Round cell battery:
This is a pure lead–acid battery which has round cells instead of rectangular cells as found in the conventional battery.
The geometrical shape assures uniform growth, while the pure lead grid provides for slower plate growth as compared to lead–calcium or lead–antimony.
The nickel–cadmium battery is constructed with pocket-plate cells for switchgear applications. The battery plates have three different construction thicknesses. Medium or thin plate construction is used for switchgear applications. The maintenance of nickel–cadmium battery is less than the maintenance for lead–acid. Its low-temperature discharge currents are higher, and it can be charged more rapidly than lead–acid batteries. The cost of the nickel–cadmium battery is higher than the lead–acid battery.