IEEE 946:2004 pdf free download – IEEE Recommended Practice for the Design of DC Auxiliary Power Systems for Generating Stations
In normal operation, the battery and charger(s) are both connected to the dc distribution bus and, therefore,operate as parallel sources to supply the connected loads.The charger,in addition to charging the battery,carries the normal continuous load.Chargers are typically provided with a current limiting circuit foroverload protection.If overloaded, the charger output voltage will drop, causing all loads in excess of thecharger rating to be supplied by the battery. In the event of: a) a failure of the ac power supply to thecharger(s) or b) a charger failure (or the removal of the charger from service), the battery must supply all ofthe power required by the loads for some designed period of time.
Specific design guidance for dc systems for nuclear generating stations are discussed fully in numerousdesign standards found in Annex A.
4.2 Number of batteries
As a minimum, one battery should be provided for each unit. If the auxiliary loads of one unit are dividedinto two or more independent systems, then each independent system should be provided with a separatebattery. If the maximum dc power requirements exceed the capacity of one battery, then the developmentof two independent systems should be considered.Cross-tics from other batteries may be provided,whereappropriate (see 7.7).
For Class 1E nuclear applications, as a minimum, a separate battery shall be provided for each EngineeredSafety Feature (ESF) Division in each unit in order to provide the required independence betweenredundant Class 1E power systems.For increased operating flexibility in designs where the reactorprotection system channels are dependent on dc,the number of safety-related batteries provided on eachunit should equal the number of independent and redundant reactor protection system (instrumentation andcontrol) channels.For example,in a unit with four reactor protection channels,four batteries should beprovided. The rated capacity of each battery should be determined as described in 5.2 of this guide.
4.3 Number of chargers and distribution panels
As a minimum，one battery charger and main distribution panel should be provided for each battery.Additional (spare or standby) chargers should be considered for increased operating flexibility and plantavailability (see Clause 8.)
4.4 System voltage and battery size considerations
The nominal system voltages of 250 V, 125V, 48 V, and 24 V are generally utilized in station dc auxiliarypower systems. The type, rating, cost, availability, and location of the connected equipment should be usedto determine which nominal system voltage is appropriate for a specific application.250 V systems aretypically used to power motors for emergency pumps，large valve operators，and large inverters. 125 Vsystems are typically used for small inverters, dc power supplies,control power for most relay logic circuitsand the closing and tripping of switchgear circuit breakers. 48 v or 24 V systems are typically used forspecialized instrumentation or communications systems.
For a given dc auxiliary power system, the connected equipment governs the maximum and minimumvoltage operating limits. Standards such as IEEE C37.16TM-1997 [B17],IEEE Std C37.90TM-1989(R1994)[B18], and NEMA MG 1-1998 [B19] establish voltage limits for various equipment and relays (see 7.3).Operating outside the voltage limits may affect the expected life of the equipment, the speed at which itoperates, the available torque, or the device’s capability to operate. When the voltage limits for a givensystem are established, the size of the battery (number of cells and capacity) and the operating proceduresshould be established as outlined in IEEE Std 485TMand IEEE Std 450TM-2002 [B7], respectively.