Power Plant 101 for DAS

Not all of us in DAS have to deal with power plants, but having some basic knowledge does not hurt. That's the purpose of this post - a basic, high level summary of how a power plant for DAS or BTS head-end works.

1. Most (active) equipment at DAS or base station require DC electricity. For this post power is same is electricity; since electricity provides power. [AC power is what you get out of a regular wall outlet. DC power is what comes out of a battery.]
2. AC power is fed into rectifiers. Rectifiers convert AC to DC power 
3. Rectifiers supplies DC power to Distribution Bus Panel and Back-up Batteries. Distribution panels helps distribute the power to the devices connected to the Power Plant. Back-up Batteries store power for future outage - in the event AC power goes out. Back-up batteries are not essential for the operation of the devices - it's mainly for power outage.
4. Distribution Bus Panels are basically 2 large metal plate – 1 positively charged and 1 negatively charged.
5. Each equipment’s positive and negative poles are connected to positive and negative metal bars (i.e., Distribution Panel).
6. Breakers complete a circuit between the positive and negative metal bar and the equipment. Technically speaking, for the distribution panel, there is only one common or shared side to the distribution bus.  Whether it’s -48dc or +24dc driven, the feed side is an isolated circuit switched by its respective breaker.
7. If for some reason AC electrical supply to rectifiers shut down, batteries start supplying DC power to the equipment. Batteries are sized in a way that it can provide power to the equipment for typically 4-6 hours.

Power calculation - the basic process
Power calculation starts with how many devices you want to support and what is their power requirement. From that we can calculate the number of rectifiers needed. Rectifier calculation includes AC to DC conversion loss, as well as the power stream needed for charging the batteries. Batteries number and sizes can also be determined similar way - from the total power requirement of the devices. Once the number and sizes of rectifiers are determined, you can basically pick the model of power plant. You can then also determine the size of AC circuit that will be needed.

Other consideration: Emergency Power
Most often DAS (and BTS) head-end will be located in a data center of the building where the power supplies are already backed up with UPS (Uninterruptible Power Supply), or e-Power (emergency power), or generator. In these cases, you may not need back-up batteries. Keep in mind, that all UPS system is also designed for certain power load. And just because there is a power outlet does not mean you should just plug your device in. Any time you are adding a device to the UPS system, you are dropping the overall back-up time of all the connected devices. Consult with the building UPS engineer, if they have enough load available to support your equipment