As covered in an earlier article, faulty DC power systems are one of the main reasons for failures in emergency power systems. A failed engine start or malfunctioning switchgear may be caused by the lack of a reliable DC power source, and more specifically, problems with the batteries supplying these emergency loads. Building redundancy into critical systems is a common design strategy, and the use of redundant battery banks is a practical approach to increasing the reliability of a DC power system. In redundant battery systems, the “best battery selector” is a key device employed to automatically select the best-available power source (battery) to connect to a load. This article focuses on basic functions and features of best battery selectors.
Redundancy Leads to Reliability
A redundant DC power system can add considerable reliability to an on-site emergency power system. A simple design may consist of two identical battery banks with battery chargers, and a “best battery selector”. As the name implies, the “best battery selector” is the device tasked with automatically selecting the best power source to maintain a realible power supply to the load (engine start motor, switchgear controls, etc.).
Basic and Enhanced Monitoring and Control Functions
A typical “best battery selector” is designed with high-power diodes to select the higher voltage of the two isolated DC power sources. These devices provide automatic selection and connection of the best-available battery to the load, while isolating the alternate (less capable) battery bank.
In some instances, it may be desirable to monitor and/or actively control some functions of the best battery selector. Enhanced features are available to allow control of power source usage, flexibility in selecting the active source of DC power, or to periodically cycle between sources. For example, from time to time, the operator may wish to alternate which battery bank is selected as the “primary” source.
Another useful feature includes the integration of an LCD display to provide voltage readings for each of the power sources. This can be very helpful in routine system checks and/or maintenance of battery systems. Best battery selectors may also be specified to include alarm annunciation for a low voltage condition (an early sign of battery trouble), and these alarms may be indicated locally and/or to a remote SCADA via ethernet or serial communication link.
Here is a typical block diagram for a redundant battery system utilizing a best battery selector:
This system illustrates two separate battery banks, each maintained by a dedicated battery charger. The output of each battery bank connects to the best battery selector, which in turn provides power to the load. In this example, the battery connections to the best battery selector are routed through circuit breakers. Circuit breakers provide overcurrent protection and allow ease of maintenance, but should be used only for constant load applications (not suitable for engine start applications due to the high in-rush currents).
Sizing Considerations
Best battery selectors are sized for the intended loads and the expected use. While engine-starting systems will experience intermittent high in-rush currents, utility/switchgear applications mostly deal with smaller continuous loads. A properly specified best battery selector should be sized to provide a long-life under the expected load profile.
In addition to the intended use, other key data to share with the equipment vendor should include: the system’s nominal voltage (24Vdc, 48Vdc, 130Vdc), the expected maximum in-rush current (if applicable), and the required continuous DC current rating.
Redundancy Also Means Higher Costs (does it really?)
How does one justify the additional cost to the facility’s owner? Is a redundant DC power system warranted for every project? Like most decisions related to investments in emergency power equipment, the initial costs must be weighed against the long-term potential risks of not adding a second layer of protection. This analysis is obviously project specific, and not all installations may require (or benefit from) added redundancy. If you have practical experience with redundant battery systems, how do you analyze their cost vs. benefit? If you have feedback on this subject, please share in the comments below.
There are many ways to design a reliable DC power system and, if you happen to come across a new project that could use some help in this area, I welcome you to reach out for my Design Advisory Services.
As always, thanks for reading!