Industrial power systems can become very complex, and the related processes need a reliable source of electricity to keep the process running, both for economic and safety reasons. There are many different kinds of distribution systems. This article will only focus on one of them: the Main-Tie-Main system configuration.
Industrial Power System Main-Tie-Main Configuration
Main-Tie-Main, also formally referred to as a "secondary selective system" consists of two independent circuits connected together at the load buses by a tie breaker. See the figure below for details.
A simple main-tie-main system configuration
Reliability
The biggest advantage that a for an industrial power system that a main-tie-main configuration has over other system configurations is reliability. Usually, the tie breaker is normally open and the system acts as two independent circuits supplied by two independent sources. For example, we will assume that there is a fault on Source 2. This fault trips CB2, cutting off all power to Load 2. Immediately after power is removed from Load 2, the Tie Breaker closes. Source 1 is now providing power to both Load 1 and Load 2, and the system is able to perform its normal functions until the fault at Source 2 is repaired. When normal power is restored and CB2 is closed, the Tie breaker opens and the system resumes normal operation. Operation is only interrupted for a very brief moment, if it is interrupted at all!
The Main-Tie-Main configuration is also good for maintenance for this very same reason: you can open CB2 and perform repairs upstream from Load 2 de-energized, while still supplying Load 2 with power.
Transformer Sizing
One characteristic of the industrial power system main-tie-main configuration is that both transformers must be sized to appropriately handle the load of both buses. In our example, we will assume that both Load 1 and Load 2 are the same size. Transformer 1 and Transformer 2 must each be sized so that in normal operation they are only loaded to 50%. This way when the tie breaker closes, the transformer that is now supplying both loads doesn't become overloaded and blow up in your face. The downside to main-tie-main is that the added reliability inherently costs more due to the system requiring larger transformers than a system that would not tie both circuits together.
Unfortunately, reliability costs money, and you'll need to oversize your transformers to add reliability to your system in a main-tie-main configuration.
You can also reduce strain on each individual transformer when it is supplying both loads by adding external cooling to the transformers (like a fan cooling system). You can also simply accept that the transformers will have a reduced life in the event of a fault on a transformer or source.
You will not have just two loads in every case. In systems with multiple loads supplied by a single transformer, transformer size (and therefore cost) can be reduced by designing the system so that only essential operational loads (such as emergency lighting) are supplied power when the tie breaker closes.
Summary
For industrial power systems, a main-tie-main configuration is an extremely reliable power system distribution model, able to maintain power during a fault with little to no interruption. Unfortunately, this added reliability has a cost, whether in the form of larger transformers, extra cooling systems, or shorter transformer lifespans. These factors should all be considered when designing an industrial power system with main-tie-main in mind.
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