Power System Design

Power system design is an extremely broad term. Without context it can be anything from the interconnected north american grid to the power system on a single board computer.

In one of these systems 245kV is considered high voltage, and in the other 12V is really high. But these are the extremes, and my expertise is really in the middle. So when I'm talking about power system design its really everything from the plant substation to the lighting panel.

I don't spend a lot of time with utility transmission and distribution, and I don't spend a lot of time in the 120/208V or 120/240V design. It's within this context that I want to discuss everything power system design. Focusing on industrial power systems.

Parts of a Plant Power System

There are five (5) main areas of the power system that I want to outline, they are:

  1. Main Substation
  2. Primary Distribution System
  3. Secondary Distribution System
  4. Critical Systems
  5. DC power distribution for protection, control and SCADA

A couple of the systems that I'm not going to talk about here is any interconnected, on-site generation. At some point I may write more about my thoughts on microgrid systems and I will make sure to link there when I do so.

To try and include the most industrial facilities I'm going to assume that the system has all of these parts. If you're plant is smaller then it may not have its own substation or primary distribution. If you want you can skip right to Secondary Distribution.

Main Substation

The main substation consists of the utility connection, transformer, metering, and connection to the primary distribution system.

The exact POI will depend on the utility, it could be a HV switch, the utility breaker, or it could be on the plant side of the transformer. I'm going to assume its on the HV switch and the plant owns and operates the HV breaker.

Along with the main electrical equipment the main substation will have a control building, or e-house for the P&C equipment, metering, etc. The entire area will be fenced to control access and ensure the safety of anyone that has access to the site.

Primary Distribution

The primary distribution system is the medium voltage system that connects all the major load centers to the Main Substation. The voltage that this distribution system operates will vary depending on the location of the facility, and the size of the plant.

Some common voltage classes for this system in North America are:

  • 5kV for small plants
  • 15kV for larger plants
  • 45kV for large plants, or plants that have facilities that are remote.

The actual operating voltage could be 4.16kV, 13.8kV, 12.46kV, 25kV, 34.5kV, etc. The voltage level that makes sense to your facility doesn't really matter for this article, but it is important to pick the operating voltages and then live with them.

Like removing nuts from cookies, its painful to change after the fact.

Secondary Distribution

Critical Systems

DC Power Distribution

Power System Studies

There are three over all time periods that a power system study cover, they are:

  • Transient
  • Dynamic
  • Static

Transient studies look at the over all power system response to an event over a very short period of time. This time period can be from a portion of a cycle to 1 or 2 seconds. An example of a transient study would be analyzing the system response of a capacitor bank being switched on to the system, and verifying that it won't introduce an instability.

Dynamic studies look at the power system response over seconds to a minute until the system has reached a new steady state. An example of a dynamic study is starting a large motor, line an ID Fan. Motors have high starting currents and can have major impacts on the system voltage.

Static studies review the system in a steady state and is used to report the values. Load Flows, also referred to as Power Flow studies are good examples of this. They are used to determine the voltages, currents, and power factor at all the buses and cables within the power system.

We have a whole page outlining the different power system studies, learn more here, we also provide consulting services to help you develop a power system model and full report with recommendations.