Intro - Substations are electrical power systems that convert from one voltage to another and distribute power out. The term substation can refer to medium voltage and low voltage systems (often with the prefix "secondary"), but, in general, the term is reserved for high voltage applications installed outdoor, like what you see in Figure 1.
Figure 1: A Transformer in an Outdoor Substation
Parts of a Substation - Substations can have all kinds of components, just like any other electrical power system. However, we usually break down substations into a few key elements:
Main Transformer: The main transformer is the centerpiece of a substation. Substations can actually have several of these devices working in parallel. The main transformer steps down voltage in a distribution substation to convert high transmission voltages to lower distribution voltages. In a generating facility the main transformer does the reverse-the voltage is stepped up from a medium voltage level to a high transmission voltage to bring power to the end users.
Circuit Breakers: Circuit breakers are the second major piece of any substation. Breakers (along with their associated relays) provide protection against a variety of adverse conditions, including short circuits.
Disconnect Switches: Disconnect switches are essential for maintenance of a substation. They provide the ability to isolate pieces of equipment, including circuit breakers, when work needs to be performed. It's common to see disconnect switches on both sides of all major equipment (e.g. breakers and transformers) in a substation.
Surge Arresters: Surge arresters are used to provide protection against both lighting and switching surge conditions. These devices are often placed around major equipment like transformers and adjacent to any incoming overhead lines. While the substation itself will likely be protected from lightning strikes, the potential for damaging surges originating form incoming lines is very real. Surge arresters don't do anything during normal plant operation, but they play an important role in protecting equipment.
Instrument Transformers: Instrument transformers are used to sense voltage or current in the substation and convey this information to the relaying and protection system. Instrument transformers are also frequently used in substations to provide metering information. Metering-class instrument transformers have a greater level of accuracy than the devices used for relaying and protection, but metering-class devices have a more limited range of measurement.
Bus (Rigid or Flexible): Rigid bus is the primary current-carrying component used in substations. It's essentially a solid piece of metal that travels between components. Rigid bus is normally either rectangular or tubular, can be hollow or solid, and can be made of aluminum or copper. In situations where rigid bus doesn't work (e.g. because of significant seismic concerns), flexible bus can be used instead.
Stranded Conductors: Stranded conductors (or jumper conductors) are normally used in substations to connect bus to equipment. The flexibility of stranded conductors makes them worse at maintaining required clearances than bus. However, this same flexibility means that there is more margin in making terminations.
Figure 2: Annotated Substation Components (Source: OSHA)
Design Considerations - Designing a high voltage substation requires careful consideration of details that are not present in a normal medium or low voltage power system.
First, substations are usually located outdoor with bare conductors and buses. Clearances have to be maintained between conductors and from conductors to ground. These clearances are driven by a number of factors and detailed calculations of the exact required distances can be a bit of a chore (See IEEE 1247 if you're interested). Fortunately two standards exist that simplify clearance requirements to simple tables: NEMA SG-6 and ANSI C37.32. Both standards are fairly short and outline the values required between metal parts and ground. In summary, clearances become large with higher BIL (Basic Lightning Insulation Level) requirements and at higher elevations. In reality, things like clearances are usually mandated by utilities and grid operators. These requirements supersede any other standards.
Figure 3 is a satellite image of a substation from Orlando, Florida. Notice how the righthand side of the image has spacings that are much wider than the lefthand side. The transformers in the middle convert power from higher voltage on the right to lower voltage on the left. This reduction in voltage means energized metal parts can be placed closer together.
Figure 3: A Satellite Image of a Substation in Orlando, Florida. Notice the differences in Spacing between the High and Low Sides of the Transformers.
Next, substations need to be designed with appropriate reliability. Will there be a single main transformer or multiple? Will tie breakers be required to feed additional buses when the normal transformer is out of service?
Figure 4, below shows how a substation can be designed with or without added reliability considerations. In both cases, a high voltage input is dropped down to medium voltage. The redundant substation makes use of multiple transformers and circuit breakers to prevent single points of failure and allow for contingency operation when failures do occur. The non-redundant design is likely a lower cost to construct, but may not meet plant reliability requirements.
Figure 4: Reliable vs. Unreliable Substation Designs
The third major design consideration for substations is grounding. IEEE 80 and 81 outline the requirements for establishing an electrically safe work condition for both touch and step potentials. These standards go beyond the National Electrical Code and with good reason. Substations often have large fault currents from remote sources. When a line-to-ground or double-line-to-ground fault occurs from an incoming line, extremely hazardous voltages could develop without a proper ground grid in place. These ground grids are generally designed with something like 1/0 AWG to 4/0 AWG bare copper conductors placed in a grid. Usually, this grid possesses even spacing to help with constructability. Uneven spacings, like the red grid in Figure 5 below, may help with step and touch potentials but are more difficult to build.
Figure 5: A Constructible Ground Grid (Even Spacing) and a Non-Constructible Ground Grid (Uneven Spacing)
Fourth, we have to think about lightning protection. Substations have many exposed conductors, leading to a high probability of lightning strikes. A proper substation design will include the necessary shielding to minimize the risk of strikes, often through the use of a lightning mast. A lightning mast is just a large grounded structure that provides a preferred path for lightning current, away from valuable equipment. NFPA 780 is an excellent resource and a typical governing code for lightning protection design. The 150' radius "rolling sphere" method can be used to find the protected area from a lightning mast.
Figure 6: A Lightning Mast and its Zone of Protection Under the Rolling Sphere
Surge arresters are also important. Surge arresters need to be installed around major equipment and adjacent to incoming overhead lines to prevent lightning strikes and switching surges causing overvoltages.
Lastly, when designing a substation it's imperative to coordinate with other engineering disciplines, field personnel, and operations & maintenance teams. Just because something works electrically doesn't mean that it will work in practice. Foundations for substation equipment may be impractical, civil design considerations may limit the buildable area and require the selection of more compact equipment types, or the type of connections being used (say welded connections instead of bolted) may be at odds with labor capabilities.
Summary - Designing substations comes with its own unique set of challenges. Outdoor and exposed equipment leads to requirements on clearances and lightning protection. Remote sources at high voltage lead to more detailed grounding requirements. However, at the end of the day, the problem isn't all that different from a low voltage or medium voltage system. Short circuit, load flow, ampacity, and other standard power engineering problems still need to be addressed. Every substation is different-each design needs to be approached individually and with a high level of care.