Understanding the Role of Substations Along the Transmission Path

Voltage transformation is essential in power systems. For balanced three-phase systems, the real power transferred is:

$$P~=~sqrt{3}V_L I_L cos (theta)$$

Equation 1.

where:

V_L is the line-to-line voltage

I_L is the line current

cos(θ) is the power factor.

For a required power P and power factor cos(θ), raising the line-to-line voltage reduces I_L in direct proportion:

$$I_L~=~frac{P}{sqrt{3}V_L cos (theta)}$$

Equation 2.

Conduction losses scale with I²R. Stepping up the voltage reduces the current, so power loss decreases with the square of the voltage. This is the central economic and thermal rationale for GSUs at the plant fence and for maintaining EHV on long corridors.

Conversely, close to loads, voltages are stepped down to levels compatible with safety. Voltage is regulated with tap changers to keep end-user voltage within statutory limits. Figure 1 illustrates how voltages change between power generation and consumption.

Figure 1. Electrical grid (generation, transmission, and distribution systems). Image used courtesy of Wikipedia

Figure 2 illustrates a representative transformation path.

Figure 2. [click to enlarge] Typical voltage transformation path. Image used courtesy of EEPower

The specific breakpoints vary by region and legacy practice, but the functional logic is universal. In all cases, the path seeks to minimize current (and losses) over distance, then adapt to utilization constraints near the load.

In this article, we’ll learn about the roles of four electrical substation types along this path:

• Step-up substations.
• Transmission substations.
• Step-down substations.
• Distribution substations.

Because step-up and step-down substations serve complementary roles in moving bulk power efficiently from generators to end-users, we’ll start by examining both of them together.

Step-Up and Step-Down Substations: Operation and Objectives

Immediately downstream of generators, step-up substations elevate the generator terminal voltage (typically 11–24 kV) to transmission levels (tens to hundreds of kilovolts). Generator step-up transformers (GSUs) and high-side breakers are key substation components. The substation control and protection objectives are:

• Maintain transmission-bus voltage by coordinated excitation control and transformer tap selection.
• Limit fault duties seen by the generator and the switchyard.
• Provide a low-impedance path to ground consistent with the generator and system protection.

Overall, step-up substations seek to reduce current for a given power transfer, thereby minimizing I²R losses and conductor ampacity requirements over long distances.

At the transmission exit, step-down substations reduce extra-high-voltage (EHV) or high-voltage (HV) levels to sub-transmission and distribution levels, and then to utilization voltages at the customer interface. These transitions, which are driven by insulation, safety, and equipment design constraints across the grid layers, are specified in standard transmission/distribution voltage bands.

Step-down substations integrate power transformers with on-load tap changers (OLTCs) or automatic voltage regulators to counter feeder voltage drops under varying load, capacitor/reactor banks for reactive compensation, and switching equipment arranged to meet reliability targets (such as ring or breaker-and-a-half schemes feeding multiple distribution feeders). OLTCs are standard practice on HV/MV transformers to hold secondary voltage within tolerance while the primary varies with system conditions.

Table 1 summarizes the attributes of step-up and step-down substations.

Table 1. Attributes of step-up and step-down substations.

Transmission Substations

Transmission substations facilitate bulk power transfer between generators and regional grids, serving both as junctures for power injection and extraction. Figure 3 shows the layout of a typical transmission substation.

Figure 3. [click to enlarge] Typical transmission substation layout. Image used courtesy of electranet

A fundamental function of transmission substations is to provide interconnections—linkages between multiple transmission lines or systems, as well as between adjacent utility grids. This enables power rerouting during maintenance or fault conditions, supports load diversity, and fosters expanded energy exchange across regions. Figure 4 illustrates North America’s regional interconnections.

Figure 4. Interconnections of North America. Image used courtesy of NERC

Leveraging such interconnectivity enhances system reliability, enables diversity in generation mix, and reduces the need for redundant capacity by allowing shared backup resources.

Transmission substations are also instrumental in voltage regulation, ensuring that grid voltages remain stable across varying loads and system dynamics. Neutral buses, on-load tap-changing transformers, or reactive compensation devices are used to maintain voltage within prescribed ranges, safeguarding equipment performance and system stability.

Voltage regulation involves manipulating the reactive power balance. Injecting Volt-Amps Reactive (VARs) supports voltage improvement, while absorbing them suppresses excessive voltage. Reactive compensation devices include:

• Shunt reactors: damp capacitive overvoltage in lightly loaded long lines.
• Shunt capacitors: boost voltage during heavy inductive loading.
• Static VAR Compensators (SVCs): dynamically manage reactive conditions, offering fast voltage correction and stabilization.

The main functions of transmission substations are summarized in Table 2.

Table 2. Functions within transmission substations.

Distribution Networks

Distribution substations serve as the critical interface where high-voltage transmission and sub-transmission systems yield to the medium-voltage distribution network, facilitating the final voltage transformation stage before electricity is delivered to end-users. These substations typically receive input from at least two transmission or sub-transmission lines and step-down voltage levels—commonly spanning from 2.4 kV to 34.5 kV—for onward distribution via feeders to residential, commercial, and industrial consumers.

From the substation, power is dispatched through distribution feeders—medium-voltage circuits extending several kilometers that branch extensively to serve customers. These feeders can be overhead or underground and typically supply multiple distribution transformers, each of which steps down the medium-voltage power to low-voltage levels (such as 120/240 V residential service in the U.S.).

When it comes to network architecture, two configurations predominate:

• Radial networks, common in rural or less-dense areas, consist of single-ended feeders that branch outward, cost-effective but with lower reliability.
• Spot network or secondary grid systems, more typical in urban centers, are networks of interconnected transformers with parallel secondary sides. These systems provide a highly reliable supply, allowing seamless load continuity during faults and localized maintenance.

Voltage Regulation and Reliability Mechanisms

Distribution substations play a vital role in voltage regulation. Load fluctuations, line impedance, and feeder length introduce voltage drop; substations mitigate this through fixed-tap or on-load tap-changing transformers, sometimes complemented by shunt capacitors for reactive support. This ensures voltage remains within permissible limits for the served community.

Below is a representative hierarchy of voltage transition at distribution substations:

• Input (sub-transmission): 66 kV, 33 kV, or up to 34.5 kV.
• Output (distribution feeders): 2.4–33 kV, often 11 or 15 kV.
• End-use service voltages: 120/240 V or 230/400 V via distribution transformers.

Fault isolation and protection are another key focus. Substations incorporate protection equipment and switchgear to detect faults—whether at the transmission or distribution level—and isolate them, preventing system-wide disruption. Rapid sectionalization supports resilience and service continuity.

Service reliability is further enhanced through network designs, such as looped or interconnected feeders, backup transformer contingencies, and selective feeder tripping, ensuring minimal outage impact.

The key functions of distribution substations are listed in Table 3.

Table 3. Key functions of distribution substations.

Key Takeaways

The different types of substations we’ve discussed are all essential building blocks of the power system. They enable the smooth transfer of electricity from generation to consumption, safely and with minimal losses, across vast networks.

Featured image used courtesy of Adobe Stock