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Writer's pictureHüseyin GÜZEL

Design Guide to Digital Control and Communication Systems for Rural Substations

This technical guide provides design #guidance for the increasing numbers of #substations necessary to meet the increasing #electrical demands in areas served by Rural Utilities Service borrowers (here and after called cooperatives). This guide is intended for the benefit of cooperatives, their #consulting and #staffengineers, and others interested in #ruralsubstation #design and #construction concerns and considerations.


Design Guide to Digital Control and Communication Systems for Rural Substations
Design Guide to Digital Control and Communication Systems for Rural Substations, photo by ABB

Substations ought to be designed, built, and managed to fulfill customer requirements at the lowest feasible cost while aligning with the desired service quality. A standard system might encompass substations for #voltage #transformation, #sectionalizing, #distribution, and #metering, occurring multiple times from #generation to utilization.

 

The Table of Contents

 

1| Purpose and Scope of The Design Guide

This guide addresses rural transmission and distribution involving air-insulated, outdoor substations rated at 345 kV (phase-to-phase) and lower.


The engineer's potential design responsibilities encompass the creation of construction drawings, specifications for materials, equipment, and labor, as well as any additional engineering design services that may be necessary.


The role of engineering extends beyond merely providing designs and specifications. This becomes particularly crucial when a cooperative engages an engineering firm to augment its staff. Refer to the U.S. Code of Federal Regulations, Title 7, Part 1724 (7 CFR 1724), which outlines "Electric Engineering, Architectural Services and Design Policies and Procedures." It is vital that the contract between a cooperative and an engineering firm explicitly defines the scope of engineering functions to be carried out. For the purposes of this guide, the term "engineer" may refer to either the cooperative’s staff engineers or those from a consulting firm.


Engineers should apply these guidelines in conjunction with their experience and expertise. A list of references provided at the end of most chapters can assist in seeking further detailed information. It is advisable for substation designers to acquire and familiarize themselves with the relevant documents when other resources like #ANSI, #IEEE, #RUS, and #ASTM are cited.


Utilizing this publication for substation design typically leads to an economical solution from a system perspective. Over time, this should foster the development of standard designs for a particular system. Standardization is a beneficial and attainable goal that ought to be strived for.


The electric power industry is constantly evolving with technical advancements and updates to codes and standards, which may render some information in this guide obsolete. It is crucial for users to actively keep abreast of these technological changes.



2| Relationship of Substation to Overall Power System

A substation functions as a component of a larger system rather than as an independent entity. Typically, a power system is engineered to ensure that the impact of an outage (triggered by the failure of a single element like a transformer, transmission line, or distribution line) leads to the least possible service disruption and impacts the smallest number of customers.


Distribution Substation
Distribution Substation

When one component in a system fails, it typically results in an increased load on the remaining components. Design criteria usually account for such contingencies to ensure system resilience.


An outage consideration for a substation might involve a transmission switching station with a basic main bus. If this bus experiences an outage, it would cause a total disruption of power flow through the substation. The engineer must take into account additional equipment within the substation, like a transfer bus or an alternative multi-bus configuration. Moreover, the engineer needs to assess the surrounding system to see if the load can be rerouted around the substation during outages, thereby reducing the amount of equipment required within the substation.

In assessing the switching arrangement of a substation, engineers must consider the system configuration that the substation integrates with. It is essential that system contingency plans allow for the outage of substation components, both for scheduled maintenance and unexpected failures.


Most substations are engineered to function autonomously. They are typically equipped with remote indicators, control mechanisms, metering, and communication methods, allowing for the monitoring of systems and their components from a centralized location.



3| Importance of Adequate Substation Planning and Engineering

Substation planning takes into account the location, size, voltage, power sources, loads, and the intended function of a substation. Without proper planning, a substation might undergo unnecessary and expensive modifications.


The engineer's meticulous work necessitates the application of valid requirements and criteria, adherence to appropriate guidelines, and the utilization of the engineer's expertise to produce construction drawings and related documents suitable for the necessary system enhancements. It is crucial for the engineer to integrate the various constraints into an acceptable design.


Substation design choices and reasons for a new modern vs retrofit and upgrade an old one
Substation design choices and reasons for a new modern vs retrofit and upgrade an old one

In the design phase, engineers should eschew personal biases when addressing technical issues and adhere to nationally recognized standards, Rural Utilities Service (RUS) guidelines, or the cooperative's established designs.


A proper engineering design offers guidance for construction, material and equipment procurement, and future maintenance needs, considering environmental, safety, and reliability factors.



4| Types of Substations

Substations can be classified into distribution, transmission, and switching substations, or a mix of these types. A common design approach is to minimize costs by decreasing the number of substations and leveraging economies of scale. However, considerations for practical system design and reliability often necessitate numerous substations. Balancing these opposing perspectives is a key objective of system studies.


4.1| Distribution Substations

A distribution substation consists of switching, control, and voltage step-down apparatus designed to convert subtransmission voltage to primary distribution voltage for residential, agricultural, commercial, and industrial consumption.


The capacity of rural distribution substations can differ significantly. Typically, these substations are equipped with transformers ranging from 1.5 MVA to 5 MVA, with either one or up to three transformers. They may receive power radially, be tapped from a subtransmission line, or have dual sources of supply. The majority of cooperative substations operate with distribution circuits of either 12,470Y/7,200 volts or 24,940Y/14,400 volts.


The term "outage" often leads to confusion due to varying definitions. For instance, an industrial company using a variable-speed drive (VSD) stipulated a minimum number of outages for the incoming feeder, as any outage would cause the drive to shut down, incur several hours of delay in restarting, and potentially lead to environmental issues. The utility company, believing their definition of an outage aligned with the customer's needs, assured compliance. However, post-installation, the customer reported numerous outages that deactivated the VSD motor. It was then discovered that the customer defined an outage as any voltage reduction of 20 percent or more lasting longer than three cycles, whereas the utility defined it as a complete service interruption after all attempts to reclose a feeder had failed. This discrepancy in the interpretation of a common term led to unsatisfactory service, necessitating changes to the installation.

A distinct category of distribution substation is the dedicated customer substation. It resembles a standard distribution substation but is designed to serve a single customer, allocating its entire capacity to that customer's needs. The secondary voltages of such a substation are tailored to meet the customer's specific requirements. Close collaboration with the customer is crucial to define the technical specifications. It is often necessary to verify the technical terminology employed, as electrical engineers across various industries might use identical terms for similar, yet distinct, technical aspects.



4.2| Transmission Substations

A transmission substation consists of a switching, control, and voltage step-down apparatus designed to lower the transmission voltage to sub-transmission levels for the distribution of electric power to distribution substations.


Transmission Substation
Transmission Substation

Transmission substations typically house multiple large transformers and serve as major power distribution hubs. Their critical role in the system often warrants bus and switching configurations that are significantly more complex than those found in distribution substations.



4.3| Switching Substations

A switching substation consists of an assembly of switching and control equipment designed to offer circuit protection and enhance system switching flexibility. Switching stations are increasingly prevalent in cooperatives' transmission systems, where adaptable switching configurations can help sustain dependable service during unusual or maintenance scenarios.


References:

  • 7 CFR 1724, “Electric Engineering, Architectural Services and Design Policies and

  • Procedures.”

  • RUS Bulletin 1724D-101A, “Electric System Long-Range Planning Guide.”

  • RUS Bulletin 1724D-101B, “System Planning Guide, Construction Work Plans.”



Document:

Design Guide for Rural Substations by The United States Department of Agriculture

Format:

PDF

Size:

9.84 MB

Pages:

764

Download:


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