Basic Power Distribution

Data Center Electrical Distribution "One-Line" Diagram
Data Center Electrical Distribution “One-Line” Diagram

Design engineers and technicians use schematics to build and troubleshoot complex circuits, while plant operators use single-line and riser diagrams to facilitate switching operations within their distribution system.

Knowing how to read and interpret various types of electrical drawings is an essential skill that all electrical workers must posses to effectively carry out their tasks. The symbols and lines within an electrical drawing speak a language that everyone involved must understand in order to design, build, and troubleshoot electrical systems.

One-Line Diagram

These drawings show the flow of electrical power or the course of electrical circuits and how they are connected. they should show all of the major components in the power system and list all important ratings. These drawings should be kept on display in the main control room of a facility to help guide switching operations by identifying feeders and the loads they serve.

Three-Line Diagram

For a more detailed view of an electrical distribution system, a three-line diagram is used to show phase relationship. In polyphase AC systems, these drawings illustrate the various connections for A, B, C, neutral and ground – each represented with their own line. This expands on the single-line by providing a basic visual guide for actual feeder cabling, instrument transformer connections, and protective devices.

Riser Diagram

To help illustrate the electrical distribution system of a multilevel building, the riser diagram is used. Riser diagrams show distribution components such as bus risers, bus plugs, panelboards, and transformers from the point of entry up to the small branch circuits on each level.

Schematic Diagram

The schematic diagram is used to emphasize circuit elements and how their functions relate to each other. Schematics are an extremely valuable troubleshooting tool that identify which components are in series or parallel and how they connect to one another. These diagrams should always be drawn with switches and contacts shown in a de-energized position.

Wiring Diagram

The wiring diagram contains all of the components in an electrical circuit and are arranged to show their actual physical location. Unlike a schematic diagram, which can be thought of as a conceptual drawing, the wiring diagram is designed for end users and installers who focus on making connections and troubleshooting components.

Bus Arrangements and Schemes

Electrical Bus Scheme Examples
Electrical Bus Scheme Examples.

The first requirement of any substation design is to avoid a total shutdown of the substation for the purpose of maintenance, or due to fault somewhere out on the line. The operational flexibility and reliability of the substation greatly depends upon the bus scheme.

Ring Bus

In the ring bus configuration, as the name implies, the circuit breakers are connected to form a ring, with isolators on both sides of each breaker. Circuits terminate between the breakers and each circuit is fed from both sides.

This scheme has good operational flexibility and high reliability, any of the circuit breakers can be opened and isolated for maintenance without interruption of service. If a fault occurs in this configuration, it is isolated by tripping a breaker on both sides of the circuit. By tripping two breakers, only the faulted circuit is isolated while all the other circuits remain in service.

The main disadvantage of the ring bus system is that if a fault was to occur, the ring is split which could result into two isolated sections. Each of these two sections may not have the proper combination of source and load circuits, this can be somewhat avoided by connecting the source and load circuits side by side.

Ring bus schemes can be expanded to accommodate additional circuits, but its generally not suited for more than six. Careful planning should be used with this scheme to avoid difficulties with future expansion.

Breaker and Half

When expansion of the substation is required to accommodate more circuits, the ring bus scheme can easily be expanded to the One and Half breaker configuration. This configuration uses two main buses, both of which are normally energized with three breakers connected between the buses.

In this bus configuration, three breakers are required for every two circuits – hence the “one and half” name. Think of it as, to control one circuit requires one full and a half breaker. The middle breaker is shared by both circuits, similar to a ring bus scheme where each circuit is fed from both sides.

Any circuit breaker can be isolated and removed for maintenance purposes without interrupting supply to any of the other circuits. Additionally, one of the two main busses can be removed for maintenance without interruption of service to any of the other circuits.

If a middle circuit breaker fails, the adjacent breakers are also tripped to interrupt both circuits. If a breaker adjacent to the bus fails, tripping of the middle breaker will not interrupt service to the circuit associated with the remaining breaker in the chain.

Only the circuit associated with the failed breaker is removed from service. The breaker and half configuration is very flexible, highly reliable, and more economical in comparison to the Double Bus Double Breaker scheme.

Protective relay schemes in this configuration are highly complicated as the middle breaker is associated with two circuits. It also requires more space in comparison to other schemes in order to accommodate the large number of components.

Double Bus Single Breaker

Each circuit is equipped with a single breaker and is connected to both buses using isolators. A tie breaker connects both main buses and is normally closed, allowing for more flexibility in operation. A fault on one bus requires isolation of the bus while the circuits are fed from the opposite bus.

Double Bus Double Breaker

Two buses and two breakers per circuit, both buses are normally energized and any circuit can be removed for maintenance without an outage on the corresponding circuit. Failure of one of the two buses will not interrupt a circuit because all of the circuits can be fed from the remaining bus and isolating the failed bus.

Main Bus and Transfer Bus

One or more circuit breakers may be used in this arrangement to make connections between the main and transfer bus. The circuit breaker to be maintained is now opened, isolated and removed for maintenance. The circuit under maintenance is transferred to the transfer bus. The switching procedure is complicated for maintenance of any circuit breaker. Failure of a breaker or fault on the bus results in an outage of the whole substation.

Levels of Power Distribution

Selective Electrical Power Distribution System
Selective Electrical Power Distribution System

In order to keep data centers running continuously and without interruption, several different sources of power and redundant systems must be kept in place and maintained at all times. The level of complexity will vary depending on the location and requirements of each data center but most large scale operations will include the following:

Medium Voltage Distribution

This is where power supplied from the local power company first enters the facility.

Low Voltage Distribution

After the medium voltage is “stepped down” to a more suitable voltage via transformers (generally 480V) it will enter some form of switchgear or switchboard where the electrical loads can be “split” to various areas around the Data Center.

Emergency Systems

Diesel/gas powered generators and automatic transfer switches are interconnected at the medium- or low-voltage distribution level.

UPS Systems

In the event of total power loss, the internal batteries can serve as a final lifeline for computer systems and exit lighting while temporary emergency power is restored.

Power Distribution Units (PDU)

Distribute electrical power to servers, networking hardware, telecom equipment, and other devices located within a data center. It does not generate or condition power

Remote Power Panel (RPP)

Provides a means of extension from PDU’s or other power sources directly to server racks. This offers added distribution capacity in the event that a PDU may have ample power capacity, but no extra breaker or panelboard capacity for additional equipment.