IEEE Power & Energy Magazine - July/August 2019 - 51

This means that adding a device or application is a matter
of publishing messages to the network and subscribing to
the appropriate messages already on the network. These
features of IEC 61850 communications make a process bus
solution future proof. Any device or application supporting
IEC 61850 can connect to the network and interoperate with
any other device on the network while using the same configuration tools.
Tools to Implement Process Bus

An IEC 61850 process bus architecture is implemented with
discrete devices connected in standard ways to provide system solutions. The discrete devices consist of process interface units (PIUs), digital instrument transformers (DITs),
communication networks, clocks, and intelligent electronic
devices (IEDs).
PIUs will exist close to the primary equipment and be
the interface between the electrical physical world and the
digital world. Their role is publish information from the primary equipment and control it from digitized control signals
on the communications networks. These PIUs will control
circuit breakers and switches, connect to traditional current transformers (CTs) and voltage transformers (VTs), and

publish both equipment signals and digital sampled values to
IEDs for protection and operation purposes.
CTs and VTs will likely be replaced by DITs, which
are nonconventional instrument transformers (NCITs)
connected through a merging unit to output digital sampled values of current and voltage. NCITs use different measurement techniques from wire-wound CTs and
VTs. Examples of NCITs are fiber-optic current sensors,
Rogowski coils, and capacitive and resistive voltage dividers. In most applications, NCITs must be DITs. The output
of an NCIT is a low-energy analog signal that must be
sent digitally to ease the interfacing with equipment and
avoid interference issues because of distance and shielding. DITs are not subject to the inaccuracies of the excitation losses of a traditional iron-core transformer and can
provide superior accuracy across a large band. The high
accuracies of DITs will simplify and unlock new methods
of protection.
Many of today's protection algorithms have limitations
regarding the accuracy of instrument transformers, and
major portions of the protection algorithms exist to deal
with those inaccuracies. A prime example of this is the traditional sloped differential characteristic that accommodates

Utility
Enterprise

Cloud

Distribution
Automation

Mobile

Microgrid

Edge Manager

Control Plane

Security Services

Data Collector

Analytics

Data Services

Apps

Bay Control
as App

Protection and
Control as App

Substation Edge Device

Process Bus
Process
Interface Unit

Digital
Instrument
Transformer

Sensing

Process
Interface Unit

figure 3. A fully digitally enabled substation.
july/august 2019

ieee power & energy magazine

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IEEE Power & Energy Magazine - July/August 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2019

Contents
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