Generator Protection
Application Guide
Introduction
This guide was developed to assist in the
selection of relays to protect a generator. The
purpose of each relay is described and related to
one or more power system configurations. A
large number of relays is available to protect for
a wide variety of conditions. These relays protect
the generator or prime mover from damage. They
also protect the external power system or the
processes it supplies. The basic principles
offered here apply equally to individual relays
and to multifunction numeric packages.
The engineer must balance the expense of
applying a particular relay against the con-
sequences of losing a generator. The total loss
of a generator may not be catastrophic if it
represents a small percentage of the investment
in an installation. However, the impact on service
reliability and upset to loads supplied must be
considered. Damage to and loss of product in
continuous processes can represent the domi-
nating concern rather than the generator unit.
Accordingly, there is no standard solution based
on the MW rating. However, it is rather expected
that a 500kW, 480V, standby reciprocating
engine will have less protection than a 400MW
base load steam turbine unit. One possible
common dividing point is that the extra CTs
needed for current differential protection are less
commonly seen on generators less than 2MVA,
generators rated less than 600V, and generators
that are never paralleled to other generation.
This guide simplifies the process of selecting
relays by describing how to protect against each
type of fault or abnormal condition. Then,
suggestions are made for what is considered to
be minimum protection as a baseline. After
establishing the baseline, additional relays, as
described in the section on Extended
Protection, may be added.
The subjects covered in this guide are as
follows:
• Ground Fault (50/51-G/N, 27/59, 59N, 27-3N,
87N)
• Phase Fault (51, 51V, 87G)
• Backup Remote Fault Detection (51V, 21)
• Reverse Power (32)
• Loss of Field (40)
• Thermal (49)
• Fuse Loss (60)
• Overexcitation and Over/Undervoltage
(24, 27/59)
• Inadvertent Energization (50IE, 67)
• Negative Sequence (46, 47)
• Off-Frequency Operation (81O/U)
• Sync Check (25) and Auto Synchronizing (25A)
• Out of Step (78)
• Selective and Sequential Tripping
• Integrated Application Examples
• Application of Multifunction Numerical Relays
• Typical Settings
• Basler Electric Products for Protection
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