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Waterpower '97
Mozina
11
Waterpower '97
August 58, 1997
Atlanta, GA
Upgrading
Hydroelectric Generator Protection
Using Digital Technology
Charles J. Mozina
Beckwith Electric Company
6190-118th Avenue North
Largo, FL 33773-3724 U.S.A. Mozina
1
Upgrading Hydroelectric Generator Protection
Using Digital Technology
Charles J. Mozina
1
Abstract
This proposed paper presents the reasons/rationale why hydroelectric generator owners should
consider upgrading the electrical protection of their generators to meet todays IEEE/ANSI stan-
dards. It specifically outlines the risks assumed by the owners in several functional protection
areas where 20+ year old generator protection is inadequate.
Introduction
Contrary to popular belief, generators do experience short circuits and abnormal electrical
conditions. In many cases, equipment damage due to these events can be reduced or prevented by
proper generator protection. Generators, unlike most other power system components, need to be
protected not only from short circuits, but from abnormal electrical conditions. Examples of such
abnormal conditions are: overexcitation, overvoltage, loss-of-field, unbalanced currents, reverse
power, and abnormal frequency. When subjected to these conditions, damage or complete failure
can occur within seconds; thus, automatic detection and tripping are required.
In the early 1990s, the IEEE Power System Relaying Committee conducted a survey to deter-
mine how major synchronous generators in North America were protected from short circuits
and abnormal electrical conditions. Survey findings indicated that despite the clear need to up-
grade older generator protection schemes to meet current standards, utilities seemed reluctant to
make needed modifications to existing power plants. This reluctance may be due to several
factors: a lack of expertise, a misunderstood belief that generators do not fail often enough to
warrant proper protection, or a belief that operating procedures will cover protection design
deficiencies.
Areas of Protection Upgrade on Older Generators
The areas of upgrading of 20+ year old generator protection fall into three broad categories:
1) Improved Sensitivity in protection areas where older relaying does not provide the level
of detection required to prevent damages. Examples of protection in this area are:
negative sequence (unbalanced current) protection
100% stator ground fault protection
2) New or Additional Protection Areas that 20 years ago were not perceived to be a prob-
lem, but operating experiences have proved otherwise. These areas are:
1
Manager Application Engineering, Protection and Protection Systems, Beckwith Electric
Company, 6190 - 118th Avenue North, Largo, FL 33773-3724, U.S.A. Mozina
2
inadvertent generator energizing
vt fuse loss
oscillographic monitoring
3) Special Protection Application Considerations that are unique to generators. These areas
include:
generator breaker failure
The IEEE/American National Standards Institute (ANSI) develop protection guides (see refer-
ences 1, 2 and 3) reflecting the need to provide the protection, which is outlined in this paper, in
the major upgrade areas cited. These guides express the views of both users (utilities/generator
owners) as well as the generator manufacturers and are reflective of in-service experience viewed
at a national level. The guides are updated on a five year basis to keep them current with both in-
service experience as well as changes in technology.
Improved Sensitivity Protection Areas
Negative Sequence (unbalanced current) Protection
There are a number of system conditions that can cause unbalanced three-phase currents in a
generator. These system conditions produce negative sequence components of current which
induce a double-frequency current in the surface of the rotor. The skin effect of the double-
frequency rotor current causes it to be forced into the surface elements of the rotor. These rotor
currents can cause excessive temperatures in a very short time.
The current flows across the metal-to-metal contact of the retaining rings to the rotor forging
wedges. Because of the skin effect, only a very small portion of this high frequency current
flows in the field windings. Excessive negative sequence heating beyond rotor thermal limits
results in failure. These limits are based on the following equation, for a given generator:
K=I22t
Where:
K = constant depending on generator design and size
t = time in seconds
I
2
= RMS value of negative sequence current in p.u.
The continuous unbalanced current capability of a generator is defined in ANSI C50.13 (refer-
ences 4 and 5). This standard states that "generator shall be capable of withstanding, without
injury, the effects of a continuous current unbalance corresponding to a negative-phase-sequence
current I
2
of the following values, providing the rated kVA is not exceeded and the maximum
current does not exceed 105 percent of rated current in any phase."
Type of Generator
Permissible I2
(percent of stator rating)
Salient Pole
With connected amortisseur windings
10
With non-connected amortisseur windings
5
These values also express the negative-phase-sequence current capability at reduced generator
KVA capabilities.
It is common practice to provide protection for the generator for external unbalanced current
conditions that might damage the machine. This protection consists of a time overcurrent relay
which is responsive to negative sequence current. Two types of relays are available for this
protection: an electromechanical time overcurrent relay with an extremely inverse characteristic,
and a static or digital relay with a time overcurrent characteristic which matches the negative
sequence current capabilities of the generator. For open conductor or open breaker pole condi- Mozina
3
tions, the negative sequence relay is usually the only protection. The low magnitude of negative
sequence currents created by this type of event (typically 10-20% of stator rating) prevents other
fault relays from providing protection. For electromechanical negative sequence relays, the mini-
mum pickup can be set to provide only 60% of stator rated current sensitivity. Thus, these relays
will provide no protection for open phase or open generator breaker pole conditions which are
frequent negative sequence events within the industry. The sensitivity of negative sequence static
or digital relays is required. Almost all 20+ year old generators are protected with electrome-
chanical negative sequence relays which make this an important upgrade area.
100% Stator Ground Fault Protection
High-impedance generator neutral grounding utilizes a distribution transformer and a second-
ary resistor. The secondary resistor is usually selected so that for a single line to ground fault at
the terminals of the generator, the power dissipated in the resistor is approximately equal to the
reactive volt-amperes in the zero sequence capacitive reactance of the generator windings, its
leads, and the windings of any transformers connected to the generator terminals. Using this
grounding method, a single line to ground fault is generally limited to 3-25 primary amperes.
R
59
N
Figure 1
High Impedance-Grounded Generator
The most widely used stator ground fault protective scheme in high impedance-grounded
systems is a time- delayed overvoltage relay (59N) connected across the grounding resistor to
sense zero-sequence voltage as shown in Figure 1. The relay used for this function is designed to
be sensitive to fundamental frequency voltage and insensitive to third harmonic and other zero
sequence harmonic voltages that are present at the generator neutral. Typically, the overvoltage
relay has a minimum pickup setting of approximately 5 V.
The 59N protective scheme is straight forward and dependable, however, this relay will pro-
vide protection for only about 95% of the stator winding. This is because a fault in the remaining
5% of the winding, near the neutral, does not produce a sufficient 60 Hz residual voltage. It is
important to protect major generators with an additional ground fault protection system so that
fault coverage for 100% of the winding is obtained. Twenty plus year old generators typically
have only 95% of the stator winding protected for ground faults. Many utilities have upgraded
protection to provide 100% stator winding ground fault protection. One method is the use of a
third-harmonics undervoltage relay. Third-harmonic voltage components are present