Generators require protection from both short circuits and abnormal operating conditions like overexcitation to prevent damage. Upgrading protection can prevent costly equipment damage and replacement costs during outages. While some abnormal conditions can be corrected by operators, automatic detection is usually required as events can be too fast for human response. Sequential tripping is used on steam turbines to prevent overspeed by first tripping the turbine valves before the generator breaker to reduce steam flow below the overspeed threshold. The paper outlines upgrading multiple functional protection areas in older generators to current standards using digital relays to cost-effectively protect generation assets.

1. Upgrading Generator Protection
Using Multiple Relay Ups

2. 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 some other power system components, need to be protected not only
from short circuits, but from abnormal operating 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 requiring
automatic detection and tripping.

3. In a properly protected generator, automatic protection against harmful abnormal
conditions is required. The bulk of this paper deals with the need to provide such protection.
Objections to the addition of such protection is not that it will fail to operate when it should,
but that it might operate improperly to remove a generator from service unnecessarily. This
concern about upgrading protection can be greatly reduced by understanding the need for
such upgrades, and how to apply them to a given generator. An unnecessary generator
tripping is undesirable, but the consequences of not tripping and damaging the machine are
far worse. The cost to the utility for such an occurrence includes not only the cost of repair or
replacement of the damaged machine, but the substantial cost of replacement power during
the periods when the unit is out of service.

4. An alert and skillful operator, at manned locations, can sometimes avoid
removing a generator from service by correcting an abnormal operating
condition. In the vast majority of cases, however, the event will occur too rapidly
for the operator to react and automatic detection is required. Operators have
also been known to make mistakes, creating abnormal conditions where
generator tripping is required to avoid damage. Inadvertent energizing and over
excitation are examples of such events. For these reasons, operating procedures
are not a substitute for proper automatic protection.

5. 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.

6. Currents in the Rotor Surface

7. Sequential Tripping
This method of shutting down a generator is used on steam
generators to prevent over speed when delayed tripping has no
detrimental effect on the generating unit. This method of generator
tripping was recommended by generator manufacturers of steam
turbines some years ago as a result of over speed generator failures and is
an upgrade item. It is used to trip the generator for prime mover
problems only where high speed tripping is
not a requirement. The first device tripped is the turbine valves. A reverse
power relay in series with the valves close position switches provides
security against possible over speed of the turbine by ensuring that steam
flows have been reduced, below the amount necessary to produce an
over speeding condition, before the generator breaker(s) are tripped. For
boiler/reactor or turbine mechanical problems, this is the preferred
tripping mode since it prevents the over speed of the machine. Figure 12
shows the block diagram for sequential tripping.

8. CONCLUSION
There are a number of functional protection areas on 20+ year old
generators which have significant shortcom- ings. This paper identifies
those protection areas and the risks of not addressing them. In addition, a
cost- effective strategy to upgrade protection to current industry
standards is outlined using multifunction digital relaying. Generation is
the single most expensive capital investment of a utility. Protecting this
investment to prevent failure should be a priority item with utilties as well
as non-utility generator owners.

9. Thank You