CHAPTER 9

Generator Protection

9.1 INTRODUCTION

The generator is the core of the Power System. A modern generation unit is a
complex system that includes the stator windings and its associated transformer, the
rotor with its field winding of the exciter, the turbine, etc. Failures can occur
varied nature within a system as complex as this, which requires a
very comprehensive protection system whose redundancy will depend on the capacity, type, and the
relative importance of the generator within the Power System, but others also influence,
as well as its connection and the types of regulation and control systems it has.

The failure rate in well-built generators is low due to modern designs.

and to the improvement of insulating materials, but defects and failures can occur and result
in severe damage and long service suspensions. For these reasons, the conditions
abnormalities must be recognized quickly and/or the area of the problem must be isolated.
7

Some of these conditions do not require the unit to be automatically disconnected.

since the problem can be corrected at serviced stations, while the machine
remains in service (They are indicated with alarms).

Generators are the elements of the Power Electric System that can be
subjected to the highest number of different abnormal operating conditions, which grants
a great diversity to its protections. These abnormal conditions may be due to the very
generator, to its primary motor, or to the electrical system to which it is interconnected, and can in
general subdivided into internal faults and regimes of conditions, the following are listed
the fundamental types of abnormal operating conditions.

ABNORMAL OPERATING CONDITIONS

INTERNAL FAILURES IN THE ABNORMAL REGIMES OF
STATOR OPERATION

A. Balanced overcurrents or
unbalanced in the stator due to
A. Short circuits between turns
external overloads or short circuits
of a phase

B. Loss or reduction of excitation

C. Overvoltage
B.Ground short circuits
Ground contacts on the rotor
E.Loss of synchronization
C. Short circuits between phases
F.Asynchronous connection
G.Loss of the primary engine
motorization
H.Sub-synchronous oscillations
I. Rotor overheating due to
overexcitement

J.Others (vibration, overspeed,

bearing problems, etc.

This chapter addresses the fundamental types of protections for a generator and
the most general criteria that are followed for their application and determination are discussed
if they should provoke the disconnection of the generator or just emit an alarm signal, the
American and European trends and the perspective on the evolution of protection
 9-3

generators as well as the application of such protections depending on the capacity of the
machine.

9.2 PROTECTION AGAINST FAULTS IN THE STATOR WINDINGS

The protection of generators against failures in the stator windings can be done with
differential or overcurrent relays. The generators with capacities exceeding certain
1000 kVA are generally protected by percentage differential relays, while in the
Small generators, primarily used in industries, have protections installed.
of overcurrent. In some cases, differential-type schemes are used consisting of
starting from overcurrent relays.

It is advisable to first analyze the basic schemes for the connection of the
generators, taking into account the influence of that connection on their protection against failures
in the stator windings; There are two basic schemes for connecting different
generators of a plant:

a) Direct connection to a common generation bus.

b) Connection in units or blocks generator - transformer.

Figure 9.1

The direct connection is shown in Figure 9.1, and in it the different

generators are connected through switches to a bar, where they in turn are
connected through switches the step-up transformers, as well as the transformer
auxiliary (Tafor the plant's own consumption. This is the typical connection of the generators.
9-4

small industrial electrical systems. Generators are generally connected

in star, however, there are some cases where the connection is in delta.

Figure 9.2
In the block connection or generator-transformer units (Figure 9.2) each generator
it connects directly to its own transformer and the switch between them is omitted. This is
the most common connection in plants formed by units of large capacities and in it
the connection of the generators is star, except in very rare exceptions. In each unit the
auxiliary transformerTais connected to the output of the generator, and the parallel connection of
All units are done on the high voltage side of the step-up transformers.

9.2.1 DIFFERENTIAL PROTECTION OF THE GENERATOR (87G)

SYMBOLISM

In section 8.4, the operating principle of differential protection was introduced and
they presented the advantages of percentage differential relays. This protection for
the case of the generator, compares the secondary currents of the current transformers
on the load side and the neutral for each phase of the generator, providing rapid detection
and sensitive to faults between phases (three-phase faults, two-phase faults) within the area of
protection. It also detects bifacial ground faults and single-phase ground faults, the latter
depending on how solidly the generator is grounded. However, the relay of
 9-5

differential percentage does not detect faults between turns in a phase, because there is no difference
between the input and output of the phase current, which is why protection should be used
separated for faults between turns.

In the differential protection of the generator, current transformers are used with
identical characteristics and it is preferable not to connect other relays or other devices to these
current circuits.

When there are split-phase generators (American tendency to use two windings
in parallel per phase) it is customary to measure the current only in one of the windings in
parallel on the neutral side, using aCTwith a transformation relationship equal to the
half; the advantage of this scheme is that it allows for the detection of excitation poles in short
an indirect way, due to the current imbalance between the split windings, being
subjected to different magnetic flows.

Percentage differential relays are not sensitive to ground faults in totality.

winding in generators grounded solidly, nor operates at all for
generators grounded through impedance. Approximately the first 10% of
Winding is not protected with this relay, however, this 10% is covered by the protection.
of failure between turns.

When the generator is connected directly to the step-up transformer without a switch in between.
medium (Block Connection, Figure 9.2), this connection is protected by two differential relays
percentages: one for the generator and another for the generator-transformer group.

The connection of the differential protection depends on whether the neutral is connected internally or
if the three terminals of the neutral (each phase) are available to place transformers of
current on both sides of the windings.

Figure 9.3 illustrates the arrangement of current transformers for the connection of
differential relay of a machine connected in star and where the 3 are available
terminals (each phase) of the neutral.
9-6

Figure 9.3
 2

Figure 9.4 illustrates the case for the connection of the differential relay when only
it has a terminal coming out of the neutral, that is, when the neutral connection is made
internally in the machine.

Figure 9.4

9.2.2 DIFFERENTIAL PROTECTION WITH OVERCURRENT RELAYS (50, 87G)

A variant of differential protection with overcurrent relays for generators of

small capacities are shown in Figure 9.5. In it, a different one is used in each phase.
toroidal core current transformer, through whose window the primary conductors are passed
the neutral and output conductors of the generator, so that both currents flow in
opposition under normal conditions. In the secondary, an overcurrent relay is connected.
instantaneous that functions as a differential with high speed and sensitivity, since the scheme
it is free from the differential error current, for having only one current transformer per
1

phase. This protection primarily responds to faults between phases or to ground, but its sensitivity
For grounding faults, it depends on the grounding impedance of the generator's neutral.

Figure 9.5

The scheme is applicable only to small capacity generators, given the need
of passing the conductors through the core of the current transformers. Another
The limitation is that the protection zone does not include the part between the
current transformers and the switch, unless the transformers are placed
After the switch, take the conductors from the neutral side to them.
generator.

9.3 Protection Against Failures Between Turns

Differential protection does not respond to short circuits between turns of the same phase in the
stator, because these faults do not cause differences between the currents that enter and exit
to that phase. It is necessary to wait for the short circuit to extend to ground or to another phase to
that can be detected, which causes additional damage to the machine, that can be avoided
with a protection against turns failure.

This protection applies almost exclusively to hydraulic turbine generators or

hydrogenerators, since steam turbine generators or turbogenerators have for the
general coils of a single turn and faults between turns that do not occur in them
involve earth. There are two trends in the design of this type of protection: the American and
the European.
 9-9

9.3.1 AMERICAN TREND

The American trend in hydraulic turbine generator design is to build the

generator with multi-circuit windings, that is, with two or more circuits per phase. This
the design of generators is called Split Phase scheme. In this scheme,
the circuit in each phase of the stator winding is split into two equal groups,
comparing then the currents of each group. Figure 9.6 illustrates a general form.
from the relay connection for the protection of phase loss for a multi-circuit generator.

Figure 9.6

A difference in these currents indicates an imbalance caused by a coil failure. The

relay used in this scheme usually consists of an instantaneous overcurrent relay and a
inverse time overcurrent relay.

When there is a normal overcurrent value between windings, the overcurrent relay
the timer does not respond until the value reaches the start threshold of the current
imbalance due to a failure between turns. The delay is used to prevent operation
for a transient current of theCTgenerated by external failures.
9-10

The start value of the instant unit must be set above the currents.
transitory ofCTthat can occur due to external failures. The resulting adjustment offers a
partial protection against turn-to-turn faults. However, it is a cost-effective backup for faults.
that involve multiple turns and phase failures.

A very common practice is to combine differential protection and short-circuit protection between
coils in a single relay. This is for the savings of current transformers and relays (Figure 9.7). Without
However, this arrangement is not as sensitive as that of the two separate relays.

Figure 9.7

9.3.2 EUROPEAN TREND

A short circuit between turns of the same phase results in a decrease in voltage at
this phase. This decrease produces an imbalance in the three-phase tension system of
generator, that is, a displacement of the generator's neutral from its equilibrium position
(earth).

The appearance of this imbalance, manifested in a zero-sequence tension, is used

to detect this type of faults. This zero sequence can be detected by means of a relay
 9-11

tension localized in the secondaries of the delta-connected potential transformers

rotation, as illustrated in Figure 9.8. This protection complements the protection
differential for faults near the neutral point of the generator.

Figure 9.8

9.4 PROTECTION AGAINST GROUND FAULTS OF THE STATOR

When the insulation of a generator fails (which is the most common cause of internal failure), the
the resulting short circuit can begin between turns and then extend to ground, or
start as a ground fault directly. The ground short circuit involves the core of the
stator, so the flow of a high-value current can melt part of the iron or
cause much greater damage than the simple failure of insulation. The repair of this type of
the breakdown is more expensive than the winding replacement, as it involves changing laminations of the
stator core in the damaged area. For this reason, in generators connected in star
measures are taken to reduce the ground short-circuit level to small values, which to its
sometimes causes the differential protection to not be sensitive enough to
detect ground faults, and additional protection is required for this purpose.

In a low resistance grounding, that resistance is selected to limit the

contribution of the generator to single-phase ground faults at its terminals to a range of
current between 200 A and 150% of the total load current. With this range of currents
available faults, the differential relay provides earth fault protection. However,
9-12

since differential protection does not provide ground fault protection for the entire winding
stator phase, it is common practice to use, as a supplement, a sensitive protection
for ground faults. This protection can be implemented with a directional current relay
polarized or with a timed overcurrent relay.

When a directional overcurrent relay is used, the biasing coil is energized.

from a current transformer in the generator's neutral while the coil of
operation is in the differential protection scheme of the relay. This application gives
sensitivity without a high coil operation 'burden' (Figure 9.9).

Figure 9.9

When an overcurrent relay is used, a sensitive overcurrent relay is connected.

timed in the neutral of the differential scheme.
 9-13

In both cases, the ground fault current protection only detects faults covered by the
differential zone, hence the need to coordinate the relay time with others is eliminated
system relays.

In practice, it is common to add a ground-sensitive timed overcurrent relay in the

neutral of the generator. This relay provides backup for earth faults of the generator and external faults.
Another type of protection against generator ground faults is with an overvoltage relay.
which is described below.

9.4.1 OVERVOLTAGE RELAY FOR GENERATOR GROUND FAULT (59G)

SYMBOLISM

When high impedance grounding is used for the generator neutral, the current
Earth fault is limited to values that the differential relay does not detect. For this reason, it is used

main and backup ground fault protection.

The most commonly used protection scheme in the grounding method with transformer of
distribution with load resistance is the time-delay overvoltage relay connected through
of a high ground impedance, which senses the zero sequence voltage and operates at a
determined time, when a specific voltage value is exceeded (Figure 9.10).

The relay used for this function is designed to be sensitive to the fundamental component of
the tension is insensitive to the third harmonic and to other zero-sequence tension harmonics that
they are presented in the neutral of the generator.

Since the ground impedance is greater than the generator impedance and other impedances.
in the circuit, it will see the entire phase-neutral voltage when there are faults between phase and ground
the terminals of the generator. The voltage at the relay is a function of the ratio of the
distribution transformer and the location of the fault. The voltage will be maximum for failure in
9-14

terminals and decreases in magnitude when the location of the fault moves away from the terminals of the
generator towards the neutral.

Figure 9.10

Typically, the overvoltage relay has a minimum setting value of approximately 5 V.

With this adjustment and with a typical distribution transformation ratio, this scheme is
able to detect faults located within 2 to 5% of the stator neutral, therefore
It is a scheme that does not allow for the detection of ground faults very close to the neutral.

The secondary winding of the distribution transformer must be grounded, whether in a

terminal of the secondary winding or at the center tap of the winding. The adjustment time of the
The voltage relay is selected to coordinate with the entire protection system. The
pertinent points are:

5 When grounded Y - Y voltage transformers are connected to the terminals of

the machine, the tension relay time must be coordinated with the fuses of the
voltage transformer, for faults in its secondary winding.

5 The voltage relay must be coordinated with the relay scheme for ground faults.
faults between phase and ground in the system will induce zero sequence voltages in the generator
due to the capacitance of coupling between the windings of the transformer unit or to
 9-15

the circulation of zero-sequence currents through the dispersion impedance of the

secondary delta of the power transformer. This induced voltage will appear in the
secondary of the grounding distribution transformer and may cause operation
of the relay.

In general, a maximum time adjustment is used to allow for a response time.

superior to that of the ground protection system. Smaller delay times are used.
when the neutral of the secondary of the voltage transformer is isolated and the phases are grounded
from the secondary or when using a high-speed ground relay in the high voltage system.

There are two trends, for the installation of the generator grounding impedance and
for the detection of the neutral potential, which will be shown below.

9.4.2 AMERICAN TREND

The American trend consists of placing a distribution transformer between the neutral and the
ground with a resistor in its secondary. The relay is placed in parallel with the resistor.
(Figure 9.10).

The nominal primary voltage of the grounding transformer is typically the voltage
nominal phase - neutral of the generator, to avoid transformer saturation during the
transient overvoltages produced by faults. The nominal secondary voltage of
Grounding transformer can be 120, 240, or 480 V, depending on the voltage.
nominal of the relay the tension that is connected in the secondary.

The value of the resistor must comply with the following expression to avoid the risk of
high transient overvoltages due to ferroresonance:
X
R≤ c Ω
3 N2
Where:
Xc: It is the total capacitive reactance phase - ground (per phase) of the generator winding and
of the power transformer, of the busbars, of the capacitors, of the
surge protectors and potential transformers.
9-16

It is the transformation ratio of the grounding transformer.

If you want to limit the current to 15 A, the resistance should be calculated like this:

V
g
R= Ω
15 3 N 2
Where:
Vg: It is the nominal value of the phase-to-phase voltage of the generator in volts.

The relationship between the kVA rating of the grounding transformer and the resistance,
it will depend on whether the overvoltage relay directly triggers the main switch of the
generator and the field, or if it only operates an alarm.

If you only want an alarm to sound, the transformer must be designed for operation
continues, at least from:
V V
g t
S (kVA) =
3 N 2R10 3
Where:
S(kVA): It is the power capacity in kVA

Vt: It is the value of the nominal primary voltage of the grounding transformer.
expressed in Volts.

Likewise, the continuous capacity of the resistanceP) must be at least (in case of
V
whatVtis equal to ): g
3

(V) 2
g
P (kW)=
3 N 2R10 3

If the relay triggers the generator switches, short circuit capacities can be used.
time both for the transformer and for the resistor. This case is very common when
These are unattended substations, where most functions are automatic.
 9-17

For cleared short circuits before 10 s, these capacities are on the order of 12% of the
continuous capacity of the transformer and the resistance.

The starting value adjustment of this voltage relay is approximately 5% of the voltage.
secondary nominal of the grounding transformer and the delay time used
It is usually between 0.3 and 0.5 s.

9.4.3 EUROPEAN TREND

This trend consists of placing a resistor between the neutral and the ground and a transformer.
of potential in parallel with the resistance. The relay is placed in the secondary of the transformer.
of potential (Figure 9.11).

Figure 9.11

The resistance must meet the following expression:

X
R≤ cΩ
3
If you want to limit the current to 10 A, the resistance must be:
V
g
R= Ω
10 3
9-18

The continuous capacity of the resistance will then be:

(V) 2
g
P (kW)=
3R
The nominal capacity of the potential transformer will depend on the consumption of the relay.
overvoltage connected in its secondary.

European manufacturers can usually provide an additional relay to cover 100%

of the generator winding. There are 2 types of 100% protection relays. One type
consists of sending to the generator windings a coded signal generated by a
alternator (with very high internal impedance) grounded on one side; if a fault appears at
ground, the current circuit is closed and the relay works.

The other type of protection consists of a third harmonic relay installed in the secondary.
the potential transformers connected in delta rotation, also including a filter for
through which the operating tension is increased to the fundamental frequency (60 Hz).
When the machine is in operation and there is no ground fault near the neutral, the relay does not
it allows the shot (the voltage at the fundamental frequency is greater than the voltage of the third
harmonic). When a ground fault occurs near the neutral, the third voltage increases.
harmonic in the neutral and the relay works.

9.5 PROTECTION AGAINST STATOR OVERHEATING

In the stator, overheating can occur due to overload or system failure.

of cooling, although normally, except in hydraulic installations, there isn't much
danger of a generator overload occurring, due to the limiters with the
What speed and voltage regulators are equipped with. However, it is necessary to anticipate
some damage in the speed and voltage regulation systems that may cause
overloads or damage in the cooling system.

Hydraulic installations are the exception given that it is common for the turbine to have, under
certain jump and flow conditions in the conduction, greater power than that supported by the
generator.
 9-19

9.5.1 PROTECTION WITH THERMAL RELAYS (49G)

SYMBOLISM

To protect the stator against this event, temperature sensing resistors are installed.
RTD or thermocouples in different parts of the winding to detect changes in
temperature. The thermal relay operates when the stator temperature, in this case, exceeds
a determined value. In chapter 4 it was described and is illustrated again in Figure 9.12, as
thermometer indicator of temperature with alarm or trigger contacts according to the temperature.

Several of these detectors can be used with a

indicator or temperature recorder, which may have
contacts for maximum temperatures and to sound the alarm. The

Temperature sensing resistors can be of

copper (value 10Ω a 25º), platinum (value 100Ω a 0º) or nickel
(value 120Ω The adjustment will depend on the temperature.
what the material it is made of can support
generator isolation.

9.5.2 THERMAL IMAGE PROTECTION

Figure 9.12

As a complement to the protection with thermal relays, an image relay can be used.
thermal connected to the secondary of a current transformer, similar to the one already described in the
chapter 4 is illustrated in Figure 4.21.

This type of relay operates on the principle of integrating the generator's current, calculating
the heating effect due to generation in the machine (I2R tIt should be noted that
this protection only detects real overloads of the machine and will not operate due to problems
thermal issues caused by deficiencies in the cooling system. It is used in generators.
small.
9-20

9.6 PROTECTION AGAINST GROUND CONTACTS IN THE CIRCUIT OF

EXCITEMENT

The excitation circuit of the generators is isolated from the ground, therefore it
that when that insulation fails at some point and a first contact with ground occurs,
Practically no current flows and there are no problems for the generator. However, the
the existence of that grounding contact increases the dielectric stress at other points of the
field development when tensions are induced in it due to transient process effects
in the stator of the machine.

When a second contact with land occurs, part of the field winding may remain
practically without excitation current, as it tends to circulate through the iron of the rotor between
the two points of failure. This results in a magnetic imbalance in the machine, which can
to provoke a very severe vibration, intolerable for the generator in critical cases. Another
the problem is the local heating experienced by the rotor at the failure points, which can
to distort it until it becomes eccentric, which is also a cause of vibrations; this
the process is slower than the previous one, and vibrations may appear after a while
on the order of minutes, and up to hours.

Some generators have their own ground contact detection systems.

excitation circuit, and mechanical type protections against vibrations. However, it is
generalized practice to equip all generators with external protection against contacts
with ground in the excitation circuit.

There are two ways to protect the rotor against this type of failure:

5 Injection of an alternating current signal through an additional circuit placed at

ground at one end, in such a way that the current can only flow through this circuit
When a ground fault occurs, this would in turn activate the protection relay. This
protection is not recommended for large generators due to the grounding capacitance
from the rotor can continuously circulate alternating current through the bearings,
contributing to their deterioration. This arrangement is illustrated in Figure 9.13.
 9-21

Figure 9.13

5 Voltage divider formed by two linear resistances and one nonlinear one, whose value
resistive varies with the applied voltage. If a failure occurs, a voltage will develop.
between point "M" and ground. The voltage developed will be maximum if the fault is in one of
the extremes. If the fault occurs at point 'M', no tension will develop and this
point is what is called the null point of the field. The function of the resistance does not

Linear is to vary this null point with the inherent variation of the field tension.

The disadvantage of this method is that there is a blind spot parallel to the central point.
of the resistive divider. Some manufacturers do not use the nonlinear resistance, but a
manual button that short-circuits part of one of the resistors and for detection
Blind spot failures require periodic pressing of the button. See the
Figure 9.14.

The schemes that have been presented are applicable to generators that have brushes in the
excitation circuit, that is to say that they have access to stationary parts of that circuit. In
static excited generators cannot apply these schemes, generally they
it has pilot brushes that are placed on rings of the excitation circuit to
9-22

purposes of insulation measurement. The appearance of voltage in these brushes or the

alteration of the field's isolation level, they can be used as a basis for protection

Figure 9.14

9.7 PROTECTION AGAINST LOSS OR REDUCTION OF EXCITATION (40)

Synchronous generators are normally operated overexcited, so that they deliver

reactive power to the system, in addition to active power. This condition corresponds to the fourth
quadrant of Figure 9.15. When the excitation is reduced to the point where the generator
it exceeds the condition of unity power factor, falling into the underexcitation zone, in which
consume reactive power (first quadrant of Figure 9.15). If the excitation is lost
completely, the machine becomes an induction generator, a condition in which
it practically maintains the delivery of active power to the system, but consumes from it a
reactive power that can be between 200 and 400% of the nominal power of a
generator.

The condition of loss of excitation, which is clearly the most critical, can be
harmful to the generator and to the system. In the generator, the operation as a machine
asynchronous leads to induced currents in the rotor; in generators with salient poles,
like the hydrogen generators, those currents can flow through the damping windings
and they are not dangerous, but in cylindrical rotor machines, such as turbogenerators, it can
there is a heating in the rotor capable of damaging it in a time frame of several minutes
 September 23

(depending on the slip value). The stator of the generator can also
overheating due to the current, which can take values from 200 to 400% of the nominal.
This process is generally slower than rotor heating, so it is not critical.

Figure 9.15

Excitement can be lost for any of the following reasons:

Open field circuit.

Opening of the field switch.
Short circuit in the field.
Poor contact of the brushes.
Damage to the voltage regulator.
Failure in the closure of the field switch.
Loss of AC power supply (static excitation).

The loss of excitation can be detected in several ways, as follows:

Minimum Current Detection:It consists of placing a low current relay in the

field or some type of directional relay. When the relay detects low or minimal current,
9-24

connect a discharge resistor in parallel with the rotor winding and just
download the winding, open the field switch.

Impedance Relay:This is the most commonly used method to protect the generator against
loss of excitation. A capacitive impedance relay can be used (relay of
Mho Off Set type distance to detect the change in the machine's operating point.
This relay is basically adjusted to: a and b, which are defined as:
X
a= d´ b=X
2 d
Where:
Xd´ Direct Axis Transient Reactance
XdDirect Axis Synchronous Reactance

Figure 9.16 illustrates various operating characteristics of the relay in a diagram.

R – X. No matter what the initial conditions are when the excitement is lost, the
The equivalent impedance of the generator traces a path from the first quadrant.
up to a certain region of the fourth quadrant. In the figure that shows the R - X diagram, it
notice that the relay operates when the measured impedance falls within the circle (the

reactance is negative, since the machine consumes reactive power). The curves1, 2y
3are for generators that were at full load at the time of losing the
excitement, and the4for a lighter initial load state.

It is advisable to complement this relay with a low voltage one. The triggering criteria
it consists of causing the generator to trip when the loss of excitation occurs
accompanied by low voltage (80 - 87%VN).

If the three relays of the scheme operate, indicating loss or reduction of excitation accompanied
with low voltage, the disconnection of the generator occurs with a delay of 0.25 to 1 s. It
prefers this last value to reduce the probability of improper operation of the protection
for stable oscillations of the generator, to which it can respond. When on the contrary, the
Loss or reduction of excitation does not result in a noticeable reduction of voltage (they operate
 9-25

only the distance and directional relays emit an alarm signal in the form
snapshot, and the generator disconnects in approximately one minute.

Figure 9.16
The system is subjected to a very severe condition when a generator loses its excitation.
not only does it lose the reactive power that it provided, but it also has to
to provide it with even higher values of reactive power. That sudden deficit of reactives
it can cause a reduction in tension such that stability in the system is lost.
The reality is that the possibility of maintaining stability depends on a set of factors.
such as: relative capacity of the generator, active power transfer, impedances of
generator and the system, inertias, duration of the disturbance and action of the voltage regulators.
If stability is ultimately lost, the time it takes for this to happen can
estimated between 2 and 6 seconds.

Modern generators generally have some type of protection built into them.
excitation system. However, given the consequences that this condition can have
For operation, it is advisable to install a protection against loss of excitation in the
generator, that duplicates and backs up the previous one, if it exists. The triggering criterion more

generalized for these protections is the following: in weak systems, which may have
stability problems, the main switches must be triggered and
generator field at a time on the order of 0.2 to 0.3 s (up to 1 s can be accepted); in
9-26

strong systems emit an alarm signal instantly to alert the operator, and
the shot is triggered (if the situation persists) with a greater time delay (between 10 s and 1
(minute). Generally, the terminal voltage reduction of the generator is taken as a criterion.
to determine whether synchronization will be lost or not. If it falls below a value
understood in the range of 80 to 90% of the nominal, a loss can be expected of
synchronism, although the specific value must be determined in each particular case.

9.8 PROTECTION AGAINST OVER-EXCITATION (24)

The generator must operate satisfactorily with the kVA, frequency, and power factor.
nominal at a voltage 5% above or below the nominal voltage. The
deviations in frequency, power factor or voltage outside of these limits, can
cause thermal stresses unless the generator is specifically designed for them
conditions. Overexcitement can cause these deviations, which is why the schemes
they have surveillance and protection for this.

The overexcitation of a generator or a transformer connected to its terminals occurs

when the relationship between the voltage and the frequency (Volts/Hertz) applied to the terminals of the
the equipment exceeds 1.1 p.u. (generator base) for a generator; and 1.05 p.u. with no load in the
high voltage terminals of the transformer. When these Volt/Hz ratios are exceeded,
Magnetic saturation of the generator core or transformers can occur.
connected and dispersed flows can be induced in non-laminated components which do not
they are designed to withstand them. The field current in the generator can also
increase. This can cause overheating in the generator or in the transformer and the
eventual breaking of the isolation.

One of the main causes of excessive Volts/Hertz in generators and transformers is

the operation of the speed regulator, which reduces the frequency generated during the
start and stop. If the voltage regulator maintains the nominal voltage while the unit
is at 95% of its speed or less, the Volts/Hertz at the machine terminals will be
1.5 p.u. or more and damage may occur to the generator or to the machine transformer.

There can also be overexcitation during a load rejection that disconnects the line.
transmission from the generation station. Under these conditions, the Volts/Hertz can
 9-27

raise to 1.25 p.u. With the excitation control in service, the overexcitation generally is
will reduce to limit values in a few seconds. Without excitation control, the overexcitation
will be maintained and damage to the generator or the transformer may occur.

The failures in the excitation system or the loss of the control voltage signal
Excitation can also cause over-excitation.

Industrial standards do not provide definitive cutting time values for

transformers and generators. However, manufacturers generally provide limit values
of overexcitation for their equipment. The primary protection against this phenomenon is provided
by the limiters or compensators of the voltage regulator, which change the setting of
excitement when determining a change in the Volt/Ampere relationship. Consequently, in
generators equipped with modern numeric voltage regulators, the installation of relays
protection externals against this phenomenon is not indispensable. For older generators,
without this type of compensation, or in which problems can arise
operation with this device without the excitation system detecting them is convenient
provide protection for this operational circumstance.

9.8.1 SIMPLE OR DUAL TIME OVEREXCITATION RELAY

There are different forms of protection available. One form uses a relay of
excitement which is set at 110% of the normal value and triggers in 6 s. A second form of
fixed time protection uses two relays, the first relay is set to 2 to 6 s. The second relay is set to
110% Volts/Hertz and powers an alarm and a timer that triggers after the time.
of permissible operation of the first relay overexcitation adjustment (for example 110%) for
the generator or the transformer. This time is typically 40 to 60 s. See Figure 9.17.

Typical excitation relays are single-phase and are connected to the transformers of
generator voltage. When a fuse in the voltage transformer fails, it can cause an
Incorrect voltage indication. A complete and redundant protection can be used.
connecting a group of relays to the voltage transformers that are connected to the
voltage regulator and connecting a second group of relays to another group of transformers
of tension that are used for measurement or for other relays.
September 28

Figure 9.17

9.8.2 REVERSE TIME OVEREXCITATION RELAY

An over-excitation relay with an inverse characteristic can be used to protect the

generator or transformer. Normally, a minimum operating level can be used.
excitement and a delay to provide an approximation of the characteristic of overexcitation
combined for the generator-transformer unit. A version of the overexcitation relay
inverse time has a unit of over-excitation separated by a delay time
adjustable. This unit can be connected to the alarm or the trigger and extend the range of the
overexcitation characteristic of the relay to the combined characteristic for the unit
generator - transformer. See Figure 9.18.

Figure 9.18
 September 29

When the nominal voltage of the transformer is equal to the nominal voltage of the generator, the scheme
previously protects the generator and the transformer. In other cases, the nominal voltage of
the transformer is lower than the nominal voltage of the generator and the relay cannot provide
protection for both teams. Hence, it is desirable to provide separate protection.
for the transformer.

Another factor that must be considered during an over-excitation is the possible operation
unnecessary differential relay of the transformer in the generator-transformer set.
This is not desirable as it indicates a false failure in the transformer. When a unit of
The transformer is connected in delta on the low voltage side, an overexcitation can
generate excitation currents of 60 Hz with some odd harmonics. At this point, the
the magnitude of the 60 Hz excitation current component can be higher than the value of
relay start and the magnitudes of the harmonics may not be sufficient to provide the
appropriate restriction.

Three options have been used to prevent unwanted operations. One option uses a relay.
of overexcitation that blocks the triggering or makes the differential relay insensitive when the
Overexcitation exceeds a specific value. The second option uses a differential scheme.
modified which extracts and uses a third harmonic of excitation current from the winding in
delta of the transformer to restrict the operation of the relay during the condition of
overexcitation. These first two options slightly reduce the range of protection.
differential that restricts the fifth and second harmonic. The fifth harmonic is the harmonic
lower than flows in the delta under balanced conditions.

9.9 PROTECTION AGAINST OVERVOLTAGE OF ALTERNATING CURRENT

SYMBOLISM

Generators should not be subjected to prolonged overvoltages, as they generally...

its design is such that they operate at a point close to the saturation elbow of the curve of
9-30

magnetization, and the overvoltages cause high values of flux density and considerable
distortion, with the consequent heating.

Under normal conditions, the generator's voltage regulator controls the current of
excitement and maintains tension within established limits. However, a failure in the
regulator, or the variation or loss of its input voltage signal (due to fuse melting of
potential transformers, for example, can result in overvoltages
elevated.

Another common cause of overvoltage is the sudden loss (total or partial) of load in the
generator. After the disconnection with delay time of nearby external short circuits
Overvoltages can also occur at the generator, but of smaller values. In all
There is a sharp reduction in the generator load, which implies the need to reduce
the excitation; if the voltage regulator does not respond with the necessary speed, the voltage
it can temporarily rise above the nominal value. The most critical case is when
the shot from the main generator switch takes place while it is near the full
load (load rejection). In this situation, the problem is aggravated by overspeed
resulting from the slow response of the speed regulator (especially in hydrogenerators),
which further increases the tension. A hydrogen generator in this case can reach speeds
up to 149% of the nominal, and at voltages around 200% of the nominal value.

It is advisable to install surge protection on the generator, based on a

Overvoltage relay connected to a potential transformer independent of the one being used
for the voltage regulator. The relay must meet the condition that its operation does not
affect with the changes in the signal frequency, so that it operates correctly even at
distinct frequencies of the nominal, which exist when the generator is disconnected from the
system.

Another possibility is to use a relay with a starting value independent of the frequency, but in
Currently, there are relays that respond to the ratio of voltage to frequency, and that have
an adjustable starting value in terms of this ratio. This type of relay is more selective,
Well, it corresponds to the value of the magnetic flux density in the generator, which is proportional.
to the ratio of tension to frequency.
 9-31

Overvoltages can be of two natures:

♪ Caused by maneuvers (switching) or atmospheric disturbances (it is protected by the)

surge protector and capacitors at the terminals before the switch
principal).

♪ Caused by the frequency of the system due to a machine problem, caused by:
• Defective operation of the voltage regulator.
• Sudden variation or loss of load.
• Over speeds.
• Overexcitement.

It is recommended to use a surge relay with two units, one with time delay and
Another snapshot; the starting voltage of the first can be adjusted to 110% of the voltage.
nominal with a delay between 1 and 3 seconds, while the second is an instantaneous unit adjusted

between 130 and 150% ofVNThere are also various opinions regarding the criterion of
shot. The conservative variant consists of the time-delayed element sending
instantly an alarm signal and about a minute later I sent to
turn off the main and field switches of the generator. Another alternative is that the
protection first initiate the action of inserting additional resistance in the excitation circuit
from the generator, and some time later, if the overvoltage persists, cause the disconnection of
the machine.

Under normal conditions, the voltage regulators associated with the generators prevent that
overvoltages occur. Therefore, very often, this protection is provided together
with the voltage regulation team.

9.10 PROTECTION AGAINST LOW VOLTAGES (27)

SYMBOLISM
9-32

A low voltage protection relay must be installed to disconnect the generator for
prevent the auxiliary service engines from suffering disturbances or, in the case of
static excitation, before the voltage level is not sufficient for the activation of the
Thyristors. The voltage drops. Normally, it is not a problem for the generator itself.
except if it involves an overcurrent (external fault, for example)

The relay must operate instantly for voltages below 60% of the voltage.
nominalVNand with time delay for voltages between 60% and 95% ofVN.

9.11 PROTECTION AGAINST UNBALANCED CURRENTS (46)

SYMBOLISM

There are several system conditions that can cause unbalanced three-phase currents in
the generator. The most common are the asymmetries of the system (untransposed lines), loads
unbalanced, unbalanced faults in the system and due to open phases in the circuits
system due to the breakage of conductors or by the action of switching equipment.

Load imbalances or faults in the system can cause negative sequence currents.
in the stator and these in turn induce double frequency eddy currents (120 Hz) in the
rotor iron of the generator. The heating of the rotor is proportional to its resistance.
an arc and even a modest value of negative sequence current can cause a serious
overheating. This component is detrimental to the axes of the generators, it tends to
flow through the non-magnetic materials that compose it, resulting in losses
the temperature rises quickly, producing vibrations in the machine, and causing a
very severe heating, which can reach the melting of certain points of the rotor.

The protection of generators against rotor overheating due to currents

unbalanced in the stator is done with an inverse time overcurrent relay that
responds to the negative sequence current, connected to the current transformers of
the terminals of the generator. The purpose of this relay is to disconnect the generator before
 9-33

that an excessive temperature is reached. It also provides support for failures that do not
They have been cleared by main protections from other elements in the system.

The correct sequence of phases should be R, S, T. A sequence of R, T, S or S, R, T means

phase inversion, which produces an inversion in the rotation direction of the generator, which is nothing
desirable. On the other hand, when a fault occurs (current above the value)
that the equipment supports), this current has positive, negative, and zero sequence components
zero; the negative sequence component of these faults can be detected by function 46 and
thus avoiding thermal stresses.

Some manufacturers consider the prolonged operation of the generator acceptable with
phase currents that do not differ from each other by more than 10% for turbogenerators and by 20%
for hydrogen generators, as long as none of the currents is greater than the nominal. This
it implies slightly higher negative sequence current values, respectively, than 5% and
10% ofINThe time during which a generator can withstand sequence currents
the negative of a greater value is given by the following expression:

Ta 2
∫0 I2 dt= A (9.1)

Wherei2it is the instantaneous value of the negative sequence current expressed in units
2 2
related to the nominal current of the generator. If the average value ofi is expressed
2 byI , the 2

Equation 9.1 takes the form:

I2 Ta= A (9.2)
2
WhereI2it is the effective value of the negative sequence current of the machine in units
related to the nominal current.

According to the standards of different countries, the parameterAit has a value of 30 for

turbogenerators and synchronous condensers with indirect cooling, of 40 for

hydrogen generators and generators powered by internal combustion engines, and from 5 to 10
for very large generators with direct cooling. It is considered that it may suffer damage
a generator that can support a negative sequence current for a time longer than
9-34

Tagiven by Equation 9.2, and it is recommended to review the surface of your rotor. Values of
time higher thanTathey involve the risk of very serious damage to the generator.
An overcurrent relay that responds to negative sequence currents is illustrated in the
Figure 9.19.

Figure 9.19

9.12 PROTECTION AGAINST MOTORIZATION OR REVERSE POWER (32)

SYMBOLISM

The motorization occurs when the turbine cannot even supply its own losses.
the unit (the mechanical power supplied by the primary engine is not sufficient to overcome
the losses due to rotor friction) and this deficiency has to be absorbed from the
system in the form of real power consumption. In other words, a generator behaves
like an engine when it does not receive sufficient mechanical power from the turbine and absorbs power
electric of the system.
 9-35

Depending on the type of turbine, certain percentages of reverse power are required for the
motorization of the generator, as follows:

PERCENTAGE OF
TURBINE TYPE Inverse Power
FOR MONITORING
Steam turbine 1 - 3%
Gas Turbine 10 - 50%
Hydraulic Turbine 0.2 - 3%
Diesel Turbine 25%

The damage that can occur under such conditions is related to the turbine and not to the
generator or the electrical system, like this:

a In steam turbines, the decrease in steam flow reduces the cooling effect.
of the turbine blades, resulting in overheating.

a In gas turbines, a great power is required for motoring to occur.

therefore, the sensitivity of reverse power protection is not very important.

a In hydraulic turbines, the motorization of the generator can produce cavitation.

the blades, especially in those that operate submerged or below the level of
the download.

a In diesel engines, during operation, great stresses occur in the shaft.

that can produce permanent deformations. In addition, there is the danger of fire or
explosion of unburned fuel.

To protect a generator against motorization, it is customary to install a relay of

power directional, which is a directional relay connected in such a way that it responds to the
active power investment in the generator. The relay must have a time delay for
avoid the incorrect operation due to the transitional investments that may be experienced by the
generator power during system disturbances or in the synchronization process. The
The connection of this relay is shown in Figure 9.20.
9-36

Figure 9.20

9.13 BACKUP PROTECTION AGAINST EXTERNAL FAILURES (51V, 21)

The generators must have protection relays against continuous power supply
short circuit to a failure in an adjacent element due to a failure in the primary protection.

Inverse time overcurrent relays cannot be used because in order to use them
coordination with the adjacent protection must have ample timing, having the
possibility of not operating as the sustained short-circuit current in a generator decreases
quickly to values below the nominal current.

A voltage-controlled inverse time overcurrent relay can be used, that is

that the relay is more sensitive when the voltage decreases, when the adjacent circuits are
protected with overcurrent relays.

51V Voltage-Dependent Overcurrent Protection: This element provides

support for the differential 87 and for the failures not cleared by the primary protections of
system. The voltage dependence ensures appropriate behavior under conditions
of overload, allowing to increase the sensitivity required by the capacity limit of
generator in short-circuit currents.
 9-37

Tension Control and Tension Restriction can be selected.

5 Control by tension, the action time is changed from a load characteristic

to a failure when the voltage falls below a set level. It is used
mainly for generators connected directly to the bus.

5Under tension restriction, the trigger current level is lowered.

proportionally with the tension when it falls below an adjusted value,
producing a continuous variation in the time characteristic. It is applied in
generators connected to the bus via a transformer.

Figure 9.21 illustrates these characteristics.

Figure 9.21

In the voltage restriction relay, the starting current varies depending on the voltage.
applied to the relay. For a type of relay, with a zero clamping voltage the current of
The start is 25% of the starting value with a restriction voltage of 100%.

In the voltage control relay, the start of the overcurrent relay is controlled by a
minimum level in a voltage relay. At normal operating voltage levels, the voltage relay
it is activated and restricts the operation of the overcurrent relay. Under fault conditions, the
voltage relay allows the operation of the overcurrent relay.
9-38

In both types of relays, the adjustable value must be lower than the fault current level of the
generator given by the synchronous reactance.

Figures 9.22 and 9.23 show the connections of the overcurrent relays, which
they are similar to the connections of distance relays.

Figure 9.22

Figure 9.23

For large generators and where adjacent circuits have distance protections
Oh pilots, a low impedance relay must be used. This relay must be set with a
scope such that it exceeds the unit transformer and even serves as backup for the
 9-39

adjacent elements. The operating time must be coordinated with the relays of
protection of adjacent circuits.

21 Low Impedance Protection: provides protection against phase-to-phase faults within

the operating area of the differential and a portion of the transformer winding. It serves as
backup protection against system faults that are not cleared by their correspondents
protections, the magnitudes to compare are the parameters in the secondaries of the CTs and
PTs, according to the formula:

Primary Impedance * CT Ratio

Secondary Impedance=
Relationship of PTs
Generally, a distance relay with a Mho characteristic is used. The relay is connected to
receive currents from the neutral current transformer and voltage from the terminals of the
generator. If there is a star-delta transformer between the generator and the system, the angle of
the phase of the input tensions to the relay must be changed so that they are in phase with the
system tensions and that the relay correctly sees the faults. It can be used to
auxiliary transformer as seen in Figure 9.22.

When a generator is directly connected to the system, the relay connections are
shown in Figure 9.23. In both cases, the relay not only provides backup for system failures,
but also provides backup protection against phase failures in the generator and the
generator zone before and after it is synchronized to the system.

In some cases, the relay is connected facing the system receiving current and voltage.
from the terminals of the generator. In this case, a Mho characteristic is also used to provide
backup protection, when system failures occur and for some failures of the
generator itself when it is connected to the system. However, this connection does not give
backup if the generator is disconnected from the system.

This relay is used with the intention of taking out the generator when there is a fault in the system.

Power should not be isolated by the transmission line switches. In some cases the
the relay is adjusted with a very long range and usually the relay trigger time is set at
one second or less. Given that modern excitation control systems have
overexcitation protection and other protections that protect the generator field, the
previous time may increase.
9-40

9.14 OTHER TYPES OF PROTECTIONS

So far, the most commonly used protections in generators have been studied,
although some of them are applicable only to generators of relatively low capacity
large. There are other less commonly used protections, either because they are intended
to generators with specific characteristics, or because they are available on the machine and not
It is advisable to install them as external protections as well. Next,
they briefly describe some of them.

9.14.1 PROTECTION AGAINST LOW FREQUENCY

SYMBOLISM

There is a possibility, when there is a loss of generation, that the frequency may drop to values below
below normal. This would cause operation at reduced frequency for a period of time
sufficient to produce overloads in gas or steam turbines.

In general, the operation of a generator turbine at low frequency is more critical than the
high frequency operation since the operator has no option to control the action. From there
It is recommended to have low-frequency protection for gas or steam turbines.

The turbine is usually considered more restricted than the generator to operate at frequency.
reduced, as this is the cause of mechanical resonance in its blades. The deviations of
the nominal frequency can generate frequencies close to the natural frequency of the
blades and therefore increase the vibrational efforts. The increases in the efforts
Vibrations can accumulate and crack some parts of the blades.

Turbine manufacturers set time limits for operations with abnormal frequency.
This data is usually given as a permissible operating time for a band of
specific frequencies. The effects of operation at abnormal frequency are cumulative. For
so, if the turbine operates 50% of the allowable time in a specific frequency band, it
 9-41

leave only 50% of the permissible time of the frequency band for the rest of the life of the
unity.

These limitations on the turbine capacity generally apply to steam turbines.

Gas turbines generally have more capacity than steam units to operate.
at low frequency. However, gas turbines are often limited by the
instability in combustion or the sudden shutdown of the turbine due to frequency drop. The
frequency limit must be provided by each manufacturer. In general, these restrictions do not
they apply for hydraulic generators.

The backup protection for low frequency is provided by the use of one or more low relays.
frequency and timers in each generator. Most schemes require the use of
a low-frequency relay for each frequency band and the relay operates if the frequency is
find within that band.

The multiple low-frequency and timed relay scheme is not used in gas turbines.
The manufacturers of these devices provide low-frequency protection that consists of a trigger.
for low frequency whose adjustment is given by the manufacturer.

Low frequency relays usually trip the generator. However, in cases

where the consequences of a machine loss are catastrophic, only the ...
protection as an alarm and the possibility of damage to the turbine is accepted.

9.14.2 PROTECTION AGAINST TRANSIENT OVERVOLTAGES

Generators must be protected against transient overvoltages of frequencies.

different from the fundamental that can affect its internal isolation. These overvoltages
they can be of external origin produced by atmospheric discharges in the system or of origin
internally produced by maneuvers or by failures. This protection is achieved through
surge protectors installed at the generator terminals.

When the overvoltage passes through the unit's transformer, it acquires an excessive
slope that is very detrimental to the generator's insulation. Such slope
9-42

it is usually reduced by means of a capacitor placed in parallel with the discharger

of overvoltage. Generally, the capacitors are 0.25µF.

9.14.3 PROTECTION AGAINST LOSS OF SYNCHRONIZATION (25)

When a generator loses synchronization with the system, it is necessary to provoke its
disconnection; however, the problem is usually in the system, as the center
the oscillation electric (see section 7.6.5) is outside the generator. Therefore, as a rule
there is no need to protect the generator against loss of synchronism, especially when there is
protection in the system lines.

With the increase in the capacity of generators and transmission voltages,

It can reach the situation where the center of the oscillations crosses through the generator.
The protection against excitation loss can incidentally detect that condition, but
its operating speed is generally insufficient. In those cases, it is necessary to install
in the generator a protection against loss of synchronization that can be of the type described
in the heading 7.6.5.

9.14.4 PROTECTION Against Burned DE THE FUSES DE THE

POTENTIAL TRANSFORMERS (60)

Generally, a generator has at least two groups of potential transformers.

one powers the voltage regulator and the other the relays and the measurement. When a fuse blows, it
some relays may fire or the regulator starts to operate incorrectly. This anomaly
It can be detected through a voltage balance relay connected between the secondaries.
of the potential transformers. This relay blocks the operation of the regulator of
tension and the triggering of some protection relays, additionally gives an alarm.

9.14.5 PROTECTION AGAINST UNINTENDED ENERGIZATION (50IE)

Unintended energization happens when someone accidentally closes the switch that
connect the generator to the network, (accidentally means that the turbine is still, that there is no
excitation) then the generator behaves like an impedance to the system and starts
to consume electrical power.
 9-43

A resting generator can be suddenly energized, causing large currents.

flow to accelerate the machine, similar to the start of an induction motor. These currents
they will quickly cause rotor deterioration due to thermal stresses on the generator.

The protection is disabled when the frequency and current are below the point
of adjustment. If a current with frequency and magnitude above the adjustment is observed by the
system, the protection will send a trigger signal.

An adequate selection of the components of the system to be isolated must be made to ensure
the correct disconnection of the generator.

9.14.6 SEPARATING LOAD PROTECTION

Small capacity generators that are interconnected with the Electric System of
Power, as occurs in many industrial systems, can at certain times
to remain isolated from the system due to a short circuit triggered by external short circuits. The load
that is connected to the generator in those cases can be very large, so it is
it is necessary to disconnect it from the external power supply.

This function is performed by a protection (generally referred to as a separator) that is

composed of a low-frequency relay and a directional power relay. The low element
frequency responds to the frequency drop associated with generator overload, and the
The directional relay operates when excessive power flows towards the system. This second relay
It is not always applicable, as its adjustment depends on the operating regime of the small one.
generating plant.

9.14.7 PROTECTION AGAINST VIBRATIONS

The vibration of the machine may be caused by electromagnetic imbalances, due to

damages in the windings of the stator or the rotor, due to mechanical imbalances caused by damage
in the machine such as detachments of rotating parts and mechanical misalignments
caused by loose elements in the machine structure or by damage to the bearings.
9-44

Two detection principles are traditionally used. Accelerometric sensors

they detect the acceleration of the piece on which they are installed while the sensors
Proximity magnetic sensors measure the relative movement between two pieces. The former are more
used for alarm and trigger functions, while the latter are more
employees for the measurement and analysis of vibrations, although they can also be used
for protection.

From a protection standpoint, vibration relays are usually installed with levels
of alarm and shot, which act when they detect that the r.m.s. value of the measured vibrations
exceeds a predetermined threshold, regardless of the vibration axis or frequency spectrum.
Some may be equipped with local or remote indication of the measured value.

The adjustment of the vibration relays is done during the commissioning of the unit, once
the results of the machine balancing are available. The adjustment must be verified every time
the machine is disassembled, since the vibration levels can vary with each
maintenance.

Traditionally, this protection has been considered optional, but it has become increasingly
more popular due to the increasing number of unattended installations or that do not have
permanent local operators, where this type of protection is indispensable.

9.14.8 OVERHEATING OF THE BEARINGS

The overheating of the bearings can be detected by a relay activated by a bulb.

thermometric type inserted in a hole of the bearing housing, or by a detector relay of
temperature with RTD, as described for the overheating of the stator. These
Detectors are placed for both the metal and the oil of the bearing.

9.14.9 PROTECTION AGAINST OVERSPEED

This protection is important to prevent damage to the generator and to the load (due to the effect of the
high frequency associated with overspeed in case the generator has stalled
with a certain load, isolated from the rest of the system). It is generally available in the turbine,
governing its speed. In exceptional cases, the generator is externally protected with
 9-45

a frequency relay, which triggers the primary and the main switch of the
generator.

9.15 EVOLUTION OF GENERATOR PROTECTIONS

The protection of generators has been characterized by its diversity, given the large number of
abnormal operating conditions that may occur in this type of machines
roundabouts.

The research activity aimed at the development of digital protections for generators
has been lower than in the case of transmission lines, however several have been achieved
commercial versions of digital relays and integrated systems for the protection of it
element.

In recent years, interest in digital protections for generators has increased,

due to the trend of interconnecting small generating plants with the system
electric, to take advantage of the cogeneration of electrical energy. On the other hand, in the systems
computerized control of large generating plants is possible and recommended
incorporate digital protection functions.

The digital protection of generators is generally done by applying classical principles.

starting from the phasor estimates of fundamental frequency. They are particularly attractive the
possibilities of digital technique for thermal simulation of the machine, applicable in the
functions for protection against balanced overloads and against rotor overheating,
due to unbalanced currents in the stator. It is also very common to incorporate algorithms from
time domain for starting and stopping regimes and algorithms of the domain of the
frequency (phasors) for the normal operation of the generator interconnected with the system.
The speed of the generator can be used as a switching criterion from one type to another.
algorithm, in the digital protection system of the generator.

By way of comparison, Figures 9.24 and 9.25 show the schemes and the
Characteristics of the generator terminal in the past.
9-46

CHARACTERISTICS
Disaggregated functions.
Great space for installation.
Continuous maintenance.
There is no record of failures.
High power consumption.
High consumption of VA's in
transformer circuits
of instrumentation (saturation
fast).
Great wiring.
Number important of
incorrect operations.

Figure 9.24

CHARACTERISTICS
Multi-area terminals.
Reduced space.
Minimum maintenance.
Communication functions,
monitoring, failure logging, curves of
load.
low power consumption.
low VA consumption in CTs and PTs.
Reduced copper wiring.
Possibility of integrating a system of
protection, control, and monitoring.
Telecommunication.
Self-diagnosis and self-supervision with
internal events report.
Adaptability.

Figure 9.25
 9-47

In summary, the evolution of generator protections, as seen in Figures 9.24 and

9.25 is mainly based on improving the existing protection functions.
in the implementation of multiple software tools, in the integration of systems
protection, control and monitoring and in telecommunications.

9.16 APPLICATION OF PROTECTIONS DEPENDING ON SIZE OF

GENERATOR

Table 9.1 is a very good guide to the most common protections depending on the
generator power capacity. (It is only a recommendation based on experiences)

Figure 9.26 presents a diagram with the protections that are achieved in a
compact protection module for generators with capacities greater than 15 MVA and in the
Figure 9.27 one for capacities less than 15 MVA.

Figures 9.28 and 9.29 illustrate the protection relays that should be used or
recommended for different capacities of Diesel generators (specific to a manufacturer).

What does the pole sliding incident consist of after a violent failure?

A fault generates very large forces. In a generator, the rotating magnetic field
produced in the stator moves at a speed and the rotor is attracted by this field, so
the way the poles of the rotor try to align with the stator, however, and according to the
generator load, such alignment is not present, and an angular displacement occurs,
this displacement is the slipping, when such slipping is very large the
generator goes out of sync, which is completely undesirable. A violent failure is a
violent load fluctuation, which causes excessive sliding.

Table 9.2 shows the loss of life of a generator due to incidents.

9-48

GENERATOR
Great
MVA 0-4 4to15 15-50
TURBOGEN
PROTECTION FUNCTION

Synchronization condition check 25 X X X X X

Generator Differential 87G X X X X
Group Differential 87TG X X X X
Abouthighandlowvoltage 59-27 X X X X X
Stator ground fault X X X X X
Loss of excitement 40 X X X X
Loss of stability X X
Inverse Power 32R X-1 X-3 X-3 X-3 X
Low impedance 21 X X X
Negative Sequence I-2 46 X-5 X-5 X X
Overcurrent 50-51 X-4 X-4
Stator overload X
Overcurrent with voltage restriction 51V X-4 X-4
Unintended energization 50-IE X X X
Switch failure 50BF X X X
Rotor Overload X
Rotor ground fault X X X X X

1 Only for diesel and steam turbines

3 It is not necessary with Pelton turbines
4 These overcurrent functions are not used in generators with self-supported excitation.
Especially important when the load imbalance significantly affects the system.

Table 9.1
 9-49

Figure 9.26
9-50

Figure 9.27

LIFE CONSUMPTION PER INCIDENT

INCIDENT

20%
False Synchronization

10%
Connection about three-phase failure

Up to 100%
Three-phase fault and reconnection unsuccessful

1%
Single-phase fault and reconnection unsuccessful

20%
Pole sliding after a failure
violent external

Table 9.2
 9-51

Figure 9.28
9-52

Figure 9.29
 9-53

BIBLIOGRAPHIC REFERENCES

[1] Introduction to Protection Relays. Carlos Felipe Ramírez G., Mejía Villegas S.A.
Pontifical Bolivarian University, Medellín, 1987.
[2] Applied Protective Relaying. J. L. Blackburn, Westinghouse Electric Corporation, 1979.
[3] Protective Relays. Application Guide
"Guidelines for the Proper Adjustment and Coordination of STN Protections". Consulting
To Elaborate Procedures Manual for the Coordination of Protections in the
CND carried out by Ingeniería Especializada S.A. for Interconexión Eléctrica S.A.
Itagüi- Antioquia, July 2000.
[5] Class notes on Protections and Stability taught by Eng. Orlando Ortiz Navas
the Industrial University of Santander. Year 2000.
The number
Conference
six. "Protection of Generators". Eng. David Páloma. Seminar New
Technologies in Medium and Low Voltage Protections, National University of
Colombia, Faculty of Engineering. July 2000.
[7] Introduction to Relays and Digital Protection Systems, Dr. Héctor Jorge
Altuve Ferrer, Autonomous University of Nuevo León. Faculty of Mechanical Engineering
and Electric. Monterrey, N.L, Mexico. Nov 1993.
[8] "Protection of Power Electrical Systems", Dr. Héctor Jorge Altuve Ferrer,
Autonomous University of Nuevo León. Faculty of Mechanical and Electrical Engineering.
Monterrey, N.L, Mexico.
[9] "Generator Protection", Article prepared by Sandra Patricia Mendoza. Bogotá
Washington D.C. October 2000.

10 Gec Alsthom T&D Protection & Control Group. Product Catalogue version 2.1 1996.