Service Manual Front Cover. Pdf

---

Application Notes
P34x/EN AP/F33
MiCOM P342, P343
Page 53/176
Where the relay is used to provide the protection required for connecting the
generator in parallel with the local electricity supply system (e.g. requirements of G59
in the UK), the local electricity supply authority may advise settings for the element.
The settings must prevent the generator from exporting power to the system with
frequency outside of the statutory limits imposed on the supply authority.
2.10
Field failure protection function (40)
Complete loss of excitation may arise as a result of accidental tripping of the
excitation system, an open circuit or short circuit occurring in the excitation DC circuit,
flashover of any slip rings or failure of the excitation power source. The field failure
protection of the P340 consists of two elements, an impedance element with two time
delayed stages and a power factor alarm element, illustrated below in Figure 17.
The elements operate from A phase current and A phase voltage signals measured
by the 
Ι
A
and V
A
inputs on the relay. The minimum phase current and voltage
required for P342/P343 field failure protection to work is 20mA and 1V (
Ι
n = 1A, Vn
= 100/120V) and 100mA and 4V (
Ι
n = 5A, Vn = 380/480V).
X
Normal machine operating impedance
R
–Xa1
Xb1
Xb2
–Xa2
Alarm angle
P2167ENa
Figure 17: Field failure protection characteristics
When the excitation of a synchronous generator fails, its internal e.m.f. will decay.
This results in the active power output of the machine falling and in an increasing
level of reactive power being drawn from the power system. As the active power
output falls, the mechanical drive can accelerate the machine so that it will gently
pole slip and run at a super synchronous speed. This results in slip frequency
currents being induced in the rotor body, damper windings and in the field windings.
The slip-induced, low-frequency rotor currents will result in a rotor flux being
produced. The machine would then be excited from the power system and hence be
operating as an induction generator. The ability to reach such a stabilised state will
be dependent on the machine’s effective speed-torque characteristic when operating
as an induction generator, and also on the power system being able to supply the
required reactive power without severe voltage depression.
Stable operation as an induction generator might be achieved at low slip (0.1-0.2%
above synchronous speed), particularly in the case of salient pole machines. The
machine may be able to maintain an active power output (perhaps 20-30% of rating)
whilst drawing reactive power from the power system (generating at a highly leading
power factor). This condition could probably be sustained for many minutes without
rotor damage being incurred and may not be detectable by traditional field failure
impedance characteristic elements. The P340, however, offers a power factor alarm

---

P34x/EN AP/F33
Application Notes
Page 54/176
MiCOM P342, P343
element in the field failure protection which can operate when the generator is
running in this condition.
Cylindrical rotor machines have a much lower output capability when operating as
an induction generator under excitation failure conditions. They are more likely to be
pushed over the peak torque level of their induction generator speed-torque
characteristic. If the peak induction generator torque level is exceeded, a machine
can stabilise at a much higher level of slip (perhaps 5% above synchronous speed).
When this happens, the machine will draw a very high reactive current from the
power system and a stator winding current as high as 2.0 p.u. may be reached. The
slip-frequency rotor currents could lead to rotor core or winding damage if the
condition is sustained.
Operation as an induction generator under field failure conditions relies upon the
ability of the rest of the system being able to supply the required reactive power to the
machine. If the system cannot supply enough reactive power the system voltage will
drop and the system may become unstable. This could occur if a large generator
running at high power suffers a loss of field when connected to a relatively weak
system. To ensure fast tripping under this condition one of the impedance elements
can be used with a short time delay. This can trip the machine quickly to preserve
system stability. This element should have a small diameter to prevent tripping under
power swinging conditions. The second impedance element, set with a larger
diameter, can provide detection of field failure under lightly loaded conditions. This
second element should be time delayed to prevent operation during power swing
conditions.
The Field Failure protection impedance elements are also provided with an adjustable
delay on reset (delayed drop off) timer. This time delay can be set to avoid delayed
tripping that may arise as a result of cyclic operation of the impedance measuring
element, during the period of pole slipping following loss of excitation. Some care
would need to be exercised in setting this timer, since it could make the Field Failure
protection function more likely to give an unwanted trip in the case of stable power
swinging. The impedance element trip time delay should therefore be increased
when setting the reset time delay.
The delay on reset timer might also be set to allow the field failure protection function
to be used for detecting pole slipping of the generator when excitation is not fully lost;
e.g. following time-delayed clearance of a nearby power system fault. This subject is
discussed in more detail in section 2.21.
DDB signals are available to indicate the start and tripping of each stage (Starts:
DDB 637, DDB 638, Trips: DDB 422, DDB 423).
The state of the DDB signals can
be programmed to be viewed in the “Monitor Bit x” cells of the “COMMISSION
TESTS” column in the relay.
Setting ranges for the field failure elements are shown in the following table:
Setting Range
Menu Text
Default Setting
Min
Max
Step Size
GROUP 1
FIELD FAILURE
FFail Alm Status
Disabled
Disabled Enabled
FFail Alm Angle
15°
15°
75°
1°
FFail Alm Delays
5s
0s
100s
0.1s
FFail1 Status
Enabled
Disabled, Enabled

---

Application Notes
P34x/EN AP/F33
MiCOM P342, P343
Page 55/176
Setting Range
Menu Text
Default Setting
Min
Max
Step Size
GROUP 1
FIELD FAILURE
FFail1 –Xa1
20/
Ι
n 
Ω
(Vn=100/120V)
80/
Ι
n 
Ω
(Vn=380/480V)
0/
Ι
n 
Ω
(Vn=100/120V)
0/
Ι
n 
Ω
(Vn=380/480V)
40/
Ι
n 
Ω
(Vn=100/120V)
160/
Ι
n 
Ω
(Vn=380/480V)
0.5/
Ι
n 
Ω
(Vn=100/120V)
2/
Ι
n 
Ω
(Vn=380/480V)
FFail1 Xb1
220/
Ι
n 
Ω
(Vn=100/120V)
880/
Ι
n 
Ω
(Vn=380/480V)
25/
Ι
n 
Ω
(Vn=100/120V)
100/
Ι
n 
Ω
(Vn=380/480V)
325/
Ι
n 
Ω
(Vn=100/120V)
1300/
Ι
n 
Ω
(Vn=380/480V)
1/
Ι
n 
Ω
(Vn=100/120V)
4/
Ι
n 
Ω
(Vn=380/480V)
FFail1 TimeDelay
5 s
0 s
100 s
0.1 s
FFail1 DO Timer
0 s
0 s
10 s
0.1 s
FFail2 Status
Enabled
Disabled, Enabled
FFail2 –Xa2
20/
Ι
n 
Ω
(Vn=100/120V)
80/
Ι
n 
Ω
(Vn=380/480V)
0/
Ι
n 
Ω
(Vn=100/120V)
0/
Ι
n 
Ω
(Vn=380/480V)
40/
Ι
n 
Ω
(Vn=100/120V)
160/
Ι
n 
Ω
(Vn=380/480V)
0.5/
Ι
n 
Ω
(Vn=100/120V)
2/
Ι
n 
Ω
(Vn=380/480V)
FFail2 Xb2
220/
Ι
n 
Ω
(Vn=100/120V)
880/
Ι
n 
Ω
(Vn=380/480V)
25/
Ι
n 
Ω
(Vn=100/120V)
100/
Ι
n 
Ω
(Vn=380/480V)
325/
Ι
n 
Ω
(Vn=100/120V)
1300/
Ι
n 
Ω
(Vn=380/480V)
1/
Ι
n 
Ω
(Vn=100/120V)
4/
Ι
n 
Ω
(Vn=380/480V)
FFail2 TimeDelay
5 s
0 s
100 s
0.1 s
FFail2 DO Timer
0 s
0 s
10 s
0.1 s
2.10.1
Setting guidelines for field failure protection
Each stage of field failure protection may be selected as ‘Enabled’ or ‘Disabled’,
within the “FFail1 Status”, “FFail2 Status” cells. The power factor alarm element may
be selected as Enabled or Disabled within the “FFail Alm Status” cell.
2.10.1.1
Impedance element 1
To quickly detect a loss-of field condition, the diameter of the field failure impedance
characteristic (“FFail1 Xb1”) should be set as large as possible, without conflicting
with the impedance that might be seen under normal stable conditions or during
stable power swing conditions.
Where a generator is operated with a rotor angle of less than 90° and never at a
leading power factor, it is recommended that the diameter of the impedance
characteristic, “FFail1 Xb1”, is set equal to the generator direct-axis synchronous
reactance. The characteristic offset, “FFail1 -Xa1” should be set equal to half the
direct-axis transient reactance (0.5X
d
’) in secondary ohms.
“FFail1 Xb1” = Xd
“FFail1 -Xa1” = 0.5 Xd’
where
Xd = Generator direct-axis synchronous reactance in ohms
Xd’ = Generator direct-axis transient reactance in ohms

---

P34x/EN AP/F33
Application Notes
Page 56/176
MiCOM P342, P343
Where high-speed voltage regulation equipment is used it may be possible to operate
generators at rotor angles up to 120°. In this case, the impedance characteristic
diameter, “FFail1 Xb1”, should be set to 50% of the direct-axis synchronous reactance
(0.5X
d
) and the offset, “FFail1 -X
a1
”, should be set to 75% of the direct axis transient
reactance (0.75X
d
’).
“FFail1 Xb1” = 0.5 Xd
“FFail1 -Xa1” = 0.75 Xd’
The field failure protection time delay, “FFail1 Time Delay”, should be set to minimise
the risk of operation of the protection function during stable power swings following
system disturbances or synchronisation. However, it should be ensured that the time
delay is not so long that stator winding or rotor thermal damage will occur. A typical
stator winding should be able to withstand a current of 2.0 p.u. for the order of 15s.
It may also take some time for the impedance seen at the generator terminals to
enter the characteristic of the protection. A time delay less than 10s would typically
be applied. The minimum permissible delay, to avoid problems of false tripping due
to stable power swings with the above impedance settings, would be of the order of
0.5s.
The protection reset (delayed drop off) timer, “FFail1 DO Timer”, would typically be
set to 0s to give instantaneous reset of the stage. A setting other than 0s can be used
to provide an integrating function for instances when the impedance may cyclically
enter and exit the characteristic. This can allow detection of pole slipping conditions,
for more information see section 2.21. When settings other than 0s are used the
protection pick-up time delay, “FFail1 Time Delay”, should be increased to prevent
mal-operation during stable power swing conditions.
2.10.1.2
Impedance element 2
The second impedance element can be set to give fast operation when the field fails
under high load conditions. The diameter of the characteristic, “FFail2 Xb2”, should
be set to 1 p.u. The characteristic offset, “FFail2 -Xa2”, should be set equal to half
the direct-axis transient reactance (0.5X
d
’). 
FFail2 Xb2 = 
kV
2
MVA
FFail2 -Xa2 = 0.5 Xd’
This setting will detect a field failure condition from full load to about 30% load.
The time delay, “FFail2 Time Delay”, can be set to instantaneous, i.e. 0s.
The protection reset (delayed drop off) timer, “FFail2 DO Timer”, would typically be
set to 0s to give instantaneous reset of the stage. A setting other than 0s can be used
to provide an integrating function for instances when the impedance may cyclically
enter and exit the characteristic. This can allow detection of pole slipping conditions,
for more information see section 2.21. When settings other than 0s are used the
protection pick-up time delay, “FFail2 Time Delay”, should be increased to prevent
maloperation during stable power swing conditions.
2.10.1.3
Power factor element
Salient pole machines can run continuously as induction generators generating
significant power and operation under these conditions may not be detectable by an
impedance characteristic. The power factor alarm can be used to signal to the
operator that excitation has failed under these conditions.

---