Energy and Power En gineering, 2010, 2, 203-207
doi:10.4236/epe.2010.23030 Published Online August 2010 (
Copyright © 2010 SciRes. EPE
Insulation State On-Line Monitoring and Running
Management of Large Generator
Qiudong Sun, Zhengxin Zhou, Weiqin Guo
School of electronic and electrical Engineering, Shanghai Second Polytechnic University, Shanghai, China
E-mail:, {zxzhou, wqguo}
Received March 23, 2010; revised May 18, 2010; accepted July 4, 2010
This study presented an insulation state monitoring method for large generator based on radio frequency (RF)
technique. As an on-line condition monitor and the precondition of condition-based maintenance (CBM), the
RF monitor used the high frequency current mutual inductor to detect the partial discharge signal from
neutral wire of stator windings. According to the magnitude of indicative value of RF monitor, a five phase
model was also proposed to manage the generator’s running better. The practices show that the proposed
method is effective.
Keywords: Large Generator, Partial Discharge, Radio Frequency Technique, On-Line Monitoring, Running
1. Introduction
Today, more and more power utilities are switching to mo-
ney-saving and effective condition-based maintenance
(CBM) programs for scheduling of machine maintenance
and testing. Such a system can overcome the disadvantage
of excess maintenance brought by the preventive
maintenance [1]. It will determine the equipment’s health,
and act only when maintenance is actually necessary.
Development in recent years have allowed extensive
instrumen- tation of equipment such as condition
monitoring to observing the state of the system, and
together with better diagnosis tools for analyzing con-
dition data, the main- tenance personnel of today are
more than ever able to decide what is the right time to
perform maintenance on some piece of equipment.
Ideally CBM will allow the maintenance personnel to do
only the right things, minimizing spare parts cost, system
downtime and time spent on maintenance.
A large generator is a complicated machine system. Its
breakdown is paroxysmal. If the accident once happens,
the imperilment will be great, and the maintenance cost
will also be great. A majority reason of its breakdown is
the short circuit caused by its insulation being destroyed
[1,2]. Due to the manufacturing and long time running of
generator, the partial discharge (PD) of its stators is un-
avoidable [1]. This state can lead to aging of its main
insulation, and eventually lead it to breakdown. There-
fore, it is necessary for generator to be equipped an on-
line condition monitoring system to observe its insulation
state. Meanwhile, the on-line condition monitoring is the
precondition of CBM. For a good CBM, it is far from
enough if there is only an on-line condition monitoring
system without being supported by a partial discharge
analyzing technique. So, it is also necessary to study the
relationship between the value of partial discharge and
the insulation state of generator.
At present, the main methods for measuring the stat-
or’s partial discharge of large generator are the neutral-
point coupling detecting, coupling capacitor detecting,
radio frequency detecting, detecting by partial discharge
analyzer (PDA) and detecting by stator slot coupler (SSC)
[3-6]. Generally, the partial discharge detecting systems
are classified into two types by their frequency band-
widths. One works in narrow bandwidth, and the other in
wide bandwidth, such as SSC. Generally, the former has
a stronger anti-jamming capability and is sensitive for
serious partial discharge, but it can not distinguish the
discharge signal occurred inside or outside of generator
[2,3,5]. Although the latter can collect plenty signals of
partial discharge for analyzing, it is unacceptable
because it needs imbedding a coupling sensor under the
stator slot wedge and changing the insulation structure of
stator windings [2,3].
In this study, we propose an insulation state monitor-
ing method for generators based on radio frequency
monitoring technique. We choose the narrow bandwidth
Copyright © 2010 SciRes. EPE
technique to design the partial discharge detecting sys-
tem, which can monitor the insulation state of generator.
We also give some advices for generator’s running ma-
nagement according to the RF signal level produced by
partial discharge and CBM experience.
This study is organized as follows. In Section 2, we
give the radio frequency monitoring technique about the
partial discharge, insulation deterioration and RF mea-
surement method. We also give the frame of RF Monitor
and the on-line monitoring architecture in this section.
Section 3 presents an on-line evaluation method of
insulation state. Section 4 gave some application exam-
ples, and shows that the proposed scheme yields more
effective through practices. Section 5 gives the conclu-
sion of this study.
2. Radio Frequency Monitoring
2.1. Partial Discharge
Partial discharge can be described as an electrical pulse
or discharge in a gas-filled void or on a dielectric surface
of a solid or liquid insulation system. This pulse or dis-
charge only partially bridges the gap between phase
insulation to ground, or phase-to-phase insulation. A full
discharge would be a complete fault between line
potential and ground [2].
These discharges might occur in any void between the
copper conductor and ground. The voids may be located
between the copper conductor and insulation wall, or in-
ternal to the insulation itself, or between the outer
insulation wall and the grounded frame [2].
These discharges also might occur at the terminal of
winding. Its surface contamination and moisture creates
the surface discharge and lightning.
2.2. Insulation Deterioration and Complete Failure
Insulation degradation is frequently linked to partial
discharges. The partial discharges are effectively small
sparks occurring within the insulation system, therefore
deteriorating the insulation, and can eventually result in
complete insulation failure [2].
At many times, the winding insulation deterioration
can be represented by the developing and forming pro-
cess of its strand breaking. Those wires of windings,
which are located at the upper winding-bar with the sa-
me slot and the same phase endured maximal magnetic
force, upmost temperature and supreme electric streng-
th, and the wires near by slot wedge, especially the wir-
es located at seamed edge, are easily deteriorated. The
deterioration process of stator windings can be describ-
ed as Figure 1. In this figure, those phenomena labeled
by symbol “*” can be detected by RF monitor or super-
2.3. Partial Discharge Measurement Method
Partial discharges are high frequency pulses originating
at various sections within an insulation system [1,2].
These pulses generate a voltage and current signal into
the insulation, returning through a ground path. The
partial discharge measurement can be implemented by
radio frequency monitoring techniques (RFMT).
We use the HF current transducer (CT), which is nip-
ped tightly at the proper unshielded position of neutral
wire of stator winding, to detect the RF current. The RF
measurement connection and technique are as shown in
Figure 2. In this figure, the RF monitor is a high
sensitive measurement meter with a magnitude of μV
quasi- peek value.
Work temperature Additional temperature
rise caused by
vibration of wires
near by slot wedge
Damage agglutinant of
enamel covered wire,
especially nearthe
slot wedge
Wire lost integrity
Wire vibrated and worn
Wire milled and
strand broken
Main insulation
HF spark
Main insulation
Gas particle*Main insulation
dielectric strength
Main insulation failure
Generator shut down
Figure 1. Deterioration process of stator windings.
On-Line Generator
HF Current
Figure 2. The measurement connection method.
Copyright © 2010 SciRes. EPE
2.4. RF Monitor
As mentioned above, the RFMT can be classified into
two types: narrow bandwidth and wide bandwidth. The
narrow bandwidth technique is applied in our monitoring
system because it has strong anti-jamming capability.
Our RF monitor (or Insulation Monitor) is composed
of receiver (includes crystal filter, RF logarithm amplify-
ier, quasi-peek value detector), logical judging process-
or and output alarming circuit as shown in Figure 3. Its
centre frequency is 1 MHz, -3 dB bandwidth is 5 kHz,
–60 dB bandwidth is 20 kHz, the dynamic input range is
10 μV ~ 10000 μV, input resistance is 50 . It also has a
greater than 60 dB suppressing circuit to attenuate the
false signals and to improve its capacity to defense ab-
rupt disturbing.
2.5. On-Line Monitoring Architecture
The on-line monitoring system is composed of RF moni-
tor, HF current transducer (CT) and telecontrol board ma-
inly as shown in Figure 4. As a probe, the HF current tra-
nsducer is used to detect the RF current. The RF Monitor
is used for on-line monitoring the state of interior faults
of generator and unwonted alarming. The telecontrol
board equipment is placed far from the generator to re-
mote monitoring its insulation state. It has the same con-
trol functions, indication buttons and the voice and light
alarming set with the main monitor. It also has a graph
recorder for plotting and saving the output signal from
RF Monitor. Finally, the telecontrol board is connected
by USB interface into a distributed control system (DCS),
which can store the RF signal timely, process and ana-
lyze the RF signal, and be accessed remotely by the users
(diagnosis personnel ) far from internet.
3. On-Line Evaluation of Insulation State
3.1. Running Experience
According to the experience from more than 360 set RF
monitoring practices, we have obtained the following
When the RF signal level is less than 300 μV or
fluctuating about this level and it is independent of
generator’s load, there is only a small discharge in the
generator system. The fact shows that the insulation state
of generator system is good or regular.
The RF signal level of greater than 1000 μV indicates
that there is a bigger discharge in the generator system.
Here, we should compare this moment data with that of
forepassed and consider the factors as follows to evaluate
the insulation state:
1) The relationship between RF signal level and
generator’s load.
2) The amplitude of RF signal is relative invariable or
changing randomly.
3) The RF signal rises tardily or abruptly.
4) We should also integrate the RF signal with the
traditional testing and the work condition change testing
to judge the insulation fault happened inside or outside
the generator.
Figure 3. Frame of insulation monitor.
Figure 4. The on-line monitoring arch itecture of RF Mon itor.
Copyright © 2010 SciRes. EPE
We have to admit that the generator need not always
shut down when the RF signal rises to a set level, and
1000 μV RF signal is not an absolute threshold to judge
the discharge happened inside the stator of generator.
In the condition-based maintenance system, it is im-
possible to judge what kind fault of generator happened
or whether the generator need shut down or not, just by
one apparatus or one signal. Therefore, we need other
testing equipment and some experienced field experts to
support the condition-based maintenance for generator.
3.2. Relationship between Insulation State and
RF Signal
According to the running experience and analysis in Sub-
section 3.1, we propose a five advice model for
generator’s running management corresponding to the
five pha- ses of insulation state as shown in Figure 5.
This model is also a result of practice statistics. It is an
eligible mo- del for most circumstances.
RF signal level detected by SJY
10 100 300 1000 2000 3000 5000 10000V
0 33.3 50 66.7 75 80 90 100%
|---------------------Good---------------------|--Aging--|--- Notice---|-Alarm-|-Danger|
Insulation state
Figure 5. Relationship between insulation state and RF signal.
3.3. Five Advice Model for Running Manage-
In Figure 5, each phase has a corresponding advice for
generator’s running management as shown in Table 1.
4. Applications
Up to now, more than 360 sets of our RF monitor (SJY)
have been launched into various power generating sets,
which include thermal power generators, water power
generators, nuclear power generators and gas turbine
generators. Their equipped capacitors are from 100 MW
to 1000 MW.
Our RF monitors have predicted various insulation
failures successfully in practices. For example, a 300
MW hydrogen-cooled generator of a Power Plant has
been run 7 years. Some day the equipped RF monitor
output an abnormal signal as shown in Figure 6 and
alarmed. The magnitude of abnormal signal was about
hundreds of μV. By analyzing the RF signal, we judged
that the terminal of stator winding has damaged to result
in the partial discharge like this. And we also gave an
advice to user to reconstruct the stator winding. After its
stator winding reconstructed, the generator got back in
order. And the RF monitor went back to indicate a nor-
mal signal as shown in Figure 7. The magnitude of RF
signal had dropped to a normal level of tens of μV.
Table 1. Insulation states of genera tor e v aluated by RF signal and its running management advices.
Insulation state RF signal level (μV) Running management of generator from field experts
Good 300 The generator is allowed long-term running.
Aging 300~1000
The insulation state of generator is in the interim from good to bad. It is allowed running unceasingly.
But we should notice to observe the trend of insulation degradation.
Notice 1000~3000 We should notice to observe the running state of generator or arrange the maintenance scheme.
Alarm 3000~5000
We should pay attention to the trend of the running state change and the work condition change
testing. The generator is needed to shut down at appropriate time.
Danger 5000 We should integrate the indicative value of RF signal with other symptoms to judge whether the
generator is necessary to shut down or not.
Figure 6. An abnormal signal of RF Monitor.
Copyright © 2010 SciRes. EPE
Figure 7. A normal signal of RF Monitor.
As mentioned in Section 1, it is necessary to install a
RF monitor into the complicated generator set for on-
line monitoring its insulation state. If the RF monitor
has been installed, but we can not unscramble its signal,
then the RF monitor like this is just an ornament and it
is useless. We should better to integrate the RF signal
with other diagnosing system to monitor the generator’s
insulation state and give a diagnosis conclusion and a
running management advice.
Also we should indicate that the RF monitor is not
enough apparatus or RF signal is not enough symptoms
to judge what kind fault of generator happened or whet-
her the generator need shut down or not. Therefore, we
need some other testing equipment and some experien-
ced field experts to support the condition-based
maintenance for generator.
5. Conclusions
This study presented an insulation state monitoring app-
roach for CBM of generators. In our approach, the RF
monitor with strong anti-jamming capability and the on-
line monitoring system were designed to detect the par-
tial discharge signal caused by the insulation degradation
of generators. In order to run generator better, we pro-
posed a five phase running management model according
to the magnitude of indicative value of RF monitor (SJY).
The practices demonstrated that the proposed method is
6. Acknowledgements
This research project was supported by the Key Disci-
plines of Shanghai Municipal Education Commission un-
der Grant No. J51801.
7. References
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