NAME : Mauro Uemori
COUNTRY : Brazil
REGISTRATION NUMBER : 1795
GROUP REF. : A1
PREF. SUBJECT : PS1
QUESTION N° : Q1.4
Paper A1-108 (India) has been difficult to categorise but has finally been left in PS1. The title leads the reader to
expect a discussion on monitoring and life assessment (both being asset management topics under PS2).
However, the main content of the paper is a mixture of a review of recent changes in temperature limits, ideas
about insulation ageing, and machine sizing criteria. The paper suggests that the parameter for temperature
monitoring should be temperature rise not temperature because it enables faults to be detected at an earlier stage
of their development.
Question 1.4
Would delegates comment on the issues raised in Paper A1-108 regarding the recent changes to the electrical
machine standards, and also give their reaction to the temperature monitoring method proposed in the paper?
Answer:
The present system of supervision of the stator winding is based on absolute temperature measured by ETD´s
and alarm and trip are set 10 to 15 deg C larger than the maximum guaranteed absolute temperature as measured
by ETD´s.
This system works well for machines operating at full load and ambient temperature close to maximum value,
but in the case machines are running at low partial load and/or the cooling medium temperature is very cold, a
large margin between actual stator winding temperature and pre-set limit for alarm and trip exists, and this may
not detect any fault at incipient stage.
Depending on the existing margin the alarm will detect any fault when it has progressed to be a more serious
fault.
Figure below shows the present system:
So, the paper proposes to change this system of supervision, monitoring the stator winding temperature rise
versus loading. The alarm would be set 5 to 10 deg C larger than the temperature rise at actual load recorded at
initial operation. In this case the cooling medium temperature shall be controlled and an alarm shall be generated
for any abnormality observed in coolant pressure, flow or temperature. Trip value shall be based on absolute
guaranteed temperature as per existing system.
According to the paper, they mention that as per convention 1% load increase results in 1% temperature rise
approximately.
The new proposition we would like to introduce here is to program the temperature rise measurements versus
any load condition, but considering a function defined by Temperature rise versus squared armature current.
A lot of site measurements show that taking temperature rise of stator winding versus squared armature current,
the behaviour of this function is a straight line for all load range. This method is also suggested by IEEE115.
The new proposition is:
• Alarm setting based on temperature rise x squared armature current;
• Continuous monitoring of the cooling medium temperature;
• Alarm to be set 10 deg C larger than original measured temperature rise;
• Trip settings shall be kept as it is today;
The figure below shows a new proposition:
•
Limits of Temperature Rise of Windings Indirectly Cooled by Air (IEC 60034-1)
According to IEC 60034-1 for:
• Machines having outputs more than 5000 kW;
• With air to water coolers;
• measured by ETD´s;
• Class B = 85 K and for Class F = 110 K, adjustments shall be made according to the armature
voltage, that means, for each 1000 V that armature voltage is larger than 12000 V, 1 K shall be
deducted from the above temperature rise.
The most usual criteria used today by a large number of technical specifications is for class F windings to use
temperature rise limits for class B when measured by ETD´s.
In the case class F is allowed for stator winding temperature rise, at the worst operating condition the maximum
hotspot temperature should not exceed class F limit.
If daily average cooling air temperature can be used (< 40 oC), the amount that it is lower than 40 oC can be
added to above temperature rise limits.
• Design of insulation system vs life expectancy
It is suggested by the authors that the insulation system shall be designed to stand at least 24 years reducing the
allowable temperature rise, but according to Klamt (2011) and Wendel (2006) mature insulation systems have
demonstrated a life expectancy superior to 40 years of service operating under temperature Class B.
So, operating on Class B means that there is no need to reduce the design temperature in order to extend the life
expectancy.
Reducing the design temperature to extend the life expectancy will represent a handicap on the commercial
competitiveness of the solution.
Wendel, C. et al - Condition Assessment Study on Stator Bars After 40 Years of Operation. Conference Record
of the 2006 IEEE International Symposium on Electrical Insulation.
Klamt T. and Hillmer, T. - Assessment of insulation condition of hydro generator stator bars after 40 years in
service. 2011 Electrical Insulation Conference, Annapolis, Maryland, 5 to 8 June 2011.