Discoveries from the Application of
On-Line Monitoring to Substation
Equipment
Contributor:
Brian Sparling, SMIEEE
Monitoring, Control and Communications for Electrical Apparatus
Transformer monitoring rationale
Global Failure rate for transformers is 1 – 2% per year
Failures are RANDOM by nature
There are 4 major sub-systems to a transformer
> Main Tank, Including
- Windings
- Oil
- Core
- Leads and connection
> On Load Tap Changer
> Cooling System
> Bushings
Transformer monitoring rationale
Where does one start?
Relative importance of each component as a
contributing factor
40
35
Percentage (%)
30
25
Australia
20
Eskom
CEA
15
10
5
0
Core and
coils
Tap
Changer
Bushings
Cooling
System
Other
Transformer monitoring rationale
Main Tank 30%
Location of Transformer Failure
OLTC 35%
Main tank
LTC
Bushing
Cooling system
Other
Bushing 15%
Cooling System 5%
Other 15%
MAIN TANK: Windings failure, Partial Discharge, Excessive H2O, surface contamination,
connection failure, screens failure, circulating currents, overloading hazard,
OLTC contact overheating and coking, desynchronization, excessive number of operations,
mechanical damage, neutral switch inactivity
Bushing moisture penetration, oil leak, Partial Discharge, ‘X’ wax formation,
Cooling System clogged coolers by pollen or dust, fan/pump failure, sludge, oil leak,
Other External, such as lightning, animal interference, operation error, that are not detectable
by on-line monitoring
Transformer monitoring rationale
What monitoring does one
install, and on which units?
Transformer Condition Assessment
Health Index
Updated continuously
Criticality Index
Updated as needed
Weighting and Ranking
Detailed condition assessment
Mitigation
Refurbishment
Replacement
Impact of age on Health Index
Large population of Power Transformer
Health Index decreases with age.
HI drop is marginal.
Condition Rating (5 Good – 1 Poor)
Initial Condition Rating: Provides some clues where to
start
Unit Number
Critical Index
Data Score
Power Plant
Double Coverage
Market place
No Spare
Owner
Monitoring vs. Diagnostics
Monitoring
Is it
Normal ?
Transformer
Maintenance &
Repair
Diagnostic
No
10%
Is it Serious ?
Yes
No
90%
8%
Yes 2%
Do
nothing
Do
something
else
else
Broadband technique
applied routinely
Focused technique
applied as required
Cigre Report No. 227, Life Management Techniques for Power Transformers. WG A2.18
Maintenance &
Repair Shop
Real Life Scenario – 1/7/2010
•
7:20 AM: Receive Emergency Hot Spot Alarm at Glade Station as the Phase A Hot Spot ultimately
exceeds 143 Degrees C.
•
Transformer did not have hot spot winding gauge prior to Transformer Monitoring Package.
•
10:30 AM: Station inspection reveals that the Glade #1 transformer (8.4/10.5MVA – 65 OA/FA), has a
single cooling fan and not the required minimum of three fans.
•
3:00 PM: 4 additional fans are added and placed on manual.
•
1/8/2010 - 7:10 AM: DDC quote to Station Management – “We checked the temperatures this morning
from SCADA and the fans definitely made a difference. The winding temp is running 20 to 40 degrees C
lower than the previous night with similar temperatures”
With Fans On
Top Oil = 74
Top Oil = 45
Example of a critical component
• Bushings on transformers are one of the highest
failure components
• Offline testing can provide a false sense of security
since many defects are:
– Temperature dependent
– Voltage dependent
– Occur very quickly
The quality of the insulation of high voltage bushings & high voltage current transformers
Measurements include
–
–
–
–
●
Imbalance current
Imbalance current temperature coefficient
• A very good indication of early stage insulation deficiency and contamination
Imbalance current trend
Calculated power factor
Imbalance current vector position; on a Polar Plot feature in the Athena Software
Easy to interpret defective phase by Users
Phase A
●
●
What is measured on-line - Bushings
Phase A
Power factor change
Capacitance change
Tanδ behaviour of a defective bushing
Power Factor as a Function of Voltage and Temperature
25
20
4
% PF
15
10
3
5
2
1
0
0
50
100
150
200
Hours
10 Kv @ 25C
10 kV @ 70C
70 kV @ 25C
70 kV @ 70C
250
An example of lesson learned
Bushing Monitor Unit installed in June 2005
Transformer failed in August 25, 2005 at 5:30 PM
Customer did not have relay alarm contacts or communications
wired to control room
Monitor was alarming, but no one knew
Result, 150 MVA actual failure
A-Phase
Components Of an Intelligent Condition Monitoring System
Information
Visualization
Data Processing
Data Exchange
Data management
Data Acquisition
Data Measurement
Now I have data, and lots of it, now what?
• Do not throw it away!
• No need to look at ‘data’ every day or hour.
• Have a response plan in place to respond to alarms (add to existing alarm
management strategy)
• Use the data to understand what is happening and why.
• What you will have is a record for the “behaviour’ of the transformer
under operating conditions!
• Now one could use advanced statistical analysis to understand normal
behaviour of the transformer
• Using the same analysis be able to spot when ‘abnormal’ behaviour begins
• Using this method has revealed problems BEFORE, alarms from a monitor
Statistical model of gassing behaviour
Single gas monitor on a transformer
Normal behaviour for many weeks before it changes
Behaviour changes 5.2 days before the transformer failed.
Operator did not respond to the alarms from the monitor (red line)
Statistical Model of behaviour of all data
Components Of an Intelligent Diagnostic System
Summary
• On-Line monitoring is an enabler for understanding transformer behaviour
in operation.
• Provides for early detection of incipient failure conditions that time based
testing methods can miss.
• Provides the opportunity to move to Condition Based Maintenance (CBM)
from time based methods.
• Now have the data to use statistical methods to make use of all the
accumulated data from monitoring systems
• Serves many masters;
Asset Managers
Operations and maintenance engineers
Systems Operators
Life extension of existing assets.
What is still Missing?
• Once the decision is made to go with monitoring;
– Develop a change management process
– Obtain ‘buy in’ from others who will be affected (IT etc.)
– Implement new and/or revise the alarm response
procedures for a net set of more specific alarms
– Do not forget, or delay communications connections, and
path for alarms.
Further Reading and Study
• The following guides will assist those interested in further study of
monitoring of transformers and how to utilize the results;
• 1) IEEE C57.143-2012, Guide for Application for Monitoring Equipment to
Liquid-Immersed Transformers and Components
• 2) C57.140-2006, Guide for the Evaluation and Reconditioning of Liquid
Immersed Power Transformers
• 3) C37.10.1-2000, Guide for The Selection of Monitoring for Circuit
Breakers
• 4) CIGRE TB 445, April 2008, Guide for Transformer Maintenance
25
Brian Sparling, SMIEEE
Dynamic Ratings
IEEE T&D Expo, Chicago IL
April 2014
26