Monitoring, Control and Communications for Electrical Apparatus
Discoveries from the Application of On-Line Monitoring to Substation Equipment
Contributor: Brian Sparling, SMIEEE
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
Global Failure rate for transformers is 1 – 2% per year
Failures are RANDOM by nature
Transformer monitoring rationale
Transformer monitoring rationale
Where does one start?
Relative importance of each component as a contributing factor
0
5
10
15
20
25
30
35
40
Core and
coils
Tap
Changer
Bushings Cooling
System
Other
Perc
enta
ge (
%)
Australia
Eskom
CEA
Location of Transformer Failure
Main tank
LTC
Bushing
Cooling system
Other
MAIN TANK: Windings failure, Partial Discharge, Excessive H2O, surface contamination,
connection failure, screens failure, circulating currents, overloading hazard,
Main Tank 30%
OLTC 35%
Bushing 15%
Cooling System 5%
Other 15%
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
Transformer monitoring rationale
What monitoring does one install, and on which units?
Transformer Condition Assessment
Detailed condition assessment
Weighting and Ranking
Mitigation
Refurbishment
Replacement
Health Index
Updated continuously
Criticality Index
Updated as needed
Impact of age on Health Index Large population of Power Transformer
Health Index decreases with age. HI drop is marginal.
Initial Condition Rating: Provides some clues where to start
Double Coverage
Market place
No Spare
Owner
Unit Number
Critical Index
Data Score
Power Plant
Conditio
n R
ating (
5 G
ood –
1 P
oor)
Monitoring vs. Diagnostics
No
8%
Diagnostic
2% Yes
Maintenance & Repair
Do something
else
Monitoring
10% No Is it Normal ?
Transformer
Do nothing
else
Yes
90%
Is it Serious ?
Cigre Report No. 227, Life Management Techniques for Power Transformers. WG A2.18
Broadband
technique
applied routinely Focused
technique
applied as required 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”
Top Oil = 74 Top Oil = 45
With Fans On
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
What is measured on-line - Bushings
Power factor change Capacitance change
Phase A
Ph
ase
A
Tanδ behaviour of a defective bushing
Power Factor as a Function of Voltage and Temperature
0
5
10
15
20
25
0 50 100 150 200 250
Hours
% P
F
10 Kv @ 25C 10 kV @ 70C 70 kV @ 25C 70 kV @ 70C
1
2
3
4
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
• 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
Now I have data, and lots of it, now what?
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
• 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.
Summary
• 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.
What is still Missing?
• 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
Further Reading and Study
Brian Sparling, SMIEEE Dynamic Ratings
IEEE T&D Expo, Chicago IL
April 2014
25
26
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