1. ea technology Effective Condition Assessment of MV Switchgear
Partners in power asset management
Effective Condition Assessment of MV Switchgear
Chris Lowsley – Technical Services Director

2. Effective Condition Assessment of MV Switchgear
Areas to be covered
- Failure and fault causes
- Appropriate diagnostic tools
- Making best use of the data
- Summary

3. Why Condition Assess? Extend maintenance intervals
Maximise availability
Maximise reliability & build confidence
Assist in refurbishment/replacement plans

4. Structured Approach to Condition Assessment is Required
Recognise developing trends in failures
Select those techniques most appropriate to cost effectively determine the extent of potential problems
Apply those techniques
Consider the results and formulate a plan of action

5. Analysis of MV Switchgear Faults
UK Fault Statistics Vacuum MV Switchgear
30% – 38%
26% – 44%

6. Analysis of MV Switchgear Faults
Reasonable to assume similar faults on SF6 equipment
However, oil equipment differs where oil is also a key indicator
Key criteria to be considered:
Partial discharge activity
Mechanical operation
Oil Condition

7. 1. Partial Discharge Testing
Electrical discharges cause deterioration and eventual failure of the insulation layer
Partial discharge breakdown of insulation produces:
light
heat
smell
sound
electromagnetic waves
A powerful technique to diagnose the condition of insulation of MV plant
Insulation Layer
Void

8. Practical Detection Methods
Electrical
Direct Measurement (intrusive)
Transient Earth Voltage (TEV) Detection (non-intrusive)
Non Electrical
Visual
Sound (Ultrasonic) Emission (non-intrusive)

9. Transient Earth Voltages (TEVs)
High frequency electromagnetic signals (TEV’s) emitted from discharge sources
TEV’s travel over switchgear surfaces
Detected using capacitively coupled probes on the switchgear metalwork

10. Internal Discharge Activity Detected by TEV
Long term discharge through cast resin resulting in failure
Sectioned Cast Resin CT

11. Ultrasonic Detection
Sound produced by P.D. is detectable with ultrasonic instruments
For the most sensitive measurements, airborne detection used
Measurement relies on an air path out of the switchgear

12. Surface Discharge Activity Detected by Ultrasonics

13. 2. Mechanical Operation Circuit Breaker Mechanism Testing
Large percentage of faults and failures in circuit breakers are caused by the mechanism
Lubrication (over or under)
Distortion
Corrosion
Non-invasive methods available for testing of mechanism operation
A large proportion of all failures within circuit breaker mechanisms are due to stiction caused by over or under lubrication of the mechanism, or distortion or corrosion of components with the circuit breaker external mechanism.
A separate method of condition assessment is therefore required to allow problems within the mechanism to be detected before they affect the safety and reliability of the equipment.
There are several instruments available for this purpose, but the most inexpensive and user-friendly of these is the Kelman Profile.

14. Circuit Breaker Trip Time Testing
Non-invasive testing of Circuit Breaker Mechanism
Requires no direct connection with the circuit breaker
Capable of monitoring first trip operation
Records current profile of trip coil
Provides information on trip coil, plunger, main and auxiliary contacts

15. Captured Profile Voltage a Coil energised b Plunger moving
c Plunger contact trip latch
d Inertia of latch overcome
e Spring released
f Aux contact open a b c d e f
Main Contacts

16. Example Circuit Breaker Mechanism Testing

17. 3. Oil Condition
Degradation processes of oil and internal components are well understood
Specific oil tests can identify oil and internal component degradation
Moisture
Acidity
Electrical Breakdown strength
Identification of particulate contamination
Over the past 12 years a wide range of projects have been completed within the Strategic Technology Program or STP at EATL has been completed in module 4, substation plant. The STP is sponsored by the majority of the Distribution Network Operators in the UK and is a means to research and expand our knowledge of HV plant. It was through this program that numerous projects relating to 11kV oil filled switchgear was completed. EA Technology has undertaken a large programme of work studying the condition and degradation of oil filled switchgear. Consequently, the performance and degradation of oil in switchgear has also been studied in detail. The work also identified that the general condition of switchgear, even aged switchgear, some 30 plus years old was good and service life times past the original manufacturers expectations could be safely and reliably achieved, assuming satisfactory maintenance.
This has led to the identification of specific tests that could be used to identify the condition of the oil and switchgear. These include the measurement of moisture, acidity and breakdown strength which are standard procedures routinely applied to oil samples. It was also identified the critical degradation of oil is linked to interaction between the oil and materials used within the switchgear. An indication of this can be obtained by filtering the sample and assessing the nature and quantity of solid material recovered. Combining the traditional measurements, moisture, acidity and breakdown strength, with this additional tests enables a comprehensive picture of the condition of oil and the switch to be achieved.
This research work completed at EA Technology and supported by individual RCM studies, revealed that oil condition is the critical factor in determining the need for maintenance for many types of switchgear.
Since the original work carried out for the Strategic Technology Programme, EA Technology have undertaken several large surveys with individual electricity companies which have confirmed the validity of the measurement criteria. This has led to a very high level of confidence that oil testing will provide a very sensitive indication of oil deterioration or switchgear deterioration, which indicates the need for maintenance.
Invasive maintenance is only required if significant oil degradation has occurred and the oil requires changing.
Prior to the introduction of condition based maintenance the maintenance period for 11kv oil filled switchgear was typically 8 to 10 years within the DNOs. There was significant pressure on all companies to reduce their running costs and increase maintenance intervals. Major savings could made if the maintenance interval is increased and this was the major driving force in the development of LTOS.
However it is also extremely important to ensure that the equipment is in a reliable and safe condition. By determining the optimum maintenance period it will ensure the risks associated with maintenance are reduced by only maintaining when required.

18. Oil Condition
Generally oil condition is the critical factor- supported by RCM studies
Large studies have confirmed the validity of the measurement criteria
Invasive maintenance is only necessary if oil needs changing
Condition based maintenance optimises the maintenance interval and ensures the safety and reliability of the network is maintained or improved
Over the past 12 years a wide range of projects have been completed within the Strategic Technology Program or STP at EATL has been completed in module 4, substation plant. The STP is sponsored by the majority of the Distribution Network Operators in the UK and is a means to research and expand our knowledge of HV plant. It was through this program that numerous projects relating to 11kV oil filled switchgear was completed. EA Technology has undertaken a large programme of work studying the condition and degradation of oil filled switchgear. Consequently, the performance and degradation of oil in switchgear has also been studied in detail. The work also identified that the general condition of switchgear, even aged switchgear, some 30 plus years old was good and service life times past the original manufacturers expectations could be safely and reliably achieved, assuming satisfactory maintenance.
This has led to the identification of specific tests that could be used to identify the condition of the oil and switchgear. These include the measurement of moisture, acidity and breakdown strength which are standard procedures routinely applied to oil samples. It was also identified the critical degradation of oil is linked to interaction between the oil and materials used within the switchgear. An indication of this can be obtained by filtering the sample and assessing the nature and quantity of solid material recovered. Combining the traditional measurements, moisture, acidity and breakdown strength, with this additional tests enables a comprehensive picture of the condition of oil and the switch to be achieved.
This research work completed at EA Technology and supported by individual RCM studies, revealed that oil condition is the critical factor in determining the need for maintenance for many types of switchgear.
Since the original work carried out for the Strategic Technology Programme, EA Technology have undertaken several large surveys with individual electricity companies which have confirmed the validity of the measurement criteria. This has led to a very high level of confidence that oil testing will provide a very sensitive indication of oil deterioration or switchgear deterioration, which indicates the need for maintenance.
Invasive maintenance is only required if significant oil degradation has occurred and the oil requires changing.
Prior to the introduction of condition based maintenance the maintenance period for 11kv oil filled switchgear was typically 8 to 10 years within the DNOs. There was significant pressure on all companies to reduce their running costs and increase maintenance intervals. Major savings could made if the maintenance interval is increased and this was the major driving force in the development of LTOS.
However it is also extremely important to ensure that the equipment is in a reliable and safe condition. By determining the optimum maintenance period it will ensure the risks associated with maintenance are reduced by only maintaining when required.

19. Live Tank Oil Sampling for Ring Main Units
Isolate & Earth one ring switch which allows access to tank but keeps customer supplied
Oil sampling via the test access cover (2 x 50ml)
Use of oil results to identify the condition of INDIVIDUAL units
Standard oil tests
Particulate analysis
Provides minimum disruption to network and gives confidence of condition to the units not able to be switched
EA Technology were approached back in 2001 by Northern electric, now CE electric to help devise a method of oil sampling for switchgear causing minimum disruption to the system.
LTOS is a method of retrieving an oil sample from oil switchgear using a modified test probe via the test probe access, which results in minimum disruption to the network. It was foreseen the regime would be used to take an oil sample from units (Ring Main Units and extensible switches) whilst part of the unit remained live, with only the earthing and isolation necessary to open a test access cover. The oil test results would then be used to identify the need for maintenance for each individual unit

20. Oil Degradation for Switchgear
Many factors influence the rate of oil degradation including maintenance practices, the original quality of the oil, oil handling and the various materials within the switchgear but the basic process of oil degradation is shown. The ageing of oil occurs in three stages.
The initial stage when new oil is placed in a switchgear unit occurs in a relatively short time period, perhaps a matter of weeks as the oil finds its equilibrium with its surroundings. The moisture content will increase and the electrical breakdown strength oil decrease with the acidity level remaining stable.
Stage 2 (A to C) occurs over a long time period and is due to the slow ageing of the oil, the relative changes in the oils properties are small.
Stage three (C to maintenance) is the relatively rapid ageing process where the moisture and acidity increases and the breakdown strength decreases and within a relatively short time scale, (approximately five years) the oil will require changing to ensure the switchgear can continue to operate safely.
It is stage three that requires identification so that maintenance of the unit can be completed. Previous work has indicated that in the majority of cases current REC maintenance intervals are less than the time taken for the oil to enter stage 3 of the degradation process and so adoption of the LTOS policy should produce savings due to extended maintenance periods. It is extremely important to recognise that extending this maintenance period is based on scientific facts and not on pure economics.

21. Classification of Oil Condition
PASS
Indicates satisfactory oil condition, which enables an extended maintenance interval to be adopted)
RETEST
Indicates evidence of some oil degradation, should be retested in 2-3 years (half Probability of Failure interval for oil degradation)
MAINTAIN
Indicates very poor oil quality, unit should be maintained within 6months
IMMEDIATE ACTION REQUIRED
Indicates EXTREMELY poor oil quality, the unit is prone to failure
The criteria for the condition assessment of the switchgear using LTOS was established. The result from each test was given a ‘weighting’ and the sum of these ‘weightings’ determines the action required on the unit. Depending on the classification for LTOS three actions are possible.
PASS - Indicates satisfactory oil condition, which enables an extended maintenance interval to be adopted.
RETEST - Indicates evidence of some oil degradation, unit should be retested in months, half PF interval for oil degradation).
MAINTAIN - Indicates very poor oil quality, unit should be maintained within 6months.
An additional classification indicating immediate action required due to a probability of failure can be used if necessary.

22. Typical Results – UK DNO
Test Results for 9 Unit Types
Maintenance interval 10 years
The LTOS procedure is used on units that are due for maintenance according to Company 1’s previous policy (10 years,)
The standard tests completed are moisture acidity and electrical breakdown strength. In addition the filtration test is completed on a selected number of units. The results of 440 units are shown (one years testing).
It can been seen that the majority of the units tested did not indicate the need for maintenance and extension of the maintenance interval was justified.
Note, even a retest result indicates an extended period of at least 30-36months when the next test is due.

23. Making Best Use of the Data
By combining Condition data from MV Switchgear we can formulate Condition Health Indices

24. Defining Condition Health Index Profiles
No. of Assets
No. of Assets
Health Index profile indicating good condition with a low stable failure rate
Health Index profile indicating poor condition with rapidly increasing failure rate

25. Making Best Use of the Data
Condition Based Risk Management
Informed Decisions
Prioritised Spending
Linkage to Corporate Risk
Condition (Health Index)
Performance
Risk
Intervention
Engineering Knowledge
Asset Data
Systematic and Objective Process

26. Conclusions
Important to consider causes of fault and failure when deciding what diagnostic tools to employ
MV Switchgear
Partial discharge testing
Mechanism tests
Oil analysis
Each of the diagnostic tools can highlight switchgear in need of immediate attention and prevent unexpected failures

27. Conclusions
Best use of data can be made by combining diagnostic information with visual inspection, maintenance data, causes of failure etc.
Derivation of a Health Index for the assets:
Allows easy comparison between assets
Links condition to Probability of Failure / End of Life
Helps evaluate future performance and effect of different intervention strategies (the 3R’s)
Replacement
Refurbishment
Retain (possibly with enhanced maintenance)

28. Conclusions Condition Based Risk Management
Effective means of linking engineering knowledge and experience to corporate decision making
Implementation has demonstrated it can deliver significant short term benefits
A vital component of successful asset management in an ever increasing regulatory and financial climate

29. Thank You