Download January 2004

ea technology
Partners in power asset management
Effective Condition Assessment of MV
Switchgear
Chris Lowsley – Technical Services Director
Effective Condition Assessment
of MV Switchgear
• Areas to be covered
•
- Failure and fault causes
•
- Appropriate diagnostic tools
•
- Making best use of the data
•
- Summary
Why Condition Assess?
• Extend maintenance intervals
• Maximise availability
• Maximise reliability & build confidence
• Assist in refurbishment/replacement plans
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
Analysis of MV Switchgear Faults
Maloperation
Mechanical
Discharge
6% 1% 8%
11%
6%
30%
3%
9%
26%
VT
CT
Cable Box
Vacuum
30% – 38%
26% – 44%
Lightning
Water
• UK Fault Statistics Vacuum MV Switchgear
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:
1. Partial discharge activity
2. Mechanical operation
3. Oil Condition
1. Partial Discharge Testing
• Electrical discharges cause deterioration and
eventual failure of the insulation layer
• Partial discharge breakdown of insulation
produces:
Void
– light
– heat
– smell
– sound
– electromagnetic waves
Insulation Layer
Practical Detection Methods
• Electrical
– Direct Measurement (intrusive)
– Transient Earth Voltage (TEV) Detection
(non-intrusive)
• Non Electrical
– Visual
– Sound (Ultrasonic) Emission
(non-intrusive)
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
Internal Discharge Activity
Detected by TEV
Sectioned Cast
Resin CT
Long term discharge
through cast resin resulting
in failure
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
Surface Discharge Activity
Detected by Ultrasonics
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
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
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 cd e
f
Main Contacts
Example Circuit Breaker Mechanism
Testing
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
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
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
Oil Degradation for Switchgear
Electrical breakdown Strength
Increasing
“Equilibrium”
“Equilibrium”
Contaminants/Moisture
Decreasing
Acidity
A
B
Time
C
Maintain
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
Typical Results – UK DNO
Test Results for 9 Unit Types
Maintenance interval 10 years
Test Result
No of Units
Total
Pass
Retest
Maintain
440
408
30
2
% of Total
Units
92.7
6.8
0.5
Making Best Use of the Data
By combining Condition data from MV Switchgear
we can formulate Condition Health Indices
Defining Condition
Health Index Profiles
600
250
500
200
assets
Assets
ofof
No.No
No. of Assets
No
of 400
as
set 300
s 200
100
100
50
0
0
1
2
3
4
5
6
7
8
9
1 2 3 4 5 6 7 8 9 10
10
Health Index
Health Index
•
150
Health Index profile indicating
good condition with a low stable
failure rate
•
Health Index profile indicating
poor condition with rapidly
increasing failure rate
Making Best Use of the Data
Condition Based Risk Management
Engineering
Knowledge
Asset Data
Condition
(Health Index)
Performance
Risk
Intervention
Systematic and Objective Process
Informed
Decisions
Prioritised
Spending
Linkage to
Corporate Risk
Conclusions
•
Important to consider causes of fault and
failure when deciding what diagnostic tools
to employ
•
MV Switchgear
1.
2.
3.
•
Partial discharge testing
Mechanism tests
Oil analysis
Each of the diagnostic tools can highlight
switchgear in need of immediate attention
and prevent unexpected failures
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)
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
Thank You