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

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

• UK Fault Statistics Vacuum MV Switchgear

8%

30%

26%

9%

3%

6%

11%

6% 1% Maloperation

Mechanical

Discharge

VT

CT

Cable Box

Vacuum

Lightning

Water

30% – 38%

26% – 44%

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 activity2. Mechanical operation3. Oil Condition

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

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 OperationCircuit 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

ba c d e f Main Contacts

Voltage

Captured Profile

a Coil energised

b Plunger moving

c Plunger contact trip latch

d Inertia of latch overcome

e Spring released

f Aux contact open

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

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 Result No of Units % of Total Units

Total 440 Pass 408 92.7

Retest 30 6.8 Maintain 2 0.5

Test Results for 9 Unit Types Maintenance interval 10 years

Making Best Use of the DataBy combining Condition data from MV Switchgear we can formulate Condition Health Indices

Defining ConditionHealth Index Profiles

0

100

200

300

400

500

600

1 2 3 4 5 6 7 8 9 10

Health Index

Noofassets

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10

Health IndexNo

of a

sset

s

• Health Index profile indicating good condition with a low stable failure rate

• Health Index profile indicating poor condition with rapidly increasing failure rate

No

. o

f A

ss

ets

No

. o

f A

ss

ets

Condition Condition ((Health IndexHealth Index))

PerformancePerformance

RiskRisk

InterventionIntervention

Informed Informed DecisionsDecisions

Prioritised Prioritised SpendingSpending

Linkage to Linkage to Corporate RiskCorporate Risk

Systematic and Objective Process

Making Best Use of the Data

Condition Based Risk Management

Engineering Engineering KnowledgeKnowledge

Asset DataAsset Data

Conclusions• Important to consider causes of fault and

failure when deciding what diagnostic tools to employ

• MV Switchgear1. Partial discharge testing2. Mechanism tests3. 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