1 Static vs. Dynamic Contact Resistance

Static vs. Dynamic Contact [email protected]

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12a-Case-study-DRM-HV-breaker

According to IEC 62271-100 and ANSI C37.100-1992 a circuit breaker is a mechanical switching device,

capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying

for a specified time and breaking currents under specified abnormal conditions such as short circuit.

What is a circuit breaker?

Definition of a circuit breaker

Automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit

Basic function: immediately discontinues electrical flow

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Ideal switch

making carrying breaking

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01-Introduction-CBR-testing

The main task of a circuit breaker is to interrupt operational + fault currents and to isolate faulty parts such as

overhead lines, cables, transformers, generators, etc. from the system. Hence a circuit breaker has a dynamic

behavior (breaking and making operational and fault currents) and a static behavior (carry operational currents

and insulating HV parts to ground).

In regards to the current the wording "break" and "make" is used whereas in regards to the action "open" and

"close" is used.

A circuit breaker is controlled by the protection device of the corresponding asset and the SCADA system

which allows controlling the grid by remote.

What is a circuit breaker?

Static behavior

Carry operational currents

Insulating HV parts to ground

Isolating faulted parts to the grid (in case the breaker is open)

Dynamic behavior

Breaking and making operational currents

Breaking and making fault currents (fault clearance)

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Depending on location of in the electrical network, network design and region several design of circuit

breakers are common – which is discussed in later more in detail. All of them consist of following three main

components

Interrupter unit(s)

Support insulator

Operating mechanism & control unit

Overview components

Example: Live-Tank circuit breaker

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Interrupter unit(s)

Breaking chamber, interrupter chamber, interrupter housing, etc.

Contains interrupter, interrupting medium

Support insulator

Line-to-ground insulator

Contains insulated pull-rod, mechanical linkage, insulating medium

Operating mechanism & control

Stored energy, secondary wiring

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The trip or close command from any control device is forwarded to the circuit breakers' main contacts via the

functional chain. The functional chain consists of the following parts:

Control: This is the control device of the circuit breaker, e.g. the protection device. For HV circuit breakers this

part is not included directly in the circuit breaker. But some MV breakers are equipped with the protection

device itself.

Operating mechanism: Coils, armatures and the drive itself (spring, hydraulic, pneumatic, magnetic)

Mechanical linkage: Linkage between the drive and the interrupter. In order to obtain synchronicity of the

breaker's main contacts the mechanical linkage can be tuned via screws.

Interrupter: Main contacts of the circuit breaker

Circuit breaker components

Components

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04a-Methods-MV-HV-breakers

Typical test on circuit breakers?

Performance of control circuits

Coil current profile analysis

Under- & overvoltage tests

Pick-up tests

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Test performance of charging motor

Motor current analysis

Current trace

Inrush current

Charging time

Under- & overvoltage tests

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Test performance of kinematic chain

Timing of main and auxiliary contacts

Contact travel (motion) of main contacts

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Identify contact wear and tear of main and arcing contacts

Static Contact Resistance

Dynamic Contact Resistance

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One very common and “simple” test is the static contact resistance test.

It is done to check the status of the main contact (and/or also bus bar junctions)

The test is done when breaker is closed using the four-point resistance measurement method. This is done to

exclude the resistance from test leads in the end value.

For the analysis, the resistance value is checked against the manufacturer specifications, comparing the

values between the phases and with the previous results if available.

Contact wear and tear of main contacts

Static Contact Resistance

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Motivation

Check the status of the main contact/bus bar junctions

The ability to carry the rated current without big losses

Analysis

Resistance value

Typical value between 10 to 100 [µΩ]

Comparing to previous results

High variations between phases

Measurement done

Using the four-point resistance measurement method

Injection current level

50[A] (IEC)

100[A] (ANSI)

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Principle

The principle of contact resistance measurement (µΩ meter test) is to measure the resistance in static

position: breaker closed.

A high current is injected between the breaking elements (direction of line) and measure the voltage drop

(line-ground). As the contact resistance is in µΩ- range a 4-wire measurement has to be applied, as otherwise

the resistance of the test leads (mΩ- range) falsify the results.

Conventional method

Disadvantage of using conventional units is the confusingly wiring effort for breakers with several interrupting

units. Connection mistakes are highly possible. Furthermore the measurement can be affected due to long

connection leads → inductive loop (antenna).

Method with CIBANO

The usage of contact modules CB MC2 reduces the effort and complexity of wiring. The screened Ethernet

cables for power supply (Power over Ethernet = PoE) and data communication to CIBANO 500 main unit are

not affected by noise.


Static/contact resistance test (µΩ)

Micro-ohm (µΩ) meter

Validates resistance of primary path

Ensures low losses of load current

Can be used for circuit breakers, bus bar joints, etc.

Measurement method

Inject a high current

Measure small voltage in a noisy environment

Use 4-wire technique for connection

Unique CIBANO 500 features

Same wiring setup as for the timing test

Short lightweight high-current cables

Eliminate capacitive interference

Allow easy wiring setup

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Performance of trip and close components

Dynamic Contact Resistance (DRM)/Timing test

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Motivation

Check the timing to ensure a safe and reliable circuit breaker operation

Checking the arcing contact wear

Often together with contact travel

Check misalignment and/or wrong assembly in the interrupter

Analysis

Operating times & Synchronicity

per phase and between phases

Resistance curves


Contact wipe

Arcing contact length


Measurement done

Using four-point resistance measurement method

Different sequences possible

O, C

OC (reclose)

CO (trip-free)

O-CO (auto-reclosing)

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What are arguments for performing a Dynamic Resistance Measurement (DRM) instead of a conventional

Static Contact Resistance measurement? Well it may become quite obvious comparing the sequences of a

circuit breaker operation with the jump of a motocross racer in the picture shown above.

Static Contact Resistance does only provide us an insight of the main contacts condition (resistance) in

defined open and close state. Represented by picture 1 = takeoff (open) and picture 9 = landing (close).

Dynamic Contact Resistance allows us to have a better insight of the main contacts condition (resistance)

during whole process of the movement. This allows us to see the condition of the arcing but also main

contacts providing information of resistance, timing and motion/travel (represented by picture 1+ picture 2 + ...

picture 9).


Why Dynamic contact resistance (DRM)?

Static Resistance Measurement = pic 1 + pic 9

Dynamic Resistance Measurement = pic 1 + pic 2 + ..... + pic 9

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A dynamic contact resistance measurement is a commonly used method, because it determines the contact

length without having to disassemble the circuit breaker.

When the arcing contact becomes shorter than the minimum requirement specified by the circuit breaker

manufacturer, a safe operation can no longer be guaranteed and the interrupter unit needs to be maintained

properly.

In order to apply that method it is important to know the components of an SF6 circuit breaker interrupter unit

and how the current is interrupted during an open operation

Principle

The principle of Dynamic Resistance measurement (DRM) is to combine all information contact resistance,

timing, coil currents and travel (motion) into one diagram for the different operations. Thereby the whole

operation can be analyzed to find out arcing problems (e.g. bouncing), contact finger problems.

Method with CIBANO

For DRM measurement CIBANO 500 main unit, contact modules CB MC2 and transducer note CB TN3 is

used. No re-wiring needed!


Dynamic contact resistance (DRM)

Records contact resistance during breaker operation (resistance over time)

Combination of contact resistance, timing and travel measurements

Use DRM to find outThe arcing contact length

Contact finger problems

Lubrication problems

ProcedureInject high current

Start recording current and voltage

Operate circuit breaker

Calculate resistance

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Contact travel

Main contact

Arcing contact length

Separation of

main contact

Arcing contact

is seperated

Resistance

Start of

contact travel

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Analyze the conductive integrity of the main contacts in closed position.

When the circuit breaker is and stays in the closed position, the current injected flows through the main

contact because it has an ohmic resistance, most of the times, lower than 100μΩ. However the arcing contact

has an ohmic resistance much higher than 100μΩ.



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Closed position Arcing Open position

7 Stationary main contact2 Stationary arcing contact

3 Moving arcing contact 8 Moving main contact

Idc R<100μΩ R>>100μΩ

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When the main contact is already opened but the arcing contact stays closed, the current injected will flow

through the arcing contact.

Tungsten copper (WCu) materials are used for arcing contacts in SF6 circuit breakers for high and medium

voltage applications. At the heart of the switching chamber, WCu arcing contacts are exposed to extreme

mechanical and thermal stresses, with temperatures ≥ 20,000 K being reached during arcing.

Tungsten-copper has unique material properties. A high level of temperature resistance is one of the most

important advantages of tungsten, while the copper content increases the electrical and thermal conductivity.



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Closed position Arcing Open position

7 Stationary main contact2 Stationary arcing contact

3 Moving arcing contact 8 Moving main contact

Idc R<100μΩ R>>100μΩ

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Above example shows a Dynamic Resistance Measurement on Open [O] operation. Overlaying the

information of contact resistance and travel (motion) over time allows to refer to the arcing contact length.



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Source: SIEMENS

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Animated Open [O sequence

Circuit breaker in closed position. The trip (open) command is send and energizing the coil (black).

Once the energy is enough the latch hits the trigger, releasing the stored energy in the spring and the

armature starts to move. Until here the main contact is still is still carrying the operational currents. In the

same time SF6 is compressed in the puffer volume

Until the main contact is separated and the arcing contacts is taking over. The arcing contacts separate and

arcing is taking place and the arc and the arc extinguishing process starts. The arcing can be easily seen in

the resistance recorded over time as the resistance value is no more stable.

The arcing is taking as long the arc is fully extinguished. The measured resistance is now very high

impendent. The coil current is interrupted by the auxiliary contact and the armature moves until it’s end

position.



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Source: SIEMENS

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Same sequence explained on a cutaway example.



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With CIBANO 500, OMICRON has introduced the first ever 3-in-1 test system that can be used to test all

types of circuit breakers: medium- and high-voltage breakers with live- or dead-tank design. CIBANO 500

is unique in that it combines a micro-ohmmeter, timing analyzer, and circuit breaker supply in just one system.

This enables both standard tests, such as contact resistance measurement and switching time

analysis, as well as advanced tests, such as motion analysis and dynamic resistance measurement, to be

carried out with ease.

Thanks to the integrated circuit breaker supply, the user is able to freely adjust DC voltage with CIBANO 500

as well as simulate a circuit breaker supply with undervoltage without having to change the cabling. Thus it is

possible to test the circuit breaker during the commissioning of switchgear systems where no station battery is

available.

CIBANO 500: 3-in-1 circuit breaker test system

Multi-channel timing and travel analyzer

Checks the mechanical performance

Detects problems during coil actuation

Reveals defects of the trip or close coils

High-accuracy micro-ohm (µ) meter

Assesses the condition of the main and arcing contacts and resistor switches

Powerful coil and motor supply of 2.4 kW

Applies a stable DC voltage level to the breaker, even if a station battery is not available

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Timing and travel analyzer

Digital micro-ohm-meter

Coil and motor supply of 2.4 kW

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07a-Hardware-CIBANO-500+accessories

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The accessories main contact module CB MC2 and transducer node CB TN3 are powered via PoE+ (Power

over Ethernet) by the main device CIBANO 500. Over the EtherCAT® interface power supply and data

commination realized where a number of units can be connected as the based on serial bus technology.

Functional scheme

CIBANO 500 Hardware OverviewPage 21

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CB

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CIBANO 500

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The idea of main contact module CB MC2 is too minimize the wiring effort by using one module for each

phase of circuit breaker. By having 2 current output and 2 voltage measurement inputs using short cables can

be used to measure contact resistance, dynamic resistance measurement and timing tests.

The main advantage is that for all test no re-wiring is required and all wires are clearly visible arranged.

Main contact module CB MC2

2 high-current outputs and 2 sensitive voltage measurement channels

Output power source: powerful “super-capacitor”

Same wiring set-up for all circuit breaker tests: no time-consuming rewiring necessary

Overall wiring minimized and clearly arranged

Short high-current cables: easier and faster measuring set-up

Light-weight digital connection cables between CB MC2 modules and CIBANO 500: transmission of measuring results without interference

Powered via PoE+ (Power over Ethernet)

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Current output 1x 200A in case of a connecting to a CB with one interrupter unit

2x 100A in case of connecting of a connecting to a CB with two interrupter

unit

Voltage input 2x voltage input, one for each interrupter unit of a CB with two interrupter

units

Visualization LED Indicating the status to identify which unit connected to which phase

EtherCAT® interface power supply and data communication with CIBANO 500 main device

Main contact module CB MC2Page 23

2x 100 A DC

current output to

interrupting unit

2x voltage/

binary input

voltage inputs

Visualization LED

status indication

EtherCAT® with PoE+

to CIBANO 500 main unit

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Here a standard connection example of 3-phase SF6 Live-Tank Circuit breaker (ganged operation) shown.

Important: first always ground all interrupter units from both sides (to busbar and asset to be protected). For

each phase (with two interrupter units) on CB MC2 is connected. Use one CB TN3 and connect all 3

transducers (rotary/linear).

Connections of trip/close coils, motor and auxiliary has to be done depending on the wiring diagram of the

circuit breaker control.

Connection concept

Live-Tank circuit breaker

Grounding on both sides of the interrupter unit

CIBANO 500 + 3 x CB MC2 : all tests on 3-phase circuit breakers with one or two interrupters

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CIBANO 500

CB MC2

CB MC2

CB MC2CB MC2

06a-Performing-MV-HV-breakers

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Case study – NorwaySF6 Live tank CB, condition assessmentTime-optimized circuit breaker diagnostics with CIBANO 500

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Inspection was scheduled for this 315kV Live-tank SF6 breaker during maintenance. Both measurement files

are in the chapter 10. Practical exercises:

2014-09-10_DRM-Norway-before-maintenance.ptm

2014-09-15_DRM-Norway-after-maintenance.ptm

Details about breaker

315 kV Live-tank SF6 breaker

Manufacturing year 1986

Rated current 3,15 kA

Rated s.c. current 40,0 kA

Pole operation ganged

Operating mechanismpneumatic

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Function principle

Interrupter unit – cutaway view

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The results of contact resistance measurement doesn’t provided much insight in the condition of the main &

arcing contacts. Just a deviation of the resistances in close position observed between the phases L1, L2, L3.

But no further assumptions can be made based on this results.

Condition assessment // 2014-09-10

Contact resistance measurement

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First timing measurement for Close operation doesn’t provide much information so far.

Condition assessment // 2014-09-10Page 29

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Timing Close [C]

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Same for timing measurement in open cycle.


Timing Open [O]

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Another DRM measurement on open sequence indicates that the second interrupter unit in shows

discontinuity of the arcing contacts which would lead to heavy arcing.


DRM Open [O] – resistances

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As a result of the DRM results it was decided to take the interrupter unit to the workshop for inspection and

maintenance. Especially on the right picture heavy arcing can be observed on the main contacts at the

location of the contact fingers.

Before maintenance

Main contacts

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As well the female arcing contact showed heavy wear.

Before maintenancePage 33

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Arcing contact

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Main contact was completely renewed and the arcing contact polished.

After maintenance

Main and arcing contact

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After re-building the interrupter units and assembling back the breaker the measurement was repeated. The

contact resistance measurement shows now equal resistance in close positon after maintenance in all 3 poles.

After maintenance // 2014-09-15

Contact resistance measurement

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Repeating the DRM measurements in open position show proper performance when the arcing contact is

taking over the current flow from the main contact. No interruption any more!

After maintenance // 2014-09-15Page 36

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DRM Open [O] – resistances

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Same measurement with view of currents over time instead of contact resistance.

After maintenance // 2014-09-15

DRM Open [O] – currents

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Assessment examplesTime-optimized circuit breaker diagnostics with CIBANO 500

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09-Assessment-CBR-measurements

This dynamic resistance measurement during opening of the circuit breaker shows that the main contact could

have something. Normally when the movement starts there will be a small change in the resistance due to

friction and a bigger increase of the resistance when the transaction from main contact to arcing contact takes

place, but in this case the bigger change is taking place immediately when the contact travel starts. This

indicates that there is either a contact problem i.e. loose main contacts (fingers) or the main contact has

contamination on the surface due to wear. But it could also be that the clamps are not correct mounted and

are moving due to the vibration during operation.


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Start of contact travel

Main contact is separated

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The dynamic resistance measurement during closing of the circuit breaker shows that the contamination of the

main contact surface can be excluded as the behavior is continuing also after that the main contact has reach

it end position. This indicates more in direction loose main contacts (fingers) or badly mounted clamps.


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This dynamic resistance measurement during opening of the circuit breaker shows that the main contact for

phase LB has something. In this measurement the resistance change is small when the contact travel starts

(due to friction), which is okay, but when the transaction from main contact to arcing contact takes place the

resistance value starts to have high peaks until the arcing contact is opened. This indicates that there is

probably a contact problem e.g. arcing contact in bad condition due to wear. The next slides shows the same

even clearer when the current trace is enabled instead.

Case study 2 – Assessment with DRMPage 44

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Phase B: worn out arcing contact?

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Above DRM measurement on a SF6 MV Circuit breaker shows a good example of discontinuity of one phase.

For a short moment the main contact losses continuity before the arcing contact is taking over which can lead

to serious arcing and heavy wear off the main contacts.

Measurement of contact resistance in both states open-close would show any problem.

09 - Assessment examples Page 46

Case study 3 – Assessment with DRM

Example 1: SF6 MV CB – discontinuity of main contact

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Source: Schneider Electric

discontinuity

on red phase

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Above shows the same before on an Open sequence and corresponding DRM measurement but just

animated.

09 - Assessment examples Page 47

Case study 3 – Assessment with DRM

Example 1: SF6 MV CB – discontinuity of main contact

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coil current (A)

travel (mm)

DRM (Ω)

Video: Schneider Electric

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1 Static vs. Dynamic Contact Resistance

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