modern design principles for numerical busbar differential...

— TEXAS A&M, 72ND ANNUAL CONFERENCE FOR PROTECTION RELAY ENGINEERS

Modern Design Principles for Numerical Busbar Differential Protection

Mike KockottABB Inc.

Wednesday, March 27, 2019COLLEGE STATION, TX

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Slide 2

Co-author acknowledgement

Zoran Gajić ABB ABHamdy Faramawy VästeråsLi He SwedenKlas KoppariLee Max

March 28, 2019

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Slide 3

Basic principles

Based on differential current measurement:

– out-of-zone fault, currents balance (i.e. Σ i =0)

– sum of currents in = sum of currents out

– in-zone fault, currents no longer balance (i.e. Σ i >>0)

– sum of currents in ≠ sum of currents out

March 28, 2019

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Slide 4

History

Circulating current differential (1900s)

High impedance differential (1940s)

Percentage restrained differential (1960s)

First generation numerical differential (late 80s / early 90s)

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Slide 5

Measurement

Traditionally, bus differential relays measure the secondary currents of magnetic core CTs

‒ non-linear devices

‒ needs to be taken into account in the measurement design of bus differential relays

Example: external fault

‒ primary currents will always be balanced

‒ on secondary side, this same balance may not be measured

March 28, 2019

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Slide 6

Measurement

CT saturation

‒ occurs for high primary current conditions

‒ no longer able to transform primary current

‒ when fully saturated, a CT is no longer a current source, but instead has just a resistance value

Remanence

‒ random parameter that can improve or reduce ability to transform primary current

March 28, 2019

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Slide 7

Measurement

Secondary dc transient

‒ occurs on interruption of primary current

‒ takes the form of an exponentially decaying dc current

‒ caused by discharge of stored energy

The non-linear phenomena of magnetic core CTs have the tendency to cause unwanted operation of bus differential relays

‒ so each type of bus differential relay (high impedance, numerical, etc.) must have the means to overcome in its measurement design this non-linear behaviour

March 28, 2019

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Slide 8

High impedance

All CT secondary circuits are galvanically connected

‒ used to overcome the non-linear phenomena

‒ a high resistance RHIGH is placed in series with the operating element

March 28, 2019

RLRL

RL

RHIGHMetrosil

I>

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Slide 9

High impedance

External (through flow) fault

‒ full CT saturation

‒ the maximum differential branch currentIdiffmax = Iunbmax*

‒ to ensure stability Idiffmax

< set I> pickup

March 28, 2019

RLRL

I>

RL

RCTRHIGHMetrosil(RL+RCT)max

RHIGH+(RL+RCT)max

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Slide 10

High impedance

Criteria to be met:

‒ CT secondary loop resistance

‒ must be low (with a similar value in all bays)

‒ CT requirements

‒ due to lack of any restraint quantity, all CTs must have the same ratio and magnetising characteristics

Requirements incur additional expense

Quite reliable and very sensitive

Gives operating times of less than one cycle for internal faultsMarch 28, 2019

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Slide 11

Percentage restrained

Developed to lessen the restrictions on CTs imposed by high impedance

All CT secondary circuits are galvanically connected

Again, it is this galvanic connection between the CT circuits that is used to overcome the non-linear phenomena of the CTs

March 28, 2019

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Slide 12


All measurement decisions are based on these three quantities

‒ operation (measurement) of the relay does not depend on the number of connected bays to the protected zone

March 28, 2019

≈ iIN≈ iD

≈ iOUT

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Slide 13


Rl = the total saturated CT secondary loop resistance (RL+RCT)

Rd = the relay differential branch resistance

March 28, 2019

RlRd

Id

Iin

Rl

Rl+RdId = Iin

operate

Idiff

Iin

stable

Stability for external faults, even with fully saturated CT, is guaranteed for differential current below the characteristic

OperateIdiff > x*Iin

StableIdiff ≤ x*Iin

x > Rl

Rl+Rd

Iout

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Slide 14


Improvements over high impedance differential

‒ less severe CT requirements

‒ can accommodate different CT ratios using auxiliary current transformers

‒ can tolerate other relays on the same CT core

‒ allows much higher resistance to be included in the secondary circuits of the main CTs

Fast operating times for internal faults

‒ detect 1 - 3ms, trip output 9 - 13ms

March 28, 2019

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Slide 15

Shortcomings of earlier generations

Unable to detect open CT

Required interposing CTs to match main CT ratios (not possible for high impedance)

Double bus (single CB) – required switching in CT circuits, bistable relays to replicate disconnector status for zone selection, trip output selection

Check zone – required to ensure stable operation for open CT (requires one set of CT cores for ZA/ZB, and a separate set of CT cores for CZ); overcome problems with disconnector auxiliary contacts

March 28, 2019

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Slide 16

Numerical

Analog inputs are galvanically isolated

Each analog input quantity is sampled and converted to a numerical number – these numerical numbers are used in the algorithms

‒ it is therefore not possible to copy and directly re-use the analog technique

March 28, 2019

RL

RELAY

CT notsaturated

RL

RCT

RELAY

CTsaturated

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Slide 17

Numerical

The analog inputs are galvanically isolated

The secondary circuit loop resistance is no longer the important factor

The critical factor now is the time available to make the measurement and decision, i.e. the short period of time, within each cycle, when the CTs are not saturated [the CTs must be able to correctly reproduce the current for a minimum time within each cycle before saturation of the CT begins]

For practical protection class CTs, time to saturation, even under extremely heavy CT saturation, is around 2ms – the design criterion used for the numerical algorithm

March 28, 2019

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Slide 18

Numerical

March 28, 2019

i1 i2 i3

i7i6i5i4

Differential Protection Zone

DifferentialProtection Zone

iin

id

ioutBus =>

iin = max(SP, SN)iout = min(SP, SN)

RMS values of the differential, incoming and outgoing currents are calculated over the last power system cycle (1 cycle long moving window)

Have IIN, IOUT, ID & iin, iout, id for the algorithms

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Slide 19

Numerical

March 28, 2019

Measured signal relationships

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Slide 20

Numerical

Internal fault

External fault without CT saturation

External fault with CT saturation

March 28, 2019

IinIout

Id

IinIout

Id

IinIout

Id

critical criterion is the time available to make the measurement [initial correct reproduction of the current before CT saturation occurs]

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Slide 21

Numerical

Zone selection

‒ adapt dynamically to changing topology for multi-zone applications (using auxiliary contacts of primary apparatuses)

‒ connection of bay currents to the appropriate 87B zone

‒ automatic zone merging (e.g. zone interconnection on double bus applications, closing of bus-sectionalizing disconnectors, etc.)

‒ selective tripping i) of only the bay CBs connected to the faulty zone, and ii) of only the bay CBs connected to the same zone as a bay CB that has failed to trip (breaker failure)

March 28, 2019

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Slide 22

Numerical

Zone selection

‒e.g. complex busbar arrangement ‒ connections

March 28, 2019

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Slide 23

Real time digital simulator testing

IED with 6 differential zones

‒ functional testing

‒ faults at different locations

‒ different switchgear configurations

‒ verify selectivity (zone selection, zone merging, etc.)

March 28, 2019

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Slide 24


‒ dynamic testing

‒ primary time constants 100, 200, 350ms; fault inception angles 0°(maximum DC offset), 30°, 60°, 90°, 120°, 150°; different combinations of main CT ratios; different remanence levels

‒ evaluate behaviour during different transient conditions (e.g. CT saturation, evolving faults, AR onto permanent external fault, etc.)

March 28, 2019

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Slide 25


March 28, 2019

Network model – functional testing

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Slide 26March 28, 2019

Functional tests –correct switchgear input status


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Functional tests –incorrect switchgear input status


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Slide 28


Evolving fault L1 (phase A) external to L2 (phase B) internal

March 28, 2019

Full test scope:

‒evolving external to internal, internal to external, after 5, 10, 15, 20ms

Z1

Z2

C1 C2 C5 C3 C4

X XX

X XO O O O

O

Z3

Z4

L2

L1

Load Source Load Source

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Slide 29


Internal L1L2 (phase AB) fault with CT saturation

March 28, 2019

Z1

Z2

C1 C2 C5 C3 C4

X XX

X XO O O O

O

Z3

Z4

L2

L1


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Open CTZ1

Z2

C1 C2 C5 C3 C4

X XX

X XO O O O

OL2

L1

Source Load Load Source

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Slide 31


Breaker failure following external fault

March 28, 2019

Z1

Z2

C1 C2 C5 C3 C4

X XX

X XO O O O

O

Z3

Z4

L2

L1


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Network model used – external fault with CT saturation

Transformer

3

IED

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External fault with CT saturation

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Slide 34

Conclusion

Numerical busbar differential protection with 6 zones, verified

– software based zone selection / merging

– fast operation for internal faults

– correct operation for internal faults with CT saturation, evolving faults, etc.

– complete stability for external faults with CT saturation

– correct timeous detection of an open CT secondary circuit

Provides additional features like disturbance and event recording, built-in circuit breaker failure protection, IEC61850 communication

March 28, 2019

modern design principles for numerical busbar differential...

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