0.1kA 1kA 10kA

1E-2s

0.1s

1s

10s

100s

1E3s

1E4s

Curva Tempo-Corrente FFF

Discrimination of protection devices on installations

Janet Roadway

Product Manager, Power Breakers

Topics of Discussion

Explaining the terminology

Degrees of discrimination

Different techniques to achieve discrimination

Backup protection

Protection devices

Any Questions?

Protection - Basics

Lets go back to basics………

•Question: Why do we use protection devices????

•Common Ans: To prevent Faults

Wrong!

Protection whether by fuse, circuit breaker or relay cannot prevent faults from happening. Only good design, high quality components, careful installation, preventative maintenance along with good working practices can prevent major faults

However, protection devices can limit the damage and inconvenience caused if faults occur.

Protection - Overload

What do we mean by a fault?

Overload

Operating condition in an electrically undamaged circuit which causes an current to flow in excess of the full load current

Example: Starting condition during DOL start

If this type of fault continues indefinitely because of an anomolous operating condition., damage begins to occur creating……….

Protection – Short Circuit

What do we mean by a fault?

Short Circuit

Operating condition in an electrically damaged circuit where there is an accidental or intentional connection by a relatively low resistance between two points of a circuit which are normally at different voltages

This type of fault can generate high current flows, arcing and fire if not cleared quickly

Discrimination

Coordinate devices to:

Guarantee safety for people and installations

Identify and exclude only the zone affected by a problem

Limiting the effects of a malfunction

Reducing the stress on components in the affected zone

Ensuring service continuity with good quality supply voltage

Achieving a valid compromise between reliability, simplicity and cost effectiveness

Explaining the terminology

Discrimination or Selectivity To make it possible to isolate a part of an installation involved in a fault

condition from the overall system such that only the device located immediately on the supply side of the fault intervenes

Discrimination

Needs

Fast Fault Detection

Fast Fault Elimination

Let-Through Energy Reduction

High Fault Current Withstanding

GO!WAIT!

FAULTCONTINUITY OF

SERVICEFAULT DAMAGE

A

B CFault occurs here

X

X X

Explaining the terminology

Degrees of discrimination

Total Discrimination This means that the isolation described occurs for all fault

levels possible at each point of the circuit

AB

Prospective Fault Current Icc

I

t

Degrees of discrimination

Degrees of discrimination

Partial Discrimination This means that above certain current levels there is

simultaneous operation of more than one protection device

AB

Prospective Fault Current Icc

I

t

Degrees of discrimination

Discrimination

Traditional solutions

Current discrimination

Time discrimination

Energy discrimination

Zone (logical) discrimination

Discrimination

Current discrimination

Discrimination among devices with different trip threshold setting in order to avoid overlapping areas.

Setting different device trip thresholds for different hierarchical levels.

Discrimination

Current discrimination

An example:

Discrimination

Current discrimination

Applications: final distribution network with low

rated current and low short-circuit current

ACB chains

Fault area: short circuit and overload

Discrimination limit current: low

Discrimination levels: low

Devices: ACBs, MCCBs and devices with time/current curves (contactors with thermal relays, fuses …)

Feasibility & discrimination study: easy

Customer cost: low

Discrimination

Time discrimination

Discrimination among devices with different trip time settings in order to avoid overlapping areas

Setting different device trip delays for different hierarchical levels

Discrimination

Time discrimination

An example:

Electronic release L (Long delay) S (Short delay) I (IST)

E4S 4000 PR111-LSI R4000

E3N 2500 PR111-LSI R2500

S7H 1600 PR211-LSI R1600

Setting: 0.9Curve: B

Setting: 1Curve: A

Setting: 1Curve: A

Setting: 8Curve: D

Setting: 10Curve: C

Off

Off

Setting: 10

Discrimination

Time discrimination

Applications: low complexity plant

Fault area: short circuit and overload

Discrimination limit current: low, depending on the Icw of the upstream device

Discrimination levels: low, depending on the network

Devices: ACBs, MCCBs and devices with adjustable time curves

Feasibility & discrimination study: easy

Customer cost: medium

Types of Discrimination

Energy Discrimination Many Low Voltage protection devices such as Circuit breakers

and Fuses have the ability to limit the peak of the current let through them to a value lower than the prospective short circuit peak.

Any protective device which clears short circuits in less than 1/2 cycle of the sinusoidal wave (i.e 10mS for 50Hz) will current limit to a certain degree

Energy based discrimination is the only way to determine true discrimination between current-limiting devices

Discrimination

Energy discrimination

Discrimination among devices with different mechanical and electrical behaviour depending on energy level

It is necessary to verify that the let-through energy of the circuit-breaker upstream is lower than the energy value needed to complete the opening of the CB downstream

Discrimination

Energy discrimination

An example:Time-currents Curve

Energy discrimination up to 24 kA

Discrimination

Energy discrimination

Applications: medium complexity networks

Fault area: Short circuit only

Discrimination limit current: medium/high

Discrimination levels: medium, CBs’ size dependent

Devices: ACBs, MCCBs, MCBs & Fuses

Feasibility & discrimination study: medium complexity

Customer cost: medium

Discrimination

Zone discrimination

Discrimination among devices in order to isolate the fault zone keeping unchanged feeding conditions of maximum number of devices

Zone discrimination is implemented by means of an electrical interlock between devices

Zo

ne

1Z

on

e 2

Zo

ne

3

Discrimination

Zone discrimination Applications: high complexity plant

Fault area: short circuit, overload, ground fault

Discrimination limit current: medium, depending on Icw

Discrimination levels: high

Devices: ACBs, MCCBs with dialogue and control features

Feasibility & discrimination study: complex

Customer cost: high

Explaining the terminology

Cascading or Backup protection Uses supply circuit breakers or fuses with current

limitation effects to protect downstream devices from damage

The amount of energy let through (i2t) by the supply device needs to be lower than that which can be withstood without damage by the device on the load side

By using this effect it is possible to install devices downstream that have short circuit breaking capacities lower than the prospective short circuit current

Back-up protection/Cascading

Back-up protection or Cascading is recognised and permitted by the 16th Edition of the IEE Wiring Regulations 434-03-01 and is covered by IEC 364-4-437 standard

Why Use Back-up Protection?

Substantial savings can be made on downstream switchgear and enclosures by using lower short circuit ratings

Substantial reductions in switchgear volumes can also result

What about Discrimination?

Backup protection should not be confused with discrimination.

Backup protection does not infer discrimination can be achieved but in practice, discrimination is normally achieved up to the maximum breaking capacity of the downstream device

GO!WAIT!

FAULTCONTINUITY OF

SERVICE FAULT DAMAGE

BACKUP Discrimination

Practical Example

Problem:

Installation requires the use of Busbar rather than cable to distribute electrical power.

Fault level calculations reveal 25kA prospective fault level at the point of installation of standard MCB distribution board

Practical Example

Solution -

Using a standard Isolator as the distribution board incoming device - all the MCBs would need to be 25kA or above

Using an MCCB as the incoming device such as an ABB Tmax T3N250TMD100, 6kA S200 MCBs could be safely used

A word of caution …...

Back-up protection can only be checked by laboratory tests and so only device combinations specified by the manufacturer can be guaranteed to provide co-ordination of this type.

Types of protection available

Fuses

Miniature Circuit Breakers

Moulded Case Circuit breakers

Air Circuit breakers

Typical fuse

Current (A)

Time (s)

Ultra Reliable

Standard Characteristic

High current limitation effects

High threshold on low overloads ( clears overloads at approx 1.45x rated FLC)

Fuseless technology

Two main types:-

Thermomagnetic protection- MCB and lower rated MCCB plus older type protection relays

Electronic protection – Microprocessor based relays fed from CTs either external to switches or integral within a circuit breaker

Thermomagnetic

Thermalcurve

Time (s)

Current (A)

Magnetic curve

Offer thermal longtime overcurrent protection using Bi-metal technology ( operates at 1.3x FLC)

Uses the magnetic effect of short circuit currents to offer shorttime short circuit protection

Electronic Relays

Time (s)

Current (A)

Overcurrent functions such as:-

Long time overcurrent

Short time instantaneous protection

Short time time delayed protection

Ground fault or Earth fault protection

Electronic Relays

Time (s)

Current (A)

Overcurrent functions such as:-

Long time overcurrent

Short time instantaneous protection

Short time time delayed protection

Ground fault or Earth fault protection

Protection releases: general features

Complete set of advanced protection functionsRc D U

OT UV OV

RV

RP

MORE

Complete set of standard protection functions

MORE

Complete set of measurements functions

MORE

A V Hz

W VA VAR

E THD

Data logger: a professional built-in fault recorder.

BACK

Data logger: a professional built-in fault recorder.

Standard in PR122 and PR123 Recording of 8 measurements (currents and voltages); Configurable trigger (i.e. During a fault); Sampling frequency up to 4.800kHz; Sampling time up to 27s; Output data through SD-Pocket or TestBus2.

Exclusive from ABB SACE.

-500

-400

-300

-200

-100

0

100

200

300

400

500

time

Voltage L1-L2

Neutral

Current phase 1

Current phase 2

Current phase 3

So what is the secret to achieving a successful discrimination study

The secret is to be aware of the capability of the technology you are using and to design your installation within the limits of the protection you have chosen

Conclusion

Double S* Used to obtain discrimination in “critical” conditions

Double G* Two different protection curves, one with the signal coming from

internal CTs and the other from an external toroid

Dual Setting* Two different set of protection parameters in order to protect in the best

way, two different network configurations (e.g. normal supply and emergency supply)

Protection releases: news on standard protection functions

* = These features are available on PR123/P

BACK

MORE

MORE

MORE

Double S “low” setting on S protection

function due to the settings on MV circuit-breaker

The circuit-breaker on LV side of the LV-LV trafo needs “high” settings due to the inrush current

Protection releases: news on standard protection functions

Protection releases: news on standard protection functions

Without double S

Protection releases: news on standard protection functions

With double SBACK

Double G It’s possible to protect the network, with the same protection

release, against earth fault both upstream and downstream the circuit-breaker

Restricted Earth Fault: the fault is upstream the LV circuit-breaker

Protection releases: news on standard protection functions

Restricted Earth Fault

MV LV

Double G It’s possible to protect the network, with the same protection

release, against earth fault both upstream and downstream the circuit-breaker

Restricted Earth Fault: the fault is upstream the LV circuit-breaker

Unrestricted Earth Fault: the fault is downstream the LV circuit-breaker

Protection releases: news on standard protection functions

Unrestricted Earth Fault

MV LV

Double G The combination of both Unrestricted and Restricted Earth Fault

protection is named “Source Ground Return”. The new PR123/P is able to detect and to discriminate both earth faults

If the fault is downstream the LV circuit-breaker the PR123/P will trip Emax circuit-breaker

Protection releases: news on standard protection functions

L1

L2

L3

NPE

Trafo secondary windings

External toroid

Emax internal CTs

Double G The combination of both Unrestricted and Restricted Earth Fault

protection is named “Source Ground Return”. The new PR123/P is able to detect and to discriminate both earth faults

If the fault is downstream the LV circuit-breaker the PR123/P will trip Emax circuit-breaker

If the fault is upstream the LV circuit-breaker the PR123/P will trip the MV circuit-breaker

Protection releases: news on standard protection functions

L1

L2

L3

NPE

Trafo secondary windings

External toroid

Emax internal CTs

BACK

Dual setting It allows to program two

different protection parameter sets in order to adapt them to the different network configurations

The most representative example is a network with supply by the utility and by emergency generator

With dual setting the discrimination between CBs is guaranteed in both network conditions

Protection releases: news on standard protection functions

Dual setting “Normal” network condition

CB “A” >>> closed

CB “B” >>> open

Discrimination is guaranteed between A and C

Protection releases: news on standard protection functions

Dual setting “Emergency” network

condition

CB “A” >>> open

CB “B” >>> closed

Discrimination is not guaranteed between B and C, due to the “low” settings (protection of the generator) of C protection functions

Protection releases: news on standard protection functions

Dual setting “Emergency” network

condition

CB “A” >>> open

CB “B” >>> closed

Discrimination is guaranteed between B and C thanks to the second set of protection parameters

Protection releases: news on standard protection functions

BACK

Protection releases: advanced protection functions

BACK

Residual current

Protection against directional short-circuit with adjustable time-delay

Protection against phase unbalance

Protection against overtemperature (check)

Protection against undervoltage

Protection against overvoltage

Protection against residual voltage

Protection against reverse active power

Thermal memory for functions L and S

Underfrequency

Overfrequency

D

U

OT

UV

OV

RV

RP

M

UF

OF

RC

Protection releases: measurements functions

BACK

Voltage (phase-phase, phase-neutral, residual). Accuracy: 1%

Power (active, reactive, apparent) Accuracy: 2,5%

Power factor Accuracy: 2,5%

Frequency and peak factor Accuracy: 0,1Hz

Energy (active, reactive, apparent, meter) Accuracy: 2,5%

Harmonics calculation (display of waveforms and RMS spectrum up to 40th @50Hz)

Current (phases, neutral, earth fault). Accuracy: 1,5%

Protection releases: measurements functions

BACK

Voltage (phase-phase, phase-neutral, residual). Accuracy: 1%

Power (active, reactive, apparent) Accuracy: 2,5%

Power factor Accuracy: 2,5%

Frequency and peak factor Accuracy: 0,1Hz

Energy (active, reactive, apparent, meter) Accuracy: 2,5%

Harmonics calculation (display of waveforms and RMS spectrum up to 40th @50Hz)

Current (phases, neutral, earth fault). Accuracy: 1,5%