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%