Discrimination of protection devices on installations

Discrimination of protection devices on installations
Janet Roadway Product Manager, Power Breakers Discrimination of protection devices on installations

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 The design of a system for protecting an electric network is of fundamental importance both to ensure the correct economic and functional operation of the installation as a whole and to reduce to a minimum any problem caused by anomalous operating conditions and/or malfunctions. The present analysis discusses the coordination between the different devices dedicated to the protection of zones and specific components with a view to: • guaranteeing safety for people and installation at all times; • identifying and rapidly excluding only the zone affected by a problem, instead of taking indiscriminate actions and thus reducing the energy available to the rest of the network; • containing the effects of a malfunction on other intact parts of the network (voltage dips, loss of stability in the rotating machines); • reducing the stress on components and damage in the affected zone; • ensuring the continuity of the service with a good quality feeding voltage; • guaranteeing an adequate back-up in the event of any malfunction of the protective device responsible for opening the circuit; • providing staff and management systems with the information they need to restore the service as rapidly as possible and with a minimal disturbance to the rest of the network; • achieving a valid compromise between reliability, simplicity and cost effectiveness. To be more precise, a valid protection system must be able to: • understand what has happened and where it has happened, discriminating between situations that are anomalous but tolerable and faults within a given zone of influence, avoiding unnecessary tripping and the consequent unjustified disconnection of a sound part of the system; • take action as rapidly as possible to contain damage (destruction, accelerated ageing, ...), safeguarding the continuity and stability of the power supply. The most suitable solution derives from a compromise between these two opposing needs - to identify precisely the fault and to act rapidly - and is defined in function of which of these two requirements takes priority.

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! FAULT CONTINUITY OF SERVICE FAULT DAMAGE To be more precise, a valid protection system must be able to: • understand what has happened and where it has happened, discriminating between situations that are anomalous but tolerable and faults within a given zone of influence, avoiding unnecessary tripping and the consequent unjustified disconnection of a part of the system; • take action as rapidly as possible to contain damage (destruction, accelerated ageing, ...), safeguarding the continuity and stability of the power supply. The most suitable solution derives from a compromise between these two opposing needs - to identify precisely the fault and to act rapidly - and is defined in function of which of these two requirements takes priority.

B C Fault occurs here X

Degrees of discrimination
Total Discrimination This means that the isolation described occurs for all fault levels possible at each point of the circuit

B Prospective Fault Current Icc I

Partial Discrimination This means that above certain current levels there is simultaneous operation of more than one protection device

B Prospective Fault Current Icc I

Discrimination Traditional solutions Current discrimination
Time discrimination Energy discrimination Zone (logical) discrimination Frequently is used time-current 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. This type of discrimination is based on the observation that the closer the fault comes to the network’s feeder, the greater the short-circuit current will be. We can therefore pinpoint the zone where the fault has occurred simply by calibrating the instantaneous protection of the device upstream to a limit value higher than the fault current which causes the tripping of the device downstream. We can normally achieve total discrimination only in specific cases where the fault current is not very high (and comparable with the device’s rated current) or where a component with high impedance is between the two protective devices (e.g. a transformer, a very long or small cable...) giving rise to a large difference between the short-circuit current values.

Discrimination Current discrimination An example:
With a fault current value at the defined point equal to 1000 A, an adequate coordination is obtained by using the considered circuit-breakers as verified in the tripping curves of the protection devices. The discrimination limit is given by the minimum magnetic threshold of the circuit-breaker upstream, T1B160 R160.

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 . This type of coordination is consequently feasible mainly in final distribution networks (with low rated current and short-circuit current values and a high impedance of the connection cables). The devices’ time-current tripping curves are generally used for the study. This solution is: • rapid; • easy to implement; • and inexpensive. On the other hand: • the discrimination limits are normally low; • increasing the discrimination levels causes a rapid growing of the device sizes.

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 This type of discrimination is an evolution from the previous one. The setting strategy is therefore based on progressively increasing the current thresholds and the time delays for tripping the protective devices as we come closer to the power supply source. As in the case of current discrimination, the study is based on a comparison of the time-current tripping curves of the protective devices.

Discrimination Time discrimination An example:
Electronic release L (Long delay) S (Short delay) I (IST) Time discrimination An example: Setting: 0.9 Curve: B Setting: 1 Curve: A Setting: 8 Curve: D Setting: 10 Curve: C E4S 4000 PR111-LSI R4000 E3N 2500 PR111-LSI R2500 S7H 1600 PR211-LSI R1600 Off Setting: 10 The following example shows a typical application of time discrimination obtained by setting differently the tripping times of the different protection devices.

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 This type of coordination: • is easy to study and implement; • is relatively inexpensive; • enables to achieve even high discrimination levels, depending on the Icw of the upstream device; • allows a redundancy of the protective functions and can send valid information to the control system, but has the following disadvantages: • the tripping times and the energy levels that the protective devices (especially those closer to the sources) let through are high, with obvious problems concerning safety and damage to the components even in zones unaffected by the fault; • it enables the use of current-limiting circuit-breakers only at levels hierarchically lower down the chain; the other circuit-breakers have to be capable of withstanding the thermal and electro-dynamic stresses related to the passage of the fault current for the intentional time delay. Selective circuit-breakers, often air type, have to be used for the various levels to guarantee a sufficiently high short-time withstand current; • the duration of the disturbance induced by the short-circuit current on the power supply voltages in the zones unaffected by the fault can cause problems with electronic and electro-mechanical devices (voltage below the electromagnetic releasing value); • the number of discrimination levels is limited by the maximum time that the network can stand without loss of stability.

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 Energy coordination is a particular type of discrimination that exploits the current limiting characteristics of moulded-case circuit-breakers. It is important to remember that a current-limiting circuit-breaker is “a circuit-breaker with a break time short enough to prevent the short-circuit current reaching its otherwise attainable peak value” (IEC , def. 2.3). In practice, ABB SACE moulded-case circuit-breakers of Isomax and Tmax series, under short-circuit conditions, are extremely rapid (tripping times of about some milliseconds) and therefore it is impossible to use the time-current curves for the coordination studies. The phenomena are mainly dynamic (and therefore proportional to the square of the instantaneous current value) and can be described by using the specific let-through energy curves. In general, it is necessary to verify that the let-through energy of the circuitbreaker downstream is lower than the energy value needed to complete the opening of the circuit-breaker upstream. This type of discrimination is certainly more difficult to consider than the previous ones because it depends largely on the interaction between the two devices placed in series and demands access to data often unavailable to the end user. Manufacturers provide tables, rules and calculation programs in which the minimum discrimination limits are given between different combinations of circuitbreakers.

Energy discrimination up to 24 kA
An example: Time-currents Curve Energy discrimination up to 24 kA From selectivity tables it can be obtained that breakers E2N1250 and S5H400, correctly set, are selective up to 55 kA (higher than the shortcircuit current at the busbar). From selectivity tables it can be obtained that, instead, breakers S5H400 e T1N160 R125 are selective up to 24kA (higher than the short-circuit current at the busbar). From the curves it is evident that between breakers E2N1250 and S5H400 time discrimination exists, while between breakers S5H400 and T1N160 there is energy discrimination.

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 Advantages: • fast breaking, with tripping times which reduce as the short-circuit current increases; • reduction of the damages caused by the fault (thermal and dynamic stresses), of the disturbances to the power supply system, of the costs...; • the discrimination level is no longer limited by the value of the short-time withstand current Icw which the devices can withstand; • large number of discrimination levels; • possibility of coordination of different current-limiting devices (fuses, circuitbreakers,..) even if they are positioned in intermediate positions along the chain. Disadvantage: • difficulty of coordination between circuit-breakers of similar sizes. This type of coordination is used above all for secondary and final distribution networks, with rated currents below 1600A.

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 Zone 1 This type of coordination is implemented by means of a dialogue between current measuring devices that, when they ascertain that a setting threshold has been exceeded, give the correct identification and disconnection only of the zone affected by the fault. In practice, it can be implemented in two ways: • the releases send information on the preset current threshold that has been exceeded to the supervisor system and the latter decides which protective device has to trip; • in the event of current values exceeding its setting threshold, each protective device sends a blocking signal via a direct connection or bus to the protective device higher in the hierarchy (i.e. upstream with respect to the direction of the power flow) and, before it trips, it makes sure that a similar blocking signal has not arrived from the protective device downstream; in this way, only the protective device immediately upstream of the fault trips. The first mode foresees tripping times of about one second and is used mainly in the case of not particularly high short-circuit currents where a power flow is not uniquely defined. The second mode enables distinctly shorter tripping times: with respect to a time discrimination coordination, there is no longer any need to increase the intentional time delay progressively as we move closer to the source of the power supply. The maximum delay is in relation to the time necessary to detect any presence of a blocking signal sent from the protective device downstream. Zone 2 Zone 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 Advantages: • reduction of the tripping times and increase of the safety level; the tripping times will be around 100 milliseconds; • reduction of both the damages caused by the fault as well of the disturbances in the power supply network; • reduction of the thermal and dynamic stresses on the circuit-breakers and on the components of the system; • large number of discrimination levels; • redundancy of protections: in case of malfunction of zone discrimination, the tripping is ensured by the settings of the other protection functions of the circuit-breakers. In particular, it is possible to adjust the time-delay protection functions against short-circuit at increasing time values, the closer they are to the network’s feeder. Disadvantages: • higher costs; • greater complexity of the system (special components, additional wiring, auxiliary power sources, ...). This solution is therefore used mainly in systems with high rated current and high short-circuit current values, with precise needs in terms of both safety and continuity of service: in particular, examples of logical discrimination can be often found in primary distribution switchboards, immediately downstream of transformers and generators and in meshed networks.

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 and is covered by IEC 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 Discrimination BACKUP WAIT! GO! CONTINUITY OF SERVICE FAULT FAULT DAMAGE

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 Ultra Reliable Standard Characteristic
High current limitation effects High threshold on low overloads ( clears overloads at approx 1.45x rated FLC) Time (s) Current (A)

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 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 Time (s) Thermal curve Magnetic curve Current (A)

Electronic Relays Overcurrent functions such as:-
Long time overcurrent Short time instantaneous protection Short time time delayed protection Ground fault or Earth fault protection Time (s) Current (A)

Protection releases: general features
Complete set of standard protection functions MORE Complete set of advanced protection functions Rc D U OT UV OV RV RP MORE Complete set of measurements functions MORE A V Hz W VA VAR  E THD

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. BACK

Conclusion 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

Protection releases: news on standard protection functions
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) MORE MORE MORE BACK * = These features are available on PR123/P

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

Without double S

BACK With double S

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 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 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 L1 L2 L3 N PE 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 L1 L2 L3 N PE 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

Dual setting “Normal” network condition CB “A” >>> closed CB “B” >>> open Discrimination is guaranteed between A and C

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

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 BACK

Protection releases: advanced protection functions
Residual current RC D Protection against directional short-circuit with adjustable time-delay Protection against phase unbalance U Protection against overtemperature (check) OT Protection against undervoltage UV Protection against overvoltage OV Protection against residual voltage RV Protection against reverse active power RP Thermal memory for functions L and S M Underfrequency UF Overfrequency OF BACK

Protection releases: measurements functions
Current (phases, neutral, earth fault). Accuracy: 1,5% 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 BACK

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