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The protection calculation module gives insight in the behaviour of the protections in case of short circuits in the network. The protection function can be incorporated with a component, such as the fuse, but can also be combined as a separate function to a switching component, such as the circuit breaker switch. The table below gives an overview of the switching components and their possible protections. The circuit breaker switch can be coupled to four different protections. Several circuit breaker switches can be coupled to a differential protection.

 

 

See also:

•Load switch

•Fuse

•Circuit breaker

•Current protection

•Earth fault protection

•Voltage protection

•Distance protection

•Differential protection

•Short circuit indicator

 

For analysing the protections and checking the correct protection settings the protection calculation module offers two types of calculations: simulation calculation and selectivity calculation. Using the simulation calculation the course of the switching actions after a single short circuit can be studied. Using the selectivity calculation the selectivity of the protections can be examined at a single glance.

It is also possible to present the protection switching characteristics in one graph. See also: Maximum current protection characteristic.

 

Simulation

Using the simulation calculation the behaviour of the protections can be studied in detail during the switch off sequence of a single short circuit. Firstly on a node a short circuit is applied. Then all flows in the network are calculated. For all the activated protections the possible switch off times are calculated. The protection which first trips opens its corresponding circuit breaker switch. Then the network is calculated again. The now fastest protection trips and the corresponding switch opens. This process is repeated until no more protections trip. The result is a number of sequences of disconnections.

Also in cables short circuits can be simulated. For that purpose Vision applies short circuits on a number of equidistance places in the selected connections (cables/lines). The number of equidistance places can be define in the Options at: Calculation | Protection.

 

Changing short circuit currents in a meshed network

When a switch opens in a meshed network, (or in separately protected parallel feeding cables) several protections are activated. In that case it can happen that after the first switching action the other protections are getting to see another flow. This new flow causes new switch off times. The behaviour of the protections on this point are modeled in a way that approaches the reality. This model has been outlined below.

 

Suppose that two feeders are feeding into the short circuit. At time T0 = 0 s the protections see currents IA and IB. If these currents are large enough, both protections are activated and the switch off times are determined according to the protection characteristics. If the current through protection A is IA = I1, the characteristic switch off time will be t1 and the ultimate switch off time will be:

TA = T1 = T0 + t1 s

 

If, however, in this example, protection B switches earlier (TB = T0 + tu) than protection A, the current through protection A changes from I1 to I2. This new short circuit current can be larger or smaller than I1. As a consequence, the switch off time for protection A has to be calculated again. The procedure follows the table below.

 

 

Remarks on this model:

•The first situation in the table implies that a protection may be reset. This is a consequence of the worst case approach.

•The thirs situation in the table is a worst case approach. When the current has been reduced (slightly), the new characteristic switch off time is added to the actual time.

•The fifth and sixth situations in the table are the most common situations. After a switching action in a parallel path the remaining short circuit current will increase in most cases.

 

 

Selectivity

The selectivity calculation module analyses the selectivity of protections of a component or a group of components.

 

An object or a group of objects is selectively protected if the consequences of a short-circuit are limited. This is the case if only the protections, located closely to the short-circuit, interrupt the short-circuit. This has been modelled in Vision so that only the fuses or circuit breakers directly surrounding the failed group will open. Switching in isolated objects does not influence the selectivity. Disconnecting an element by switching of a fuse or a circuit breaker influences the selectivity if defined in the Options (at: Calculation | Protection | Selectivity | Influence element protection).

 

One way of determining the selectivity of protections in simple cases is to compare the protections characteristics. The protections are selective if their characteristics do not cross and if there is enough margin between them. For more complex cases with parallel paths, network meshes or dispersed generation, the selectivity calculation module will be preferred.

 

The selectivity calculation module automatically determines the selectivity of protections. It uses the simulation module calculations. The procedure calculates all protection actions for all possible short circuit situations on a selected component.

 

In the calculation several faults with various resistances are applied on a node or in a branch (cable/line). The range of fault resistances is established automatically.

For each fault all currents in the network are calculated. The protections, for which the current is larger than the threshold value, will be activated. For these protections the switch off times are calculated. These times are saved and can be viewed with Results | Detailed graph. Next the tripped circuit breakers will be opened and the switching times of the remaining protections (especially in meshed networks) will be calculated. This action will be repeated until the short circuit has ended.

 

The result is a curve of possible protection switch off times as a function of the short circuit current. Presented in a graph, these curves give a good insight in the protections selectivity. The curves show:

•Intended switches off: selectivity curves for the initially activated protections that intend to switch off

•Real switches off: switch off curves for the actually switching circuit breakers or fuses.

 

See also:

•Maximum current protection characteristic

•Calculation

•Result