COORDINATION OF LOW VOLTAGE PROTECTION
COORDINATION OF LOW VOLTAGE PROTECTION
Sona Rzayeva
Head of laboratory Department of Electromechanics, Azerbaijan State University of Oil and Industry,
Azerbaijan, Baku
Seymour Beylarzadeh
Master of the Department of Electromechanics Azerbaijan State University of Oil and Industry,
Azerbaijan, Baku
ABSTRACT
The article deals with the design and operation of modern low voltage (LV) power supply systems. The most important task in this case is to ensure the selectivity of protection devices, i.e. coordination of their performance characteristics for any type of damage. All these features and requirements are fully met by the devices of the Masterpact NT and NW series of the Merlin Gerin trademark of the Schneider Electric company, which have been deservedly considered number one devices in the world for many years. The article allows you to better understand the technical and economic problems and phenomena that arise when using Schneider Electric electrical equipment and automation tools.
Keywords: low voltage, device selectivity, cascade connection, power supply, protection coordination.
Introduction
When solving design and operation problems, there are three characteristic levels of the power supply system (see Fig. 1). Each level has different features and imposes its own requirements on protection devices [1].
Figure1. Low voltage power supply system
Level A. Main distribution board
The MDB is the most important part of the LV network, which is characterized by:
- high requirements for uninterrupted power supply, since a false operation of the
device at this level leads to the disconnection of a large number of users;
- relatively high values of short-circuit currents (SC) due to proximity to the power source;
- high rated currents, since the entire load of the downstream network is powered
by the main distribution board sections.
At this level, non-current-limiting air circuit breakers (Air Circuit Breakers - ACB) are most often used as input devices. These devices, according to GOST R 50030.2 and IEC 60947.2, belong to the category of application "B", for which the value of the short-term permissible through short-circuit current is normalized (Icw, kA actual value) [3]. This allows these devices to operate with a specified time delay, which is set on the monitoring and control unit. The task of the manufacturer is to ensure that the circuit breaker of category "B" has an Icw value close to or equal to the value of the ultimate breaking capacity (Icu, kA actual value), that is, that it provides time selectivity with downstream devices at short-circuit currents up to up to the value of its ultimate breaking capacity.
At the same time, it is equally important that the operating breaking capacity of the device (Ics, kA actual value) be equal to the limiting one (Icu), that is, Ics=100% Icu. This allows the device to break a short-circuit current equal to the maximum breaking capacity of the circuit breaker at least three times.
All these features and requirements are fully satisfied by the devices of the Masterpact NT and NW series of the Merlin Gerin trademark of the Schneider Electric company, which have been deservedly considered the number one device in the world for many years [2]. Most of their modifications, in particular, N1 and H1, have characteristics Icw=Ics=Icu, which allows them to meet all the most stringent requirements at the level of the main distribution board.
In some cases, it is necessary that close short circuits with respect to input devices (for example, short circuits on the main busbars of the main distribution board) are turned off earlier than the short circuit protection time delay set on the input device. In this case, Schneider Electric proposes to use the so-called "logical" selectivity, which is implemented by transmitting information along the control wire. This wire connects the devices covered by the "logical selectivity". In emergency mode, the circuit breaker located above the fault detects it and sends a blocking signal to the upper level, i.e. the upstream circuit breaker. In this case, the upstream device will operate with the time delay set on the release. In the event that the upstream circuit breaker does not receive a blocking signal, for example in the event of a short circuit on the main distribution board busbars, it trips instantly.
Thus, Masterpact incoming devices of category “B” used on the main distribution board provide time selectivity at short-circuit currents up to their ultimate breaking capacity, have the function of logical selectivity and the characteristic Ics = Icu.
Level B. Intermediate distribution boards (secondary distribution)
The features of this level of the low voltage power supply system are still increased requirements for uninterrupted power supply, high values of expected short circuit currents, the need to reduce the thermal and electrodynamic effects of short circuit currents on the electrical installation (especially on cable lines). Therefore, at this level, current-limiting circuit breakers in a molded case (Molded Сase Сircuit Breakers - MCCB), which, as a rule, belong to category “A”, are most often used.
In Schneider Electric's offer, MCCB devices include the Compact NS series, which has a number of fundamental technical advantages compared to similar devices from other manufacturers.
Level C. Final distribution
The main requirements of this level, as a rule, are to ensure effective current limitation and electrical safety (since devices of this level most often protect the direct end user). Therefore, at this level, modular current-limiting circuit breakers (MCB) belonging to category "A" are used.
In Schneider Electric's offer, MCB devices include the Multi 9 series, which is a very wide range (circuit breakers, RCDs, differential circuit breakers, lighting control devices and much more) and has a number of patented technical solutions similar to the Masterpact and Compact NS series.
As noted above, between devices of category "B" on the main distribution board and downstream devices, temporal (less often logical) selectivity is most often used. This type of selectivity is provided by shifting or shifting the time-current characteristics of series circuit breakers along the time axis (see Fig. 2).
Figure 2. Temporal selectivity between series circuit breakers D1 and D2 Ir1, Ir2 - adjustable overload protection settings, Isd1, Isd2 - selective current cutoff settings, IiD1- instantaneous current cut-off setting
If it is necessary to check the provision of selectivity between the Masterpact NT H1 630A Micrologic 2.0 and Compact NS 400NSTR23SE devices, two simple operations are performed:
1) In the tables of selectivity is the considered combination of superior and lower standing apparatus;
2) At the intersection of the corresponding column and row in the table, the result is indicated:
- "T" means full selectivity (i.e. the selectivity of the pair of devices under consideration is ensured at short-circuit currents up to the limiting breaking capacity of the downstream device IcuD2);
- “Number” means that selectivity is partially provided, that is, only if the expected short-circuit current is less than the value indicated in the table (according to GOST 50030.2, the value indicated in the table means the limiting current of selectivity - Is);
- "empty cell" means no selectivity.
Selectivity of this kind can be checked by overlaying the time-current characteristics of the devices or by using selectivity tables. A fragment of the selectivity table is shown in Fig.3. These tables are provided in the "Additional technical information" section. They indicate whether or not selectivity is provided between Merlin Gerin circuit breakers.The data indicated in the tables are the results of tests or comparisons of the characteristics of the considered apparatus [4].
Figure 3. A fragment of the selectivity table
Figure 3 shows that the selectivity between the devices under consideration is complete (T - Total).
The most difficult type of coordination of protective devices is the case when the pair of circuit breakers under consideration is current-limiting. In this case, the analysis of selectivity by superimposing time-current characteristics does not give an accurate and complete picture, since at relatively high values of the short-circuit current, the curves have a so-called uncertainty zone. In this zone, that is, at high short-circuit currents, current-limiting devices can already be in the “arc” zone, that is, their contacts, due to a special design, will be repelled to introduce additional arc resistance into the short-circuit circuit and thus limit the short-circuit current flowing.
Conclusion
Therefore, the coordination of current-limiting devices according to IEC 60947.2 (GOST 50030.2) can only be guaranteed by the manufacturer, who is obliged to test and thus confirm this type of coordination. The result of these tests and the guarantee of selectivity between current-limiting devices are the mentioned selectivity tables.
References:
- Elektricheskie i elektronnie apparati / Pod. red. Yu.K.Rozanova.-M.:Informelektro, 2001.
- Koordinasiya zashit nizkoqo naprajeniya // Rukovodstvo po nizkomu naprajeniyu, № 5. – Schneider Electric, 2005.
- Roland Talon. Electrical installation guide /According to IEC International Standards. – Schneider Electric, 2005.
- https://www.se.com/ww/en/