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Abstract


Сontent

Introduction

Advances in technology and increasing demands for electrical caused a significant change in the systems to ensure reliable and uninterrupted operation of electrical equipment. Current trends in the automation of technological processes in enterprises, require the use of modern technology supply.

Modern systems require the installation of electrical protection at all voltage classes. In this case, protection, low voltage is one of the components of total electricity supply. Coordination of protection at a low voltage is one of the fundamental quality and reliability of the power system in the enterprise.

The purpose of the master's work is the study and development of modern methods of protection coordination at a low voltage.

1. Safety and uninterruptible єlektrosnabzheniya

In applying the protections of the above damage must be considered:

    1. The regulation and safety of staff;
    2. The technical and economic requirements.

The equipment used to protect must:

    • Resistant against fault currents, remove them from the economic cost in relation to the required specifications;
    • To limit the negative impact of fault currents on electrical installation and to provide reliable (uninterrupted) power supply.

The above requirements are provided under the coordination of the performance of protection, it is necessary to:

    • Safety and increase the life of electrical systems by limiting the thermal and electrodynamic effects;
    • Ensure the security of electricity supply by switching off only the damaged part, that only triggered an automatic switch that directly protects the damaged area.

The following types and methods of co-ordination between the switches:

    • Cascade connection (according to GOST)[1];
    • Selectivity.

If your network has the means of differential protection, you must also provide the selectivity of the differential protection.


2. The structure of the low-voltage power supply

Структура электроснабжения низкого напряжения
A simplified diagram of a typical setting, the majority of cases encountered in practice.
Levels of low-voltage power supply system.

Each of the three levels of the power supply system imposes its own requirements for security and business continuity.

3. Basic functions and operation of protection devices

Protection devices and their coordination should take into account the particular installation.

    1. At the level of the main switchboard, low voltage the most important requirement - is uninterrupted power supply;
    2. At the sub-distribution boards is important to limit the thermal and electrodynamic effects on the electrical installation;
    3. At the level of the final distribution of the most important requirement - it is electrical.

The function and purpose of circuit breakers.

These switching devices are able to turn on and shut off the circuit for any value of current, ie until his breaking capacity.

The main functions performed:

    • switch circuit;
    • provision of current flow;
    • open circuit and power cut; 
    • disconnect function.

Selection of circuit breakers is determined by:

    • requirements and electrical characteristics;
    • requirements to ensure the smooth and safe power supply;
    • cost optimization.

Level A: the main low voltage switchboard

The main switchboard - this is the most important part of the low voltage network.

Reliability (continuity) of power supply is a major requirement at this level.

Short-circuit currents at this level reaches high values​​:

    • due to the proximity to the power supply;
    • because cross-section of conductors (busbars, cables) is sufficiently large and is intended for the flow of high currents.

This is the level where the circuit breakers are used for high currents.

These circuit breakers are designed to distribute large currents:

    1. Installed in the switchboard that suspending NN as input devices, and may be designed to protect the powerful feeders (feeder);
    2. Short-circuit some time they should remain in the ON state to the downstream circuit breakers were able to repair the damage. Therefore, as a rule, devices on the main switchboard triggered delayed.

Resistance to thermal circuit breakers and electrodynamic effects determined by short-time withstand short-circuit current Icw (according to IEC 50030.2 [2], according to IEC 50030.1 [3]).

The main characteristics of circuit breakers for large currents:

    1. used in industrial and civil objects according to IEC 6094772;
    2. large values ​​of the breaking capacity Icu: 40 kA to 150;
    3. Switches category B:
      • large values ​​of short-time withstand short-circuit current Icw: 40 to 150 kA for 1 second;
      • high electrodynamic stability;
    4. breakers can be equipped with a motor drive for the remote control, which allows for the ATS.

Reliable and uninterrupted power supply is provided by a full selectivity:

      1. Together with upstream protection of power transformers, for example, fuses;
      2. Together with downstream protective devices feeders (generally used, time selectivity).

It should be noted that the most important to ensure a complete protection of the selectivity between the devices 6, 10,20 kW and 0.4 kW, so as to restore the power side higher voltage requires greater effort and expense.

Level B: intermediate distribution panel

Features of this level of power supply:

    1. Paspredelenie electricity carried over cable lines and track system;
    2. Distance from the power supply is small, so the short-circuit currents can reach values ​​of up to 100 kA
    3. At this level is also very important to ensure uninterrupted power supply.

Thus, the circuit breakers at the level of B to limit thermal and electrodynamic effects, and to be fully coordinated with the upstream and downstream protection devices.

This is the level where the circuit breakers are used in molded case.

Automatic molded case circuit breakers typically are fast.

Major task – a reduction of the thermal and electrodynamic effects on guides (cable lines, busbars) as well as the consumer. This is achieved by current limit, ie by repulsion of contacts (moving away from the fixed contact parts) that allows you to disable the short circuit current before it reaches its expected steady-state value.

Level B: the final distribution

At this level, circuit breakers protect the end-user directly. Thus, should be provided with higher selectivity protections.

For this level is characterized by low values ​​of short-circuit currents.

This is the level where applicable MCBs.

These switches are intended to protect the final distribution circuits where necessary to limit the thermal and electrodynamic effects both on the conductors (cables, connection device) and the users. MCBs can satisfy the above requirements. Limiting capacity of MCBs is partly dependent on the electromagnetic element (actuator). After his release, he is working with (strikes) at the moving contact, reporting last initial speed. Thus, the arc voltage begins to develop early and very quickly. Circuit breakers with lower rated current has a greater resistance to the pole, which further contributes to the stall prevention. Miniature circuit breakers are designed for domestic use and for the protection of secondary circuits: in this case, they correspond to the stand main characteristics of MCBs:

    1. breaking capacity, depending on the application;
    2. rated current: 1.5 to 125 A, according to customer;
    3. generally intended for domestic use and comply with IEC 60898.

These circuit breakers must provide:



Thus the use of such protective systems will improve the quality of power supply and protect the enterprise from non-normal modes of operation of the enterprise caused by the accident in the electricity system.

In writing this essay master's work is not yet complete. Final completion: December 2013. Full text of the work and materials on the topic can be obtained from the author or his manager after that date.

References

  1. ГОСТ Р 50030.2   «резервная защита»;
  2. ГОСТ Р 50030.2 «номинальным кратковременно выдерживаемым током»
  3. ГОСТ Р 50030.1 «номинальным кратковременно допустимым током»
  4. Стандарты Международной электротехнической комисии  – IEC
  5. Амельницкая Е.В. Совершенствование управления локальными электрическими сетями как субъектами хозяйствования // Наукові праці Донецького національного технічного університету. – Донецьк, ДонНТУ, 2005. – с.101-115.
  6. Мукосеев Ю.Л. Электроснабжение промышленных предприятий М:, "Энергия", 584 с.
  7. Федоров А.А., Сербиновский Г.В. Справочник по электроснабжению промышленных предприятий: Электрооборудование и автоматизация. 1981. М.: Энергоиздат  – 624 с, ил.