Circuit Breakers for Direct Current Applications

Introduction

Direct current, which was once the main means of distributing electric power, remains widespread today in electrical plants supplying particular industrial applications. The advantages offered by the use of DC motors and supply through a single line make direct current supply a good solution for railway and underground systems, trams, lifts and other transport means.

In addition, direct current is used in conversion plants (installations where different types of energy are converted into electrical direct energy, e.g. photovoltaic plants) and, above all, in those emergency applications where an auxiliary energy source is required to supply essential services such as protection systems, emergency lighting, wards and factories, alarm systems, computer centers, etc. Accumulators are the most reliable energy source for these services, both directly as direct current as well as by means of uninterruptible power supply units (UPS), where loads are supplied in alternating current.

This technical application paper is intended to explain the main aspects of the most important applications in direct current and to present the solutions offered by ABB products. This paper also has the goal to give precise information to provide a rapid choice of the protection/disconnection device, paying particular attention to the installation characteristics (fault types, installation voltage, grounding arrangement).

Generalities on Direct Current

By definition, direct current has a unidirectional trend constant in time. Analyzing the motion of the charges at a point crossed by a direct current, the quantity of charge (Q) flowing through a cross section is always the same.

Batteries or dynamos can provide direct current. It is also possible to convert alternating current into direct current through a rectifying process. However, a "pure" direct current, a current which does not present any periodic fluctuation, is generated exclusively by batteries (or accumulators).

In a DC system, respecting the current direction has a remarkable importance. Therefore it is necessary to correctly connect the loads by respecting the terminals, as operation and safety problems could arise if the terminals should be connected incorrectly. For example, if a DC motor were supplied by switching the terminals, it would rotate in reverse and many electronic circuits could suffer irreversible damage.

Applications of Low Voltage Direct Current

Low voltage direct current is used for different applications, which have been divided into four macrofamilies:

Conversion of Alternative Energies into Electrical Energy (Photovoltaic Plants)

A photovoltaic plant converts the energy associated with solar irradiation into DC electrical energy. These plants are made up of semiconducting panels which can generate electrical power once exposed to the rays of the sun.

Photovoltaic plants can be grid-connected or supply a single load (stand alone plant). In this last case an accumulator battery is present to provide power in case of a lack of solar radiation. The basic element of a photovoltaic plant is the photovoltaic cell made of semiconducting material. Exposed to the rays of the sun, this cell is able to supply a maximum current Impp at a maximum voltage Vmpp, which corresponds to a maximum power called Wp.

A stand alone photovoltaic plant includes:

• Photovoltaic array: photovoltaic cells suitably interconnected
• Charge regulator: an electronic device able to regulate charging and discharging of accumulators
• Accumulator batteries: to provide power supply in case of lack of solar radiation
• DC/AC inverter: to turn direct current into alternating current

A grid-connected photovoltaic plant may leave out the accumulator battery since the user is supplied by the network when solar irradiation is unavailable.

Electric Traction

The particular torque/speed characteristic curve and the ease with which the speed itself can be regulated have led to the use of DC motors for electric traction. Direct current supply also gives the great advantage of having the contact line consisting of a single conductor as the rails provide the return conductor.

Currently, direct current is used primarily in urban transport like trolleybuses, trams and underground railways, with a supply voltage of 600 V or 750 V, up to 1000 V. Direct current is not limited only to vehicle traction but also represents a supply source for the auxiliary circuits on board vehicles.

Supply of Emergency Services or Auxiliary Services

Direct current is used (directly or indirectly through accumulator batteries) in those plants for which service continuity is fundamental. Plants that cannot tolerate a power failure caused by a loss of energy need a ready-to-use supply source which is able to cover the time needed to start an emergency generating set.

Examples include:

• Industrial applications (process control systems)
• Safety and emergency installations (lighting, alarms)
• Hospital applications
• Telecommunication
• Applications in the data processing field (data centers, work stations, servers, etc.)

Accumulator batteries are the most reliable electric energy source for the supply of such services, both directly in direct current as well as in alternating current by using an inverter. This is carried out by uninterruptible power supply units (UPS).

Particular Industrial Applications

The use of direct current is often required in many industrial applications such as arc furnaces, electro welding plants, graphite manufacturing plants, and metal production and refining plants (aluminum, zinc, etc.). In particular, many metals such as aluminum are produced through an electrolytic process where the service currents are very high, greater than 3000 A. Another very common application is represented by galvanizing plants where processes are carried out to obtain the plating of metallic surfaces with other metals or alloys.

Interrupting Direct Current

Interrupting direct current presents different problems than alternating current as the arc extinction is particularly difficult. With alternating current there is natural passage of current through zero at each half cycle, which corresponds to the quenching of the arc during the circuit opening. With direct current there is no such natural passage and therefore the current must decrease to null to guarantee arc extinction (forcing the current passage through zero).

To guarantee arc extinction, it is necessary that the arc voltage (Va) is so high that the first part of the formula becomes negative. It is possible to conclude that the extinction time of a direct current is proportional to the time constant of the circuit T = L/R and to the extinction constant.

To summarize, in order to guarantee breaking of a short-circuit current in a DC system it is necessary to employ circuit breakers that can ensure:

• rapid tripping with adequate breaking capacity
• high fault current limiting capacity
• overvoltage reduction effect

Types of DC Networks

In order to break a short-circuit current in a DC system, it is necessary to connect the circuit breaker poles in a suitable way. To do this, it is necessary to know the grounding type of the plant. This information allows any possible fault condition to be evaluated and consequently the most suitable connection type to be selected.

Network Insulated from Ground

This type of network represents the easiest connection to carry out as no connection between the battery terminals and ground is provided. These types of systems are widely used in those installations where grounding is difficult, but above all where service continuity is required after an initial ground fault. With this type of network, the fault type which affects the version and connection of the circuit breaker poles is fault A (between the two terminals).

Network with One Terminal Grounded

This type of network is obtained by connecting one terminal to ground. This connection type allows the overvoltages due to static electricity to be discharged to ground. With this type of network, the fault type which affects the version of the circuit breaker and the connection of the poles is fault A (between the two terminals). However it is also necessary to take into consideration the fault between the non-grounded terminal and the ground itself because a current could flow at full voltage.

Network with the Middle Point of the Supply Source Connected to Ground

This type of network is obtained by connecting the middle point of the battery to ground. This type of connection reduces the value of static overvoltages, which could otherwise be present at full voltage in an insulated plant. With this type of network, the fault which affects the version of the circuit breaker and the connection of the poles is fault A (between the two terminals). However, the fault between a terminal and ground should also be taken into consideration because a current could flow at a voltage equal to V/2.

Choice of the Protective Device

For the correct sizing of a circuit breaker in a direct current network, some electrical parameters which characterize the device itself must be evaluated:

Rated operational voltage (Ue) – the value of the application voltage of the equipment and to which all the other equipment parameters are referred.

Rated uninterrupted current (Iu) – the value of current which the equipment can carry for an indefinite time. This parameter is used to define the size of the circuit breaker.

Rated current (In) – the value of current of the trip unit mounted on the circuit breaker and determines the protection characteristic.

Rated ultimate short-circuit breaking capacity (Icu) – the maximum short-circuit current value which the circuit breaker can break twice at the corresponding rated operational voltage.

Rated service short-circuit breaking capacity (Ics) – the maximum short-circuit current value which the circuit breaker can break three times at a defined rated operational voltage.

Rated short-time withstand current (Icw) – the current that the circuit breaker in the closed position can carry during a specified short time.

Sizing Criteria

To size the circuit breaker, it is necessary to know the following characteristics of the network:

• The type of network – to define the connection of the circuit breaker poles according to possible fault conditions
• The rated voltage of a plant (Vn) – to define the operational voltage (Ve) by verifying Vn ≤ Ve
• The short-circuit current at the installation point (Ik) – to define the circuit breaker version by verifying Ik ≤ Icu
• The rated current absorbed by the load (Ib) – to define the rated current (In) by verifying Ib ≤ In

ABB Offering for DC Applications

ABB offers a comprehensive range of products for the protection and disconnection of DC networks, including miniature circuit breakers, molded case circuit breakers and air circuit breakers.

Miniature Circuit Breakers

Miniature circuit breakers available for use in direct current include the S280UC, S800S UC and S800 PV series. The S280UC series complies with IEC 60947-2 and differs from standard versions in that they are equipped with permanent magnetic elements on the internal arcing chambers, allowing the electric arc to be broken up to voltages equal to 440 VDC. These circuit breakers are available with characteristics B, C, K and Z and rated currents from 0.5A up to 63A.

Tmax Molded Case Circuit Breakers

Tmax MCCBs are available for DC applications with rated uninterrupted currents from 160A up to 800A and voltages up to 750V DC. They are available with thermal-magnetic or electronic trip units and can be configured with various pole connections to suit different network types.

Emax Air Circuit Breakers

For higher power DC applications, Emax air circuit breakers are available with rated currents up to 5000A and voltages up to 1000V DC. These are equipped with PR122-PR123/DC electronic trip units specifically designed for DC applications.

Tmax PV Molded Case Switches

For photovoltaic applications, Tmax PV molded case switches are available with rated currents up to 1600A and voltages up to 1100V DC.

Conclusion

Direct current applications continue to grow across multiple sectors, from renewable energy and electric traction to emergency power systems and industrial processes. Understanding the unique challenges of interrupting direct current, the different network grounding arrangements, and the appropriate selection of protective devices is essential for ensuring safe and reliable operation.

ABB's comprehensive range of products, from miniature circuit breakers to high-power air circuit breakers, offers solutions for virtually any DC application. By following the sizing criteria and pole connection guidelines provided in this guide, engineers can select the optimal protection devices for their specific DC networks.

This article summarises key concepts from the ABB technical application paper 'ABB circuit breakers for direct current applications'.

Download the full free document here: ABB Circuit Breakers for Direct Current Applications (PDF)

References: ABB Inc. (2009). ABB circuit breakers for direct current applications. Technical Application Paper, 1SXU210206G0201. ABB Low Voltage Products & Systems. IEC 60947-2: Low-voltage switchgear and controlgear – Part 2: Circuit-breakers. IEEE 551: Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems. IEEE 141: Recommended Practice for Electric Power Distribution for Industrial Plants.

Disclaimer: This article provides a summary of the ABB technical application paper 'ABB circuit breakers for direct current applications'. While efforts have been made to accurately represent the content, please refer to the official document for complete technical details. Final design, specification, and implementation decisions should be made by qualified professionals in accordance with all applicable regulations and standards.