Direct Current System Application and Generation

Direct current, which was once the main means of distributing electric power, is still 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. 

Related Article: Plant DC System

By definition, the 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 flowing through a cross-section is always the same. 

Direct Current Applications

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

1. conversion into other forms of electrical energy (photovoltaic plants, above all where accumulator batteries are used); 
2. electric traction (tram-lines, underground railways, etc.); 
3. supply of emergency or auxiliary services; 
4. particular industrial installations (electrolytic processes, etc.)

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. 

Generally speaking, a stand-alone photovoltaic plant includes the following devices: 

• Photovoltaic array: photovoltaic cells suitably interconnected and used for the conversion of sunlight energy into electrical energy; 
• 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 the direct current into alternating current by controlling it and stabilizing its frequency and waveform.

Photovoltaic Plant | Source: ABB

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.

Read: Safety Measures for Electric Vehicle Charging

The applications of circuit breakers in DC circuits for electric traction can be summarized as follows: 

• Protection and operation of both overhead and rail contact lines; 
• Protection of air compressors onboard subway and train cars; 
• Protection of distribution plants for services and signaling systems; 
• Protection of DC supply sources (accumulator batteries) 
• Protection and operation of DC motors

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. 

Here are some examples of this type of user plant: 

• 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.)

UPS | Source: ABB

DC Generation

Direct current can be generated: 

1. by using batteries or accumulators where the current is generated directly through chemical processes; 
2. by the rectification of alternating current through rectifiers (static conversion); 
3. by the conversion of mechanical work into electrical energy using dynamos (production through rotating machines).

Storage Batteries

A storage battery, or accumulator, is an electrochemical generator able to convert chemical energy into direct electrical current. The structure of a storage battery is analogous to that of a normal battery. The main difference is that the discharging/ charging process for accumulator batteries is reversible. By using a DC generator, it is possible to restore the initial state of the electrodes which have been altered during discharge. This process cannot be carried out with a normal battery. 

Battery

The main electrical characteristics of storage batteries are: 

• Nominal voltage: potential difference existing between the negative and positive plates immersed in the electrolyte. The voltage value reported is usually related to every single cell (2V, 4V, 6V, 12V). To obtain the required voltage it is necessary to use several cells in series. 
• Capacity: quantity of electricity that a battery can deliver for a defined time. Capacity is expressed in ampere-hours (Ah) and can be obtained by multiplying the value of the intensity of the discharge current (amperes) by the discharge time (hours). 
• Internal resistance: the value of the internal resistance of the battery. This value is given by the manufacturer. 
• Power: power which the battery can deliver. It is obtained from the average discharge voltage multiplied by the current and it is expressed in watts (W).

Static Conversion

Direct current can be supplied by using electronic devices (rectifiers) able to convert alternating current input into direct current output. These devices are also called static converters. The operating principle of rectifiers exploits the properties of the electronic components made of semiconductor materials (diodes, thyristors, etc.), their capacity of carrying currents only when positively polarized.

3 Phase Bridge Rectifier

Dynamo

A dynamo is a direct current generator used to convert kinetic energy into a direct electrical current. As shown in the figure, these devices consist primarily of a stationary structure (called the inductor system), which generates a magnetic field, and of a moving part (called the rotor), made up of a system of conductors, which are “struck” by the magnetic field generated by the inductor.

Dynamo

Source:

• ABB Circuit Breakers for Direct Current Applications | pp. 2 - 11
• Publisher: ABB
• Download Here