Arc detection topologies comparison

The electrical installations are found in all sectors of industry. To meet the need for power supply, the industry generally uses distribution centers of energy and loads and sets of switching and control. These devices are usually subject to operational problems. On the face of it, the steps for the design, installation, operation and maintenance of these devices require a special attention in the matters related to human and property safety. [1], [2].

Among the problems which electrical circuits are subjected, we can stand out the electric arc, also known as voltaic arc. The electric arc is an alternative path to the flow of electric current, such as air, for example. If any event is capable of going beyond the insulation of a material, called dielectric resistance, this, which was insulated, shall conduct electrical current.

An electric arc is the result of a sequence of physical phenomena. For the happening of a arc, the air between the arc points needs to be ionized by corona effect. If the electric field between the points is large enough, the ionized fluid (air) becomes conducting. When the dielectric resistance is broken, the current starts to flow at high speed (approximately 100 m/s) and a high amount of energy is released in the form of electrical, thermal, acoustic, chemical, radiation and mechanical [3].

In this way, a path of low resistance is made up, the current starts to flow [4] and there is freed visible light up. Ultraviolet radiation starts being released during the ionization of the air, that is, before the flow of electrical current. If, for some reason, the electric field between the points of the arc is suddenly removed during the ionization of the air, the current cannot flow and therefore the arc does not happen.

The temperature at the point of origin of an electrical arc can reach 20,000°C, much higher than the supported temperature of any known material, almost four times greater than the temperature of the sun. Pressure waves are other destructive aggravating an electric arc, created by the thermal expansion of air and vaporization of conductors, can cause invisible damage as injuries in brain, lungs, ear and so on.

Therefore, the protection of human potential is essential. Professionals are exposed to the risk of electric arc during measurements of power quality, thermographic inspection, maintenance and switchgear operations, troubleshooting, faults and during other activities nearby panels. The control and switchgear are relatively resistant to the effects of internal arcing due to exhaustion and gas extinction systems. However this level of protection is not enough.

In order to minimize the harmful effects of electrical arcs, the industry has been established safety practices to protect workers and equipment involved in electricity services. Protective clothing or PPE (Personal Protective Equipment) are used in order to minimize possible injuries suffered by workers, such as third degree burns. Such devices should be easily removable by the user and should not be flammable. [5] Currently, PPEs are classified according to the risk level of the activity to be performed. The level of risk is defined through the calculation of amount of incident energy in an electric arc. In electrical arcing, PPEs should be able to provide a minimum level of protection to workers in order to prevent serious injuries such as third degree burns. However, if an object is thrown, the PPE will probably be unable to protect workers [6].

Due the strong expansion of air within the panel, moving parts may be thrown by large distances and may fatally injure people nearby the place. Accordingly, the arc resistant panels have been designed and used. These panels have special designed features to withstand the strong expansion of air and high temperature from the arc [7]. The panel is designed in order to target the pressure and temperature for upper outer part through flaps and air ducts, as shown in Fig. 1.

The effects of the electric arc in a panel can be split into four stages, which also happen sequentially. First, the arc energy is discharged into the air within the panel and therefore the pressure increases. This step, which is called compression, lasts between 5 and 15 milliseconds. The pressure increasing opens the relief ducts and the air starts to be expelled from the panel, decreasing the internal pressure. This step is called expansion and lasts between 5 and 15 milliseconds. As the pressure decreases, the air continues to be expelled and the temperature increases. This step is called expulsion, lasts between 40 and 60 milliseconds, until the ambient temperature reaches the arc temperature. The last step is called thermal, at this stage the arc temperature reaches thousands of Celsius degrees and the internal panel materials begin to fuse. There is not a well-defined range for the thermal stage; it keeps going until all the arc energy be dissipated. To increase the level of protection to workers, and also protect the equipment, protection systems have been developed. These systems are capable of openning the circuit breakers right after the identification of the arc.

Initially, the light detection systems have been used, in this topology sensors capable of detecting visible light from the arc are used [8]. There are two basic types of visible light sensors: the spot sensors and optical fiber sensors. The spot sensors can detect visible light from a particular point, that is, each sensor covers a relatively limited area. The optical fiber sensors, due its design, can detect light throughout its length, that is, a fiber sensor can be installed to detect the presence of light in a large area.

Regarding the safety level to workers and equipment, the visible light detection system isquite effective, but unreliable when compared to other systems. The major problem of this detection system is that visible light is not unique of arcing. When the panel is opened, the sensors tend to detect visible light. In this case, a false arc can be detected and the system acts in an improper way.

To let the system more reliable, double detection topologies could be used. Light and current detection systems or light and sound systems have been used.

The light and current detection system consists of electric current measure units and also visible light sensors, as shown in Fig. 2. The sensors are strategically placed in the panel and a trip signal is sent if both devices are triggered.

In general, the circuits subjected to electrical arcs are inductive, thereby, the equivalent impedance at the point of arc does not allow high current variations (relatively instantaneous). Due to the difficulty of measure the level nominal and fault current, these systems tend to use relative measures as shown in Figure 3. Thus, if the device responsible for current monitoring identifies high relative changes, and, simultaneously, visible light is detected, the system sends a trip signal [9].

The efficiency of this type of topology is very good when compared with other methods presented, however, as it is always required two devices being fired, the total time for to send the trip signal tends to be relatively large. Usually, the topology aforementioned needs 2 milliseconds to send the trip signal. This interval is increased by operating time of the disconnecting device and, although it looks small, it can be large enough to damage equipment or for a person exposed to the arc suffers untreatable burns or even death.

An alternative to arc protection is the use sensors that combine detection of light and sound. The difference between the speed of light and sound generates a delay of single time, sufficient to distinguish an event arc from other sources of light and sound [10]. In this topology, the trip signal takes about 1 millisecond, considerably lower than the current and light systems, however, still large enough injury the workers.

The Zyggot Arc line performs the detection of arcs in an innovative way, in this topology is not necessary current or sound measurements, the arc is detected only by ultraviolet radiation. The intelligent sensors, which detect just ultraviolet radiation, are strategically placed on the panel as shown in Fig 4 bellow. The monitoring process is based on the environment ultraviolet radiation level, as the release of ultraviolet radiation happen before the current flow, the system can perform the detection of electric arcs before their harmful effects. The sensors are able to quickly identify the presence of an electric arc and send the trip signal in less than 300 microseconds.

In traditional arc detection topologies, the number of arc identification points is usually limited. In large panels, where a large number of arc identification points is needed, it is necessary to include auxiliary devices in the system, increasing the price of the product and the complexity of maintenance.

The Zyggot Arc line allows many sensors to be installed on the same relay, if needed is possible to use up to 100 sensors [11]. Moreover, installation is easy due the type of connection between the sensors and cables, connection is made by mini USB output so that the power and communication are done by same cable. Each sensor has two parallel mini USB outputs, one for communication with the relay or previous sensors and the other is an extension to the next sensor, as shown in Fig 4.

As the installation is oversimplified, the system may be quickly implemented in any panel. The only one configuration required is the addressing of each sensor, which is done by a Varixx freeware software.

Since this is a technological innovation, Zyggot Arc line presents a differentiated solution and its cost is on average 20% lower than the solutions in the market. Another interesting aspect of Zyggot Arc line is that it can be installed even in panels contemplated with other detection topologies, increasing the level of security of these panels. Owing to its design, smart sensors identify just the ultraviolet radiation present in the arc, in this way, the Zyggot Arc system does not affect the operation of other devices, improving time performance and reducing the incident energy of an electric arc.

A second aspect to be highlighted is the output contact of the relay, the conventional systems uses dry contact in the tripping signal sending device. In this case the response time is relatively high, when an event is signaled, the signal takes at least 7 milliseconds to be sent [12]. When the device gets old, the contacts tend to oxidize, and, therefore the response time may be higher. The Zyggot Arc system uses a static contact in parallel with the dry contact, the response time is 9 nanoseconds, and there is no problems related to the aging of the device.

Conclusions

A study about electric arcs and a comparative of the protection methods in electrical systems was presented. The study indicates that the protection systems are essential in all types of electrical installation, even in arc resistant panels.

The protective clothing has been a good way to minimize the effects of the electric arc, for workers, in the electricity services. However, for performances in high risk areas, the protection level of PPE is not sufficient to protect the workers. For performances of risk relatively lower, it is possible to size safe clothes, but often these devices are heavy and uncomfortable. The calculation of the of PPE protection level involves the protection system operating time. If an effective protection system is used, there is a possibility to reduce the level of risk and consequently the level of PPE protection, so workers could use more light and comfortable clothes.

The data have shown that the ultraviolet radiation detection system, Zyggot Arc, is currently the fastest arc detection system. The complexity of installation and maintenance is lower in Zyggot Arc system, when compared to current and visible light detection system as well as the costs of implementation and maintenance.

In low voltage, the current and visible light detection system and has detection failures. There is large difficulty to parameterize the current detection loop, in these cases, since the conditions of empty and full load are very different. There are cases where electrical arcing occurs without the bring current to a level clearly detectable, in these cases the incident energy can damage the current detection loop, making the system miss its reference and remain inoperative.

Thus, the Zyggot Arc system was superior to conventional systems, available in the market, in every sense.

References

[1] Vestimenta de proteção contra queimaduras por arco elétrico. In: O setor Elétrico, ed. 45. 2009.

[2] A natureza e os riscos do arco elétrico. In: O Setor Elétrico, ed. 72. 2012.

[3] Arco Elétrico na Indústria Petroquímica. In: O Setor Elétrico, ed. 37. 2009.

[4] Modelagem do arco elétrico no ar. Alessandra de Sá, Benevides Câmara. Tese de doutorado. COPPE. UFRJ. 2010.

[5] Principais normas sobre os riscos do arco elétrico. In: O Setor Elétrico. ed. 73. 2012.

[6] A NFPA 70E e os requisitos de segurança para arco elétrico - Seleção de EPIs. In: O Setor Elétrico. ed. 74. 2012.

[7] Painéis resistentes a arco elétrico. In: O Setor Elétrico. ed. 77. 2012.

[8] Dispositivos de proteção contra arco elétrico – sensores de luminosidade. In: O Setor Elétrico. ed. 78. 2012.

[9] Dispositivos de proteção contra arco elétrico - arquiteturas e equipamentos usuais para proteção de arco elétrico. In: O Setor Elétrico. ed. 79. 2012.

[10] Dispositivos de proteção contra arco elétrico - relés de proteção digitais com detecção de arco integrada. In: O Setor Elétrico. ed. 80. 2012.

[11] Manual Zyggot Arco. Disponível em: www.varixx.com.br/site/static/uploads/products/ 652472081f8604f311fbc652bf22de5101eb.pdf

[12] Manual Vamp 221. Disponível em: http://wwwfi.vamp.fi/Manuals/English/VM221.EN018.pdf

Authors

SÉRGIO CARLOS MAZUCATO JÚNIOR is student of electrical engineering in Federal Technological University – Parana (UTFPR), researches electric power systems stability and optimization.

CASTELLANE SILVA FERREIRA is electrical engineer, graduated by Univ Estadual Paulista (UNESP), master in electrical engineering by Universidade Estadual Paulista (UNESP) and commercial director of Varixx company.