ZŁ-2 - Arc fault protection

     Protects human life and equipment
 

     Instantaneous shutdown
 

     
     Light detection via fiber optic sensors
 

     Self-testing for circuit integrity and fiber continuity
 

     Autonomous
 

     Many years of experience

The essence of the problem

The essence of the problem
According to the official statistics of the State Labour Inspectorate, several people are killed and dozens of people are seriously injured by arc faults throughout the country every year. Electrical equipment in which an arc fault has occurred is damaged to such an extent that it is usually necessary to replace it with a new one. In specific cases the losses including the costs of downtime amount to many millions of PLN.

The most common cause (60%) of arc faults is human error: incorrect connections, poor organization of work. To protect people and equipment from the effects of short circuits in the technical standards PN-EN 62271-200-20x, PN-88/E-05150, PN-IEC 439-1 specifying the requirements for MV and LV switchgear and transformer stations, it is recommended, in addition to maintaining the relevant design features of equipment, to use additional measures to reduce the effects of arc faults. One of the recommended methods are arc flash sensitive protections. Such a solution is the fastest and most effective way to minimize the effects of arc faults.

Our proposal

In the ZŁ-type protections the arc fault identification is based on two criteria: detection of the arc light and voltage drop in the protected area. The switching impulse is generated within time below 10 ms. The arc is extinguished within 30-50 ms, depending on the type and condition of the breaker. Such a shutdown time guarantees full protection of human life and health and limits the damage to the device to a minimum.

Application of the voltage drop criterion guarantees protection of the whole switchgear (there are no dead/unprotected zones). It enables simple and correct operation of protection in systems supplying one section of switchgear from several different power supply sources. Protections based on current criterion do not have those features.

ZŁ-2 Protection of dry-type transformers and other equipment

The Arc Protection System ZŁ-2 is intended for protection of dry-type transformers and electrical equipment in switchgear cabinets where the danger of arc faults exists such as converters, rectifiers etc. The optical front sensors are installed in the transformer enclosure or in the cabinet of the protected device. The condition for using the protection is the access to the voltage measurement and possibility of remote power-off of the device. In case of simultaneous detection of the arc light and voltage drop below 0.7 Un the power supply is disconnected.

Design

The Arc Protection device ZŁ-2 comprises a relay, a power supply unit and front sensors attached to it. The relay is enclosed in housing CN 100 AK (100x75x105 mm) with protection degree of Ip40.

Basic functions of the ZŁ-2 protection relay:

• detection of voltage drop on switchgear busbars;
• detection of arc light;
• receipt of information about status of optical inputs;
• generation of switching-off impulses;
• blocking of ATS automatics;
• signaling of single-criteria operation;
• Indication of device malfunction;
• indication of failure of the device;
• 5 tripping contacts;
• 2 signaling contacts on common potential.

Fiber optic sensors

The sensors are characterized by an appropriate light absorption coefficient. The sensitivity of the light detection path is set individually depending on the length of the sensors and the conditions on site. The sensitivity must be selected so that the system does not react to sunlight or headlights used by the operator. Standard lengths of fiber optic cable with sensors are 3,5,15 m. Protection versions with three or four sensors are available.

The effects of arc faults in electrical systems not protected by lightning arcing protection

Accidents in the category of "electric shock" result in 70% from arc faults. Arc faults are most common in medium-voltage electrical systems, where the proportion is up to 90%. Short-circuit current in medium-voltage equipment ranges from several to several tens of kA. The most common cause (60%) of arc faults in addition to technical reasons (design, material, workmanship defects) and environmental reasons (aging, corrosion, overvoltage, vibrations, rodents) are human errors: incorrect connections, poor organization of work. During a long-lasting arc fault, the temperature reaches 20,000°C, a powerful bang is generated and a strong shock wave appears, tearing the equipment apart. Under such conditions a person is exposed to loss of life or serious injury.

The thermal energy of a short circuit depends on two variables: its duration and the level of the short circuit current, according to the formula Q=I2 x t. We can only influence one of these factors - time. The action of an electric arc in a period of less than 100 ms does not cause serious damage, while with a short circuit longer than 500 ms irreversible damage occurs (melting of copper and steel) causing the necessity of replacing the damaged equipment, which forces business interruption. Reducing the duration of a short circuit is therefore the only effective way to reduce the adverse effects of arc faults. It is worth remembering that in traditional overcurrent protections, operating sequentially, the setting times can reach even 2.5 seconds.

In Poland, each year there are several hundred accidents caused by arc faults in which people are injured. There is no data on the number of arc faults in general. It is obvious that information about such incidents is not disseminated. It can be predicted that there are several thousand short circuits caused by arc faults per year. The following are some examples that illustrate the destructive potential of arc faults.

Benefits of Fiber Optic Arc Protection System type ZŁ

Arc protections type ZŁ have been used for 10 years. More than 1000 systems have already been installed. We know of many cases in which they have protected devices and lives of specific people. The Arc Protection System identifies a short-circuit and generates switching impulses to the circuit breakers within a time shorter than 10 ms, which with modern circuit breakers allows to cut off the power supply of a short-circuit even within less than 30 ms. Thus, the effects of arc faults are reduced to a minimum, which significantly increases the safety of operators and the reliability of equipment operation.

The requirement of simultaneous occurrence of two criteria (light and voltage drop) ensures correct tripping of the protection. The algorithm of short circuit identification is very reliable. We are not aware of any cases of failure to operate or incorrect operation of protections. Below we present selected examples of positive effects of ZŁ protections.