ZL-4A - Arc fault protection

     Protects human life and equipment
 

     Instantaneous shutdown



     Selectivity of exemptions
 

     
     Light detection via fiber optic sensors
 

     Light detection via fiber optic loops
 

     Self-testing for circuit integrity and fiber continuity



     Voltage-free connection of sensors
 

     Autonomous
 

     Many years of experience

The essence of the problem

According to official statistics from the National Labour Inspectorate, several dozen people are killed and dozens suffer severe injuries during the year across the country as a result of arch-shortening. Electrical equipment in which an arc has occurred is destroyed to such an extent that it is usually necessary to replace it with new ones. In specific cases the losses, including downtime costs, amount to many millions of zlotys.

The most common cause (60%) of arc shorts are human errors: wrong connections, bad organisation of works. In order to protect people and equipment from the effects of short circuits, the technical standards PN-EN 62271 -200 - 20x, PN-88/E-05150, PN-IEC 439-1 defining the requirements for MV and nn switchgears and transformer stations, it is recommended, apart from maintaining the appropriate design features of the equipment, to apply additional countermeasures limiting the effects of arc short circuits. One of the recommended ways are protections reacting to arc light. Such a solution is the quickest and most effective way to minimize the effects of arc shorts.

Our proposal

In protection types ZŁ, the identification of an arc short circuit is based on two criteria: detection of arc light and voltage drop in the protected area. The switching impulse is generated in less than 10 ms. The arc is extinguished in 30-50 ms, depending on the type and condition of the breaker. Such a switch-off time guarantees full protection of human life and health and minimises damage to the device.

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

ZŁ-4A - Protection of hooded switchgears Full of selective breakdowns

The protection ZŁ-4A is dedicated to the protection of MV/nn hooded switchgears. It has dispersed construction. The central unit is usually installed in the voltage measurement bay, while the field units are installed in the switchgear fields. Optical frontal sensors (4 pieces) are placed in particular bay compartments.

In case of simultaneous detection of an electric arc and voltage drop of 0.7 Un, depending on the place of short circuit, it is switched off:

the switch of a given bay - a short-circuit in zone 2 (connection compartment)
switchgear power supply bay breaker - short circuit in zone 1 (breaker or busbar compartment)
the nearest circuit breaker in the supply line of this switchgear - a short circuit in the supply bay.
The possibility of selective breaking down is a key feature of the protection ZŁ-4 very valuable from the operational and economic point of view.

Construction

The protection ZŁ-4A consists of the central unit ZŁ-4A JC, power supply unit and field units ZŁ-4 JP. Each field unit is equipped with 4 optical sensors. Communication between the units takes place via the high-speed CAN bus. Parallel to the CAN bus, power supply circuits are distributed. The central and field units are installed directly on the TS bus. Dimensions of the central unit: 159 / 90 / 58 mm, field unit dimensions: 53 / 90 / 58 mm, protection degree of the central unit and field unit: Ip20. 99 field units can be connected to one central unit. Due to the use of a distributed design in the case of ZŁ-4, the unit cost of the enclosed switchgear field protection is relatively low.

Central unit ZŁ-4A JC

Basic functions of the central unit:

• Voltage drop detection on switchgear rails
• supervision of the correct operation of the systems
• security configuration
• sharing information on security status
• generation of cut-off pulses
• blocking ATS automatics
• single-criteria operation signalling
• Rs485 communication (Modbus RTU)
• signalling the activation of the device
• signalling of equipment malfunctions
• 6 programmable switch-off contacts
• 2 signal contacts on common potential
• possibility of testing the sensors of the selected field unit and setting the self-test time

Field units ZŁ-4A JP

Basic functions of the field units:

• Arc light detection within the protected part of the switchgear
• generating the shutdown pulse
• generating alarm signals to the central unit
• user can test fiber optic sensors and run the self-test in cycles, every 24 hours
• Optical frontal sensors

The sensors are characterised by a suitable light absorption coefficient. The sensitivity of the light detection path is set individually depending on the length of the sensors and the conditions in the object. The sensitivity must be selected so that the system does not react to sunlight or headlights used by the operator. The standard lengths of light guides with sensors are 3, 5 and 15 metres respectively.

Effects of arc short circuits in electrical systems not protected by fiber optic arc protection

Accidents in the category "electric shock" are 70 % caused by arc shorts. Arc shorts most often occur in medium voltage electrical systems, where this proportion reaches 90%. Short-circuit current in medium-voltage devices ranges from several to several tens of kA. The most common cause (60%) of arc shorts, apart from technical (construction, material, workmanship defects) and environmental (aging, corrosion, overvoltage, vibrations, rodents) are human errors: wrong connections, bad organization of works. During a long-term arcing short circuit the temperature reaches 20 000°C, a powerful bang is generated and a strong shock wave bursts out the equipment, a large amount of gases, often toxic with temperatures reaching several hundred degrees, are emitted. 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 operation of an electric arc in a period of less than 100 ms does not cause more serious damage, while with a short circuit longer than 500 ms irreversible damage occurs (melting of copper and steel) causing the need to replace the damaged equipment, which forces interruptions in business activity. Shortening the duration of a short-circuit is therefore the only effective way to reduce the adverse effects of arcing. 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 a short circuit in which people are affected. There is no data on the number of short circuits in general. It is obvious that information about such events is not disseminated. It can be predicted that there are several thousand short circuits a year. Here are some examples to illustrate the destructive potential of short circuits.

Benefits resulting from application of fiber optic arc protection type ZŁ

The protection type ZŁ has been used for 10 years. Over 1000 systems have already been installed. We know many cases in which they have protected devices and the lives of specific people. WRONG type protections identify a short circuit and generate disconnecting impulses to the circuit breakers in less than 10 ms, which with modern circuit breakers allows the power supply to be cut off a short circuit in even less than 30 ms. Thanks to this, the effects of arc shorts are reduced to a minimum, which significantly increases operating safety and reliability.

The requirement of simultaneous appearance of two criteria (light and voltage drop) ensures correct operation of the protection. The short-circuit identification algorithm is very reliable. We do not know of any cases of failure or faulty operation of protections. Below we present selected examples of positive effects of the protection ZŁ.