SLOVAK UNIVERSITY OF TECHNOLOGY Faculty of Material Science and Technology in Trnava ELECTRICAL ENGINEERING AND ELECTRONICS Róbert Riedlmajer TRNAVA 2007
Unit 14 - Fundamentals of power system protection Electrical Fuses The fuse is a simple and reliable safety device. It is second to none in its ease of application and its ability to protect people and equipment. The fuse is a currentsensitive device. It has a conductor with a reduced cross section (element) normally surrounded by an arc-quenching and heat-conducting material (filler). The entire unit is enclosed in a body fitted with end contacts. Ratings Most fuses have three electrical ratings: ampere rating, voltage rating, and interrupting rating. The ampere rating indicates the current the fuse can carry without melting or exceeding specific temperature rise limits. The voltage rating, ac or dc, usually indicates the maximum system voltage that can be applied to the fuse. The interrupting rating (I.R.) defines the maximum short-circuit current that a fuse can safely interrupt. If a fault current higher than the interrupting rating causes the fuse to operate, the high internal pressure may cause the fuse to rupture. It is imperative, therefore, to install a fuse, or any other type of protective device, that has an interrupting rating not less than the available short-circuit current. A violent explosion may occur if the interrupting rating of any protective device is inadequate. In accordance with IEC 364-4-41 (1992) a low voltage (LV) distribution system is usually characterized from electricity source to terminal equipment with the following: grounded power supply source (e.g. low voltage connection of local network transformer); grounded system of exposed conductible parts in consumer electrical installations. Through this three basic types of systems could be defined as distribution systems: TN system, TT system and IT system. Used abbreviations have the following meaning: The first letter (T or I) describes grounded system of power-feeding electricity source. T direct grounding of power supply source single point (primarily connecting point of transformer winding); I isolation of all active parts from ground or connection electricity source single point to ground over some sort of impedance. The second letter (T or N) describes grounded system of exposed conductible parts of electrical installation. T exposed conductible part is directly grounded independently of eventual existing grounded feeding point. N exposed conductible part is directly connected to grounding electrode (grounding resistor). Further letters (C or S) describe an arrangement of neutral and protective conductor. S neutral and protective conductor are separated. C neutral and protective conductor are connected. Hence it follows that there are three possible varieties of TN system: TN-S, TN-C and TN-C-S. Protective devices, that can be built in into different systems are flux protective device, RCD (Residual Current Device), isolation control device and surge error protective device. As mentioned above, an arrangement between system form and protective device is necessary. The following protective devices are used in different systems: TN system - flux protective device and RCD; TT system - flux protective device, RCD and surge error protective device; IT system - flux protective device, RCD, isolation control device and surge error protective device. 2
A TN system (Fig.14.1) always provides a return path for faults in the LV grid. The grounding conductors at the transformer and at all customers are interconnected. This ensures a distributed grounding and reduces the risk of a customer not having a safe grounding. Also better lightning safety is assured. However faults in the electrical network at a higher voltage level may migrate into the LV grid grounding causing touch voltages at LV clients. The consequences of faults in LV and MV grids should be taken into account in the design of LV network. The utility is not only responsible for a proper grounding but also for the safety of customers during disturbances in the power grid. TN systems has the advantage that in case of an insulation fault, the fault voltages are generally smaller than in TT systems. Reasons: voltage drop in the phase conductor; earthing of the PEN conductor with a lower impedance than the consumer grounding in TT systems. Advantages of an TN system: in case of an insulation fault the fault- and touch-voltage usually remain below a few ten volts; high reliability of disconnection of a fault by overcurrent devices; lower grounding resistances of the PEN-conductor (overall resistance of the network and resistance of the consumer grounding electrode); compensation of the grounding effect of old gas and water pipes, which are now made of plastic materials; good protection against overvoltages of atmospheric origin due to resulting low grounding resistance of the PEN-conductor; highest attainable protection level (overcurrent device and RCD in special cases). A fault in the LV network may cause touch voltages at other LV clients. Therefore a fast switch-off is required. Most critical are faults at the ends of the branches, where the circuit impedance is the highest. In the design of LV-grids, this circuit impedance should be limited. It reduces the touch voltage and increases the earth-fault current, resulting in a faster switch off. The maximum length of an outgoing cable is therefore limited. A limited cable length increases the cost of the low-voltage grid. An additional connection between separate LV-cables, a separate PE-wire parallel to the cable or a special fast fuse extend the maximum cable length. During a fault in the MV grid, touch voltages may occur in the LV grid when a TN system is used. Most relevant parameters are the amplitude of the fault current, the duration of the fault and the type of cable. In a TN-S system neutral and PE conductor are separated. This system has the best EMC properties for 50Hz and HF currents, certainly when LV cable with a grounded sheath is applied. For low frequencies the 50Hz circuit (DM-circuit) and ground (CM-circuit) are separated and the current distribution is well defined. Additional electrodes in the LV grid, preferably at each user, divert external induced (lightning) currents. In a TN-S system five conductors are required. Appropriate short circuit protection should be carried out, which means that the calculated values of maximum and minimum short circuit current should be available for the checking of breaking capacity and the short circuit protection activation. Fault voltage protection should be carried out for all levels, even less than 50V, by the provisions stated in STN IEC, which means that by using of neutral wire the protective wire should be a special conductor not conducting the operation current, and 3
it should be connected to the main distributor outside danger zone, and also to the neutral point earthing of the power transformer. Fig.14.1. TN-C network system For clarification, it should be said that the network protection with neutral wire connection with combined neutral conductor and protective conductor in one conductor (TN-C network system) is not allowed in hazardous area. A system with the neutral conductor partly separated from the protective conductor (TN-C/S system) in a hazardous area may be used in areas of zones 1 and 2 only if TN-S system (neutral conductor and PE conductor are separated) is used in hazardous area e. For the supply via plug-socket, the use of residual current protective device is required. The TN-S network and apparatus grounding system are allowed in areas with zone 1 and zone 2, provided that a fault is tripped in case of earthfault i.e. at single-pole short circuit. If a single-pole short circuit (minimum current) cannot be tripped in the required time, then that part of network must be protected with residual current protection. If insulation checking is carried out before voltage closing, for zone 1 it must be carried out with insulation measuring current which cannot ignite the explosive atmosphere. The protective grounding system with separate grounding for live parts and conductive parts exposed to contact (TT network system) is not allowed for hazardous areas, but it can be used in areas containing zones 1 and 2, only in the networks which are fitted with momentary acting sensitive residual current earthfault protection device. In addition to the above mentioned measures in a danger zone all metal structures should be connected to the common main grounding or additional potential equalizing system. The casings of electrical apparatuses need not be separately connected to the potential equalizing system, if the casings or apparatuses are fixed to the plant or to the piping system through which they are connected by the metal structure to the potential equalizing system. The surface piping should be earthed even if the piping ends are outside hazardous area. The connection points should be bridged. The IT network system can be used without limitation with momentary tripping in case of an earthfault or insulation resistance drop below 20 Ohm/V. Voltage closing is enabled if the insulation resistance is bigger than 40 Ohm/V, while the inadequate insulation is signalled till the insulation resistance becomes higher than 100 Ohm/V. 4
Potential equalizing is obligatory for zone 1 (and recommendable also for zone 2). For an intrinsically safe circuit with galvanic separation it is not required. Fuses used for short circuit protection should be marked (according to IEC 60269...) as follows: "gg" - intended for general application ("gl" are intended for the protection of lines and cables). "am" - intended for the protection of motor circuits. "ar" and "gr" - intended for the protection of semiconductor devices (depending on fuse type and system). When the installation circuit breakers are applied, the characteristic type, B, C or D, should be indicated. For TN-EIE the following should be ensured: impedance of power supply network, single-line diagrams of installation in danger zone, parameters of cable installation and the protection of apparatuses applied, parameters of loads or TN-URE and parameters of earthing network and earthers. Protective measures in the TN-system In case of TN-system it appears on faulty device, e.g. with short circuit between operating conductor and the body, a fault voltage, which magnitude is about half the phase voltage as long as crosssections of the forward and return conductor are the same. Because the value lies over the permissible touch voltage 50 V, a switch off is urgent required. Application of overcurrent protection equipment and residual-current-operated protective device is here permissible. The impedance of fault circuit Z s (Fig.14.2), which is determined by line lengths and line cross-sections, can be in practical application so large, that the fault current is recognised and switched off within the time of 0,2 s: U 0 Z s. I a where U 0 is phase voltage, I a - operating current of protection device. The application of residual-current-operated protective devices is limited only to the TN-S-system and TN-C-S-system, in which the protection conductor PE is laid at least partially separately from the neutral conductor N. In the TN-C-system is however a sensible installation not possible. Moreover, in the TN-C-system, interruption of the PEN-conductor or mix-up of outer conductor and the PEN-conductor can lead to danger, which is why this network configuration takes off in its use. L 1 v A P, I v 1 TN-C RT PEN v 2 V Sp Fig.14.2. Measurement of the impedance of fault circuit Z s for TN-C-system 5