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still achieving very rapid fault elimination (of the order of a few milliseconds) for fault currents near to the low voltage network's short-circuit capacity (see fig.21 ). t MV circuit-breaker LV fuse 1 2 10 20 I/I n Fig. 21: operation relative to overload protection and internal faults by an MV circuit-breaker. 6.2 Advantages of earthing protection The behaviour observed during an internal ground fault depends on the MV network neutral earthing arrangement. Residual current detection can cover all or part of these earth faults. Furthermore, detection of residual current is also sensitive to the faults between primary and secondary windings, corresponding to earth faults detected by the upstream network. Such protection is useful for a distribution transformer, apart from in earthed and distributed neutral networks. Its operating threshold must be as low as possible; in fact there are certain limitations since: c it must allow the “normal” residual currents to flow. In fact, in certain network operation situations, the imbalances of simple voltages - in relation to earth - can generate a residual current that is not zero across the transformer's stray capacitances and any connecting wires. Even outside of a fault situation, all parts of the network have a “natural” capacitive imbalance generating a residual current. c it can be limited by the errors of instrument transformers in the case of a summation of three phase current measurements. The technological limitations of the current transformers and protection devices require the use of threshold detection generally greater than 10 % of the rated current to avoid spurious tripping on the occurrence of transitory phenomena or in short-circuit. In certain cases, “earthed tank” type detection, which implies being able to install the transformer insulated from the earth, can be considered. Nevertheless, this type of protection poses difficulties in implementation related to the physical installation of the transformers and to the possible distance between these and the protection device. It is never used for distribution transformers. 6.3 Independent protection devices: Time Fuse Links (TFL) and relays In many situations, particularly in public distribution and of course in small installations, it is not always conceivable to use an auxiliary power sypply to achieve protection. In fact, direct use of low voltage from the transformer does not enable a simple response to all fault hypotheses and the presence of an auxiliary source leads to a more expensive installation and unacceptable maintenance. Several types of protection devices without an auxiliary source exist, and fuses belong to this category. Regarding the opening of a circuit-breaker, one finds three categories of mechanisms: c Direct relays, in which the monitored current activates the release mechanisms by a thermal Cahier Technique Schneider n° 192 / p.24
or magnetic effect, without current transformation. This is the case of many low voltage circuitbreakers, but the direct relays are also suitable to medium voltage devices. They are tending nevertheless to disappear, basically due to their simple nature, their mediocre accuracy and their limited adjustment capacity. c “Time Fuse Links” are mainly used by the British (see fig.22 ). Under normal operating conditions, the coil is short-circuited by a low voltage fuse which is used to determine the protection parameters. In case of a fault, there is fusing and the current transformer's secondary current activates the coil. This basic principle is simple and efficient. Nevertheless, it implies having replacement fuses and it only offers a choice of limited characteristics related to the fuses' fusing curves. Earthing protection can be achieved using a coil placed in the common conductor of current transformers. The current normally being zero in this section, there is no parallel connected fuse on this coil. c Self powered electronic relays, in which the energy required for electronic and circuit-breaker tripping operation is taken from the sensors' secondary. These relays are combined with low energy release devices, generally with magnetic latching, which requires rearming by the circuitbreaking mechanism itself. These relays are often combined with sensors, specially designed for this type of application, less voluminous and less costly than standardized current transformers. The protection chain so formed can be integrated in a given switchgear, which enables a global solution to be offered with greater possibilities than the direct relay or TFL solutions. The performance levels offered cover virtually all installation cases, using standardized curves or manufacturer's curves, and with very wide setting ranges. The principle is nevertheless limited on low threshold values, due to the lack of available energy in the case of low MV current unless voluminous current sensors are used whose cost would be prohibitive. Current limits (1998) for autonomous operation are approximately 10 amperes. Lower “ground fault” threshold values can be used, but will not be activated unless a load current - phase current - exists above the autonomous operating limit. Fig. 22: wiring principle for a Time Fuse Links type protection device with two “phase” coils and one “earth” coil. 6.4 Protection devices with auxiliary power supply: GRPT, temperature sensors and relays When an auxiliary source is decided upon to supply all or a part of the protection functions, it is possible to use other information than merely measurements of electrical values. The monitored transformer's low voltage can supply these functions if protection against the shortcircuits is guaranteed by an independent mechanism. Two widely used applications are dedicated to faults not yet causing a noticeable overcurrent and to overload situations: the GRPT and temperature sensors. Cahier Technique Schneider n° 192 / p.25
Page 1 and 2: Technical collection Cahier techniq
Page 3 and 4: n° 192 Protection of MV/LV substat
Page 5 and 6: Protection of MV/LV substation tran
Page 7 and 8: 1.2 A review of transformer technol
Page 9 and 10: 2 Operating stresses and failure mo
Page 11 and 12: 2.3 Overloads General The acceptabl
Page 13 and 14: According to the current that passe
Page 15 and 16: 2.6 Faults related to technology ty
Page 17 and 18: Lightning arrestors Lightning arres
Page 19 and 20: protected transformer. For the same
Page 21 and 22: Selection criteria The ability of l
Page 23 and 24: 5.3 Using a fuse switch combination
Page 25: voltage short-circuit level varies
Page 29 and 30: 7 Conclusion The choice of distribu
Page 31 and 32: Appendix 1: Rules governing selecti
Page 33 and 34: coefficient α varies from 2.2 to 5
Page 35: Overvoltage protection Internal fau

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