resistors > Grounding resistors

Technical explanation

For permanent or temporary low resistance neutral grounding in medium voltage networks

Neutral connection

The connection mode of the neutral in three-phase systems determines the extent of the operating frequency dependent voltage rise on the outer conductor without ground fault, in the event of an earth leakage. The ratio between the effective value of the highest operating frequency dependent conductor-earth voltage ULF of an outer conductor without ground faults and the effective value of the conductor-earth voltage UL which is present without faults at the relevant place, is called ground fault factor. The ground fault factor is a determining factor for the level of insulation in accordance with DIN57111/VDE0111.
Neutral connectionw
e=ULF/UL

Direct connection, Z0/Z1 = 0
1,0

Low resistance Z0/Z1 ~ 1…5
1,1…1,4

High resistance Z0/Z1 ~ 20…100
1,75…1,8

compesated -> infinite
1,75…1,85

Insulated Z0/Z1 ~ 100…200
1,75…1,85

Z0 = neutral impedance [1]
Z1 = Symmetrical Net impedance
A direct neutral grounding has the disadvantage that a unipolar earth leakage constitutes also a unipolar short-circuit diverting the short-circuit current, which is only limited by the impedance at the fault location. There is no operating frequency dependent voltage rise in the non-affected conductors. In net with an insulated neutral, an earth connection bridges the earth capacity of the affected conductors. The earth leakage current equals the sum of the capacitive currents of the other two outer conductors, which have the combined voltage value (conductor-conductor) at the earthing point. We have compensated Netwhen the neutral is grounded via a reactance coil, the inductive impedance of which equals the amount of the capacitive conductor impedance at the earthing point. The compensation of the conductor-earth capacity generates a voltage vector at the blow-out coil, directed in the opposite sense of the exciting voltage of the deficient phase and makes the fault arc go out. However,automatic choke only functions in a condition of almost complete compensation and is therefore only practicable in networks of a limited scale. Because of the complex voltage ratio it is very difficult to detect a continuous earth leakage.Low resistance neutral groundingis chosen for extensive networks. The neutral point is grounded via a resistor which, in case of faults, limits the earth-leakage current to a predetermined value until the moment of release. The extent of the fault current depends on the ohmage value of the resistor and the impedance at the fault location. The maximum earth leakage current in case of an earth leakage can only flow near the transformer. In such cases the neutral voltage assumes more or less the voltage of the conductor-earth tension. This does not affect the remaining operating frequency dependent conductor voltages. A short-time low resistance neutral grounding is used to detect a constantly reoccurring ground fault in compensated net. This makes it possible to ground a transformer neutral during a short period of time by means of a switch system via a resistor. In contrast with a permanent low resistance neutral grounding this system requires only one resistor for several transformers or generators.

General observations

In net with an accumulated current switch-off system the selected maximum ground fault current may be relatively low, which implies that the ground resistance is determined in order to limit the earth fault current to a value lower than the nominal current. In net provided with overcurrent release, the earth leak current value must be higher than the nominal current in order to be identified as overcurrent. It is determined at a value between 1.5 and a multiple of the nominal current. It is essential that the chosen value is high enough, on the one hand to ensure the safe detection of an earth connection to the grid periphery, and on the other hand to allow a trouble-free control of a ground fault current near the generator or transformer. The grid structure and safety cut-out devices are important in this context, which is why there are no general regulations or instructions available. If there are several transformers or generators connected to the grid, all grounding resistors actually have the same rating, which is equal to the set value of the safety device. Although the safety devices often cut off an earth leakage in fractions of a second, a longer switch-on cycle is chosen for the resistor to allow several switch-on attempts. Since in most cases the earth leakages occur as a result of spark-overs near outdoor insulators of which the electric arc is put out, a short switch-on time is required in order to reduce the interruption time. In such cases, a continuous earth leakage results in a new resistance load. Normal values for acceptable load time of a grounding resistor are 5…10…15…20…30 seconds, 10 seconds being the most common load time. The requirement for a 30-second switch-on cycle goes back to the era of the liquid resistors in which the load time was dependent, among other things, on the number of electrolytes. A switch-on cycle of 30 seconds for air-cooled metal resistors is not cost-effective, since this type of resistors has a relatively short cooling-off time compared to liquid resistors and the switch-on time largely determines the price of the resistor. Oil-cooled metal resistors are effective only with high safety ratings and/or a long switch-on time, since the resistor material is used insufficiently as a result of the relatively low admissible oil temperature. Indoor resistors are manufactured according to IP00 and IP20 protection classes. Outdoor resistorsrequire at least protection class IP23. Higher protection classes are problematic, considering the restricted ventilation as a result of the temperature stress on components, insulators and housing. The insulation is provided for system voltages 12, 24, 36, 52 kV, with requirements for larger air and/or creepage distances, depending on the place of installation, due to the climatic conditions, the pollution hazard or the installation height. Applicable standards and regulations: DIN40050 Protection classes DIN57101/VDE0101 Installation of high-tension systems exceeding 1kV DIN57111/VDE0111 Co-ordination of insulation for industrial installations in three-phase lines exceeding 1 kV DIN57141/VDE0141 Grounding in alternating current systems exceeding 1 kV IEC 273 Characteristics of indoor and outdoor post insulators IEEEStd 32-1972 Requirements, Terminology and Test Procedure for Neutral Grounding Devices

Specifications of GINO grounding resistors

GINO grounding resistors consist of resistor packs with resistor elements of siliceous cast iron with or without surface protection (e.g. zinc dust primer) or grid plates composed of various resistor materials. It is possible to integrate several resistor packs into one plug-in unit which is insulated from the casing, and one cabinet can contain up to three plug-in units. For integrating one or more plug-in units into an existing switch device, it is also possible to mount these units onto a basic frame, thus constituting a resistance device of protection class IP00. One of the essential factors in the construction of the casing is the selection of the location where it will be installed. GINO/ESE grounding resistors of protection class IP23 are suitable only for installation in an electrical working environment. In case of installation outside an electrical working environment it is necessary to provide a special encapsulation so that no dangerous components can be touched with a straight wire. In case of installation in public areas this wire must be as thin as possible. Therefore it is necessary to take the appropriate protective measures, in addition to the protection classes formulated in DIN 40050.

Program of the grounding resistors

After sandblasting the frame surfaces all devices for indoor installation are grounded and coated with a high-grade synthetic-resin lacquer. The housings for outdoor installation are painted with a weatherproof PUR coating which consists of two layers: a two-component PUR priming coat and a two-component PUR finishing coat. The standard colour is RAL 7032. For on-site installation an even base is required, provided with the necessary cable conduits. The bottom of the housing consists of a perforated sheet and is provided with detachable floor panels in the right places for feeding in cables later on. The connections inside the housing are made on standard copper cable rails. The cables are fed into the housing through the floor, as described above, or through PG side inlets. On payment of a supplement, the devices can also be provided with indoor/outdoor lead-throughs or angular inserts for N connections. The insulation of the operational grounding connection depends on the conditions applying for installation of the ground connection. If the earth fault voltage UE at the connecting point exceeds 3000V, according to VDE0141, condition 4, it is advisable to insulate also the earth connection of the system voltage or 1/3 of the system voltage. In all other cases, it is not only possible to insulate for lower voltages but also to use low-voltage transformers instead of the more expensive medium-voltage transformers. The optimal conditions for applying this solution is when the tolerated fault current is only a few hundredths of amperes. A transformer that has to be integrated into the grounding resistor, substitutes a (lower) resistor pack, which is a factor to be taken into account when the type of housing is chosen. According to the VDE 0141 provisions, all conductive housing and frame sections that are not part of the operating current circuit, must be interconnected. Doors and detachable cover plates have a separate earth connection. To establish the earth connection the framework is provided with at least one M10 earth screw or several M8 earth screws.

Routine tests

Each separate resistor device is subjected to a thorough inspection, which includes – besides the visual inspections of the construction, and a verification of the dimensions and thicknesses of the coats of paint – the performance and registration of the following electrical tests : o Inspection of resistor packs for compliance with IEEE Std 32-1972, by applying the 2.25-fold of the normal longitudinal voltage + 2kV, during 1minute. o Measurement of the DC resistance at ambient temperature. o The insulating capacity is established by the use of approved insulators and the compliance of the regulations regarding minimal air-leakage distances in accordance with VDE 0101 and VDE 0111.

Available special designs and accessories

o Zinc-plated casing, thermogalvanised frame, Sendzimir galvanised sheathing, coated with two-component PUR lacquer. o Resistors with punched plate elements of rust- and acid-proof chrome nickel steel 18 9, item number 1.4301/AISI304 o Higher protection class, IP3x, IP4x, IP5x. o Transformer, supporting transformer or low-voltage transformer on the earth connection side (see remarks above). o Special version with larger air and creepage distances through the application of C-supports, in accordance with IEC 273. o Unipolar isolating switch, ring drive o Unipolar isolating switch, crank drive o Unipolar isolating switch, motor-drive o Separate low-voltage niche or junction box Author and ©: Dieter Brockners, GINO Gielen + Nothnagel GmbH 1998, copying, even partially, is only allowed with acknowledgement of the source. Email: info@gino.de

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