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Electrical system and Earthing System (교류전원 공급방식과 접지체계)

작성자 admin
2014-09-05 12:04:14
조회 4155
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Three Phase Electricity

In general we are delivering the electrical in three phases even though in last usage we use only one phase of them. Three phase electricity has the same sinusoidal wave shape but has phase difference of 120 degree with respect to each other. It is convenient to display the phase with three point of equilateral triangle in a plane, which has the crossing angle of 120 degree. Three points indicate each phase and the distance from one point to the other indicates voltage. Now consider the center point as neutral point. Then the ratio of the distance from center point to each triangle point and the distance from one angle point to the other angle point is 1.7321, that is the root value of 3. This means that the phase to phase voltage is 1.7321 of phase to neutral voltage.

Load Connection (Wiring System)

There are two types of load connection methods. One is to connect the load between phase to phase and the other is to connect between phase and neutral. In general the method of using phase to phase voltage is called as ‘delta (Δ) system’ and the method of using phase to neutral voltage is called ‘wye (Y) system.’ Energy carrying conductors in delta system are three phase lines, and in Y system needs three phase lines and one neutral line. The neutral line is the return paths of three phases. In neutral line, the sum of each phase current is zero if the loads of each phase are the same. This is the reason why we call the return path as neutral. Sometimes Δ system is called “3 phase 3 wire system” and Y system is called “3 phase 4 wire system”.

Transformer Connection

As you know well, to carry the electric energy for long distance, we need high voltage to lessen the wire size. Because we use low voltage of AC, eventually, there are stepdown transformers in the AC distribution process. Transformer winding wire connection methods can be distinguished as similar as prediscussed load connections, that is, delta and wye. Because the transformer winding consisted of primary and secondary, connection method can be denoted by primary connection secondary connection, for example, Δ₋Δ, Δ₋Y, YY and Y₋Δ. Transformer types vary by country and era when the distribution system has been established. In general, the distribution technology is modernized year by year, and nowadays there are many countries choosing deltawye and wyewye system for residential and official buildings.

Electrical Earthing

The main purpose of the electrical earthing is to make potential reference for safety control of the electrical fault. Because between all electrical wires and surface of the earth has a certain amount of electric capacitance and the earth can be considered as conductor, AC currents could flow through the earth’s surface a little even if the electrical is not earthed. But when one point of the electrical circuit has been connected to earth’s surface and the accident of the electrical fault to touch the earth by itself or by touching with human body or other conductive objects, then we can monitor the fault along the circuit path and can disconnect the circuit from the power source to prevent any damage to personnel or building quickly, because current sensor through the electrical wire and earth’s surface can be experienced the fault current easily. The system of electrical earthing is different by country and by the electrical distribution system. In earthing system, there are two major concerns. One is how to connect the source side, generator or transformer and the other is how to connect the return path of fault current in user sides, chassis or metallic enclosures of electric appliances.

Earthing system

In IEC 60364 standards, earthing systems are distinguished by two letter, such as ‘TT’, ‘TN’ and ‘IT.’ The first letter indicates about the source side treatment and the second letter indicates about user side treatment. “T” denotes terra which means the earth (terra in Latin), “I” denotes isolation and “N” denotes Neutral. (In the case of “Δ” distribution and single phase power source, earthed phase is called as “Neutral” also.)

The ‘IT’ means that any of phase or neutral connected with earth in high impedance for AC mains frequency but in low impedance for monitoring signal frequency and in user side conductors, which must be protected such as chassis or metallic enclosures, should be connected with earth directly and independently. ‘TT” means the neutral or any of phases is to be earthed in source side, and in user side the protective conductor is to be connected with earth conductor independently for the source side earth. And ‘TN’ means in source side one point of phases or neutral is earth and in user side the protective wire is to be connected to neutral wire which is supplied from the source side. In Δ system, TN is rarely used and TT is the mostly used.

In lowvoltage, residential area, and the beginning of TN system may give problems that “live” and “neutral” might be accidentally reversed in the wiring, or the neutral to earth connection might fail or be installed improperly. And in single phase, even normal load current the neutral might generate hazardous voltage drops and so in this case the neutral might be no more reference earth in safety. This type of earthing, called as TNC is rarely used. And TN is modified to supply a protective earth (P.E) conductor separately. There are two types as follows.

TNS gives PE and N and these are connected together in source (lowvoltage transformer) side.

TNCS gives PE and N and these are connected together in user (consumer) side. This system is developed in UK. In consumer location the PE could be earthed to real earth at many user locations, to reduce the risk of broken neutral. The US also has safety standard of TNCS which allows connecting the neutral conductor with PE and the PE earthed to real earth at only one point of first service switch board in every user sides.

In TT system the fault return current is dependent mainly on the sum of source and user side earth resistances. Comparing with TN, the earth fault current of TT is no more than one over tenth of TN’s. IT allows the earth fault current only when occurring two faults in same time and in this case the current flow is similar with TT, but in case of single fault no current flows at all. But IT needs an expensive insulation monitoring system in their distribution system. TT has an advantage in installation cost saving for the electrical company because they might need not to make the neutral size bigger than that of TN system. And for user side, IT and TT system can provide noise free earth because the earth is independent form the various kinds of load return currents. TT is preferable for special applications like telecommunication site and research center because they need an interferencefree earthing. Nowadays that the fault current is low is no problem on safety because the RCD (residual current device) can protect any fault very effectively.

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