Pagsulay sa Kasalaan sa Elektrisidad | Positibo Negatibo Ug Zero Sequence Impedance

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Larangan: Basic Electrical Basikong Elektikal

China

Electrical Fault Calculations Zero Sequence Impedance

Bago mag-apil sa maong IEE-Business electrical protection system, kinahanglan nga adunay kompletong kaalam sa kondisyon sa electrical power system samtang may fault. Ang kaalam sa kondisyon sa electrical fault mahimong gamiton aron mas sayon ang pagbutang og mga protective relays sa iba't ibang lugar sa electrical power system.

Ang impormasyon bahin sa maximum ug minimum fault currents, voltages samtang may fault, ug ang ilang relasyon sa magnitude ug phase sa current sa iba't ibang bahin sa power system, kailangan nga makolekta aron mas sayon ang pag-apil sa protection relay system sa mga iba't ibang bahin sa electrical power system. Ang pagkolekta sa impormasyon gikan sa iba't ibang parametro sa sistema gitawag og electrical fault calculation.

Ang fault calculation nagpasabot sa pagkuha sa fault current sa anumang electrical power system. Usa ra ka proseso ang pagkuha sa faults sa usa ka sistema.

Pagpili sa impedance rotations.
Pagreduce sa komplikado nga electrical power system network ngadto sa single equivalent impedance.
Pagkuha sa electrical fault currents ug voltages pinaagi sa paggamit sa symmetrical component theory.

Impedance Notation of Electrical Power System

Kon atong tingali sa anumang electrical power system, makita nato nga adunay daghang voltage levels. Pwede ta magsulti og typical power system diin ang electrical power gibuo sa 6.6 kV, ug ang 132 kV power gipadala sa terminal substation diin gi-step down niini ngadto sa 33 kV ug 11 kV levels, ug ang 11 kV level pwede pa mobaba ngadto sa 0.4 kv.

Gikan sa maong halimbawa, malinaw nga ang sama nga power system network mahimo nga adunay daghang voltage levels. Kini nagpadako sa hirap ug komplikado sa pagkuha sa fault sa anumang lugar sa sistema kon mag-atempto kita nga pagkuha sa impedance sa iba't ibang bahin sa sistema batasan sa ilang voltage level.

Kini nga hirap mahimo nga mapugos kon atong pagkuha sa impedance sa iba't ibang bahin sa sistema batasan sa usa ka base value. Kini nga teknika gitawag og impedance notation sa power system. Sa uban pa, bago ang electrical fault calculation, ang mga parameter sa sistema, kinahanglan nga irefer sa base quantities ug ipresentar isip uniform system of impedance sa ohmic, percentage, o per unit values.

Ang electrical power ug voltage kasagaran gitawag isip base quantities. Sa three phase system, ang three phase power sa MVA o KVA gitawag isip base power ug ang line to line voltage sa KV gitawag isip base voltage. Ang base impedance sa sistema mahimo nga makalkula gikan sa base power ug base voltage, as follows,

Per unit is an impedance value of any system is nothing but the radio of actual impedance of the system to the base impedance value.

Percentage impedance value can be calculated by multiplying 100 with per unit value.

Again it is sometimes required to convert per unit values referred to new base values for simplifying different electrical fault calculations. In that case,

The choice of impedance notation depends upon the complicity of the system. Generally base voltage of a system is so chosen that it requires minimum number of transfers.
Suppose, one system as a large number of 132 KV over head lines, few numbers of 33 KV lines and very few number of 11 KV lines. The base voltage of the system can be chosen either as 132 KV or 33 KV or 11 KV, but here the best base voltages 132 KV, because it requires minimum number of transfer during fault calculation.

Network Reduction

After choosing the correct impedance notation, the next step is to reduce network to a single impedance. For this first we have to convert the impedance of all generators, lines, cables, transformer to a common base value. Then we prepare a schematic diagram of electrical power system showing the impedance referred to same base value of all those generators, lines, cables and transformers.

The network then reduced to a common equivalent single impedance by using star/delta transformations. Separate impedance diagrams should be prepared for positive, negative and zero sequence networks.

There phase faults are unique since they are balanced i.e. symmetrical in three phase, and can be calculated from the single phase positive sequence impedance diagram. Therefore three phase fault current is obtained by,

Where, I f is the total three phase fault current, v is the phase to neutral voltage z 1 is the total positive sequence impedance of the system; assuming that in the calculation, impedance are represented in ohms on a voltage base.

Symmetrical Component Analysis

The above fault calculation is made on assumption of three phase balanced system. The calculation is made for one phase only as the current and voltage conditions are same in all three phases.

When actual faults occur in electrical power system, such as phase to earth fault, phase to phase fault and double phase to earth fault, the system becomes unbalanced means, the conditions of voltages and currents in all phases are no longer symmetrical. Such faults are solved by symmetrical component analysis.

Generally three phase vector diagram may be replaced by three sets of balanced vectors. One has opposite or negative phase rotation, second has positive phase rotation and last one is co-phasal. That means these vectors sets are described as negative, positive and zero sequence, respectively.
positive negative zero sequence voltage
The equation between phase and sequence quantities are,

Therefore,

Where all quantities are referred to the reference phase r.
Similarly a set of equations can be written for sequence currents also. From, voltage and current equations, one can easily determine the sequence impedance of the system.

The development of symmetrical component analysis depends upon the fact that in balanced system of impedance, sequence currents can give rise only to voltage drops of the same sequence. Once the sequence networks are available, these can be converted to single equivalent impedance.

Let us consider Z1, Z2 and Z0 are the impedance of the system to the flow of positive, negative and zero sequence current respectively.
For earth fault

Phase to phase faults

Double phase to earth faults

Three phase faults

If fault current in any particular branch of the network is required, the same can be calculated after combining the sequence components flowing in that branch. This involves the distribution of sequence components currents as determined by solving the above equations, in their respective network according to their relative impedance. Voltages it any point of the network can also be determine once the sequence component currents and sequence impedance of each branch are known.