The sequence analyzer block outputs the magnitude and phase of the positive, negative, and zerosequence components of a set of three balanced or unbalanced signals. They are called symmetrical components because, taken separately, they transform into symmetrical sets of voltages. If an alternator is subjected to an unbalanced load situation, connected to nonlinear loads or an asymmetrical fault condition, then the. The presence of negativesequence and zerosequence components indicates unbalanced operation of the power system and power system faults. Negative sequence represents impedances of the system to currents with. Negative phase sequence or unbalanced overcurrent protection function 46 purpose negative sequence component of current is similar to the positive sequence system, except that the resulting reaction field rotates in the opposite direction to the dc field system. Ac circuit phase sequence phase sequence is the order in which the generated voltages in the phase winding of an alternator reach or attain their peak or maximum values. Considerations in choosing directional polarizing methods.
With no zerosequence current present, the i a1 and i a2 components cancel to produce zero aphase current. Advantages responds to true negative sequence voltage component. Negative phase sequence protection can have a pickup setting lower than. The operate quantity is typically either measured or calculated residual ground current 3i0 or negative sequence current i2.
Zero sequence represents impedances of the system to equal inphase currents in all three phases. Phase and zerosequence phasors for a singlephasetoground fault negativesequence directional elements use negativesequence voltage as the polarizing quantity and negativesequence current as the operate quantity. Ideally, the positivesequence set is the only one present during balanced operation. The positive and negative sequence bphase and cphase components combine to produce equal and opposite b and c current of the linetoline fault. This can be protected by negative sequence current filter with over current relay. Negative sequence three phasors, equal in magnitude, 120o apart, with the opposite sequence acb of the original phasors. Pdf novel microprocessor based negative phase sequence. Modern digital protection relays often offer both negative phase sequence protection and overcurrent protection. Negative sequence overcurrent relay and protection assemblies 1mrk 509 045ben revision. The threephase sequence analyzer block outputs the magnitude and phase of the positive denoted by the index 1, negative index 2, and zerosequence index 0 components of a set of three balanced or unbalanced signals. For the threewire case, we can see that the expected negative phase sequence component is dependent on the impedance of the load in the broken conductor scenario. Hence, a flux is produced which cuts the rotor at twice the rotational. Broken conductors and negative sequence overcurrent. Besides, an unbalance can also be quantified by comparing the intensity of negative sequence currents in comparison to the positive sequence currents.
The negative sequence relay protects the generator and motor from the unbalanced load. Negative sequence current monitoring is one of the wellknown condition monitoring applications that capable of identifying stator faults in induction motor as well as producing fast and reliable results using a simple measurement technique. Considering a threephase system, symmetrical components positive sequence, negative sequence, and zero sequence allow one to analyze power system operation during unbalanced conditions such as those caused by faults between phases andor ground, open phases, unbalanced impedances, and so on. Index 1 denotes the positive sequence, index 2 denotes the negative sequence, and index 0 denotes the zero sequence. Negativesequence overcurrent considerations for induction motor. Ieee pes boston chapter technical meeting september, 19. Almost all negative sequence voltage polarized relays compare the. Implementation of negative phase sequence protection in. Unbalance may cause due to single phase fault or unbalanced loading and it gives rise to negative sequence current. Negativesequence overcurrent considerations for induction motor loads miles dupuis cleco power llc.
Negative phase sequence protection is effective at detecting certain faults, and is. These are positive, negative and zerosequence components. An negative phase sequence current is usually caused by asymmetrical loading of the three phases, but may also be the an opencircuit phase singlephasing. The three component variables v1, v2, v 0 are called, respectively, positive sequence, negative sequence and zero sequence. The lack of negative sequence detection in this method leads to the phaseoscillation in case of single line faults. Acb, and in the third set the phasors a, b and c are in phase with. The basic theory of symmetrical components is that phase currents and voltages in a threephase power system can be represented by three singlephase components. Rebirth of negativesequence quantities in protective.
However, the transformer representation in the zerosequence network can be more complex and is dependent on the type of transformer connection. The mrs1 relay is a negative sequence protection relay with universal application. A relay which protects the electrical system from negative sequence component is called a negative sequence relay or unbalance phase relay. For unbalanced systems, such as an openphase there will be positive, negative and possibly zerosequence currents. Pdf extracting negative phase sequence components from the threephase ac voltages and currents has been a long standing challenge for protection. Normally ac three phase voltages and currents may be shown and represented by three vectors which could be balanced or unbalanced in nature. The siemens pti recommendation was to alter the phase position of one particular doublecircuit 345 kv line. Nick mahoney, aadityaa padmanabhan, and krithika bhuvaneshwaran. For our example, the negative sequence current for the reverse fault was 3 negative phase sequence protection a distinct advantage over traditional overcurrent techniques and means that protection coverage can be extended further into the network without the need for line reconducting or the installation of fuses. Negative sequence overcurrent relay and protection. Positive sequence represents impedances of the system to normal balanced currents in all three phases. The latter can represent a hazard for the rotor and this is the reason for applying nps protection. The phasebyphase addition to reconstitute the phase currents is shown in figure 6.
Voltage unbalance mitigation using positive sequence. The negative phase sequence flux induces currents in the rotor and these results in additional rotor losses and increased rotor temperature. These sets of phasors are called the positive, negative, and zerosequence components. Figure 1 positivesequence, negativesequence, and zerosequence components figure 1 illustrates the three sets of phasors. Pdf the demand for power is increasing day to day and is becoming prime requirement. In this method the polarizing reference quantity used is the negative sequence voltage v2. A simplified logic diagram of the rxiik 4 relay is shown in figure 2. Negativesequence overcurrent or current unbalance relays are also. The voltage waveform above starts at zero along the horizontal reference axis, but at that same instant of time the current waveform is still negative in value and does not cross this reference axis until 30 o later.
An asymmetrical load on a generator produces a magnetic filed, which rotates in the opposite direction to the positive sequence field. As with zerosequence directional elements, the relay calculates torques to determine the direction of the fault. Current and voltage unbalance causes and counter measures. In both cases a solution is possible using the normal method adopted for a balanced threephase. Multi phase system plays an important role in this regard and six. The concept of negativesequence current is rooted in symmetrical component methodology. Rxiik 4 comprises mainly of three functional units, a measur ing unit, hmi unit and transformer unit.
Detailed investigation of negative sequence current. Negativesequence currents flowing in the balanced network produce only negativesequence voltage. Therefore, the actual resultant negative sequence current depends on. The differing definitions of voltage unbalance are analyzed in order to. The properties of these components can be demonstrated by tranforming each one back into phase variables. These components allow for the simple analysis of power systems under faulted or other unbalanced.
It serves for negative sequence protection of threephase generators. The final set of balanced phasors is equal in magnitude and in phase with each other, however since there is no rotation sequence figure 3 this is known as a zero sequence. Compute positive, negative, and zerosequence components. What is negative sequence current and how does it affect.
Definitions of voltage unbalance ieee power engineering. It is represented by the letters a, b, and c or the numbers 1, 2, 3 or by using double letter as ab, bc and ca or an, bn and cn. Be147n negative sequence voltage relay automation now. The negative sequence currents in the generator are independent of the positive sequence load of the generator. In a threephase system, voltage unbalance takes place when the magnitudes of phase or line voltages are different and the phase angles differ from the balanced conditions, or both. If we know the phase sequence and one voltage in the set, we. A balanced system is one where the three vectors are equal and at 120 degrees out of phase with each other. Pdf logic based detection of negative sequence currents for six.
A short circuit between twophases will give a large negative sequence current, but these faults are normally cleared by the short circuit protection in much. The negative sequence relay protects the generator and motor from the unbalanced load which mainly occurs because of the phasetophase faults. Pdf a novel numeric relay algorithm for extracting negative phase. If an alternator is subjected to an unbalanced load situation, connected to nonlinear loads or an asymmetrical fault condition, then the resulting state is described as having a negative phase sequence component. As an example, we will calculate the positive and negativesequence components of the following three phasevoltage conditions.
Triplen harmonics 3rd and 9th shown in this table which dont rotate at all because theyre in phase with each other, are called zero sequence. Agn 016 negative phase sequence currents stamford avk. So far as the positive and negativephase sequence components are concerned, the corresponding equivalent circuits are similar to the normal balanced threephase case except for the opposite phase rotation of the negativephase sequence voltages. Summary fault conditions balanced threephase fault. Phasetophase fault was right at pickup of phase timeovercurrent element 512 a. The negative sequence voltage at the 345 kv bus increases with the load on the lines. Then there exists a phase difference between the two waveforms as the current cross the horizontal reference axis reaching its maximum peak and zero values after the voltage waveform. Harmonics such as the 5th, which rotate in the opposite sequence as the fundamental, are called negative sequence. Why does a balanced three phase system contain only. In a threephase system, one set of phasors has the same phase sequence as the system under study positive sequence. Instead, these faults appear as phasetophase faults on the delta side of the transformer. Zero sequence along with negative would be needed if system is grounded three phase four wire system.
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