1. Positive order, negative order and zero order.
At present, the AC power system in the world is generally ABC three-phase, and the positive, negative and zero-sequence components of the power system are determined according to the order of ABC three-phase.
Positive sequence: phase A is 120 degrees ahead of phase B, phase B is 120 degrees ahead of phase C, and phase C is 120 degrees ahead of phase A.
Negative sequence: phase A is 120 degrees behind phase B, phase B is 120 degrees behind phase C, and phase C is 120 degrees behind phase A.
Zero sequence: ABC three-phase phase is the same, which phase is not ahead, nor behind.
1. When the three-phase short circuit fault and normal operation occur, the system is in positive sequence.
2. When a single-phase grounding fault occurs, the system has positive sequence, negative sequence and zero sequence components.
3. When the two-phase short circuit is faulty, the system has positive sequence and negative sequence components.
4. When the two-phase short circuit grounding fault occurs, the system has positive sequence, negative sequence and zero sequence components.
2. Negative sequence and zero sequence current.
Negative sequence current
When the generator is operating normally, the negative sequence current in the system is zero, and when there is asymmetry in the system, there is a large negative sequence current in the system.
1. Principle of generation
The direction of the rotating magnetic field generated by the negative sequence current is opposite to the direction of movement of the rotor, the rotor is cut at twice the synchronous speed, and the frequency doubling current is induced in the rotor, and the main part of the frequency doubling current flows along the axial direction on the surface of the rotor, this current can reach a maximum value, and will cause high temperature in some contact parts of the rotor surface, and serious electrical burns occur, and at the same time, the local high temperature may also make the danger of loosening the ring guard; in addition, the double alternating electromagnetic torque generated by the negative sequence magnetic field makes the unit produce 100HZ vibration, causing metal fatigue and mechanical damage。
2. The occurrence of positive sequence, negative sequence and zero sequence is to analyze the decomposition of the asymmetric components of the three phases into symmetrical components (positive and negative sequences) and the zero sequence components in the same direction when the system voltage and current are asymmetrical. As long as it is a three-phase system, the above three components can be decomposed (a bit like the synthesis and decomposition of forces, but in many cases the value of one component is zero). For an ideal power system, both the negative and zero sequence components are valued at zero due to the symmetry of the three (which is why we often say that there are only positive sequence components in the normal state). When the system fails, the three phases become asymmetrical, and the magnitude of the negative and zero sequence components can be decomposed.
3. China's relevant regulations on the normal operation of generators negative sequence current: the long-term allowable negative sequence current of steam turbine generator is 6% ~ 8% of the rated current of the generator, and the long-term allowable negative sequence current of hydro generator is 12% of the rated current of the generator. For the overload of the rotor surface caused by asymmetrical load, non-full-phase operation and asymmetric short circuit, generators with a value of 50MW or more (the constant of the rotor surface's ability to bear negative sequence current) greater than or equal to 10 shall be equipped with negative sequence overload protection within a set time limit.
1. Zero sequence current:
In a three-phase four-wire circuit, the sum of the phasors of the three-phase current is equal to zero, ie
Ia+Ib+IC=0
If a current transformer is connected to a three-phase four-wire system, the induced current is zero. When an electric shock or leakage fault occurs in the circuit, there is a leakage current flowing through the circuit, and then the phasor and unequal zero of the three-phase current passing through the transformer are: Ia+Ib+Ic=I (leakage current)
In this way, there is an induced voltage in the secondary coil of the transformer, which is added to the electronic amplification circuit of the detection part, and compared with the predetermined action current value of the protection area device, if it is greater than the action current, even if the sensitive relay acts, it acts on the actuator tripping. The transformer connected here is called a zero-sequence current transformer, and the phasor sum of the three-phase current is not equal to zero, and the generated current is the zero-sequence current.
2. Zero-sequence reactance:
The zero sequence parameter (impedance) is related to the network structure, especially the wiring method of the transformer and the neutral grounding method. In general, the zero-order parameters (impedance) and the structure of the zero-sequence network are different from those of the positive and negative order networks. In the case of transformers, the zero-sequence reactance is related to its structure (three single-phase transformers or three-column transformers), the connection of the windings (△ or Y), and whether they are grounded or not. When one side of a three-phase transformer is connected into a triangle or a star with no neutral point grounded, the zero sequence reactance of the transformer is always infinite from this side. Because no matter how the connection method of the other side is, when the zero sequence voltage is applied on this side, the zero sequence current can not be sent to the transformer. So only when the windings of the transformer are connected to the star and the neutral point is grounded, the zero sequence reactance is limited (although sometimes large) when looking at the transformer from the star side. For transmission lines, the zero sequence reactance is related to the number of loops of parallel lines, whether there is an overhead ground wire and the conductivity of the ground wire. The zero-sequence current is in-phase in the three-phase line, and the mutual inductance is very large, so the zero-sequence reactance is larger than the positive sequence reactance, and the zero-sequence current will return through the ground and the overhead ground wire, and the overhead ground wire plays a shielding role on the three-phase wire, so that the zero-sequence flux is reduced, even if the zero-sequence reactance is reduced. When the zero-sequence current passes through the same direction in the two-round three-phase overhead transmission line erected in parallel, not only the mutual inductance of any two of the first circuit relative to the third phase produces a magnetic-aiding effect, but also the mutual inductance of all three of the second circuit relative to the third phase of the first circuit also produces a magnetic-aiding effect, and vice versa.
There are two conditions for generating a zero-sequence current:
1. Whether it is a longitudinal fault, a transverse fault, or an asymmetry between normal and abnormal times, as long as there is a zero sequence voltage;
2. There is a path for the zero sequence current.
The above two conditions are indispensable. Because without the first one, there is no source, and without the second one, we usually talk about the question of "whether there must be a current if there is a voltage."
Zero-order formula:
3U0=UA+UB+UC
3I0=IA+IB+IC
The occurrence of positive sequence, negative sequence and zero sequence is to analyze the decomposition of the asymmetric components of the three phases into symmetrical components (positive and negative sequences) and zero sequence components in the same direction when the voltage and current of the system are asymmetrical. As long as it is a three-phase system, the above three components can be decomposed (a bit like the synthesis and decomposition of forces, but in many cases the value of one component is zero). For an ideal power system, both the negative and zero sequence components are valued at zero due to the symmetry of the three (which is why we often say that there are only positive sequence components in the normal state). When the system fails, the three phases become asymmetrical, and the magnitude of the negative and zero sequence components can be decomposed (sometimes only one of them), so by detecting these two components that should not be normal, it is possible to know that something is wrong with the system (especially the zero sequence component when the single phase is grounded). The following is a method used as a graph to simply derive the amplitude and phase angle of each component, and the prerequisite is that the voltage or current (vector value) of the three phases is known, and of course, the actual engineering is to measure each component directly. Since you can't get a picture, please draw a picture on paper according to the text instructions.
Draw a vector diagram of the three-phase current of the system (in the case of current, the same is true for voltage) from known conditions (not too extreme for the sake of clarity).
(1) Finding the zero ordinal component: add the three vectors to sum them. That is, phase A does not move, and the origin of phase B is translated to the top of phase A (arrow), note that phase B is only translational and cannot be rotated. In the same way, the C phase is translated to the top of the B phase. At this time, the vector from the origin of phase A to the top of phase C (sometimes arrow-to-arrow), this vector is the sum of the vectors of the three phases. Finally, take one-third of the amplitude of this vector, which is the amplitude of the zero-order component, and the direction is the same as this vector.
(2) Finding the normal order component: the original three-phase vector diagram is processed as follows: phase A does not move, phase B turns 120 degrees counterclockwise, and phase C turns 120 degrees clockwise, so a new vector diagram is obtained. According to the above method, the three phases of this vector diagram are added and one-third is obtained, so that the A phase in normal order is obtained, and the amplitude of the A phase vector is used to draw the B and C phases respectively according to the method of 120 degrees apart. This gives us the positive ordinal component.
(3) Finding the negative order component: Note that the processing method of the original vector graph is different from that of the positive order. The A phase is not moving, the B phase is rotated 120 degrees clockwise, and the C phase is rotated 120 degrees counterclockwise, so a new vector diagram is obtained. The following method is the same as in normal order.
Through the above methods, we can analyze the general situation of various system faults, such as why the zero-sequence protection will act when there is a single-phase ground, and there is basically no zero-sequence current when the two-phase short circuit occurs.
Here we will talk about the relationship between each component and harmonics. Due to the special relationship between the harmonic and the frequency of the fundamental wave, it will exhibit positive order, negative order and zero order characteristics when synthesized with the fundamental wave. But we can't equate harmonics with these components. As mentioned above, the reason why the fundamental wave should be decomposed into three components is to facilitate the analysis of the system and the state of the discrimination, such as the occurrence of zero sequence in many cases is the occurrence of single-phase grounding, these analyses are based on the fundamental, and it is the harmonic superimposed on the fundamental wave and the error of the measurement, so the harmonic is an external interference quantity, and its value is not what we want when we analyze, just like the interference of the third harmonic on the zero sequence component.
Zero-sequence current protection
The device that uses the zero-sequence current generated when grounding to make the protection action is called zero-sequence current protection. Special zero-sequence current transformers are used on the cable route to achieve grounding protection.
[1] Zero-sequence current protection: When a grounding short circuit occurs in the neutral point direct grounding system, a large zero-sequence current will be generated, and the zero-sequence current component is used to form protection, which can be used as a major grounding short-circuit protection. The zero-sequence overcurrent protection does not respond to three-phase and two-phase short circuits, and there is no zero-sequence component during normal operation and system oscillation, so it has good sensitivity. However, the zero-sequence over-current protection is greatly affected by the change of the operation mode of the power system, and the sensitivity is therefore reduced, especially in the short-distance line and the complex ring network, because the protection range of the quick-moving section is too small, or even there is no protection range, the performance of each section of the zero-sequence current protection is seriously deteriorated, so that the protection action time is very long, and the sensitivity is very low.
Zero-sequence current protection with and without directionality is a simple and effective way to protect against ground, and its advantages are:
1. The structure and working principle are simple, and the correct action rate is higher than that of other complex protections.
2. There are few intermediate links in the whole set of protection, especially for near faults, which can achieve rapid action, which is conducive to reducing developmental faults.
3. Under the condition that the zero sequence network of the power grid is basically stable, the protection range is relatively stable.
4. The absolute value of the zero sequence current of the protection reaction is less affected by the fault transition resistance.
5. The protection value is not affected by the load current, and it is basically not affected by the short-circuit fault of other neutral point non-grounded power grid, so the sensitivity of the protection delay period is allowed to be set higher.
In order to prevent the malfunction of zero sequence current protection when oscillating in the non-full-phase operation state of the three-phase contact in the three-phase closing process or the single-phase reclosing process, the four-stage protection composed of two first sections is often used.
The sensitive section is set to the maximum zero-sequence current that occurs when avoiding a single-phase or two-phase ground short circuit at the end of the protected line. Its operating current is small and the protection range is large, but it is locked in the non-full-phase operation state after the single-phase fault is removed. At this time, if the other phases fail again, it is necessary to wait for the reclosing to reclose, and then accelerate the tripping after the recloser. The tripping time is long, which may cause the adjacent lines of the system to trip due to the lack of protection. Therefore, a set of insensitive section protection is added.
The insensitive section is set according to the maximum zero-sequence current that occurs when it escapes the non-full-phase operation and produces oscillation, and its action current is large, and it can avoid the zero-sequence current in the above-mentioned non-full-phase case, both of which are instantaneous actions in the case of calculation of power system imbalance, the symmetrical component method is cited, that is, any three-phase unbalanced current, voltage or impedance can be decomposed into three balanced phasor components, namely the positive phase sequence (UA1, UB1, UC1), the negative phase sequence (UA2, UB2, UC2) and the zero phase sequence (UA0, UB0, UC0), i.e.,
UA=UA1+UA2+UA0
The following issues need to be paid attention to in the operation of zero-sequence current protection:
(1) When the current circuit is disconnected, it may cause protection malfunction. This is a common weakness of generally more sensitive protection, and it needs to be prevented during operation. In terms of the probability of disconnection, it is much less likely to be disconnected from the protective voltage circuit. If necessary, the zero-sequence current lockout method of adjacent current transformers can also be used to prevent this malfunction.
(2) When the power system appears to be in symmetrical operation, there will also be zero sequence current, such as the asymmetrical operation caused by the same three-phase parameters of the transformer, the two-phase operation in the single-phase reclosing process, the three-phase circuit breaker in the three-phase reclosing and manual closing periods, the parallel process of the circuit breaker and the disconnector or the normal loop operation of the circuit breaker when the bus reversing operation, the zero sequence circulation current occurs due to the inconsistency of the three-phase contact resistance of the disconnector or the circuit breaker, and the unbalanced excitation inrush current generated during the airdrop transformer, especially in the case that the bus where the airdrop transformer is located has a neutral point grounding transformer in operation, there may be unbalanced excitation inrush current and DC components for a long time, etc., all of which may cause zero-sequence current protection to start.
(3) When one of the parallel lines is faulty, it may cause the induced zero-sequence current to appear on the other line, resulting in the malfunction of the relay in the zero-sequence direction of the reverse branching side. If this is possible, you can use a negative sequence direction relay instead to prevent the misjudgment of the above direction relay.
(4) Because the AC circuit of the zero-sequence direction relay usually does not have zero-sequence current and zero-sequence voltage, the circuit is not easy to be found; when the zero-sequence voltage of the relay is taken from the triangle side of the opening of the voltage transformer, it is not easy to check the correctness of its direction with a more intuitive simulation method, so it is easier to cause protection rejection action and malfunction when the power grid is faulty due to the problem of AC circuit.