变压器零序阻抗计算

变压器零序阻抗计算 The zero sequence impedance of transformer (2009-05-17 16:07:47) reprint labels: zero sequence impedance of three column connection in winding When transformer is running, there are two kinds of symmetrical and unsymmetrical running. Asymmetric operation including emergency operation, such as single-phase or two-phase short circuit, three-phase asymmetric load, the asymmetry is a single-phase load, distribution transformers often have this type of load, yn low voltage connection line, and the midpoint between the single-phase load is asymmetric load. The three-phase asymmetrical three-phase operation of three-phase transformer and single-phase transformer is related to the core structure and the winding connection group of magnetic circuit. Asymmetric operating conditions include transient faults (such as single phase grounding), momentary disturbances (such as three-phase currents with different instantaneous values) and asymmetrical continuous loads, which can cause asymmetric operation: (1) instantaneous or continuous loss of three-phase symmetrical voltage, including winding and core loss; (2) the current instantaneous or continuous asymmetric load, especially through the midpoint of the current and voltage will affect the stability of, such as voltage asymmetry, midpoint voltage offset, will produce magnetic leakage and core excitation. In order to make the transformer meet the requirements of asymmetric operation, some core structures can not be fitted with the winding coupling group, so it is necessary to analyze the asymmetric operation. In the study of asymmetric operating conditions, the first hypothesis: three-phase voltage is synchronous and sinusoidal, three-phase current and has equivalent constant impedance or admittance is associated with the linear equation, calculated by symmetrical component method. The voltage, current and impedance voltage are decomposed into three components, the positive sequence, the negative sequence and the zero sequence. Positive sequence voltage and current refer to symmetrical voltage and current components of three opposite angles of 120 degrees of electrical rotation, which are A, B and C, and have the positive sequence component under normal symmetrical load condition. The positive sequence impedance is the impedance of the positive sequence current. Negative sequence voltage and current is built under the conditions of the asymmetric component, symmetric operation without this component is counterclockwise three rotating symmetric mutual difference of voltage and current components of 120 degrees electric angle, but the rotation of the order of A, C, B. Negative sequence impedance is the impedance of negative sequence current. Zero sequence voltage and current are single phase components, which are the residual components built up under asymmetric conditions. The zero sequence component is the same phase and the same amplitude. Zero sequence impedance is the impedance of zero sequence current. The sum of the positive sequence component and the negative sequence component is zero at every instant, but the sum of the zero sequence components is not zero, and the amplitude in each phase is 1/3 of the zero sequence component. After the above decomposition, the instantaneous value is not equal to zero of the wrong weighing (phasor diagram of asymmetrical stars, the three phasor does not form a closed triangle connection), you can calculate. The characteristics of each component in the actual transformer: (1) Y connection (star connection, such as high voltage insulated neutral winding 10kV is often used in this connection) because there is no return grounding wire or two wire leads to the midpoint of the line current, and must be equal to zero in the system, according to the symmetrical component decomposition, contains only positive sequence and negative sequence current component but, without the presence of zero sequence current component. The winding current from the system to the angular connection also has this characteristic. (2) YN connection, Neutral earthing when the current lead wire current flow through the midpoint or midpoint (such as the four wire distribution system), the phase current system with zero sequence current; for zero sequence current components in phase three phase with amplitude 1/3, zero sequence current component for neutral point current. This is a condition when there is a single-phase load between the line and the ground of the four wire distribution system. For high voltage power transmission and transformation system, generally, there is no larger midpoint load current. Asymmetrical load generally refers to the load between line and line, and can be decomposed into negative sequence current component without zero sequence component. (3) the connection of three line voltage of three-phase angle winding is zero by D connection. Because it is a closed connection, it will not contain zero sequence voltage component. But in the corner winding, there is zero sequence current and short circuit current, which are induced from the other winding. The other winding has no zero sequence current component, and there is no zero sequence current component in the angle winding. A fillet after winding, zero sequence current component can be balanced with each other, to the magnetic, finally zero sequence current can reduce the ampere turn balance, zero sequence impedance. Transformers and reactors are static electrical appliances, so has the characteristics of positive sequence impedance is equal to the negative sequence impedance, the impedance is positive impedance transformer, so the positive sequence impedance can be measured in the test, the zero sequence impedance depends on the circuit configuration and winding connection method, winding relative position, magnetic flux leakage channel. Different transformers with same positive sequence impedance may have different zero sequence impedances. For example, the corrugated oil tank made of high quality carbon steel (boiling steel) has different zero sequence characteristics with the flat fuel tank made of ordinary carbon (killed steel). In some cases, even nonlinear zero sequence impedance can be found. Zero sequence impedance measurement is a special test. The Yyn connection, in asymmetric load, only one side (yn side) with zero sequence current component, so to excitation in the iron core when the core is different, there are different zero sequence magnetizing impedance (zero sequence impedance in parallel): (1) three-phase three column core; Zero sequence magnetic flux part of the loop is air, so the reluctance is large, equivalent to parallel zero sequence magnetoresistance, resistance is relatively small, about 60%. (2) three-phase core with three-phase five column core, three phase shell core and single phase core. Zero sequence magnetic flux can form a loop in the iron core, so the reluctance is small, and the parallel zero sequence excitation impedance is very large, sometimes 10000%, such as zero sequence flux saturation, and also lead to current distortion. Zero sequence magnetic induction zero sequence voltage component causes midpoint deviation. Therefore, for Yyn connection, three-phase five column core and three phase shell core should not be adopted. Single-phase transformer also cannot use the three-phase Yyn group linked up. The YNd connection, such as unbalanced operation, high voltage and low voltage winding can contain zero sequence current component, which can achieve the ampere turn balance, so the zero flux is very small, the zero sequence impedance series impedance, the impedance value approximately equal to the voltage of 90% ~ 100%. The core structure does not affect the zero sequence impedance. The zero sequence impedance values of the general connection are listed below. (1) YNy, three-phase three column core, high voltage winding excitation, zero sequence impedance typical value is 50%; (2) Yyn, three-phase three column core, low voltage winding excitation, zero sequence impedance typical value is 60%; (3) YN, YN, y, Three phase three column core, high voltage winding excitation, zero sequence impedance typical value is a, 1, Z 12, a 1 is the coefficient, 0.8 < a 1 < 1, Z 12 is high voltage and medium voltage impedance voltage; (4) YN, YN, y, three-phase three column core, medium voltage winding excitation, zero sequence impedance typical value is a, 2 Z 12, a 2 is the coefficient; a 1, < A, 2 < 1. (5) YN, YN, y, three-phase five column core, high voltage winding and medium voltage winding excitation, zero sequence impedance typical value is Z 12. However, the high voltage and Mid Voltage midpoint of this connection must be grounded simultaneously, otherwise the zero sequence current component can not balance with each other. (6) YNd, three-phase three column core, high voltage winding excitation, zero sequence impedance typical value of a 1, Z 12. (7) YNd, three-phase five column core, high voltage winding excitation, zero sequence impedance typical value of Z 12. (8) Dyn, three-phase three column core, low voltage winding excitation, zero sequence impedance typical value of a 2, Z 12. (9) Dyn, three-phase five column core, low voltage winding excitation, zero sequence impedance typical value of Z 12. (10) YNynd, three-phase three column core, high-voltage excitation winding, typical of zero sequence impedance value is: a 1 [Z 1 +Z 2, Z 3, Z 1, Z 2 and Z 3 were high, medium, low voltage single winding impedance. When the medium voltage winding is excited, the typical value of zero sequence impedance is: A 2 (Z 2 +Z 1, Z 3) (11) YNynd, three-phase five column core; When the HV winding is excited, the typical value of zero sequence impedance is Z 1 +Z 2, Z 3 When the medium voltage winding is excited, the typical value of zero sequence impedance is: Z 2 +Z 1, Z 3 In the typical values above, a 1 and a 2 are the coefficients of winding pairs, and the coefficient is close to 1 when the exciting windings are close to the core. It should also be noted that the concept of zero sequence exciting parallel impedance is different from that of symmetrical operation. When there is a zero sequence current component in the high voltage and low voltage windings, the zero sequence excitation parallel impedance is small, while the exciting current always exists, which is the function of voltage and nonlinearity, so the excitation impedance is also nonlinear.