Differential protection method without magnetizing inrush current blocking based on transformer state recognition

Abstract

The present application discloses a differential protection method without excitation inrush current blocking based on transformer state identification, including: collecting currents on each side of the transformer, identifying the transformer state, if it is in a no-load closing state, then putting into the excitation inrush current criterion, if it is in an operating state, then collecting the transformer voltage, calculating the voltage change, identifying the voltage mutation, and then judging the transformer core state when the voltage mutation occurs, if the core is saturated, then putting into the voltage excitation inrush current criterion, otherwise the fault in the area is judged by the current sampling value differential, if the fault in the area occurs, the sampling value differential action, exit the excitation inrush current criterion, otherwise the sampling value differential does not act, and the excitation inrush current criterion is put into. The present invention can reliably and quickly identify the transformer operating state, and adopt different excitation inrush current blocking strategies according to the transformer operating state, cut off the transformer area fault, and make the transformer protection accurate and reliable.

Description

Technical Field

[0001] The invention belongs to the technical field of power systems and relates to a differential protection method based on transformer state identification without excitation inrush current blocking. Background Art

[0002] Ultra-high voltage direct current transmission has low power transmission loss and has unique advantages over AC transmission in long-distance and large-capacity transmission scenarios. In recent years, ultra-high voltage direct current transmission technology has been vigorously developed, and the traditional pure AC power grid has gradually transformed into an AC / DC hybrid power grid. The application of power electronic equipment in the converter station affects the power quality of the AC power grid. Even if harmonic suppression measures are taken, rich high-order harmonics will still be injected into the AC power grid.

[0003] In transformer differential protection, the second harmonic braking method is still the main basis for identifying the excitation inrush current. When an in-zone fault occurs, the high second harmonic content in the differential current will cause the differential protection to be incorrectly locked, resulting in the risk of refusal to operate.

[0004] The generation of magnetizing inrush current depends on whether the transformer core is saturated. In normal operation and external short circuit, the transformer core will not be saturated. When the transformer is closed at no-load or the voltage is restored after the external fault is removed, the transformer voltage suddenly rises from a very small value to the operating voltage, and the transformer core may be severely saturated. In addition, DC bias magnetization can also cause core saturation.

[0005] The current transformer protection does not strictly distinguish the transformer operating status, and the excitation inrush current lockout criterion has always been put into use. Therefore, different excitation inrush current lockout strategies can be adopted according to the different operating states of the transformer to avoid the protection failure caused by the increase of secondary harmonic content due to non-excitation inrush current, which affects the stable operation of the system. Summary of the invention

[0006] In order to address the deficiencies in the prior art, the present application provides a differential protection method based on transformer state recognition without excitation inrush current blocking, which can reliably and quickly identify the operating state of the transformer, and adopt different excitation inrush current blocking strategies according to the operating state of the transformer, thereby cutting off faults in the transformer area and making the transformer protection operate accurately and reliably.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] The differential protection method without excitation inrush current blocking based on transformer state recognition comprises the following steps:

[0009] Step 1: Collect the current on each side of the transformer and identify the transformer status. If it is in the no-load closing state, the excitation inrush current judgment criteria is put into operation. If it is in the running state, go to step 2;

[0010] Step 2: Collect the voltage on each side of the transformer, calculate the voltage change, identify the voltage mutation, and then judge the transformer core state when the voltage mutation occurs. If the core is saturated, the voltage excitation inrush current criterion is used, otherwise, proceed to step 3;

[0011] Step 3: The fault in the zone is determined by differential current sampling value. If a fault in the zone occurs, the sampling value differential is activated and the excitation inrush current judgment criterion is exited. Otherwise, the sampling value differential is not activated and the excitation inrush current judgment criterion is activated.

[0012] The present invention further includes the following preferred embodiments:

[0013] Preferably, in step 1, currents on the high, medium and low voltage sides of the transformer are obtained;

[0014] If there is current on only one side of the high and medium voltage sides and there is no current on the low voltage side, the transformer is considered to be in the no-load closed state, otherwise the transformer is in the operating state.

[0015] Preferably, in step 1, if at least one phase current among the three-phase currents on this side is greater than a current threshold, it is considered that there is current on this side.

[0016] Preferably, the flow threshold is 0.05I n , where I n It is the secondary rating of CT (current transformer).

[0017] Preferably, in step 2, when the voltage change Δu of n consecutive sampling points is greater than the set value U set When , the voltage is considered to have changed suddenly;

[0018] Among them, Δu=|u(t)-2*u(tT)+u(t-2T)|;

[0019] u is the voltage sampling value, and T is a power frequency cycle.

[0020] Preferably, the U set The value is 0.2U n , U n It is the rated value of the secondary voltage of PT (voltage transformer).

[0021] Preferably, the value of n is 4.

[0022] Preferably, in step 2, the core state is determined at the moment of voltage mutation:

[0023] First calculate:

[0024]

[0025] Among them, S1(k) is the sum of the absolute values ​​of the voltage in the half cycle before the voltage mutation, which is used to represent the steady-state component of the magnetic flux;

[0026] S2(k) is the sum of the voltage changes within the half cycle starting from the moment of voltage mutation, which is used to represent the non-periodic component of magnetic flux;

[0027] S e It is the sum of the absolute values ​​of the sampling points within the rated voltage half cycle, used to represent the rated flux linkage;

[0028] N is the number of sampling points per cycle, u(k) is the voltage value of the kth sampling point, u e is the rated voltage sampling value.

[0029] Then we get:

[0030]

[0031] When S>S set , it is judged that the transformer core is in a saturated state.

[0032] Preferably, the S set The value ranges from 1.2 to 1.5.

[0033] Preferably, in step 3, the method for distinguishing faults in the area according to the sampling value differential is as follows:

[0034] Calculate the sampled value differential current i d , sampling value braking current i r :

[0035]

[0036] Among them, i1,…,i n It is the sampling value of the current on each voltage side of the transformer converted to the high voltage side;

[0037] When more than M points among the consecutive N / 2 sampling points meet the following action zone criteria, it is considered that an in-zone fault occurs:

[0038]

[0039] Where N is the number of sampling points per cycle;

[0040] The value range of M is: 0<M≤N / 2;

[0041] I cd is the sampled value differential starting current, I e is the secondary rated current on the high voltage side of the transformer;

[0042] k is the sampling value differential ratio braking coefficient.

[0043] Beneficial effects achieved by this application:

[0044] The present invention can adopt different excitation inrush current strategies according to the operating state of the transformer. When the transformer is in operation, if the core saturation is not determined from the moment of voltage mutation and the sampling value differential is operated, the excitation inrush current judgment criterion is exited, so that the transformer differential protection is not affected by the externally injected harmonics and refuses to operate, thereby realizing the differential protection of the AC power grid transformer under the trend of flexible DC transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 The present invention is a flow chart of a differential protection method based on transformer state identification without magnetizing inrush current blocking. DETAILED DESCRIPTION

[0046] The present application is further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present application.

[0047] like Figure 1 As shown, the differential protection method based on transformer state recognition without excitation inrush current blocking of the present invention comprises the following steps:

[0048] Step 1: Collect the current on each side of the transformer and identify the transformer status. If it is in the no-load closing state, the excitation inrush current judgment criteria is put into operation. If it is in the running state, go to step 2;

[0049] When the transformer is operating in a steady state, the no-load current is very small. However, when the transformer is closed at no-load, a large impact current will be generated on the power supply access side. Therefore, the feature that only one side has current can be used to determine the no-load closing state.

[0050] During specific implementation, the current amplitudes on the high, medium and low voltage sides of the transformer are collected and obtained.

[0051] Calculate whether the three-phase currents of A, B, and C are greater than 0.05I for each of the high, medium, and low voltage sides respectively. n , I n is the secondary rated value of CT;

[0052] If at least one phase current is greater than 0.05I n It is considered that there is current on this side, and the number of current-carrying sides and current-carrying sides are recorded.

[0053] If there is current on only one side of the high and medium voltage sides and there is no current on the low voltage side, the transformer is considered to be in the no-load closed state, otherwise the transformer is in the operating state.

[0054] Step 2: Collect the voltage on each side of the transformer. When a fault or disturbance occurs in the system, the voltage suddenly changes. Calculate the voltage change and identify the voltage sudden change. Then, when the voltage suddenly changes, determine the state of the transformer core. If the core is saturated, use the voltage excitation inrush current criterion. Otherwise, proceed to step 3.

[0055] That is, when a fault or disturbance occurs in the system, first determine whether the voltage has a sudden change. When the voltage has a sudden change in the transformer operation state, then determine whether the transformer core is saturated;

[0056] From the sinusoidal characteristics of AC voltage, we know that the magnitudes of two instantaneous values ​​separated by a power frequency cycle are equal, that is,

[0057] u(t)=u(tT)

[0058] Among them, u is the voltage sampling value, and T is a power frequency cycle.

[0059] When the voltage changes, the voltage change Δu can be expressed as

[0060] Δu=|u(t)-u(tT)|

[0061] To reduce the impact of frequency changes, the following calculation method can be used

[0062] Δu=||u(t)-u(tT)|-|u(tT)-u(t-2T)||

[0063] When the frequency changes, the angular difference between u(t) and u(tT) is basically the same as the angular difference between u(tT) and u(t-2T), so after rearranging the above equation, Δu can be expressed as follows:

[0064] Δu=|u(t)-2*u(tT)+u(t-2T)|

[0065] When the voltage change Δu of n consecutive sampling points is greater than the set value U set When , the voltage is considered to have changed suddenly;

[0066] The U set The value is 0.2U n , U n is the PT secondary voltage rating.

[0067] Considering the sudden change of voltage when crossing zero point, preferably, n takes the value of 4.

[0068] Determine the core state at the moment of voltage mutation:

[0069] After a voltage mutation, the flux is composed of a steady-state component and a non-periodic component. Therefore, the ratio of the flux after the mutation to the flux before the mutation can be used to determine the core saturation.

[0070] First calculate:

[0071]

[0072] Among them, S1(k) is the sum of the absolute values ​​of the voltage in the half cycle before the voltage mutation, which is used to represent the steady-state component of the magnetic flux;

[0073] S2(k) is the sum of the voltage changes within the half cycle starting from the moment of voltage mutation, which is used to represent the non-periodic component of magnetic flux;

[0074] S e It is the sum of the absolute values ​​of the sampling points within the rated voltage half cycle, used to represent the rated flux linkage;

[0075] N is the number of sampling points per cycle, u(k) is the voltage value of the kth sampling point, u e is the rated voltage sampling value.

[0076] Then we get:

[0077]

[0078] When S>S set , it is judged that the transformer core is in a saturated state.

[0079] Since the saturation flux multiple of modern transformers is usually between 1.2 and 1.5, preferably, the S set The value ranges from 1.2 to 1.5.

[0080] In specific implementation, S set Take 1.2.

[0081] Step 3: The fault in the zone is determined by differential current sampling value. If a fault in the zone occurs, the sampling value differential is activated and the excitation inrush current judgment criterion is exited. Otherwise, the sampling value differential is not activated and the excitation inrush current judgment criterion is activated.

[0082] If the core saturation is not determined from the moment of voltage mutation and the sampling value differential action is not performed, the excitation inrush current judgment criterion is exited.

[0083] Specifically, the method for distinguishing faults in the area according to the sampling value differential is as follows:

[0084] When a proximal external fault occurs, the CT may be saturated, which will generate second harmonics that have an adverse effect on the determination of the excitation inrush current. Since the sampling value differential has a certain anti-saturation capability, the half-cycle sampling value differential can be used to determine the fault within the zone.

[0085] Calculate the sampled value differential current i d , sampling value braking current i r :

[0086]

[0087] Among them, i1,…,i n It is the sampling value of the current on each voltage side of the transformer converted to the high voltage side;

[0088] In order to ensure the reliability of the sampling value differential judgment, a multi-point repeated judgment method is required, that is, more than M points among the consecutive N / 2 sampling points meet the action zone judgment, and the action conditions are met. N is the number of sampling points in one cycle, and the value range of M is: 0<M≤N / 2.

[0089] In specific implementation, when more than 8 points out of 12 consecutive sampling points meet the following action zone criteria, it is considered that an in-zone fault occurs:

[0090]

[0091] Among them, I cd is the sampled value differential starting current, I e is the secondary rated current on the high voltage side of the transformer;

[0092] k is the sampling value differential ratio braking coefficient.

[0093] In summary, when the transformer is in operation, if the core saturation is not determined from the moment of voltage mutation and the sampling value differential is not operated, the excitation inrush current criterion is exited. Under other operating conditions and no-load closing state, the excitation inrush current criterion is activated.

[0094] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, but not to limit the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A differential protection method without magnetizing inrush current blocking based on transformer state recognition, characterized in that: The method comprises the following steps: Step 1: Collect the current on each side of the transformer and identify the transformer status. If it is in the no-load closing state, the excitation inrush current judgment criteria is applied. If it is in the normal operation state, proceed to step 2. Specifically, if there is current on only one side of the high and medium voltage sides and there is no current on the low voltage side, the transformer is considered to be in a no-load closed state, otherwise the transformer is in a normal operating state; Step 2: Collect the voltage on each side of the transformer, calculate the voltage change, identify the voltage mutation, and then judge the transformer core state when the voltage mutation occurs. If the core is saturated, the voltage excitation inrush current criterion is used, otherwise, proceed to step 3; Specifically, the voltage mutation criterion is: the voltage change △u of four consecutive sampling points is greater than 0.2U n , U n is the PT secondary voltage rating; The transformer core condition is determined based on the following parameters: Among them, S1(k) is the sum of the absolute values ​​of the voltage in the half cycle before the voltage mutation, which is used to represent the steady-state component of the flux; S2(k) is the sum of the voltage changes in the half cycle starting from the moment of the voltage mutation, which is used to represent the non-periodic component of the flux; S e is the sum of the absolute values ​​of the sampling points within a half cycle of the rated voltage, used to represent the rated flux; N is the number of sampling points in a cycle, u(k) is the voltage value of the kth sampling point, u e is the rated voltage sampling value; Step 3: The fault in the zone is judged by the differential of the current sampling value. If the fault in the zone occurs, the sampling value differential is operated and the excitation inrush current judgment is exited. Otherwise, the sampling value differential is not operated and the excitation inrush current judgment is entered. Specifically, the criterion for faults within the zone is: more than M points among the consecutive N / 2 sampling points meet the following action zone criterion: Where N is the number of sampling points per cycle; M value range: 0<M≤N / 2; I cd is the sampled value differential starting current, I e is the secondary rated current of the transformer high voltage side; k is the sampling value differential ratio braking coefficient; i d 、i r are the sampled value differential current and sampled value braking current.

2. The differential protection method based on transformer state recognition without excitation inrush current blocking according to claim 1 is characterized in that: In step 1, the currents on the high, medium and low voltage sides of the transformer are obtained; If at least one phase current among the three-phase currents on this side is greater than the current threshold, it is considered that there is current on this side.

3. The differential protection method based on transformer state recognition without excitation inrush current blocking according to claim 2 is characterized in that: The flow threshold is 0.05I n , where I n is the secondary rated value of CT.

4. The differential protection method based on transformer state recognition without excitation inrush current blocking according to claim 1, characterized in that: In step 2, Δu = |u(t)-2*u(tT)+u(t-2T)|; u is the voltage sampling value, and T is a power frequency cycle.

5. The differential protection method based on transformer state recognition without excitation inrush current blocking according to claim 1, characterized in that: In step 2, according to S1(k), S2(k), S e The calculation results are: When S>S set , it is judged that the transformer core is in a saturated state.

6. The differential protection method based on transformer state recognition without excitation inrush current blocking according to claim 5, characterized in that: The S set The value ranges from 1.2 to 1.

5.

7. The differential protection method based on transformer state recognition without excitation inrush current blocking according to claim 1, characterized in that: In step 3, the sampled differential current i d , sampling value braking current i r The calculation formula is: Among them, i1,…,i n It is the sampling value of the current on each voltage side of the transformer converted to the high voltage side.

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