CN100420114C - Generator Differential Protection Method Against TA Transient Unbalance - Google Patents

Abstract

The invention relates to a method for relay protection in the field of power systems. In order to improve the safety and reliability of generator differential protection, a generator differential protection method against transient unbalance of current transformers on both sides of the generator (abbreviated as TA, the same below) is disclosed. This method is composed of phasor ratio braking characteristics and multiple fixed value sampling value ratio braking opening criteria. It adopts variable data window algorithm and cycle blocking principle. In order to ensure correct protection action behavior in the transient process of the system, this method also It has generator grid-connected detection criterion, external fault detection criterion, intermittent disconnection criterion of the secondary circuit of the current transformer on both sides of the generator, and TA saturation criterion.

Description

Generator Differential Protection Method Against TA Transient Unbalance

technical field

The present invention relates to the field of power systems, and more particularly to a method for relay protection.

Background technique

Differential protection is a fast main protection for internal faults of generators. In principle, it has natural selectivity, high sensitivity and fast action speed, and is widely used. Field operation shows that the generator differential protection still malfunctions from time to time. In particular, when the external fault is removed or the generator is connected to the grid asynchronously, the differential protection of the generator will malfunction. At present, the action time of the generator differential protection is generally around 20ms, and the time constant of the large unit is relatively large. When there is a large disturbance, the P-level current transformers on both sides (TA for short) Under the action of current, it is difficult to maintain the consistency of the transmission characteristics, and it is inevitable that there will be a large transient unbalanced differential current. This unbalanced differential current poses a serious threat to the reliable operation of the generator differential protection. If the generator differential protection does not take measures against TA transient unbalance, it may malfunction.

The operation shows that the mold-in circuit is one of the weakest links in the differential protection. Due to damage to components, errors in the A/D circuit, instantaneous voltage drop of the power supply under interference, etc., there may be problems in the sampling circuit of the differential protection. move. The safety and reliability of generator differential protection need to be improved urgently.

To improve the reliability of differential protection, in addition to selecting TA with higher conversion accuracy, matching the secondary load of TA, selecting reliable hardware for the protection device and using dual A/D "two out of two" outlets, etc., the protection scheme can also be Take measures.

The sampling value differential is a differential protection that directly uses the current sampling value. Generally, the protection action criterion is used to compare the S points among the R points to meet the conditions. Compared with conventional phasor differential protection, it has the characteristics of less calculation and faster speed, and is often used in occasions that require relatively high action speed, such as busbar protection. However, this method has a shortcoming. Due to the randomness of the initial sampling time, there is a certain fuzzy area in the differential protection of the sampled value, so that the relay protection staff has a certain gap in the measurement and setting of the threshold and slope of the differential action characteristic of the sampled value. It's an inconvenience. Since the generator itself is a power supply system, there is still a rather long transition process when an internal fault occurs, so there is no need to pursue ultra-fast action speed one-sidedly for the generator differential protection, and more importantly, ensure the reliability of the protection . Aiming at the problems existing in the current generator differential protection, the invention combines the advantages of sampling value differential and phasor differential, and proposes a generator differential protection method with multi-fixed value sampling value ratio braking open.

Contents of the invention

In order to improve the safety and reliability of generator differential protection, the invention discloses a generator differential protection method against TA transient unbalance. This method uses the phasor ratio braking characteristic as the main criterion, and relies on the multiple fixed value sampling value ratio braking open criterion to achieve the prevention of inconsistent TA transient characteristics on both sides of the generator, the prevention of TA transient saturation and the prevention of protection device A/ Purpose of bad data in D sampling. The method also has generator grid-connected detection criteria, out-of-area fault detection criteria, intermittent disconnection criteria of TA secondary circuits on both sides of the generator, and TA saturation criteria.

The present invention comprises steps:

The generator differential protection device samples the current waveform of the current transformer at the generator end and the neutral point and the voltage waveform of the voltage transformer at the generator end to obtain the instantaneous value of current and voltage;

The complex number form of each current and voltage is obtained through the variable data window recursive Fourier algorithm;

Calculate the differential current and brake current according to the following formula:

In the formula: I _dz , I _zd are the differential current and braking current respectively; I _T ^& , I _N ^& are the current phasors at the generator end and the neutral point side respectively;

According to the phasor ratio braking characteristic criterion equation described in the following formula, plus the generator grid-connected detection criterion, the external fault detection criterion, the TA secondary circuit gap disconnection criterion on both sides of the generator, and the TA The saturation criterion enters the phasor ratio braking characteristic criterion equation for discrimination:

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}$

In the formula: I _CD is the initial value of the differential current, K _res is the braking coefficient;

After the above phasor ratio braking characteristic is activated, enter the sampling value ratio differential braking characteristic judgment, and calculate the sampling value differential current and sampling value braking current according to the following formula:

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span>\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\end{array}\right\}$

In the formula: i _dz , i _zd are sampled value differential current and brake current respectively; i _T , i _N are current sampled values of generator terminal and neutral point respectively;

According to the following formula, enter the multi-fixed value sampling value ratio brake release criterion. When the phasor ratio brake characteristic criterion and the multi-fixed value sample value ratio brake release criterion are simultaneously in the action area, the generator differential protection operates instantaneously:

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}$

In the formula: $k_{\mathrm{cd}}\&Element;(0,\sqrt{2})$ is a constant, called the differential fixed value coefficient.

The invention greatly improves the ability of the generator differential protection to prevent the generator from being asynchronously connected to the grid, when the external fault is removed, and when the external fault is removed, the TA on both sides is transiently unbalanced, and prevents the A/D sampling bad data from causing the protection to malfunction. The ability to move improves the reliability of the protection without reducing the sensitivity and operating speed of the generator differential protection.

Description of drawings

Figure 1 shows the definition of the installation position and polarity of current transformers and voltage transformers;

Figure 2 shows the operating characteristics of the generator differential protection;

Fig. 3 shows a logic block diagram of the generator differential protection of the present invention.

specific implementation plan

The present invention implements generator differential protection against TA transient unbalance according to the following formula. Please refer to Figure 1 for the installation position and polarity definitions of current transformers and voltage transformers. Sex point TA.

The calculation method of differential current and braking current is shown in formula (1):

In the formula: I _dz , I _zd are the differential current and braking current respectively; I _T ^& , I _N ^& are the current phasors of the generator terminal and the neutral point respectively, and they all point to the system as positive.

In order to improve the action speed of serious internal faults and ensure that external faults do not malfunction, the calculation of differential current and braking current adopts the variable data window algorithm. The differential current and braking current are calculated by Fourier algorithm of half cycle data window firstly, at this time, the fixed value of the minimum operating current is automatically increased by 18%. After adopting the variable data window algorithm, the fastest action time of the protection is within 12ms. If the result obtained by the half-cycle algorithm cannot act, the protection will automatically switch to the full-cycle Fourier algorithm, which can improve the reliability of the protection.

The action equation of the ratio brake differential protection is shown in formula (2), and the realized ratio brake action characteristics are shown in Figure 2:

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Where: I _CD is the initial value of the differential current, K _res is the braking coefficient.

When the above phasor ratio braking characteristic is activated, it enters the sampling value ratio braking criterion, where the sampling value differential current and braking current are calculated as shown in formula (3):

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span>\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\end{array}\right\}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

In the formula: i _dz , i _zd are sampled value differential current and braking current respectively; i _T , i _N are current sampled values of generator terminal and neutral point respectively.

The sampling value ratio braking criterion is shown in formula (4):

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

In the formula: $k_{\mathrm{cd}}\&Element;(0,\sqrt{2})$ is a constant, called the differential fixed value coefficient.

If there is an S point among the R points that satisfies the formula (4), the sampling value ratio braking criterion acts. For generator differential protection, R can be selected as the number of sampling points of one cycle, that is, R is equal to 24 for a protection device that samples at 24 points in one cycle. According to the k _CD value determined in advance, the value of S can be determined by sampling the sine function. Due to the randomness of the initial sampling moment, there is a certain fuzzy area in the sampling value ratio brake protection. However, this method uses sampling value ratio braking as an open criterion to improve protection reliability, and properly reduces the value of S, so that when the phasor ratio braking characteristic is at the critical operating point, the sampling value ratio braking can reliably fall in the action area , so as to avoid the defect of blurred area inherent in sampling value ratio braking.

In order to further improve the reliability of the differential protection of the generator, the multi-fixed sampling value ratio braking characteristic can be adopted. That is, first determine m k _CD values: k _CD1 , k _CD2 , ΛΛk _CDm , and then correspond to m S values S ₁ , S ₁ , ΛΛS _m . If for each k _CD value, the number of points satisfying the formula (4) in the R point sample value data is greater than the corresponding S value, then the multi-fixed value sample value ratio braking characteristic is satisfied.

For example: take m=2, k _CD1 =0.8, k _CD2 =1.2

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>\mathrm{<span\; class="notranslate">\; 0.8</span>}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

$\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>\mathrm{<span\; class="notranslate">\; 1.2</span>}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

If the number of R sampling points satisfying formula (5) is greater than S1, and the number of points satisfying formula (6) is greater than S2 (S2<S1), then the multi-fixed value sampling value ratio braking characteristic is satisfied. Among them, formula (5) is the criterion of low fixed value, and formula (6) is the criterion of high fixed value. The reason for adopting multi-fixed value sampling value ratio brake feature is that when there is bad data in the sampling, the sampling value at this point may be the positive maximum or negative maximum that A/D can represent, and its absolute value is large but the number of bad data points may be At this time, since the normal data is the load current, the difference is very small, so that the braking characteristic of sampling value ratio with many action points and low action setting can be relied on to ensure that the protection does not malfunction; and when an external fault occurs Cut off, when the TA transient characteristics on both sides of the generator are inconsistent, the generated differential current may be longer but the amplitude is small, and the sampling value ratio braking characteristic with high action setting and few action points can be relied on to ensure correct protection move. In addition, when TA is saturated, there is a TA linear transmission area after the current zero crossing, so that some points of sampling value ratio braking are located in the braking area, so the sampling value ratio braking characteristic also has a certain ability to prevent TA saturation.

This differential protection has generator grid-connected detection criteria. When there is no current at the generator terminal and the neutral point TA for a long time, it is judged that the generator is not connected to the grid. If a certain amount of current suddenly appears at the generator terminal and the neutral point TA at the same time, it means that the generator is connected to the grid at this time. After it is judged that the generator is connected to the grid, the slope of the ratio brake can be appropriately increased to prevent the protection from malfunctioning during non-synchronous closing.

This differential protection has detection criteria for out-of-area faults. When the current mutation starts for a period of time, if the protection is in the non-action area, it means that the out-of-area fault is activated. At this time, the number of judgments can be increased to prevent out-of-area faults from causing Protection against misoperation.

The action logic of the protection will be described below in combination with Figure 3 . Since the generator differential protection only protects phase-to-phase faults, the protection adopts the principle of cycle blocking, which can improve reliability. The principle of loop blocking is shown in the logic block diagram of the first part in Figure 3, and the protection will exit only when two phases and above are in differential action.

As shown in the second part of the logic block diagram in Figure 3, in order to protect the two-point ground fault with one point inside the zone and one point outside the zone, a negative sequence voltage element at the machine terminal is also provided. When only one phase of the differential protection meets the operating conditions, in addition to increasing the number of calculations and the number of TA abnormal judgments, and at the same time when the negative sequence voltage at the machine terminal is large, the protection will issue a trip command.

As shown in the logic block diagram of the third part in Figure 3, this method has the gap disconnection criterion of the secondary circuit of the current transformer on both sides of the generator, and the protection will not malfunction when the TA secondary circuit is in poor contact due to wiring. But when the maximum phase current is greater than 1.2 times the rated current, the differential protection should be opened. In addition, this method also has a TA saturation criterion. When a serious fault outside the area makes the TA saturation on the side of the generator, the protection will not malfunction.

As shown in the fourth part of the logic block diagram in Figure 3, when the differential protection does not operate, but the differential current of one phase or more exceeds the limit value of the differential current, the protection sends a differential current limit signal to remind the operator to check the wiring of the secondary circuit .

Since microcomputer protection can realize resource sharing, adding a protection principle will not increase hardware complexity and reduce reliability. For large generators that can lead to two sets of TAs from the neutral point, this method considers the time-division multiplexing of the high-performance CPU to complete a set of generator longitudinal differential protection and a set of split-phase horizontal protection in the same protection device. Differential protection, two sets of incomplete longitudinal differential protection, so as to realize the comprehensive differential protection of the generator. Among them, split-phase transverse difference protection and incomplete longitudinal difference protection can protect the inter-turn faults of stator windings. Except that the cycle blocking method is not necessary, other implementation methods are the same as generator longitudinal difference protection.

The invention greatly improves the ability of the generator differential protection to prevent the generator from being asynchronously connected to the grid, when an external fault occurs and when the external fault is removed, the TA on both sides is transiently unbalanced, and prevents the A/D sampling bad data from causing the protection to malfunction. The ability to operate improves the reliability of the protection without reducing the sensitivity and operating speed of the generator differential protection.

Claims (4)

1. A method of anti-TA transient unbalance generator differential protection, the method comprising steps: The generator differential protection device samples the current waveform of the current transformer at the generator end and the neutral point and the voltage waveform of the voltage transformer at the generator end to obtain the instantaneous value of current and voltage; The complex number form of each current and voltage is obtained through the variable data window recursive Fourier algorithm; Calculate the differential current and brake current according to the following formula: $\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span>\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\end{array}\right\}$ In the formula: I _dz , I _zd are the differential current and braking current respectively; I _T , I _N are the current phasors at the generator end and the neutral point side respectively; According to the phasor ratio braking characteristic criterion equation described in the following formula, plus the generator grid-connected detection criterion, the external fault detection criterion, the TA secondary circuit gap disconnection criterion on both sides of the generator, and the TA The saturation criterion enters the phasor ratio braking characteristic criterion equation for discrimination: $\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}$ In the formula: I _CD is the initial value of the differential current, K _res is the braking coefficient; After the above phasor ratio braking characteristic is activated, enter the sampling value ratio differential braking characteristic judgment, and calculate the sampling value differential current and sampling value braking current according to the following formula: $\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span>\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\end{array}\right\}$ In the formula: i _dz , i _zd are sampled value differential current and brake current respectively; i _T , i _N are current sampled values of generator terminal and neutral point respectively; According to the following formula, enter the multi-fixed value sampling value ratio brake release criterion. When the phasor ratio brake characteristic criterion and the multi-fixed value sample value ratio brake release criterion are simultaneously in the action area, the generator differential protection operates instantaneously: $\left.\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cd</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; dz</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; res</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; zd</span>}}\end{array}\right\}$ In the formula: $k_{\mathrm{cd}}\&Element;(0,\sqrt{2})$ is a constant, called the differential fixed value coefficient. 2. The method as claimed in claim 1, wherein according to m k _CD values determined in advance: k _CD1 , k _CD2 , ΛΛk _CDm , the corresponding m S values S ₁ , S ₁ can be determined by the method of sampling the sine function , ΛΛS _m , if for each value of k _CD , the number of points satisfying the above multi-fixed sampling value ratio braking equation in the R point sampling value data is greater than the corresponding S value, then the multi-fixed sampling value ratio braking characteristic satisfies . 3. The method according to claim 1, the generator grid-connected detection criterion is: when the generator terminal and the neutral point TA have no current for a long time, it is judged that the generator is not connected to the grid, if the generator terminal and the neutral point TA are suddenly simultaneously The presence of a certain amount of current indicates that the generator is connected to the grid at this time. 4. The method as claimed in claim 1, the detection criterion of the external fault is: when the current mutation is activated for a period of time, if the protection is in the non-action area, it means that the external fault is activated.

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