CN105974173A - Current detection method of one-phase inversion grounding device - Google Patents

Abstract

The invention discloses a current detection method of a single-phase inverter type grounding device. The main body of the device is a single-phase inverter connected between a neutral point of a distribution network and the ground. In the normal operation of the power system, two power frequency currents with equal amplitude and different phase angles are injected into the neutral point of the distribution network through the inverter circuit in time-sharing, and the neutral point voltage value when the current is injected twice is measured in real time. , and calculate the parameters of distribution network to ground (distribution network equivalent impedance and equivalent potential), and then obtain the amplitude and phase of distribution network fault arc suppression and three-phase unbalance compensation current. Injecting this current into the neutral suppresses the ground fault arc and maintains the three-phase voltage balance. This method can be implemented when the system is running normally, effectively avoiding the impact of the transient process on the detection accuracy when the ground fault occurs. The advantage of simplicity is realized.

Description

A current detection method of a single-phase inverter grounding device

technical field

The invention relates to a current detection method of a single-phase inverter type grounding device, which is particularly suitable for related fields such as single-phase grounding faults of distribution networks and three-phase unbalanced overvoltage suppression.

Background technique

The distribution network is at the terminal of the power system, has a complex and changeable structure, and is closely connected with users, which easily leads to various faults. Single-phase ground fault is the most common fault in the distribution network. When the fault current is too large, arc light will be generated, and the arc combustion will bring great harm such as fire and equipment damage. Three-phase unbalanced voltage is a long-term existence in the distribution network. It is difficult to solve the problem completely. The existence of three-phase unbalanced overvoltage will affect the normal operation of single-phase loads in the distribution network, endanger the insulation of the power grid, and bring difficulties to fault detection, and even cause the protection device to malfunction and cause power accidents.

In order to solve the above problems in the distribution network, domestic and foreign distribution networks generally adopt the method of changing the neutral point grounding to limit the ground fault current and unbalanced overvoltage. The development process such as grounding through small resistance. The neutral point ungrounded system can still run with the fault for 1~2 hours after a single-phase ground fault occurs, and the power supply reliability is high, but the insulation of each phase must be based on the phase voltage. Double-fold design, which requires high insulation; the neutral point through the arc suppression coil grounding system can effectively reduce the ground fault current, but it can only compensate for the fault current of the power frequency, and the connection of the arc suppression coil will lead to fault selection When a ground fault occurs in a small-resistance grounding system, the short-circuit current is very large, and the relay protection device will act at the moment of the fault to cut off the faulty line, but at the same time it also reduces the power supply reliability of this method.

In order to fully meet the operation requirements of the distribution network in various states, make up for the defects of different grounding methods, comprehensively utilize the advantages of various grounding methods, and flexibly select the grounding method according to the operating status of the power grid, a new distribution network grounding method is proposed. Way. That is, a single-phase inverter device is used to connect between the neutral point and the earth, replacing the traditional non-effective neutral point grounding method, injecting a controllable current into the neutral point of the distribution network through the inverter, and regulating the amplitude of the current. Value and phase, so as to achieve the zero-sequence voltage of the control system, and achieve the purpose of suppressing faults and unbalanced overvoltages.

Contents of the invention

Aiming at the single-phase grounding faults and three-phase unbalanced overvoltages that often occur in the distribution network, the present invention proposes a current detection method for a single-phase inverter grounding device. The injected current is calculated to obtain the grounding parameters of the system, from which the amplitude and phase of the injected current under different fault conditions can be obtained. The technical scheme that the present invention solves the problems of the technologies described above comprises the following steps:

1) In the case of normal operation of the distribution network, the new grounding device connected between the neutral point and the earth injects current into the neutral point twice, which are I _i1 and I _i2 respectively, and the corresponding neutral points are measured and recorded. Point voltage U _N1 , U _N2 ;

2) Using I _i1 , I _i2 , U _N1 , U _N2 to calculate the equivalent ground impedance Z _eq of the distribution line and the equivalent potential E _eq of the three-phase power supply of the distribution network;

3) When a single-phase ground fault occurs in the distribution network, calculate the corresponding injection current amplitude and phase according to the Z _eq and E _eq obtained in step 2), and inject it into the neutral point of the distribution network to suppress the generation of fault arc ;

4) When the three-phase unbalanced overvoltage occurs in the distribution network, calculate the corresponding injection current amplitude and phase according to the Z _eq and E _eq obtained in step 2), and inject it into the neutral point of the distribution network to maintain the three-phase balance.

The beneficial effects of the present invention are:

1) During the normal operation of the distribution network, the current is injected twice to calculate the line-to-ground parameters, and there is no need to inject current for calculation after the fault occurs, which can eliminate the influence of transient parameters on the measurement results during the fault, and significantly improve the current detection accuracy;

2) This method only needs to inject two currents into the neutral point to realize the detection of fault arc suppression current and three-phase unbalance compensation current, which has the advantages of high real-time and simple implementation.

The present invention will be further described below in conjunction with the accompanying drawings.

Description of drawings

Figure 1 is a topological diagram of a single-phase inverter grounding device.

Figure 2 is an equivalent circuit diagram of a single-phase inverter grounding system.

Figure 3 shows the three-phase unbalanced two injection current values and the calculated values of distribution network parameters.

Figure 4 is a dynamic waveform diagram of three-phase unbalanced overvoltage suppression.

Figure 5 shows the two injected current values and the calculated values of distribution network parameters for single-phase ground faults.

Figure 6 is a dynamic waveform diagram of single-phase ground fault voltage suppression.

detailed description

Figure 1 is a topological diagram of a single-phase inverter grounding device, , , is the three-phase power supply of the power distribution system, , , , , , Represent the three-phase ground resistance and capacitance, R _d represents the ground resistance when a single-phase ground fault occurs, U _N is the neutral point voltage of the system. The single-phase inverter grounding system is grounded through an active inverter. The inverter is powered by a DC power supply U _DC , and the amplitude and phase controllable injection current I _i is generated through PWM inversion and injected into the neutral point. T represents Inject the transformer.

Figure 2 is the equivalent circuit diagram of the new grounding device and distribution network. Since the active inverter circuit is fully controllable, it can be equivalent to a controllable current source, and the three-phase power supply of the distribution network is equivalent to the potential , (in , , ); The three-phase ground impedance is equivalent to the impedance , .

Taking the A and B-phase line voltage as the reference, the current I _i1 and I _i2 are injected into the neutral point of the distribution network twice through the injection current source, where ,and . After sequentially injecting I _i1 and I _i2 and recording the corresponding neutral point voltages U _N1 and U _N2 respectively, the neutral point voltage can be expressed as:

(1)

The equivalent impedance to ground of the distribution network can be obtained by subtracting the two formulas , the equivalent potential .

At the same time, according to Figure 2, it can be seen that the relationship between the injection current and the neutral point voltage and the equivalent potential and impedance of the distribution network is:

(2)

The value of the injected current is analyzed for the three-phase unbalanced condition and the single-phase ground fault respectively as follows:

( 1 ) Three-phase imbalance occurs in the distribution network:

The new grounding device suppresses the three-phase imbalance by injecting current into the distribution network to clamp the neutral point voltage to zero. At this time, the control target , into formula (2), we can see that the injected current is:

(3)

Injecting the current shown in formula (3) into the neutral point can suppress the neutral point voltage to zero and realize the suppression of three-phase unbalance.

Figure 3 shows the neutral point voltage values corresponding to the two injected currents when there is a three-phase imbalance in the distribution network, as well as the corresponding distribution network parameters and the reference value of the injected current. Figure 4 shows the dynamic waveform diagram of three-phase unbalanced overvoltage suppression.

( 2 ) A single-phase ground fault occurs in the distribution network:

The equivalent power supply of the power distribution system after a fault is assumed to be , the equivalent impedance is . The inverter device forces the fault phase voltage to be zero by injecting current into the neutral point to achieve the purpose of fault arc suppression. At this time, the control target is (assuming that the fault occurs in phase C), and , , into formula (2), we can see that the injected current is:

(4)

Injecting the current shown in formula (4) into the neutral point can realize that the neutral point voltage is the opposite number of the fault phase voltage, and at the same time the fault phase voltage is suppressed to zero, realizing single-phase ground fault arc suppression.

Figure 5 shows the neutral point voltage values corresponding to the two injection currents during the normal operation of the system, as well as the corresponding distribution network parameters and injection current reference values. Figure 6 shows the dynamic waveform diagram of single-phase ground fault voltage suppression.

Claims (2)

1. an electric current detecting method for single-phase inversion type grounding device, comprises the steps:

1) in the case of power distribution network is properly functioning, to neutral point timesharing, the single-phase inversion type grounding device being connected between neutral point with the earth injects that amplitude is equal, phase angle two power currents not etc., is designated as respectively I _i1、 I _i2, record and record corresponding neutral point voltage U _N1、 U _N2；

2) utilize I _i1、 I _i2、 U _N1、 U _N2Calculate distribution line equivalence earth impedanceZ _eqAnd power distribution network three phase mains equivalent potential E _eq；

3) when power distribution network generation singlephase earth fault, according to step 2) gainedZ _eqAnd E _eqCalculate corresponding injection current amplitude and phase place, and be injected into power network neutral point, the generation of suppression fault electric arc；

When power distribution network produces three-phase imbalance overvoltage, according to step 2) gainedZ _eqAnd E _eqCalculate corresponding injection current amplitude and phase place, and be injected into power network neutral point, maintain three-phase equilibrium.

The electric current detecting method of a kind of single-phase inversion type grounding device the most according to claim 1, described step 3) in injection current amplitude with phase calculation method be: if there is singlephase earth fault, injection current is(set fault to occur in C phase,E _cFor faulted phase voltage)；If generation three-phase imbalance, then injection current is。