CN107346003B - Voltage source converter fault detection and positioning method and system - Google Patents

Abstract

The invention discloses a voltage source converter fault detection and location method and system, and specifically provides a grid-side grounding fault location method in a voltage source converter topology connected to a DC grid in a flexible direct current transmission system without a transformer. The first method is to detect the fault current, compare the current of the bridge arm reactor and the current measured by the grid-side current transformer, and calculate the difference current between the two according to the current direction of the bridge arm and the grid-side current. The difference current is greater than the given value. The value is considered to be the fault side. Under the condition of short-time blocking of the converter, the zero-sequence voltage of the converters on both sides is detected. If it is a full-bridge structure, the short-time full blocking or semi-blocking method can be used to ensure that the zero-sequence grounding current will not be transmitted to the pair through the DC side. In the side converter station, the half-bridge structure can detect the output zero-sequence voltage of the single-side converter by means of short-time blocking. If it is greater than a given value and exceeds a certain anti-jitter delay, it is judged as the fault side, and this side is considered as the fault side.

Description

A method and system for detecting and locating faults in a voltage source converter

technical field

The invention belongs to the technical field of flexible AC and DC power transmission in power systems, and in particular relates to a method and system for locating a ground fault on a grid side in a voltage source converter topology connected to a DC grid in a transformerless manner in a flexible DC power transmission system.

Background technique

In the modern AC distribution network, on the one hand, the load density of large cities continues to increase, and the scale of the transmission and distribution network continues to expand. Subject to problems such as short-circuit capacity and electromagnetic ring network, the distribution network generally adopts high-voltage partitioned and medium-voltage open-loop operation. On the other hand, customers' requirements for power supply reliability and power quality continue to increase, and the sensitive loads in large cities are gradually increasing, and short-term power supply interruption will also bring greater damage. Economic losses and even serious social impacts. Flexible DC transmission uses voltage source converters, which can independently adjust the transmission of active and reactive power. Flexible DC technology is one of the effective means to solve the problems of existing distribution networks. Flexible DC transmission can be used without increasing short-circuit capacity. Realize the interconnection of AC power grids, realize the closed loop operation of AC power grids, improve the reliability of power grid operation, improve power quality and power grid operation efficiency.

The AC-DC interconnected power grid based on flexible DC transmission and distribution is one of the trends in the development of power grids. Based on the development of modular multi-level technology, it solves the problem of voltage equalization and large losses that plague the development of two-level flexible DC transmission. AC system harmonics are reduced, making it possible to connect AC systems without a transformer. This makes it possible to reduce the overall investment and volume of the converter station.

When the voltage source converter of the flexible DC transmission system is connected to the power grid in a transformerless way, when the AC system fails, the fault characteristics of the half-bridge MMC and the full-bridge MMC are similar, and the flexible ring network control device cannot isolate the zero sequence. For example, for AG (single-phase-to-ground) fault, the non-fault phase voltage has the phase voltage raised to the line voltage, and the ABC three-phase current remains symmetrical. Converter station 2 also shows the fault characteristics of single-phase ground fault. The DC voltage has a power-frequency common-mode AC component, and the control systems on both sides cannot predict which side the fault occurs on. Therefore, the fault control strategy will be activated to suppress the zero-sequence component. It will lead to the co-regulation suppression results on both sides. Since the zero-sequence current components on both sides are actually of the same nature, it may eventually cause the suppression effects on both sides to cancel each other due to the asynchronous relationship between the current loops on both sides. Therefore, it is necessary to locate the fault. On the basis of accurately locating the fault side, the method of suppressing the zero-sequence component of the cooperation on both sides is determined.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for locating the ground fault on the grid side in the topology of the voltage source converter connected to the DC grid in a transformerless way in a flexible DC power transmission system. The magnitude of the grid-side current, according to the current direction of the bridge arm and the current direction of the grid-side, calculate the difference current between the two, and determine the actual ground fault occurrence side by comparing the difference current of each station. If the faulty side cannot be determined by the differential current calculation method, and if the zero-sequence voltage on the AC side is detected at the same time, the zero-sequence voltage of the converters on both sides is detected under the condition of short-term blocking of the converters. By comparing the zero-sequence voltage of each station, the actual ground fault occurrence side is determined.

In order to achieve the above object, the technical scheme adopted in the present invention is:

By calculating the difference current between the bridge arm reactor current and the grid side current, the difference current is greater than the given value △I _set , and when it exceeds a certain anti-jitter delay △t ₁ , it is judged as the fault side, and the difference current is less than the given value △ The I _set side is judged as the non-faulty side, and this positioning method can be carried out under the condition of not blocking the converter, where Δt ₁ ranges from 0 to 5 s.

The above-mentioned method of locating the fault side is applicable to the following neutral point grounding methods of the AC system: 1) the neutral point is not grounded; 2) the neutral point is grounded by resistance; 3) the neutral point is grounded by the arc suppression coil. The converter topology can be a half-bridge, full-bridge or full-bridge-like structure or a hybrid bridge arm structure of half-bridge plus full-bridge or full-bridge-like structure.

In the above grid-side grounding fault location method, when a single-phase grounding fault occurs at one end of the converter, a two-phase grounding fault and a three-phase grounding fault can be detected by differential current detection to locate the faulty side.

The above grid-side grounding fault location method, if it is a full-bridge structure, can use short-time full-blocking or semi-blocking methods to ensure that the zero-sequence grounding current will not be transmitted to the opposite-side converter station through the DC side, and the half-bridge structure can pass short-time full-blocking or semi-blocking methods. In the blocking mode, the output zero-sequence voltage of the AC side is detected. If it is greater than the given value △U _set and exceeds a certain anti-jitter delay △t, the ₂ side is judged as the fault side, and the other side is the non-fault side. Among them, the value of Δt ₂ ranges from 0 to 5s.

A grid-side grounding fault location system includes a current detection unit, a zero-sequence voltage detection unit, a logic comparison unit and a fault detection and location unit. The current detection unit detects the grid side current and the bridge arm current. The zero-sequence voltage detection unit detects the magnitude of the output zero-sequence voltage. The logic comparison unit calculates the magnitude of the differential current between the two and calculates the magnitude of the zero-sequence voltage output by the converter under blocking conditions. The logic comparison unit calculates the difference current between the two. If the difference current is greater than the given value △I _set , and exceeds a certain anti-jitter delay △t ₁ , it is judged as the fault side, and the difference current is less than the given value △I _set . It is the non-faulty side, and under the condition of short-term blocking of the converter, compare the zero-sequence voltage of the AC side, if it is greater than the given value △ _Uset and exceeds a certain anti-jitter delay △t2, the side is judged as the faulty side, and the other side is the non-faulty side. On the fault side, the value of Δt2 ranges from 0 to 5s. Combining the above two comparison results, the fault detection and location unit firstly detects the magnitude of the differential current to determine the faulty side. If the faulty side cannot be determined by the differential current calculation method, the short-term blocking converter is immediately activated to detect the zero-sequence voltage of the converters on both sides. size. By comparing the zero-sequence voltage of each station, the actual ground fault occurrence side is determined.

After adopting the above scheme, the beneficial effects of the present invention are:

(1) A variety of composite detection methods can easily detect the one end of the converter with a ground short circuit on the AC grid side in the DC network, and it is convenient for the converter side to start the zero-sequence voltage or current suppression strategy.

(2) After the method determines the fault side, the converters can cooperate with each other in the control system to jointly suppress the zero-sequence current or voltage. It can better suppress the fault current component in the system and provide guarantee for the normal operation of each converter in the DC grid.

Description of drawings

Figure 1 is a schematic diagram of the fault points k1 and k2 occurring on the grid side when the converters at both ends are connected in a transformerless DC mode.

FIG. 2 is the topology of the converter in the present invention, wherein the bridge arm module can be a half bridge (HBSM), a full bridge (FBSM) or an inner full bridge (SFBSM).

Detailed ways

The technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

The purpose of the present invention is to provide a method for locating the ground fault on the grid side in the topology of the voltage source converter connected to the DC grid in a transformerless way in a flexible DC power transmission system. One method is to detect the fault current and compare the magnitude of the bridge arm reactor current. Measure the current with the grid-side current transformer, and calculate the difference current between the two according to the current direction of the bridge arm and the grid-side current. In the state of short-term blocking of the converter, the output zero-sequence voltage of the single-side converter is directly detected. If it is greater than the given value and exceeds a certain anti-jitter delay, it is judged as the fault side. Under the condition of short-term blocking of the converter, the detection of both sides is carried out. The zero-sequence voltage of the converter, if it is a full-bridge structure, can use short-time full-blocking or semi-blocking methods to ensure that the zero-sequence grounding current will not be transmitted to the opposite-side converter station through the DC side, and the half-bridge structure can be short-term blocking. In this way, the output zero-sequence voltage of the single-side converter is detected, and it is judged as the fault side if it is greater than the given value and exceeds a certain anti-jitter delay.

In order to achieve the above-mentioned purpose, the solution of the present invention is:

A method for locating the ground fault on the grid side of the voltage source converter connected to the DC grid is proposed. By detecting the fault current, comparing the magnitude of the bridge arm reactor current and the grid side current, according to the current direction of the bridge arm and the grid side current direction, calculate the two By comparing the difference current size of each station, the actual ground fault occurrence side is determined. The specific scheme is as follows: by calculating the magnitude of the differential current between the two, the differential current is greater than the given value △I _set , and the side that exceeds a certain anti-jitter delay △t ₁ is judged as the fault side, and the differential current is less than the given value △I _set side If it is judged as the non-faulty side, this positioning method can be carried out under the condition that the converter is not blocked, and the value of Δt ₁ ranges from 0 to 5s.

If the faulty side cannot be determined by the differential current calculation method, and if the zero-sequence voltage on the AC side is detected at the same time, the zero-sequence voltage of the converters on both sides is detected under the condition of short-term blocking of the converters. By comparing the zero-sequence voltage of each station, the actual ground fault occurrence side is determined. The specific implementation is as follows: if it is a full-bridge structure, it can be ensured that the zero-sequence grounding current will not be transmitted to the opposite-side converter station through the DC side by means of short-term full-blocking or semi-blocking, and the half-bridge structure can be short-term blocking. Detect the output zero-sequence voltage of the AC side, if it is greater than the given value △U _set and exceeds a certain anti-jitter delay △t, the ₂ side is judged as the fault side, and the other side is the non-fault side. Among them, the value of Δt ₂ ranges from 0 to 5s.

The above-mentioned method of locating the fault side is applicable to the following neutral point grounding methods of the AC system: 1) the neutral point is not grounded; 2) the neutral point is grounded through a resistance; 3) the neutral point is grounded through an arc suppression coil. The converter topology can be a half-bridge, a full-bridge or a full-bridge-like structure, or a hybrid bridge arm structure of half-bridge plus full-bridge or full-bridge-like.

A specific embodiment of a grid-side ground fault location system is as follows: the system includes a current detection unit, a zero-sequence voltage detection unit, a logic comparison unit and a fault detection and location unit.

The current detection unit detects the grid side current and the bridge arm current. The zero-sequence voltage detection unit detects the magnitude of the output zero-sequence voltage. The logic comparison unit calculates the magnitude of the differential current between the two and calculates the magnitude of the zero-sequence voltage output by the converter under blocking conditions. The logic comparison unit calculates the difference current between the two. If the difference current is greater than the given value △I _set , and exceeds a certain anti-jitter delay △t ₁ , it is judged as the fault side, and the difference current is less than the given value △I _set . It is the non-faulty side, and under the condition of short-term blocking of the converter, the output zero-sequence voltage of the single-side converter is detected, which is greater than the given value △U _set and exceeds a certain anti-jitter delay △t ₂ side is judged as the fault side , the other side is the non-faulty side, where Δt ₂ ranges from 0 to 5s. Combining the above two comparison results, the fault detection and location unit firstly detects the magnitude of the differential current to determine the faulty side. If the faulty side cannot be determined by the differential current calculation method, and if the zero-sequence voltage on the AC side is detected at the same time, it will immediately start the short-term blocking replacement. Check the zero-sequence voltage of the inverters on both sides. By comparing the zero-sequence voltage of each station, the actual ground fault occurrence side is determined.

Claims (6)

1. A voltage source converter fault detection and positioning method is characterized in that: by detecting the fault current, comparing the size of the bridge arm current and the grid side current, and calculating the distance between the two according to the current direction of the bridge arm and the grid side current direction. The difference current size is determined by comparing the difference current size of each station to determine the actual ground fault occurrence side; the comparison of the difference current size of each station specifically refers to comparing the difference current with the predetermined current threshold; the difference current is greater than the set threshold △I _set , and if the set anti-jitter delay Δt ₁ is exceeded, the side is judged to be the fault side; the differential current is less than the set threshold ΔI _set , and the side is judged to be the non-fault side; When the differential current is less than the _set threshold △Iset, the side is judged as the non-faulty side, that is, the faulty side cannot be judged by the differential current calculation method, and if the zero-sequence voltage on the AC side is detected at the same time, the commutation is blocked for a short time. Under the condition of the inverter, the zero-sequence voltage of the converter is detected, and the actual ground fault occurrence side is determined by comparing the zero-sequence voltage of each station; the comparison of the zero-sequence voltage of each station specifically refers to comparing the zero-sequence voltage with the preset voltage threshold Compare. 2. A voltage source converter fault detection and positioning method as claimed in claim 1, characterized in that: in the fault detection and positioning method, the comparison of the magnitude of the differential current of each station is performed under the condition that the converter is not blocked, wherein The value range of Δt ₁ is 0～5s. 3. A voltage source converter fault detection and positioning method as claimed in claim 1, characterized in that: the fault detection and positioning method is applicable to the following neutral point grounding methods of the AC system: 1) The neutral point is not grounded; 2) The neutral point is grounded by resistance; 3) There are three ways to ground the neutral point through the arc suppression coil; The converter topology is a half-bridge structure, a full-bridge structure, or a full-bridge-like structure, or a half-bridge plus a full-bridge-like hybrid arm structure, or a full-bridge plus a full-bridge-like hybrid arm structure. 4. A voltage source converter fault detection and location method according to claim 1, characterized in that: the fault detection and location method is used to deal with single-phase grounding faults, two-phase grounding faults, two-phase grounding faults, etc. Three-phase ground fault. 5. A voltage source converter fault detection and location method according to claim 1, characterized in that: when the converter topology is a full-bridge structure, a short-time full-blocking or semi-blocking method is used to ensure zero-sequence The ground current will not be transmitted to the opposite-side converter station through the DC side; when the converter topology is a half-bridge structure, the output zero-sequence voltage of the single-side converter is detected by a short-time blocking method. When the voltage is greater than the set value The threshold ΔU _set , and exceeds a certain anti-jitter delay Δt ₂ , the side is judged to be the faulty side, and in other cases, the side is judged to be the non-faulty side. 6. A voltage source converter fault detection and location system, characterized in that: the system comprises a current detection unit, a zero-sequence voltage detection unit, a logic comparison unit and a fault detection and location unit; The current detection unit detects the grid side current and the bridge arm current; The zero-sequence voltage detection unit detects the magnitude of the zero-sequence voltage on the AC side; The logic comparison unit calculates the difference current between the grid side current and the bridge arm current. When the difference current is greater than the set threshold ΔI _set and exceeds a certain anti-jitter delay Δt ₁ , the side is judged as: Fault side; if the differential current is less than the given value △I _set , it is judged that the side is the non-fault side; When the differential current is less than the set threshold _ΔIset , the side is judged as the non-faulty side, that is, when the logic comparison unit cannot determine the faulty side through the differential current calculation method, the logic comparison unit receives the zero-sequence voltage detection unit. The zero-sequence voltage obtained under the condition of blocking the converter is judged according to the zero-sequence voltage: when the zero-sequence voltage is greater than the set threshold △U _set and exceeds a certain anti-jitter delay △t ₂ , it is judged as a fault side, the other side is the non-faulty side; In combination with the above comparison results, the fault detection and positioning unit first detects the magnitude of the differential current to determine the faulty side, and when the differential current is less than the set threshold _ΔIset , the logical comparison unit cannot determine the faulty side through the differential current calculation method, and starts immediately. The converter is blocked for a short time, the zero-sequence voltage of the AC side is detected, and the actual ground fault occurrence side is determined by comparing the zero-sequence voltage of each station.

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