CN107478950A - A kind of discrimination method of the bipolar short trouble of DC line - Google Patents

Abstract

The invention relates to a method for identifying bipolar short-circuit faults of DC lines, comprising: negative busbar voltage, current, voltage change rate and current change rate; judging whether the positive and negative busbar voltage change rates and current change rates meet the set fault identification Pre-judgment conditions: If the fault identification pre-judgment conditions are met, it is judged whether the positive and negative bus voltage and current meet the set fault identification and judgment conditions; if the fault identification and judgment conditions are met, it is determined that the system has a bipolar short-circuit fault. The present invention firstly judges whether a bipolar short-circuit fault occurs in the system according to the voltage change rate and current change rate of the positive and negative bus bars, and then further determines whether the bipolar short-circuit fault occurs in the system through the voltage and current of the positive and negative bus bars. The fault identification can be completed quickly and accurately within the system, which can meet the requirements of fast and accurate detection of the double-pole short-circuit fault of the DC line of the DC circuit breaker.

Description

An identification method for double-pole short-circuit faults in DC lines

technical field

The invention relates to a method for identifying bipolar short-circuit faults of DC lines, and belongs to the technical field of DC transmission in power systems.

Background technique

With the development of VSC-HVDC power transmission technology to VSC-HVDC grid, higher requirements are put forward for the reliability, stability and safe operation of the whole system. The huge challenge is the breaking of short-circuit current in VSC-HVDC power grid. Compared with the AC system, the time constant in the VSC-HVDC power grid is small, the short-circuit current rises quickly, and at the same time causes the drop of the DC voltage, and even causes the inverter and the short-circuit current to run out of control, and the DC current is difficult to start due to the lack of a natural zero-crossing point. broken. HVDC circuit breakers capable of quickly disconnecting or isolating short-circuit faults have become a bottleneck in the development of VSC-HVDC power grids.

At present, the hybrid high-voltage DC circuit breaker combining mechanical circuit breakers and power electronic switches has become a research hotspot. Its on-state loss is low, and at the same time it has a fast breaking speed. The current shutdown time under line faults only needs 3ms , has practical engineering application value.

When a DC line fault occurs, it is necessary to quickly cut off the line fault current. It is required to complete the fast fault detection of the DC line within 0.1ms, and output a protection action signal to complete the opening of the DC circuit breaker. Therefore, the fault detection of the DC circuit breaker line needs to be fast, and at the same time, it needs to be reliable, so as to prevent the fault detection from causing the circuit breaker to be disconnected incorrectly.

For the flexible HVDC system, when a bipolar short-circuit fault occurs at the outlet of the converter valve, the positive and negative bus voltages rapidly drop to about zero at the same time, and the fault current first drops to zero and then rapidly increases in the opposite direction. The Chinese patent document with the publication number CN104820159A discloses a DC bipolar short-circuit fault identification method for a flexible direct current transmission system. The method determines the occurrence of a bipolar circuit breaker fault based on the characteristics of voltage and current changes during the development of a bipolar short-circuit fault. This method needs about 10ms to determine the fault, which obviously cannot meet the requirement of the DC circuit breaker to detect the fault of the DC line less than 0.1ms.

Contents of the invention

The purpose of the present invention is to provide a method for identifying bipolar short-circuit faults of DC lines, which is used to solve the technical problem in the prior art that the identification speed of bipolar short-circuit faults is too slow to meet application requirements.

In order to solve the above technical problems, the present invention provides a method for identifying bipolar short-circuit faults of DC lines, which includes the following steps:

Obtain positive and negative bus voltage, current, voltage change rate and current change rate;

Judging whether the voltage change rate and current change rate of the positive and negative busbars meet the set fault identification and prediction conditions;

If the fault identification and pre-judgment conditions are met, judge whether the positive and negative bus voltage and current meet the set fault identification and judgment conditions;

If the fault identification and judgment conditions are met, it is determined that a double-pole short-circuit fault has occurred in the system.

Further, the pre-judgment condition for fault identification is:

The maximum of the absolute values of the positive and negative bus voltage transformation rates is greater than the DC voltage change rate threshold, or the maximum of the absolute values of the positive and negative bus current change rates is greater than the DC current change rate threshold.

Further, the fault identification and judgment conditions are:

(1) The absolute value of both the positive and negative bus voltages is not greater than the minimum threshold of the DC-to-ground voltage;

(2) The maximum value of the absolute values of the positive and negative bus currents is not less than the maximum DC current threshold or the minimum value of the absolute values of the positive and negative bus currents is not greater than the minimum DC current threshold.

The beneficial effects of the present invention are:

By judging whether the voltage change rate and current change rate of the positive and negative busbars meet the set fault identification and pre-judgment conditions, first pre-judge whether the system has a bipolar short-circuit fault. If the fault identification and pre-judgment conditions are met, the system is likely to A bipolar short-circuit fault occurs; then, combined with the positive and negative bus voltage and current amplitudes, it is accurately judged whether the system actually has a bipolar short-circuit fault. The invention not only can quickly and accurately complete fault identification within 0.1ms, which meets the requirement of rapidity, but also avoids misjudgment, and can meet the requirement of fast and accurate detection of double-pole short-circuit faults of the DC line of the DC circuit breaker.

Description of drawings

Fig. 1 is the flowchart of bipolar short-circuit fault identification method of the present invention;

Fig. 2 is a circuit structure diagram of the flexible direct current transmission system;

Figure 3 is the characteristics and identification results when a bipolar ground fault occurs at the DC outlet of the DC line valve;

Figure 4 shows the characteristics and identification results when a bipolar ground fault occurs at the smoothing reactance of the DC line.

detailed description

The specific implementation manner of the present invention will be described in detail below with reference to the drawings and specific embodiments.

As shown in Figure 1, the identification method of bipolar short-circuit fault of DC line includes the following steps:

Step 1, obtain positive and negative bus voltage, current, voltage change rate and current change rate.

Specifically, optical transformers are used to obtain identification data such as positive and negative DC bus voltages, currents, voltage change rates, and current change rates. In order to ensure the rapidity of data transmission, in this embodiment, the upload data cycle of optical transformers is 20us .

Step 2, judging whether the voltage change rate and current change rate of the positive and negative busbars meet the set fault identification and pre-judgment conditions.

Among them, the pre-judgment conditions for fault identification are:

The maximum of the absolute values of the positive and negative bus voltage transformation rates is greater than the voltage change rate threshold, or the maximum of the absolute values of the positive and negative bus current change rates is greater than the current change rate threshold.

The mathematical expression of the above fault identification and pre-judgment conditions can be expressed as follows:

Among them, U _dcP and U _dcN are DC positive voltage and negative voltage respectively, U _{rate_set} is DC voltage change rate threshold, I _dcP and I _dcN are DC positive current and negative current respectively, and I _{rate_set} is DC current change rate threshold.

Step 3, if the fault identification pre-judgment conditions are met, judge whether the positive and negative bus voltages and currents meet the set fault identification and judgment conditions.

Among them, the fault identification and judgment conditions are:

(1) The maximum value of the absolute values of the positive and negative bus voltages is not greater than the minimum threshold of the DC-to-ground voltage;

(2) The maximum value of the absolute values of the positive and negative bus currents is not less than the maximum DC current threshold or the minimum value of the absolute values of the positive and negative bus currents is not greater than the minimum DC current threshold.

The mathematical expressions of the above fault identification and judgment conditions (1) and (2) can be expressed as follows:

Among them, U _dcP and U _dcN are DC positive voltage and negative voltage respectively, I _dcP and I _dcN are DC positive current and negative current respectively, U _{dcgmin_set} is the minimum threshold of DC voltage to ground, I _{dcgmax_set} is the maximum threshold of DC current, I _{dcgmin_set} is the minimum threshold of DC current.

If the above-mentioned fault identification and judgment conditions (1) and (2) are all satisfied, it is considered that the fault identification and judgment conditions are met.

Step 4, if the fault identification and judgment conditions are met, it is determined that a bipolar short-circuit fault occurs in the system.

It should be noted that the above U _{dcgmin_set} , I _{dcgmax_set} , I _{dcgmin_set} , U _{rate_set} and I _{rate_set} are all configuration parameters. According to the different fault points of the DC line, the fault characteristic parameters of the fault point are obtained through the system simulation method, and combined with each fault point This method is more universal and flexible.

Under the condition of meeting the DC circuit breaker’s requirement that the DC line fault detection and judgment time is less than 0.1ms, in order to further improve the reliability of fault identification, the parameters uploaded by the optical transformer can be identified for 4 consecutive cycles, that is, 80us in a row, and the fault judgment The time is less than 0.1ms.

In order to verify the feasibility of the bipolar short-circuit fault identification method, as shown in Figure 2, a VSC-HVDC flexible direct current transmission system model at both ends is built. The system parameters are set as follows: DC voltage is ±200kV, rated capacity is 400MVA, DC current protection The threshold is 1200A. Configuration parameters DC-to-ground voltage minimum threshold U _{dcgmin_set} = 80kv, DC-to-ground voltage maximum threshold U _{dcgmax_set} = 280kv, DC current minimum threshold I _{dcgmin_set} = 0A, DC current maximum threshold I _{dcgmax_set} = 1200A, DC voltage change rate threshold U _{rate_set} = 9.04kv/us, DC current change rate threshold I _{rate_set} = 2A/us.

In Figure 2, when the bipolar grounding fault occurs at fault point 1 (at the DC outlet on the valve side), the voltage and current changes of the positive and negative buses are shown in Figure 3. The positive and negative bus voltages drop rapidly to about zero at the same time. Since the fault occurs at the outlet of the converter valve, the fault current appears to drop to zero first and then rapidly increase in the opposite direction. Within 0.1ms of the fault, the circuit breaker completes the fast Fault identification and implementation of breaking protection.

In Figure 2, when the bipolar ground fault occurs at fault point 2 (line side of the smoothing reactor), the voltage and current changes of the positive and negative buses are shown in Figure 4. The positive and negative bus voltages drop rapidly to about zero at the same time. Since the fault occurs on the line side of the smoothing reactor, the power flow of the fault station does not reverse before and after the fault, so the fault current increases rapidly. Within 0.1ms when a fault occurs, the circuit breaker quickly completes fault identification and performs section protection.

The present invention starts the fault detection logic based on the change of the positive and negative pole voltage and current of the DC bus, and uses the positive and negative voltage change rate and current change rate of the DC bus as the criterion for fault detection, and compares the fault identification parameters with the corresponding threshold values It can quickly and accurately complete fault identification in 0.1ms, avoid misjudgment, and meet the requirements of rapidity, which can meet the requirements of fast and accurate detection of bipolar short-circuit faults of DC circuit breaker DC lines.

Claims (3)

1. A method for identifying a bipolar short-circuit fault of a DC line, comprising the following steps: Obtain positive and negative bus voltage, current, voltage change rate and current change rate; Judging whether the voltage change rate and current change rate of the positive and negative busbars meet the set fault identification and prediction conditions; If the fault identification and pre-judgment conditions are met, judge whether the positive and negative bus voltage and current meet the set fault identification and judgment conditions; If the fault identification and judgment conditions are met, it is determined that a double-pole short-circuit fault has occurred in the system. 2. The method for identifying a bipolar short-circuit fault of a DC line according to claim 1, wherein the pre-judgment condition for the fault identification is: The maximum of the absolute values of the positive and negative bus voltage transformation rates is greater than the DC voltage change rate threshold, or the maximum of the absolute values of the positive and negative bus current change rates is greater than the DC current change rate threshold. 3. The method for identifying bipolar short-circuit faults of DC lines according to claim 1 or 2, wherein the fault identification criteria are: (1) The absolute value of both the positive and negative bus voltages is not greater than the minimum threshold of the DC-to-ground voltage; (2) The maximum value of the absolute values of the positive and negative bus currents is not less than the maximum DC current threshold or the minimum value of the absolute values of the positive and negative bus currents is not greater than the minimum DC current threshold.