Several problems of safety automatic device in the

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Several problems of safety automatic device in substation automation system

1.1 summary description

synchronous closing is a common operation in substation, which plays an important role in reducing impact and improving system stability. There are three conditions for the same period: frequency difference, pressure difference and angular difference are qualified

the synchronization requirements are safe, accurate and fast. The treatment of building leakage by the national construction authority lasted for more than 20 years. Among the three conditions, safety is the most important. The synchronous device must have a perfect locking function, and it is better to refuse to operate without misoperation. For differential frequency synchronization, when the system angle difference is 0, the impact on the system is the smallest; As a generator in the power plant, the rapidity is also very important. Capturing the first 0-angle closing can save a lot of energy. Due to the particularity of the fixture structure, sometimes it is difficult to be sure which kind of fixture is more suitable for the sample

1.2 ring paralleling and differential frequency synchronization

differential frequency synchronization refers to the paralleling of two systems without electrical connection, including the paralleling of generators and the paralleling of two unconnected electricity; Since the frequencies on both sides are different, it is possible to capture the closing time of 0 angle. Ring paralleling refers to two systems that have already been electrically connected, and then an interconnection switch is added at this point; The frequency on both sides is the same, and the phase angle difference is the power angle of the system between these two points. This angle basically remains unchanged when the network topology and load do not change greatly

in China, some call it check synchronization and capture synchronization, some call it check synchronization and quasi synchronization, and some call it same frequency synchronization and difference frequency synchronization. If the frequency difference between the two systems is within the measurement error range, it is the same frequency, but it cannot be synchronized according to the same time. In order to clarify the physical concept, this paper defines these two methods as ring juxtaposition and differential frequency synchronization

the goal of differential frequency synchronization is to capture the first zero phase angle difference opportunity to switch on, that is, automatic quasi synchronization; The ring parallel phase angle difference is the power angle at both ends, which is only a locking function of pressure difference and power angle

1.3 synchronous remote control mode and adaptive recognition

the requirements of ring juxtaposition and differential frequency synchronization are different. Although the device can adaptively judge whether it is the same frequency or the different frequency, for the system with small frequency difference, doing so means sacrificing some time to judge, which will delay the closing time. The dispatcher understands the operation structure of the system and knows whether the circuit breaker to be closed is in the same frequency or different frequency synchronization position. It will be better to distinguish the same frequency synchronization, different frequency synchronization and remote closing command when sending the command. The automatic identification function of the device means that after the closing command is issued, it automatically determines whether it is in the state of differential frequency, same frequency or no voltage, and operates under different constraints

1.4 calculation of time before switching on

the action time from the outlet of the device to the closing of the circuit breaker is directly related to the accuracy that the two ends of the synchronous point angle sample are clamped at the same time. The conventional method is to calculate the time from sending the closing order to the signal displacement through the way of input, that is, by connecting the auxiliary contact of the circuit breaker. This method is direct and easy to implement; However, the problem is that when the circuit breaker closes the current, the time difference inconsistent with the auxiliary contact displacement will introduce errors. In addition, contact jitter will also affect the accuracy

this paper presents a method to detect the lead time with analog quantity, that is, the detection of current from scratch. If the sampling rate of the sampling device can reach 64 points/cycle (DF1700 module sampling rate), the time resolution is about 0.3 milliseconds, which can meet the requirements. This method requires the introduction of current detection. The distributed synchronization system generally integrates the synchronization function into the measurement and control unit of the circuit breaker, which can meet this requirement. The physical concept of this method is clearer: only when it changes from no flow to current (rather than auxiliary contact displacement), can it be considered as a real success of closing

1.5 synchronization algorithm

synchronization is an operation with high reliability requirements. The large angle closing in case of maloperation will bring great impact to the generator and system, reduce the service life of the generator, or bring system oscillation and disconnection. The delay of the first best period of the same period should also be prevented as much as possible. Therefore, high reliability, high precision, multi-level locking and fast control algorithms and measures must be considered

considering the reliability of the device, it is a good idea for some manufacturers to adopt the mode of dual microcomputer control. Other methods on hardware can also be used. Algorithmic multiplicity and locking are also important

there are generally two calculation methods, one is the method of hardware shaping pulse phase comparison, and the other is the method of comparing amplitude and phase through sampling points. The two methods have their own advantages and disadvantages, and mutual cooperation can produce perfect and stable results

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