This paper proposes a frequency measurement method based on TMS320F28335 for monitoring the power quality of power systems. The method uses the DSP's eCAP module and a general-purpose timer to capture the rising edge of the input signal, and obtains the frequency of the input signal by recording the trigger time of the two rising edges. Compared with the software frequency measurement method, the hardware circuit is simple, the reliability is high, and the real-time performance is good. Theoretical analysis and experimental tests show that the method has high frequency measurement accuracy and satisfies the requirements of power quality monitoring devices.
introduction:
Frequency is an important indicator to measure power quality, and it is also an important basis for judging power system failure. In general, the frequency of the power system will vary with load fluctuations. Under normal conditions, the frequency of the grid changes slowly. Even in the event of a system accident, the amount of change in the grid frequency is small in a short period of time (such as a power frequency cycle). Frequency measurement If the grid frequency can be measured continuously in real time, the measured frequency error can be reduced to a small extent.
The digital frequency measurement methods mainly include: (1) measuring the time of a certain integer cycle of the voltage waveform to calculate the frequency; (2) estimating the frequency using waveform recognition or curve fitting techniques. The latter method can not suppress the harmonic components very well, and the calculation amount is too large. It is necessary to calculate each cycle once, which will take up too much processor time, which cannot balance the calculation accuracy and real-time.
The measurement accuracy of the former is greatly affected by the voltage zero crossing.
This paper proposes to use the zero-crossing detection circuit to shape the fundamental wave of the power grid into a square wave. The frequency measurement method of capturing the rising edge of the square wave with the capture module of TMS320F28335 (DSP) suppresses the influence of voltage zero-crossing to a certain extent. Good measurement accuracy and real-time.
1 general purpose timer and capture module
The TMS320F28335 has an instruction cycle of 6.67 ns. The main frequency is up to 150 MHz; the high-performance 32-bit CPU, single-precision floating-point arithmetic unit (FPU), uses the Harvard pipeline structure to quickly perform interrupt response. And has a unified memory management mode. The frequency measurement method proposed in this paper mainly uses the capture unit (eCAP) and general-purpose timer (GPT) unit in TMS320F28335.
1. 1 general purpose timer
The general-purpose timer is a commonly used PIE interface of the TMS320F28335. Its core is the counter, 32-bit count. General-purpose timers have multiple modes of operation to meet different needs. Each timer can work independently or in sync with each other. The register can be set in advance to implement the corresponding function.
The global general-purpose timer control registers GPTCON A (in EVA) and GPT CONB (in EVB) specify the actions taken by the general-purpose timers in different events and specify their counting direction. In order to perform the functions required for frequency measurement, it is necessary to set the GPT count register T xCN T, the timer compare register Tx CMPR, the timer period register Tx PR, and the timer control register T xCON ( x = 1, 2, 3, 4). .
1. 2 capture module
The eCAP module is a complete capture channel that enables multiple time capture tasks. The eCAP unit structure is shown in Figure 1.
Figure 1 capture unit structure
The TMS320F28335 has six capture units, divided into two groups, each with a dedicated capture input pin that reacts to changes in the input pin and captures the time at which the level change occurred. When the pin level changes, the trigger event will be triggered: The count value of the specified general-purpose timer is pressed to the two-stage FIFO of the capture unit, and the capture interrupt request is triggered when the number of data of the FIFO is greater than or equal to 2. The interrupt response can be used to calculate the frequency and its corresponding operation.
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