A digital single-phase sine wave variable frequency power supply based on MSP430F2002

1 Principle of Through-Casing Resistivity Logging

Formation resistivity is an essential factor in evaluating the hydrocarbon content of a reservoir. Formation resistivity mainly depends on the liquid it contains. The resistivity of a formation containing conductive brine is much lower than that of a formation filled with hydrocarbons. Therefore, resistivity measurement has irreplaceable engineering value for locating hydrocarbon strata. Traditional resistivity measurement is carried out in open hole wells. If the oil well is penetrated by metal casing, the resistivity of the metal casing is extremely small compared to the formation resistivity (the resistivity of the formation is between 1Ω·m and 1000Ω·m, while the typical value of the resistivity of the metal casing is 2×10-7Ω·m). Therefore, traditional resistivity logging instruments cannot measure formation resistivity, and it is necessary to develop through-casing formation resistivity logging instruments. The instrument measures the formation resistivity by measuring the tiny voltage drop on the casing. The principle of through-casing resistivity logging is to measure the formation resistivity by measuring the voltage drop on the casing in the casing. The measurement principle is shown in Figure 1. If current is injected into the casing, most of the current will flow up or down along the casing, and only a small part of the current will leak into the surrounding formation. If we can estimate the magnitude of the leakage current △I within the length range of △z, we can calculate the resistivity of the formation, as follows:

In order to obtain the formation resistivity, it is required to provide a single-phase AC variable frequency excitation power supply with a voltage range of 0 to 30V, an adjustable frequency between 1 and 70Hz, and a maximum load current of 1A. The purpose of this design is to use the MSP430F single-chip microcomputer to realize a low-power, low-cost, low-distortion, high-reliability single-phase AC variable frequency excitation power supply that is easy to power with batteries and carry on site, so as to meet the urgent needs of the site, which is of great significance for the promotion and application of the through-casing resistivity logging method on site.

2 Schematic diagram and performance indicators of the design

In summary, the principle block diagram of the design is shown in Figure 2.

The circuit diagram constructed in the PSIM simulation environment according to the principle block diagram shown in Figure 2 and its simulation results are shown in Figures 3 and 4. The simulation results show that the principle block diagram is feasible and has guiding significance for design. [page]

The performance indicators of this design are:

System input:
Rated voltage: 36VDC (battery)
System output:
(1) Output voltage: 0~30V
(2) Output current: IRMS≥1A
(3) Output frequency: 1~70Hz
(4) Frequency accuracy: ±1%
(5) Harmonic distortion: ≤3%
(6) Efficiency: ≥85%
Working environment: Temperature 0℃~65℃, humidity ≤90% (25℃)