Application of MC68HC11K4 in UPS-EEWORLD

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1 Introduction

During the operation of UPS, it is very important to accurately grasp the values of input and output voltage, output current and output power (active power and apparent power) to ensure the normal operation of UPS and provide timely protection. The values of input voltage and output voltage are one of the important parameters for UPS mains and inverter conversion and STS conversion; the value of output current is one of the important parameters for UPS to judge overload amplitude and short circuit, which is crucial for protecting power devices; the value of output power (active power and apparent power) is an important parameter for UPS to determine load type, output power factor and load degree. The following introduces how MC68HC11K4 calculates the values of voltage, current and power in UPS, as well as the flowchart and annotations of related programs.

2 Basic calculation methods

MC68HC11K4 calculates the input voltage, output voltage, output current and output power (active power and apparent power) through the interrupt subroutine CALRMS_PRC. It uses Time Output Compare2 to select the next interrupt according to the inverter count, and calls CALRMS_PRC to calculate the relevant values. The basic method is as follows:

(1) The input voltage, output voltage, and current are read from the A/D buffer, squared, and then divided by the number of A/Ds and stored in the corresponding TEMP0.

(2) The active power W is also read from the A/D buffer. The current and voltage are multiplied to determine the positive and negative signs, added up, and then divided by the number of A/Ds and stored in WATTV_TEMP0.

(3) In each cycle, calculate the RMS value of the inverter, take the square root of INVV_TEMP0 and filter it to get INVVOLT. Similarly, calculate LINEVOLT every two cycles and CURVOLT every 32 cycles.

(4) Every 32 cycles, multiply CURVOLT and LINEVOLT or INVVOLT to obtain VOLT_AMP.

(5) Every 32 cycles, multiply WATTV_TEMP0 by 2, take the high byte, filter it, and store it in WATTVOLT.

[page]3 Flowchart and annotations of CALRMS_PRC

The flowchart and annotations of CALRMS_PRC are shown in Figures 1, 2 and 3.

Figure 1 Flowchart and annotations 1

Figure 2 Flowchart and annotation 2

Figure 3 Flowchart and annotation 3

[page]4 Advantages of the calculation method

The advantage of the calculation method is that the UPS CPU calculates the RMS value of the mains voltage every 2 cycles, the RMS value of the inverter voltage every 1 cycle, the RMS value of the inverter current every 32 cycles, and the output power every 32 cycles, so as to timely and accurately understand the input voltage, output voltage, output current and output power of the UPS. It not only ensures the normal operation of the UPS, but also responds quickly to abnormalities, so that the UPS can be transferred to the corresponding working state in time, and safely protect the load and the UPS itself. For example:

(1) Input voltage abnormality protection: The CPU can detect instantaneous power failures and calculate the A/D value of the mains power read in the most recent cycle every 4ms. If it is less than 140V, it is considered a power failure and the UPS immediately switches to battery inverter state.
(2) Output voltage abnormality protection: When the inverter output voltage value is lower than 160V or higher than 280V for 80ms continuously, it is considered an inverter output voltage abnormality and the UPS immediately switches to bypass and alarms.
(3) Output current abnormality protection: When the inverter output current value exceeds 3.6 times the rated value, the current limiting protection circuit immediately shuts down the PWM to protect the power device; only when the output current value is less than 3.6 times the rated value will the PWM work again.
(4) Output power protection: If the UPS detects that the load exceeds 110% before switching from bypass to inverter output, the UPS cannot switch to inverter output, and the CPU controls the buzzer to beep every 0.5s. If the load increases to 110% to 130% after startup, the CPU controls the buzzer to beep every 0.5s, and the UPS switches to bypass after 10s. If the load increases to more than 130% after startup, the UPS will immediately switch to bypass.

5 Conclusion

The calculation method is scientific and reasonable, the program structure is rigorous, and the calculated input voltage, output voltage, output current and output power (active power and apparent power) values have a short cycle and are accurate. It can effectively improve the technical level of UPS. I hope we can learn from it. ■

Keywords：68HC11K4 UPS Reference address：Application of MC68HC11K4 in UPS

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