Relationship between wiring and voltage phase sequence of DX8 type three-phase reactive energy meter

0 Overview

When the electricity meter is wired correctly, the measured electrical energy will exactly equal the energy consumed by the load or transmitted through the line. Conversely, if the wiring is incorrect, the measured electrical energy will not match the energy consumed by the load or transmitted through the line, and the degree of miscalculation varies depending on the wiring configuration. Therefore, ensuring correct wiring of the electricity meter, accurately analyzing the wiring configuration, and precisely deriving the mathematical expression for measuring electrical energy under various meter wiring configurations are extremely important.

As we know from basic electrical engineering knowledge, electrical energy is the product of power and time (this article only studies reactive energy), that is...

AQ = Qt where

AQ represents the reactive power consumed by the load, in kvar·h;

Q represents the reactive power of the load, in kvar;

t represents the power consumption time of the load, in hours.

From the above formula, we can determine whether an electricity meter can accurately measure the electrical energy consumed by the load. We need to examine whether the power measured by the electricity meter is equal to the actual power consumed by the load to determine if the meter's wiring is correct. In other words, if the expression for the three-phase reactive power measured by the electricity meter under a certain wiring configuration conforms to the following form, the wiring is correct; otherwise, the wiring is incorrect.

A few years ago, the Daxing Power Station, which was connected to our grid, discovered during electricity meter calculations that the active energy meter was running in the forward direction while the reactive energy meter was running in the reverse direction, and the reactive energy consumption was higher than the actual reactive energy generated by the plant (calculated based on the reverse direction). Upon investigation, it was found that the A and C phases of the reactive energy meter's power supply sequence had been swapped. The analysis based on this is as follows.

1. Correct wiring

The reactive energy meter in this station's metering device is a DX8 type two-element three-phase reactive power meter (60° phase angle difference three-phase reactive energy meter). A resistor is connected in series with the voltage coil inside the meter, causing the current in the voltage coil to lag the voltage by 60°. When the load is inductive, the correct wiring of this meter is shown in Figure 1. Its phasor diagram is shown in Figure 2.

Since the angle between the voltage and magnetic flux in an active energy meter is 90°, while the angle in this meter is 60°, it is equivalent to shifting the voltage forward by 30° compared to a regular active energy meter. The power calculation formulas for each component are as follows.

Power measured by component 1:

As can be seen, when connected according to Figure 1, the power P measured by the two components is exactly equal to the reactive power of the perfectly symmetrical three-phase circuit, and multiplying it by time gives the reactive energy in the three-phase circuit.

2. Incorrect wiring

2.1 In Figure 1, phases A and C are interchanged, while phase B remains unchanged. The current and voltage coils connected to the energy meter remain the same. The phasor diagram is shown in Figure 3. The power measured by the two elements of the energy meter are as follows:

It is evident that this wiring causes the reactive energy meter to reverse, and the measured result is not the reactive energy within the three-phase circuit.

2.2 Interchange the power supplies of phase A and phase B in Figure 1, leaving phase C unchanged. The current and voltage coils connected to the energy meter remain the same. The phasor diagram is shown in Figure 4. The power measured by the two elements of the energy meter are as follows:

2.3 If the power supply of phase B and phase C in Figure 1 is interchanged, while phase A remains unchanged, and the current and voltage coils connected to the energy meter remain unchanged, the measurement results will be the same as in 2.1.

3. Conclusion

The above analysis shows that the wiring of the DX8 type reactive power meter is closely related to the voltage phase sequence. Only by wiring according to Figure 1 can the reactive power in the three-phase circuit be accurately measured. Therefore, the metering device of this power station must verify the phase sequence of the three-phase voltage. Only by confirming that the voltage is in the correct phase sequence can the accuracy of the meter wiring and measurement be guaranteed.