Three-phase transformer connection Y-Y (Wye-wye connection of Potential Transformer)

    Potential Transformer, hereafter referred to as PT in this article, the wiring methods used by three-phase PT can be Y-Y, V-V, △-△. The above wiring methods are mainly to avoid phase angle differences and are the more commonly used wiring methods. It is Y-Y, and the correct proportional voltage value (three-phase voltage) of the primary side voltage can be obtained, and the phase angle can also be accurately proportioned to the secondary side. However, Y-Y does not provide a zero-sequence impedance path. The correct statement should be that it has extremely high zero-sequence impedance (Zero-Sequence Impedance). If the three-phase transformer is a core type, or three single-phase transformers, Its zero-sequence impedance is very high, and the third harmonic cannot circulate normally. It can only form a loop through the space magnetic flux through the outer box of the machine. If the primary side power supply is a sine wave voltage, the transformer core must also appear on the secondary side. The proportional sine wave voltage value, however, the excitation current of the transformer must contain the third harmonic, as shown below:
    Since the third harmonic exists in each phase, and there is no phase angle difference (0) between the third harmonics, a low impedance must be provided to the third harmonic. This low impedance can be a magnetic circuit (four-arm iron core or five-arm iron core). Arm core), or it can be a circuit (transformer Y connection and neutral point grounded). This low impedance must be enough to form a third harmonic circulation loop (Circulation loop), which can be called the zero sequence impedance of the transformer. The following two are summarized: situation:
-CASE I : For high zero-sequence impedance such as Y-Y wiring, the phase voltage contains about 30%~60% of the third harmonic (it varies according to the core saturation curve), and the line voltage does not measure the third harmonic.
-CASE II : Low zero-sequence impedance such as Y-△ wiring, phase voltage and line voltage do not contain third harmonic voltage, but the delta winding (Winding) forms a third harmonic circulating current, and its magnitude value is the third harmonic current of each phase three times.
      "Harmonics" have always played an important role in power quality issues. The power pollution caused by harmonics and the power problems that may be caused have been widely discussed in articles or papers published by many scholars and advanced experts in the past, and will not be discussed in this article. This article only discusses the problems that may be encountered in the application of traditional passive filters.
     According to the above explanation, when measuring voltage, if the PT adopts three-phase Y-Y connection, if the electric meter uses the 3∅4W method, the third harmonic voltage of the phase voltage will be measured, but if the 3∅3W method is used, it will not be measured. third harmonic voltage.

Measured data-1

  For a single-phase three-winding transformer, the Y connection on the primary side is not grounded, the Y connection on the secondary side is not grounded, and the tertiary side is open delta wiring (does not constitute a low zero sequence impedance). The power supply is 3phase 3Wire 220V (L-L). The wiring diagram is as follows :

The power supply voltage is 220V sine wave, three-phase three-wire non-grounded system,
● The neutral point on the primary side of the transformer is not grounded, and the neutral point on the secondary side is not grounded. The tertiary side is open delta, so both the primary side phase voltage and the secondary side phase voltage contain third harmonics. However, the primary side and secondary side lines The voltage has no third harmonic.
● The neutral point on the primary side of the transformer is not grounded, and the neutral point on the secondary side is not grounded. The tertiary side is changed to delta connection (Delta). In this way, there is no third harmonic in the primary side phase voltage and the secondary side phase voltage. , and at the same time, the line voltage has no third harmonic.
● When the tertiary side is used as a delta connection, the polarity must be paid attention to. If there is any reverse connection, a huge short-circuit current will occur. Usually, the open circuit voltage can be measured before closing. The basic wave voltage should be 0V for the correct connection. When the primary When the side is connected in star connection, it will be a little troublesome to check the open circuit voltage, because the sum of the third harmonic voltage of each phase (3xUo) appears in the open circuit voltage. You must pay attention to this situation. If you have a harmonic analyzer, you can easily interpret the open circuit voltage. Is it correct.
● PT usually does not have a third winding. Therefore, if the electric meter uses a 4-wire measurement, the phase voltage can measure the third harmonic, but the line voltage does not. Therefore, the voltage harmonic distortion is relatively high. The third harmonic of the phase voltage is The third harmonic voltage value caused by the PT excitation current depends on the saturation point of the core hysteresis loop. Setting the meter to three-wire measurement can correctly display the harmonic voltage value.

Measured data-2

  The primary side phase voltage of the transformer contains 30% of the third harmonic voltage. The basic 60Hz voltage is 134V, 132V, 132V. The third harmonic (Hz) voltage is 42.9V, 39.3V, 41.5V respectively. The voltage waveform and its corresponding harmonic spectrum The picture is as follows :

Measured data-3

  The secondary side phase voltage of the transformer also contains 30% of the third harmonic. The fundamental wave 60Hz voltage is 7.27V, 7.15V, 7.19V. The third harmonic (Hz) voltage is 2.38V, 2.14V, 2.24V respectively. The voltage waveforms correspond to them. The harmonic spectrum diagram is as follows :

Measured data-4

  At the same time, the open circuit voltage of the third side open triangle is measured. The fundamental wave is 0.2V and the third harmonic is 6.80V. AV= Phase L1 (add phase L1 waveform as frequency comparison) DV=open delta voltage


 

Measured data-5

  If the tertiary winding is connected into a delta connection, a low zero sequence impedance is formed, and the transformer wiring forms Y-Y-△. The third harmonic current forms a loop in the tertiary winding. At this time, the secondary side phase voltage is measured and the voltage of each phase of the three phases is found. The third harmonic has dropped to a very low level, and the total harmonic distortion rate (THDv) has also dropped a lot.

Measured data-6

  Observing the current on the tertiary side (Delta), only the third harmonic current circulates in the winding. This current also has the function of stabilizing the voltage under unbalanced conditions, so the third winding is usually also called stabilized winding.