JP7615012B2 - Transformer Control Device - Google Patents

Description

The present invention relates to a control device for a transformer that supplies power to a load, and in particular to a control device for a transformer that handles voltage imbalances.

In general, an on-site power distribution system receives power from a higher-level power source via a transformer, and supplies electricity to various loads such as electric motors through circuits that branch out like a tree. In such on-site power distribution systems, if a voltage imbalance occurs due to a failure in a transformer or a failure in the higher-level power source system, this voltage imbalance mainly causes problems for electric motors, and even an imbalance of a few percent can have a thermal effect on the motors, leading to burnout and a shortened lifespan.

Therefore, in current on-site power distribution systems, a relay that detects voltage imbalance is installed within the station to deal with this problem. However, when using a relay to deal with this problem, a relay that detects voltage imbalance is generally installed at each load, and when there is a voltage imbalance, the power supply circuit breaker to the load is opened, making it difficult for that load to continue operating. When there is an imbalance in the upstream power supply itself, the upstream circuit breaker of the transformer is opened when the relay is activated, and the power supply is isolated to provide protection, making it difficult for all loads connected downstream of the transformer to continue operating.

Motors are generally designed on the assumption that the power supply is three-phase balanced. If the three-phase voltage becomes unbalanced due to external factors, reverse-phase current may flow through the windings, causing overheating damage. For this reason, motor design standards (such as the NEMA standard) stipulate that the voltage unbalance rate of the motor's power supply must be 1% or less, and since motors are vulnerable to unbalance events, appropriate protection is required.

On the other hand, in recent years, there has been discussion about the effects of motor burnout and other problems caused by imbalances in the power supply within a station due to a single-phase break in an overhead transmission line, which occurred in the United States. This has been addressed both in Japan and overseas by installing a dedicated relay on the primary side of the transformer. However, even if this relay detects an abnormality, the final response is to cut off the power supply, making it difficult to continue operating the load.

Patent Document 1 discloses a detector installed on the primary side of a transformer that detects a single-phase break and outputs an alarm. When this detector detects a fault, it disconnects the power supply system, causing a power outage in all power supply systems that receive power from that power source.

Patent No. 6927892

If an unbalanced voltage occurs on the secondary side of the transformer due to an internal fault in the transformer or a fault in the upper power supply, an unbalanced voltage will be applied to all motors connected to the secondary side of the transformer, causing a negative-phase current to flow.

To prevent this, it is necessary to monitor the imbalance in the upper power supply voltage and disconnect the power supply if an abnormality occurs. However, if the power supply is disconnected, the load connected to the power panel will suffer a power outage, making continued operation difficult.

It is also possible to switch to a different power system, but this would require the existence of a different power system in the first place, and additional equipment such as circuit breakers and electrical circuits to receive power from a different power source would be required. In addition, the operation of the power switchover would also need to be considered.

The present invention was made in consideration of the above-mentioned problems, and aims to provide a transformer control device that can continue to operate even when the power supply voltage is unbalanced.

In order to solve the above problems, the present invention provides a control device for a transformer whose secondary side is delta-connected, in which each phase of the secondary side of the transformer is formed by a series circuit of a secondary winding and a circuit breaker, and the control device detects an imbalance in the secondary side voltage, opens the circuit breaker of the series circuit, and continues operation with two windings.

The present invention provides a transformer control device that can continue to operate even when the power supply voltage is unbalanced.

According to an embodiment of the present invention, when a power supply is unbalanced, the winding that is causing the unbalance is disconnected, thereby reducing the impact of the unbalance on the motor and allowing the motor to continue operating as a symmetrical three-phase power supply.

1 is a diagram showing an example of the overall configuration of a transformer control device according to an embodiment of the present invention; 13 is a diagram showing an example of a process flow of a transformer control device. FIG. 13 is a diagram showing an example of an analysis showing the transformer secondary voltage before and after the fault winding is isolated. 13 is a diagram showing an example of analysis showing an unbalance detection signal before and after disconnection of an abnormal winding.

The following describes an embodiment of the present invention with reference to the drawings. Note that in each drawing, the same components are given the same reference numerals, and duplicate descriptions will be omitted as appropriate.

The transformer according to the present invention can be applied to a transformer installed in a power transmission and distribution system, but the following examples show an application to an on-site power supply circuit in a power generation plant, general factory, etc. Note that, in addition to the on-site power distribution system, it may be, for example, a substation or a pole-mounted transformer, but it is a three-phase transformer with a delta connection on the secondary side (secondary side in this case includes the tertiary and quaternary sides).

In the power distribution system shown below, power is received from an upstream power source and then stepped down by a transformer to the voltage required by each load (motor). Since power generation plants generally have ungrounded or high-resistance grounded systems, delta winding transformers are used for both the primary and secondary windings to prevent the inflow of harmonics.

An example of the overall configuration of a transformer control device according to this embodiment is shown in Figure 1. A three-phase transformer Tr for stepping down the voltage of a higher-level power supply is configured to include a primary winding 101 on the power supply side, a secondary winding 102 on the load side, and a circuit breaker 105 for each phase connected in series to the secondary winding 102 of each phase, and at least the secondary winding 102 on the load side is in a delta connection with a series circuit of the secondary winding and the circuit breaker. In this configuration, each phase on the secondary side of the transformer is formed by a series circuit of the secondary winding 102 and the circuit breaker 105.

In addition, for the protection and control of the transformer Tr, it is equipped with a measuring instrument 103 that measures the secondary voltage, and a control device 104 that determines whether or not there is a voltage imbalance from the signal from the measuring instrument 103. Of these, the measuring instrument 103 measures the line voltage of each phase on the secondary side of the transformer. The line voltage detected by the measuring instrument 103 is sent to the control device 104 and processed according to the processing flow in Figure 2.

In the process within the control device 104 shown in FIG. 2, first, in process step S201, the detected line voltage is input, and in process step S202, the unbalance rate of each phase is calculated in an unbalance rate calculation circuit based on the main line voltage. The unbalance rate can be calculated, for example, based on the NEMA standard, by dividing the absolute value of (maximum value of each line voltage - average value) by the average value, but since the calculation method for the voltage unbalance rate varies depending on the standard, it is best to apply a calculation method that corresponds to the applicable standard.

Next, in processing step S203, if the calculated unbalance rate is equal to or greater than a threshold value (for example, 1%) that has been previously evaluated based on the motor's tolerance, it is determined to be an abnormality. Note that since the unbalance rate may exceed the reference value for a short period of time and then immediately return to normal, it is best to determine that an abnormality exists if it occurs continuously for a long period of time that is longer than the set time. In this case, an abnormality such as a short circuit in the transformer winding is suspected.

If no abnormality is found in processing step S203, the process returns to processing step S202 and executes the loop to calculate the unbalance rate again, but if an abnormality is determined, the process moves to processing step S204 and the winding causing the abnormality is disconnected.

The winding causing the abnormality can be determined by, for example, calculating the following for the three phases U, V, and W: (maximum value of U-V phase (Vu-Vv) - average value of three-phase line voltage) ÷ average value of three-phase line voltage, (maximum value of V-W phase (Vv-Vw) - average value of three-phase line voltage) ÷ average value of three-phase line voltage, and (maximum value of W-U phase (Vw-Vu) - average value of three-phase line voltage) ÷ average value of three-phase line voltage.

If we assume that Vu-Vv = Vv-Vw = 1.0 p.u. and Vw-Vu = 0.9 p.u., the average value of the three-phase line voltage is 0.97 p.u., and the absolute value of (maximum value of U-V phase (Vu-Vv) - average value of three-phase line voltage) ÷ average value of three-phase line voltage = 0.03, the absolute value of (maximum value of V-W phase (Vv-Vw) - average value of three-phase line voltage) ÷ average value of three-phase line voltage = 0.03, and the absolute value of (maximum value of W-U phase (Vw-Vu) - average value of three-phase line voltage) ÷ average value of three-phase line voltage = 0.07. As a result, if the threshold is set to 0.01 (1%), it is determined that there is a voltage imbalance, and the W-U phase, which has the largest imbalance rate, is determined to be the abnormal phase.

After disconnecting the winding causing the abnormality in process step S204, the process proceeds to process step S205, where the remaining two phases of the transformer Tr continue to operate as a V-connection, thereby continuing operation as a symmetrical three-phase power supply. As a result, the unbalance rate calculated by the above formula also becomes 0%.

On the other hand, if the determination result in processing step S205 is that the abnormality continues even after disconnecting the abnormal winding, there is a possibility of a multi-phase fault, so in processing step S206, the power supply itself is disconnected (all circuit breakers 105 on the secondary side are opened).

After disconnecting the winding causing the abnormality in process step S204, or after disconnecting the power supply itself in process step S206, the winding causing the abnormality is replaced or repaired. Then, by closing the circuit breaker 105 for the disconnected winding, the three-phase delta connection is restored, and normal three-phase operation can be resumed.

Figure 3 shows an example of an analysis of voltage waveforms before and after disconnecting an abnormal winding by a transformer control device. Before disconnecting the abnormal winding, the voltage dropped in only one phase, causing a voltage imbalance, but by disconnecting the winding causing the abnormality with circuit breaker 105, it can be seen that a symmetrical three-phase voltage is produced by the remaining two windings.

Figure 4 shows the unbalance signal that is the output of the control device 104 during the analysis in Figure 3, and outputs "1" if the unbalance rate is equal to or greater than a preset threshold, and "0" if the unbalance rate is less than the threshold. Before disconnecting the abnormal winding, "1" is output, indicating an unbalance rate equal to or greater than the threshold, but after disconnecting the abnormal winding, "0" is output, indicating an unbalance rate less than the threshold, and it can be seen that the unbalance rate of the power supply has been improved.

The present invention is not limited to the above-described embodiment, but includes various modified examples. For example, the above-described embodiment has been described in detail to easily explain the present invention, and is not necessarily limited to an embodiment having all of the configurations described. Since the load affected by voltage imbalance is mainly an electric motor, the present invention focuses on electric motors, but is effective not only for electric motors but also for all loads affected by voltage imbalance.

In addition, after the abnormal phase is disconnected, power is supplied from the remaining two windings, so the available power supply capacity is reduced. Therefore, if the capacity required for each winding after disconnecting the abnormal phase exceeds the equipment capacity, it is possible to add a control mode that does not disconnect the abnormal phase in order to prevent damage to the transformer.

In this case, the control device will compare the equipment capacity on the load side with the power supply capacity that can be supplied by two windings to determine whether or not to perform control that does not disconnect the abnormal phase. In this case, control that does not perform control that does not disconnect the abnormal phase means that three-phase operation will continue while leaving the voltage unbalanced state as it is.

From the viewpoint of load protection against voltage imbalance, it is preferable to supply power using a V-connection, while from the viewpoint of preventing transformer damage due to exceeding the equipment capacity, it is preferable to continue three-phase operation. Therefore, depending on the degree of voltage imbalance or exceeding the equipment capacity, it is possible to take the more acceptable countermeasure, or if both are severe, it is preferable to stop operation (three-phase opening) in order to satisfy both.

In the above example, it is assumed that the abnormal phase itself is identified by the control logic, but for a simpler solution, it is also possible to only calculate the three-phase unbalance rate and open the circuit breakers one phase at a time in sequence to improve the unbalance rate.

In this case, when an unbalance is detected by calculating the three-phase unbalance rate, the circuit breaker of a given phase (for example, U-phase) is first opened, and if the unbalance rate improves as a result, it can be determined that the abnormal winding was the U-phase, and operation can continue with the V-connection.

If the unbalance rate is not improved, it can be determined that the abnormal winding was not U-phase, so after re-closing the U-phase circuit breaker, the circuit breaker for any other phase (for example, V-phase) is then opened. If the unbalance rate is improved as a result, it can be determined that the abnormal winding was V-phase, so operation can continue with the V-connection.

If the unbalance rate has not improved even after this, it can be determined that the abnormal winding was not U-phase or V-phase, so the circuit breaker for V-phase is closed again, and then the circuit breaker for W-phase is opened, and operation continues in this V-connection mode. Note that if the unbalance rate does not improve even after opening each of the three phases, the control device will determine that a multi-phase fault has occurred.

The steps up to this point should improve the unbalance rate, but if this does not work, processing step S206 will be executed to disconnect the power supply itself (open all circuit breakers 105 on the secondary side).

101: Primary winding 102: Secondary winding 103: Voltage measuring instrument 104: Control device 105: Circuit breaker Tr: Transformer

Claims (6)

A control device for a transformer having a delta-connected secondary side,
The transformer forms each phase on the secondary side by a series circuit of the secondary winding and the circuit breaker,
A transformer control device characterized in that the control device detects an imbalance in the secondary voltage, opens the breaker of the series circuit, and continues operation using two windings. The control device for a transformer according to claim 1,
A transformer control device characterized in that the control device determines which phase is causing an unbalance and controls the opening of the circuit breaker for that phase. The control device for a transformer according to claim 1,
A transformer control device characterized in that the control device detects an unbalance and controls the opening of a first phase circuit breaker, and continues operation as is depending on the unbalance after opening, or controls the opening of a second phase circuit breaker after the first phase circuit breaker is reclosed, and continues operation as is depending on the unbalance after opening, or controls the opening of a third phase circuit breaker after the second phase circuit breaker is reclosed. The transformer control device according to any one of claims 1 to 3,
A control device for a transformer, characterized in that the control device determines whether or not to open a circuit breaker by comparing the equipment capacity on the secondary side of the transformer with the power supply capacity that can be supplied by two windings. The control device for a transformer according to claim 3,
A transformer control device characterized in that, when the unbalance rate is not improved even after opening each of the three phases, the control device determines that a multi-phase fault has occurred. The control device for a transformer according to claim 1,
A transformer control device characterized in that the control device has the function of selecting and controlling either continuing operation with two windings, continuing operation with three windings, or opening all three phases when it detects an imbalance in the secondary voltage.

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