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How to solve the inverter undervoltage fault

Date: 2020-01-07 00:51
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Abstract: The drive (VFD) undervoltage protection of ordinary low-HP drives is based on the measured voltage on the DC bus of the drive unit rather than the input AC voltage. It is important to be aware of this fact to help troubleshoot. Any event that may cause the DC link voltage to drop may cause a DC link undervoltage fault event. This guide will help you troubleshoot VFD issues with undervoltage fault codes. Under idle (not running) conditions, the ideal DC bus voltage for a 3-phase VFD should be approximately the square root of 2 times the AC rms voltage. Therefore, for a 480V, 3-phase system, the expected idle DC bus voltage should be approximately 678V. For a 415V three-phase system driver, the idle DC bus voltage should be about 586V. The actual value of the DC bus voltage depends on many parameters, including: AC input peak ...

The drive (VFD) undervoltage protection of ordinary low-HP drives is based on the measured voltage on the DC bus of the drive unit rather than the input AC voltage. It is important to be aware of this fact to help troubleshoot. Any event that may cause the DC link voltage to drop may cause a DC link undervoltage fault event. This guide will help you troubleshoot VFD issues with undervoltage fault codes.

3相VFD 的理想直流母线电压应约为2的平方根乘以交流有效值电压。 Under idle (not running) conditions, the ideal DC bus voltage for a 3-phase VFD should be approximately the square root of 2 times the AC rms voltage. Therefore, for a 480V, 3-phase system, the expected idle DC bus voltage should be approximately 678V. For a 415V three-phase system driver, the idle DC bus voltage should be about 586V. The actual value of the DC bus voltage depends on many parameters, including:

加载到驱动器上 交流线路电抗器的存在 DC扼流圈的存在 存在隔离变压器 由于上述因素,正常情况下的直流母线电压变化很大。 The AC input peak voltage is loaded on the driver. The presence of AC line reactors. The presence of DC chokes. The existence of isolation transformers. Due to the above factors, the DC bus voltage under normal conditions varies greatly. 是: Typical nominal values are:

415V系统:560 VDC 480V系统:648 VDC 600V系统:810 VDC 测量直流母线电压需要了解预期的直流电压幅度和额定承受该电压的设备的可用性。 230V system: 310 VDC 415V system: 560 VDC 480V system: 648 VDC 600V system: 810 VDC Measuring the DC bus voltage requires knowledge of the expected DC voltage amplitude and the availability of equipment rated to withstand that voltage. Most drivers will provide the magnitude of the DC bus voltage on the driver display, or it can be measured at designated terminals on the driver.

The undervoltage threshold that triggers a fault varies by manufacturer. The following are the undervoltage fault thresholds for a few manufacturers:

VFD undervoltage trip threshold

Comparing the undervoltage trip threshold with the nominal DC bus voltage, it can be seen that the undervoltage trip setting for most drivers is approximately 60% of the nominal DC bus voltage.The corresponding approximate AC voltage at which an undervoltage fault occurs can be calculated by dividing the above value by 1.35 .

Typical cause of DC bus undervoltage fault on VFD

瞬间电压骤降(下降) 失去一个阶段 预充电电路存在问题 VFD直流电压检测电路故障 以下讨论这些中的每一个。 Instantaneous voltage dips (drops) at steady state under-voltage . Loss of phase. Precharge circuit has problems. VFD DC voltage detection circuit failure . Each of these is discussed below.

VFD schematic with precharge circuit

:在这种情况下,设施总线上的电压小于标称值。 1. Steady state undervoltage : In this case, the voltage on the facility bus is less than the nominal value. When the VFD input voltage is low, the DC bus voltage will drop accordingly. Ripple on the DC bus will also increase. If the DC voltage is below the threshold, the drive will fail in the event of undervoltage.

:稳态欠压的解决方案是联系供电公司,检查是否有可能改变供电变压器分接位置或调节稳压器Solution : The solution for steady-state undervoltage is to contact the power supply company to check whether it is possible to change the tap position of the power transformer or adjust the voltage regulator . 以增加电压的可能性。 If the facility has a step-down transformer, investigate the possibility of changing transformer taps to increase voltage. 之前,建议进行额外的检查。 Before adding any power factor correction capacitors to increase the system voltage , additional checks are recommended. When the system has a large non-linear load (VFD, etc.), the capacitor can interact with the harmonics generated by the driver to generate harmonic resonance problems. Resonance can cause capacitors and / or VFDs to malfunction.In these cases, detuned capacitors are recommended .

驱动电压设置参数可以更改为该值。 If the steady-state voltage of the driver is slightly lower than the nominal voltage, the driving voltage setting parameter can be changed to this value. For example, if the input voltage is 460V instead of 480V, setting the drive voltage parameter to 480V will cause the undervoltage trip threshold to be higher than the threshold set to 460V. This is because the undervoltage trip setting is based on the programmed input voltage.

:根据IEEE Std 1159定义,电压下降或电压骤降是持续时间大于周期且小于1分钟的均方根电压的变化,保留电压在标称值的10%和90%之间。 2. Instantaneous voltage dip (drop) : According to the definition of IEEE Std 1159, a voltage drop or voltage dip is a change in the root mean square voltage that lasts longer than the period and is less than 1 minute. %between. A momentary voltage drop may be detrimental to the operation of the drive. Voltage sags can cause several problems. These can be divided into two categories:

Main impact

次要影响 Minor impact of undervoltage fault

VFD整流二极管/电容器故障 主要影响 :瞬间电压骤降可能导致直流母线电压变低并达到故障阈值并触发驱动器关闭。 The main effect of a fuse / breaker trip VFD rectifier diode / capacitor failure : an instantaneous voltage drop may cause the DC bus voltage to fall to a fault threshold and trigger the drive to shut down.

:根据VFD的负载,瞬间电压下降可能导致直流母线电压低于故障阈值。 Minor impact : Depending on the load of the VFD, a momentary voltage drop may cause the DC bus voltage to fall below the fault threshold. During a voltage sag (descent), the motor's energy comes from the DC bus capacitors, causing it to drain faster than normal. When the voltage is restored (when the voltage drop ends), a large current inrush occurs on the AC input side to charge the drive bus capacitor. Depending on the size of the driver, this current peak may be 5-20 times the normal peak current of the driver. This large current surge may cause the fuses or circuit breakers of the drive to trip. In addition, inrush current can also damage the diode rectifier circuit and capacitors, especially if the driver is near a large kVA source transformer.

Solutions for voltage sags:

System-level voltage sags: Use of sag mitigation techniques can reduce facility-level voltage sags. Some of them are:

静止无功补偿器[SVC] 防护变压器 VFD的电压下降解决方案:许多现代VFD都能够通过功能执行某种下陷 Dynamic Voltage Restorer [DVR] Static Reactive Compensator [SVC] Protects the voltage drop solution of the transformer VFD: Many modern VFDs are able to perform some kind of sag by function . This means that the drive is capable of degrading performance with a voltage dip for a short duration. These are manufacturer-specific and it is recommended to consult the appropriate drive data sheet for more information. Some common options for modern drives include:

At the beginning of the voltage dip, the driver will continue to operate from the energy stored in the DC bus until the DC voltage drops to a preset value. This allows the drive to power the motor during a brief voltage dip event. If the droop persists and the DC bus voltage drops to a predetermined threshold, the driver will trip on undervoltage. At the beginning of the voltage dip, the drive will stop powering the motor and will attempt to maintain the DC bus voltage as long as the motor inertia is available. By doing so, the VFD can remain online without tripping, although the motor will begin to slow down. If the drive has an undervoltage fault delay setting, it can be used to delay the occurrence of a fault. Solutions to mitigate the secondary impact of voltage sags on VFDs:

Prevent fuse operation, diode failure: Input fuse or circuit breaker operation after a voltage dip (drop) is caused by the current drawn by a discharged DC capacitor. By applying the following methods, the magnitude of the inrush current can be reduced, thereby reducing the possibility of fuse operation and diode failure:

直流扼流圈 隔离变压器 上面列出的所有组件都会在驱动器的电源侧增加额外的阻抗,从而降低峰值充电电流的大小。 All of the components listed above for a line reactor DC choke isolation transformer add extra impedance to the drive's power supply side, reducing the peak charge current. Reducing the peak charge current after a voltage drop will help prevent fuse operation and diode failure or even premature damage to the capacitor.

:如果在风暴过程中电力设施的一个阶段已经打开或者由于熔断器操作或某些其他原因导致设施单相,则可能出现这种情况。 3. One-phase loss : This situation may occur if a phase of the power facility has been turned on during the storm or the facility is single-phase due to fuse operation or some other reason. When the driver inputs the input voltage, it will detect an input phase failure or undervoltage fault according to the design.

If the driver that lost one phase continues to operate (by cancelling the phase loss parameter setting or other means), the driver will draw a larger peak current in the other two healthy phases. This will also increase the ripple voltage on the DC bus and may cause the rectifier diode or capacitor on the drive to fail. It is not recommended to turn off the phase loss parameter.

:没有什么可以做的,以防止电力线路上的单相。 Solution : There is nothing you can do to prevent single phase on the power line. 设备上安装缺相继电器,并且分流主断路器。 However, the facility can install phase-loss relays on incoming service switchgear and shunt main circuit breakers. The drive phase loss parameter needs to be set to ON to protect the drive from damage in the single-phase state.

4. Problems with precharge circuit:

On many modern drives (not all drives have this feature), there is a pre-charge relay that charges the DC bus capacitor through a series resistor. See the figure at the beginning of this article. This circuit is used to prevent a large amount of current from flowing into the discharge DC bus when the driver is initially turned on. After a preset time delay, at this time the DC bus will be charged to the required voltage, the contactor is used to bypass the resistor, and the resistor is no longer part of the circuit.

If there is any problem with the contactor switching back from the resistor, the entire current consumption will pass through the resistor and the DC bus voltage will be low to support the load and may trip on undervoltage. This usually happens at the beginning of a VFD. If the drive is restarted several times, this problem usually causes the precharge resistor to fail quickly.

5. Problems with VFD DC voltage detection circuit

The drive's undervoltage and overvoltage fault settings are based on the voltage measured by the drive control circuit from the DC bus. Any failure of this circuit may cause a malfunction or overvoltage trip.

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