What's is the difference between Kinetic Buffering and Flying restart feature available with VFD at MV or LV Level ? ...

Ajith,

I am sure you are facing problemsof VFD tripping during voltage sag.Kindly let us know what actions are being proposed by the VFD manufacturers.

Voltage Loss ride through with flying restart:
In this method, when the voltage sag causes the VFD to reach its undervoltage trip level, the drive will shut off the inverter section and thus remove power from the motor instead of tripping. The motor will coast down during the duration of the sag and, as soon as the voltage recovers, the VFD will start into the still-spinning motor and ramp up to set speed. How much the motor speed will drop depends on the inertia of the load and the duration of the sag.

Kinetic backup
This option, which is also provided by some manufacturers of VFDs, uses the energy stored in the mechanical load to keep the DC bus voltage from dropping down to the trip level. This is accomplished by running the inverter section during a voltage sag at a frequency slightly below the motor frequency, causing the motor to act as a generator. Similar to the flying restart option, the motor speed will drop while it is acting as a generator, however the advantage is that the motor is never disconnected from the drive. This option works best for those high-inertia loads .

Thanks Sudipto,

Please clarify my below queries with reference to your briefed answer:

1. What is the difference between Voltage Loss ride through & Power Loss ride through?

2. Is Voltage Loss ride through always associated with Flying restart ?

3.As you mentioned the inverter section is cut-off during under voltage conditions but how is that achieved practically in a Power Cell ?

4.You mean to say in kinetic buffering that this feature of kinetic back up is applicable only till the voltage is within the threshhold of under voltage tripping .. is it ?

.1.What is the difference between Voltage Loss ride through & Power Loss ride through?
No difference in VFD terminology

2. Is Voltage Loss ride through always associated with Flying restart ?

Not always.You have to configure the VFD for flying restart.During low input voltage the inverter section is cut off to maintain the DC bus voltage.If the voltage restores before the DC bus voltage goes below the tripping value, the inverter is again put on but the driven load speed has already reduced due to brief period of no volatge at the motor terminal.Flying restart feature enables the VFD to restart the Motor at the same speed at which the motor is operating thus preventing any high current.So it is basically catching a spinning motor.Without flying restart high current will be observed once the inverter section is put ON.Flying restart feature is also helpful if you want to restart a motor which is already spinning.

3.You mentioned the inverter section is cut-off during under voltage conditions but how is that achieved practically in a Power Cell ?

Inverter section cut off means that the power semiconductor devices eg.. IGBTs are switched off .There is no physical seperation.

4.You mean to say in kinetic buffering that this feature of kinetic back up is applicable only till the voltage is within the threshhold of under voltage tripping .. is it ?

Yes.Kinetic buffering is a feature to prevent the VFD from tripping during voltage sags.If the VFD trips due to DC bus undervoltage there is no need for kinetic buffering.

Few more queries Sudipto ..

1. When is DBR used ? and in which all application ?(Constant Torque/Variable Torque/..) and why ?

2.I am little confused about use of use of DBR & Regenerative concept ..so want to clarify that ?

3.When is Active front end suggested ? and is Active Front end available for Large drives like for 1800kw Motor ?

If the VFD setting at kinetic buffering and during voltage dip the VFD trips on overvoltage?