Method and device for on-line monitoring DC-bus capacitor

1. Method for on-line monitoring a DC-bus capacitor comprised in a three-phase inverter having a diode rectifier front end, the three-phase inverter controlling the three-phase power provided to a load at each switching cycle using a space vector modulation, characterized in that the method comprises the steps of:

determining, within the switching cycle, when current is not provided to the DC-bus capacitor,

determining, for each switching cycle, the sector of a reference vector, active vectors durations and null voltage vectors durations,

determining from the determined durations, sampling instants,

sampling the currents through the three phases and the DC-bus voltage at the determined sampling instants,

determining, from the phase currents sampled at the determined sampling instants, the current flowing through the DC-bus capacitor during the active vectors,

estimating the capacitance value and/or the equivalent series resistance value of the DC-bus capacitor,

comparing the capacitance value and/or the equivalent series resistance value to a threshold and determining if the DC-bus capacitor reaches its end of life according to the comparison result.

2. Method according to claim 1 , characterized in that the method comprises steps of:

detecting current flowing from the diode rectifier to the DC bus capacitor, and

determining the period when current is not provided to the DC-bus capacitor based on the detected current.

3. Method to claim 1 , characterized in that the diode rectifier includes a plurality of diodes without including a switching element.

4. Method according to claim 1 , characterized in that each switching cycle is divided into a first half switching cycle and a second half switching cycle and in that the sampling, the estimating and comparing steps are executed at each half switching cycle.

5. Method according to claim 4 , characterized in that sampling instants are determined as in the middle of the durations of the active vectors or as the beginning of half switching cycles.

6. Method according to claim 1 , characterized in that the period when current is not provided to the DC-bus capacitor is determined using a current sensor located at an output of the front end rectifier or by detecting a voltage drop of the DC-bus voltage.

7. Method according to claim 1 , characterized in that the threshold is determined during a calibration period.

8. Method according to claim 7 , characterized in that the threshold is selected among a plurality of thresholds using an obtained temperature of the DC-bus capacitor.

9. Device for on-line monitoring a DC-bus capacitor comprised in a three-phase inverter having a diode rectifier front end, the three-phase inverter controlling the three-phase power provided to a load at each switching cycle using a space vector modulation, characterized in that the device comprises:

means for determining, within the switching cycle, when current is not provided to the DC-bus capacitor,

means for determining, for each switching cycle, the sector of a reference vector, active vectors durations and the null voltage vectors durations,

means for determining from the determined durations, sampling instants,

means for sampling the currents through the three phases and the DC-bus voltage at the determined sampling instants,

means for determining, from the phase currents sampled at the determined sampling instants, the current flowing through the DC-bus capacitor during the active vectors,

means for estimating the capacitance value and/or the equivalent series resistance value of the DC-bus capacitor,

means for comparing the capacitance value and/or the equivalent series resistance value to a threshold and determining if the DC-bus capacitor reaches its end of life according to the comparison result.

10. Method for on-line monitoring a DC-bus capacitor comprised in a three-phase inverter having a diode rectifier front end, the three-phase inverter controlling the three-phase power provided to a load at each switching cycle using a space vector modulation, characterized in that the method comprises the steps of:

determining, within the switching cycle, when current is not provided to the DC-bus capacitor,

determining, for each switching cycle, the sector of a reference vector, active vectors durations and null voltage vectors durations,

determining from the determined durations, sampling instants,

sampling the currents through the three phases and the DC-bus voltage at the determined sampling instants,

determining, from the phase currents sampled at the determined sampling instants, the current flowing through the DC-bus capacitor during the active vectors,

estimating the capacitance value and/or the equivalent series resistance value of the DC-bus capacitor, vectors according to the following formula:

where the voltage Vc(n−1) is the voltage at the beginning of a half switching cycle,

Cest

=

I

avg

⁢

⁢

1

·

Δ

⁢

⁢

T

⁢

⁢

1

+

I

avg

⁢

⁢

2

·

Δ

⁢

⁢

T

⁢

⁢

2

Vc

⁡

(

n

-

1

)

-

Vc

⁡

(

n

)

Vc(n) is the voltage at voltage at the end of the half switching cycle, Iavg 1 and Iavg 2 are determined depending on the position of reference vector within one of the sectors of the space vector modulation, ΔT 1 is the duration of a first active vector and ΔT 2 is the duration of a second active vector,

comparing the capacitance value and/or the equivalent series resistance value to a threshold and determining if the DC-bus capacitor reaches its end of life according to the comparison result.