1. A method for monitoring DC link capacitance in power converters comprises the following steps:

- measuring current of the phase a, b, c,

- measuring voltage of the DC link unit 3 and setting initial capacitance Cint of the DC link, where the initial capacitance of the DC link is given by the user,

- determining inverter switch states of the inverter and rectifier switch states of the rectifier for each phase a, b, c, where the switching states have a value equal to 0 or 1 ,

- generating a trigger signal STRIG when the value of the inverter switch states of the inverter for each phase a, b, c have an equal values and determining a triggered time duration t for the generated trigger signal,

- determining the rectifier switch states of the rectifier unit 2 for each phase a, b, c at the triggered time duration t and calculation current of the phase a, b, c and voltage of the DC link at the triggered time duration

- reconstructing the DC link current c(t) at the triggered time duration t using the following formulae:

°aREC (t)^■ /_α(ί) + SbREC °CREC

- where:

la(t) - is a measured current for phase a at the triggered time duration t,

- Ib(t) - is a measured current for phase b at the triggered time duration t

- Ic(t) - is a measured current for phase c at the triggered time duration t,

- SaREc(t) - is a value of switching signal of the rectifying unit for phase a at the triggered time duration t,

- SbREc(t) - is a value of switching signal of the rectifying unit for phase b at the triggered time duration t, - ScREc(t) - is a value of switching signal of the rectifying unit for phase c at the triggered time duration t,

- calculating an indicator ERS (Equivalent Series Resistance) for the DC link voltage at the triggered time duration t according to the formulae:

where:

UDc(t) - is a measured DC link voltage at the triggered time duration t,

Cinit - is initial DC link capacitance given by the user and stored in the converter control unit, lDc(t) - is a reconstructed current od DC link at the triggered time duration t, Ton - is the start time of the triggered signal STRIG,

Toff - is the stop time of the triggered signal STRIG,

- calculating a total DC link capacitance C according to the formulae:

where: toc(t) - is calculated DC link current at the triggered time duration t, UDc(t) - is measured DC link voltage at the triggered time duration t, ESR(t) - is calculated equivalent series resistance indicator at the triggered time duration t,

- comparing the value of total DC link capacitance C with the initial DC link capacitance Cint and triggering an alarm when the initial DC link capacitance Cint is bigger than a threshold value given by user.

2. A system for monitoring DC link capacitance in power converter comprising a power converter circuit (1) with a rectifying unit (2) , a DC link unit (3), an inverter unit (4) which are connected with a converter control unit (5) and with a DC monitoring unit (6) and with means for measuring electrical parameters of the system, characterized in that it further comprising:

- means for setting initial capacitance Cint of the DC link, where the initial capacitance of the DC link is given by the user in a computer device,

- means for determining inverter switch states of the inverter unit (4) and means for determining rectifier switch states of the rectifier unit (2) for each phase a, b, c, implemented in the computer device,

- means for comparing the value of inverter switch states and for generating a trigger signal STRIG when the value of the inverter switch states of the inverter unit (4) for each phase a, b, c have an equal values, implemented in the converter control unit (5),

- means for determining a triggered time duration t, implemented in the converter control unit (5),

- means for determining the rectifier switch states of the rectifier unit (2) for each phase a, b, c at the triggered time duration t and calculation current of the phase a, b, c and voltage of the DC link unit (3) at the triggered time duration t,

- means for reconstructing the DC link current at the triggered time duration t, - means for calculating an indicator ERS (Equivalent Series Resistance) for the DC link unit (3) voltage at the triggered time duration t and means for calculating a total DC link unit (3) capacitance C,

- means for comparing the value of total DC link unit (3) capacitance C with the initial DC link capacitance Cint,

- means for triggering an alarm when the initial DC link capacitance Cint is bigger than a threshold value given by user.

3. A system according to claim 2, characterized that the converter circuit 1

comprises solid switches (2a,2b,2c) and (3a, 3b, 3c) connected to the switching legs of the rectifying unit (2) and inverter unit 4 respectively.

4. A system according to claim 3, characterized in that the solid switches

(2a, 2b, 2c) and (3a, 3b, 3c) have a form of diodes or transistors or thyristors.

5. A computer program for monitoring DC link capacitance in power converter system, which computer program is loadable in and executable on a data processing unit (13) of a computer device having a form of a DC monitoring device (6) and which computer program performs when being executed by the data processing unit (13) of the computer device, the method according to claim 1.