METHOD FOR MONITORING DC LINK CAPACITANCE IN POWER CONVERTERS

Method and apparatus for measuring current of phases a, b, c and voltage of the DC link; determining inverter switch states of the inverter and rectifier switch states of the rectifier for each phase a, b, c; generating a trigger signal 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 for the trigger signal; determining the rectifier switch states of the rectifier for each phase a, b, c at the triggered time duration 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 at the triggered time duration; calculating an indicator ERS for the DC link voltage at the triggered time duration; calculating a total DC link capacitance C and comparing it with the initial DC link capacitance.

The subject of the invention is a method for monitoring DC link capacitance in power converters which is useful in condition monitoring of electronic equipment.

BACKGROUND OF THE INVENTION

Many of converters applications are being considered as critical for plant production process. Offering enhanced functionality in modern power converters is a way to gain a competitive advantage in worldwide market. One of the values to the customers comes from low cost condition monitoring of critical equipment and from optimized maintenance costs. DC link is one of the elements of power converters circuits. Although the designers pay a lot of attention when designing DC link, breakdowns and failures of the DC link can still happen. Unfortunately this means that the whole converter must be shutdown in emergency mode to be repaired. Nowadays, monitoring of the DC link is becoming one of the desired features from customer side, as well as from the converter manufacturer.

The ever-increasing demands for greater efficiency and reliability of power converters, force the introduction of novel methods for improving their performance. One of the ways is to provide a detailed condition monitoring features for the elements of the converter, ensuring tracking of entire device status. Assuring proper monitoring of the DC link capacitance level is one of the solutions for converter status determination. The capacitor deterioration usually manifests by an increase in ESR and decrease in an effective capacitance. Earlier detection of the ESR increase allows for preventive mitigation actions i.e. scheduling shutdown and repairs. Moreover it also helps with evaluation of the expected operational time until failure may happen. Preventing the growth of ESR values over a specified level provides maintaining the quality of converted power and protects the converter from DC-link failure.

There is known from patent US 2010/0295554 A1 a method and system for monitoring the condition of the capacitor arrangement of the DC-voltage intermediate circuit of a power electronics appliance, such as of a frequency converter, at the place of usage, in which method the discharge voltage over the capacitor arrangement as a function of time is measured, and in which method the intermediate circuit is pre-charged With a pre-determined DC voltage, the pre-charging is removed from the intermediate circuit, the voltage of the intermediate circuit is measured by sampling at regular intervals, the voltage drop as a function of time is determined on the basis of the measured voltage of the intermediate circuit, the capacitance or the relative change in it is determined on the basis of the voltage drop, the value of the determined capacitance or of the relative change in it is compared to a pre-determined limit value on the basis of the voltage drop, and the necessary condition monitoring procedure is performed when the value determined with the measurement reaches the pre-determined limit value or is close to it. The disadvantage of the method is the need for performing initial measurement of the discharge curve and performing the measurements each time before starting the converter, thus it cannot be used online continuously.

There is known from patent U.S. Pat. No. 6,381,158B1 a system and method for monitoring DC link capacitance in three level inverters where a signal is injected into a neutral point regulator of an inverter drive and the response to that injected signal is monitored as an indication of the capacitance of the inverter drive. The disadvantage of the method is the need of additional passive elements used for additional device which is responsible for signal injecting.

SUMMARY OF THE INVENTION

The presented invention provides a diagnostic method for monitoring DC link in power converters according to claim1.

The presented invention is advantageous relative to the previously described existing methods as it allows for monitoring of the DC link using less measurement devices inside the power converter, namely only DC link voltage measuring device is used. Moreover in the method according to the invention the calculation process is simplified and thus does not required a significant calculation power. Also by using synchronization with inverter switching signal further simplification in software by reducing algorithm complexity and in hardware by reducing number of elements is achieved, so the inventive method allows for saving time during monitoring DC link capacitance.

A power converter system comprises a power converter circuit1having a rectifying unit2, a DC link unit3, an inverter unit4, and the power converter circuit1is connected with a converter control unit5and a DC link monitoring unit6. The converter control unit5and the DC link monitoring unit6may be combined in single unit what is not presented in the drawing. The power converter circuit1, the control unit5and DC link monitoring unit6are powered by three phase power supply lines a, b, c. The rectifying unit2has at least two solid state switches2a,2b,2c,forming an inverter switching leg for each phase a, b, c, for converting the AC input voltage into DC output voltage. The solid-state switches of the rectifying unit2is one of the possible known kinds e.g. diodes, transistors, thyristors, etc. The DC link unit3includes at least one DC link circuit7and a DC link voltage measuring device8. DC link circuit7comprises capacitor and an equivalent of series resistance ESR presented inFIG. 3. The inverter unit4has at least two solid state switches3a,3b,3c, forming an rectify switching leg for each phase a, b, c, for converting the DC input voltage into AC output voltage. The solid state switches of the inverting unit4can be one of the possible known kinds—e.g. transistors, thyristors etc.

The converter control unit5is a computer device, having a data acquisition module9for receiving and processing input signals such as: current Ia, Ib, Ic, measured by the sensors10mounted on power supply lines a, b, c, and a voltage UDCof DC link unit3, connected to the circuit7. The converter control unit5is also equipped with an output module11for generating switching signals for solid switches2a,2b,2cof the rectifying unit2and for solid switches3a,3b,3cof the inverting unit3. The converter control unit5is also equipped with an alarm generating module12, for comparing the initial capacitance Cintand total capacitance C, and for generating alarm if certain condition is met. The DC link monitoring unit6is a computer device, having processing unit13for receiving and processing input signals such as: current Ia, Ib, Icand a voltage UDCof DC link unit3, and signals from switches2a,2b,2c,3a,3b,3c, The DC link monitoring unit6is equipped with the calculation module14for calculating an indicator of ESR (Equivalent Series Resistance) and for calculation a total capacitance C of the DC link unit3.

The inventive method is implemented according to the following steps20-28presented inFIG. 2.

Checking if there is Cinit value provided by the user.

Measuring a DC link voltage UDCby the DC link voltage measuring device8and measuring current Ia, Ib, Icof three phase power supply lines a, b, c, by the sensors10.

Calculation inverter switching signals SaINV, SbINV, ScINVof the inverter unit3and rectifier switching signals SaREC, SbREC, ScRECof the rectifier unit2in the converter control unit5, using known control methods for such calculation e.g. field oriented control method, scalar control method, direct torque control method, and sending calculated data to the rectifier unit2, the inverter unit3and DC link monitoring unit6. The calculation is realized in the converter control unit5. The switching signals can take the following values, according to the formulae's:

Generating a trigger signal STRIGby comparison each individual switching signal with all remaining switching signals and when SaINV==SbINV==ScINVthen the trigger signal STRIGhas a value equal to 1.

Determination of triggered time duration t for the trigger signal STRIGand indicate measured current Ia, Ib, Icin the time t as Ia(t), Ib(t), Ic(t), where triggered time duration t is defined as the time between start Tonand stop Toffof the trigger signal STRIG, where k-n are consecutive number of triggered time duration t, what is presented in theFIG. 4and described according to the formulae:

Determination switching signals SaREC, SbREC, ScRECof the rectifying unit2from the converter control unit5at the triggered time t as SaREC(t), SbREC(t), ScREC(t) and determination of the DC link voltage Udcand current Ia, Ib, Icof three phase power supply lines a, b, c at the triggered time t as Udc(t), Ia(t), Ib(t), Ic(t).

Reconstructing the DC link current by multiplying phase current Ia(t), Ib(t), Ic(t) at the triggered time t by corresponding rectify switching signal SaREC(t), SbREC(t), ScREC(t) at the triggered time t and making a sum of all results what is presented inFIG. 5and described according to the formulae:

IDC(t)=SaREC(t)·Ia(t)+SbREC(t)·Ib(t)+ScREC(t)·Ic(t)   (3)where:Ia(t)—is a measured current for phase a, at the triggered time t,Ib(t),—is a measured current for phase b, at the triggered time t,Ic(t),—is a measured current for phase c, at the triggered time t,SaREC(t),—is a value of switching signal of the rectifying unit for phase a, at the triggered time t,SbREC(t),—is a value of switching signal of the rectifying unit for phase b, at the triggered time t,SbREC(t),—is a value of switching signal of the rectifying unit for phase c, at the triggered time t.

Calculating an indicator of an Equivalent Series Resistance (ESR) in monitoring unit6, which requires the DC link voltage UDCmeasured voltage and IDCcurrent obtained using equation 3 and is initial DC link capacitance provided by the user, according to the formula:

ESR(t)=∫TonToffUDC(t)t-IDC(t)CinitIDC(t)tt(4)where:UDC(t)—is measured DC link voltage at the triggered time t,Cinit—is initial DC link capacitance stored in the converter control unit5,IDC(t)—is calculated DC link current, at the triggered time t,Ton—is the start time of the triggered signal STRIG,Toff—is the stop time of the triggered signal STRIG.

Calculating the total DC link capacitance C (in percent's), which requires using IDCcurrent obtained using equation 3, ESR indicator obtained using equation 4 and measured DC link voltage, according to the formulae:

C[%]=(IDC(t)·ESR(t)-UDC(t))2UDC2(t)·100%(5)where:IDC(t)—is calculated reconstructed DC link current,UDC(t)—is measured DC link voltage,ESR(t)—is calculated equivalent series resistance.

Comparing the initial capacitance Cinitwith the calculated total capacitance C in alarm generating module12, according to the formulae:

Where the value of k is a threshold value given by user depended on a nominal power of the converter, for example for the converter of 1 MW power the value of kpis preferably 0.7.

Generating an alarm if the threshold value is exceeded and sending a command from alarm generating module12to the output module11where rectify switching signals SaREC, SbREC, ScRECof rectifying unit2and inverter switching signals SaINV, SbINV, ScINVof inverter unit4, are set to 0, which signals automatically switched off the converter circuit1. In all other situations steps22-27are repeated.