Temperature of a link capacitor
To determine the temperature of a link capacitor (C) of a link converter ( 1 ) more accurately with less expenditure, a device and a method are described, in which the link capacitor (C) is modeled as a series interconnection of an equivalent capacitance (CS) and an equivalent series resistance (ESR), wherein a modeled capacitor current (i Cm ) flows across the equivalent series resistance (ESR). A modeled capacitor power loss (P C ), from which the capacitor temperature (T C ) is determined by means of a specified temperature model, is calculated from the modeled capacitor current (i Cm ) and the value of the equivalent series resistance (ESR) by means of a first relationship of the form P C =f(i Cm , ESR). Direct measurement of the capacitor temperature (T C ), of the capacitor current (i C ), or of the capacitor power loss (P C ) is not required. For example, a measurement of the capacitor voltage (u C ) and a further calculation of the modeled capacitor current i Cm and finally of the capacitor power loss (P C ) are sufficient. The method can be used for the monitoring and processing of the capacitor temperature (T C ), particularly the switching-off of an element, preferably at least part of the link converter ( 1 ), when a maximum temperature, such as a preset maximum temperature, is exceeded. The method can also be used to determine the temporal progression of the capacitor temperature (T C (t)) and also to determine the remaining service life (RL) of the link capacitor (C) of a specified relationship, preferably by means of the Arrhenius formula.
1. A method for determining a capacitor temperature of a link capacitor of a link converter, which contains at least one n-phase inverter, comprising:
modeling the link capacitor as a series interconnection of an equivalent capacitance and an equivalent series resistance (ESR), wherein a modeled capacitor current (i Cm ) flows across the equivalent series resistance (ESR);
calculating a modeled capacitor power loss (P C ) from the modeled capacitor current (i Cm ) and a value of the equivalent series resistance (ESR) by a first relationship P C =f(i Cm ,ESR); and
determining, from the modeled capacitor power loss, the capacitor temperature via a specified temperature model, wherein the modeled capacitor current (i Cm ) is modeled by using a measured capacitor voltage (u C ) and the equivalent capacitance (CS) via a second relationship of the form i Cm =f(u C ,CS).
2. The method according to claim 1 , wherein the first relationship is P C =i Cm 2 ESR.
3. The method according to claim 1 , wherein the second relationship is
i
Cm
=
CS
d
dt
u
C
4. The method according to claim 1 , wherein the modeled capacitor current (i Cm ) is split into a low-frequency component and a high-frequency component, wherein the low-frequency component of the modeled capacitor current is used to determine a first modeled power loss and the high-frequency component of the modeled capacitor current is used to determine a second modeled power loss and the modeled capacitor power loss (P C ) is calculated from the sum of the first modeled power loss and the second modeled power loss.
5. The method according to claim 4 , wherein the low-frequency component of the modeled capacitor current is determined by mean-filtering the capacitor voltage (u C ) and/or a time derivative of the capacitor voltage
(
d
dt
u
C
)
and/or the modeled capacitor current (i Cm ).
6. The method according to claim 4 , wherein the high-frequency component of the modeled capacitor current is calculated from an inverter current caused by the inverter.
7. The method according to claim 6 , wherein a square of the high-frequency component of the modeled capacitor current is calculated by determining a square of an arithmetic mean of the inverter current and an arithmetic mean of a squared inverter current and subtracting the square of the arithmetic mean of the inverter current from the arithmetic mean of the squared inverter current.
8. The method according to claim 7 , wherein the arithmetic mean of the inverter current is calculated in a time-discrete manner by sampling the inverter current at a first sampling rate in a first time span, wherein a first number of sample values of the inverter current is produced and a sum of the sample values of the inverter current and is divided by the first number.
9. The method according to claim 7 , wherein the arithmetic mean of the squared inverter current is calculated in a time-discrete manner by sampling a squared inverter current at a second sampling rate in a second time span, wherein a second number of sample values of the squared inverter current is produced and the sum of the sample values of the squared inverter current and is divided by the second number.
10. A device for determining the capacitor temperature of a link capacitor of a link converter, comprising:
a calculating unit, which models the link capacitor as a series interconnection of an equivalent capacitance and an equivalent series resistance (ESR), in which a modeled capacitor current (icm) flows across the equivalent series resistance (ESR),
wherein a modeled capacitor power loss (P C ) is calculated from the modeled capacitor current (icm) and a value of the equivalent series resistance (ESR) by a relationship of the form P C =f(i Cm ,ESR), and
wherein the capacitor temperature is determined by a specified temperature model from the modeled capacitor power loss, wherein the modeled capacitor current (i Cm ) is modeled by using a measured capacitor voltage (u C ) and the equivalent capacitance (CS) via a second relationship of the form i Cm =f(u C ,CS).
11. The device according to claim 10 , further comprising a differentiating unit and a voltage-measuring unit, wherein the voltage-measuring unit determines a capacitor voltage present at the link capacitor and feeds said capacitor voltage to the differentiating unit and the differentiating unit determines the modeled capacitor current.
12. The device according to claim 10 , further comprising a low-frequency unit, which is designed to calculate a low-frequency component of the modeled capacitor current in that a number of low-pass filters is provided before and/or after the differentiating unit.
13. The device according to claim 10 , further comprising a high-frequency unit, which calculates a high-frequency component of the modeled capacitor current from an inverter current caused by the inverter.
14. The device according to claim 13 , further comprising a number of mean calculators, which calculate a mean of the inverter current or a mean of the squared inverter current.
15. A method for monitoring and processing of capacitor temperature, comprising:
determining the capacitor temperature of a link capacitor by:
modeling the link capacitor as a series interconnection of an equivalent capacitance (CS) and an equivalent series resistance (ESR), wherein a modeled capacitor current (i Cm ) flows across the equivalent series resistance (ESR);
calculating a modeled capacitor power loss (P C ) from the modeled capacitor current (i Cm ) and a value of the equivalent series resistance (ESR) by a first relationship P C =f(i Cm ,ESR); and
determining, from the modeled capacitor power loss, the capacitor temperature via a specified temperature model, wherein the modeled capacitor current (i Cm ) is modeled by using a measured capacitor voltage (u C ) and the equivalent capacitance (CS) via a second relationship of the form i Cm =f(u C ,CS); and
monitoring and processing the determined capacitor temperature.
16. The method according to claim 15 , further comprising switching-off of an element of the link converter when a maximum temperature is exceeded.
17. A method to determine a temporal progression of a capacitor temperature and to determine a remaining service life of a link capacitor from the temporal progression of the capacitor temperature by a specified relationship, comprising:
determining the capacitor temperature of a link capacitor by:
modeling the link capacitor as a series interconnection of an equivalent capacitance (CS) and an equivalent series resistance (ESR), wherein a modeled capacitor current (i Cm ) flows across the equivalent series resistance (ESR);
calculating a modeled capacitor power loss (P C ) from the modeled capacitor current (i Cm ) and a value of the equivalent series resistance (ESR) by a first relationship P C =f(i Cm ,ESR); and
determining, from the modeled capacitor power loss, the capacitor temperature via a specified temperature model, wherein the modeled capacitor current (i Cm ) is modeled by using a measured capacitor voltage (u C ) and the equivalent capacitance (CS) via a second relationship of the form i Cm =f(u C ,CS);
determining the temporal progression of the determined capacitor temperature; and
determining the remaining service life of the link capacitor from the determined temporal progression of the capacitor temperature.
18. The method according to claim 17 , wherein the specified relationship comprises the Arrhenius formula.