Method and arrangement for an internal failure detection in a Y-Y connected capacitor bank

1. A method for detecting a number of internal failures in a capacitor bank connected to a power system, wherein the capacitor bank comprises a plurality of capacitor units that are divided into two Y sections, each phase in each of the Y sections defining a leg, each leg including series and/or parallel-connected capacitor units, and each internal failure event may occur in one or more capacitor elements or units or involve one or more legs, the method comprises:

measuring the phase current in one of the phases,

calculating the Root Mean Square value, denoted by RMS, of the measured phase current,

measuring the unbalanced current between the two sections,

calculating the RMS value of the measured unbalanced currents, and

detecting the phase angle between the measured phase current and the measured unbalanced current, characterized in that the method further comprises:

calculating the per unit value of the unbalanced current based on the calculated RMS value of the measured phase current and the calculated RMS value of the measured unbalanced current,

recognizing each internal failure event by tracking the change in the unbalanced current,

tracking and detecting a change of the unbalanced current between the present calculated per unit value of the unbalanced current and the previous calculated per unit value of the unbalanced current,

determining the vectorial step change of the unbalanced current for each internal failure event,

normalizing the phase angle of the determined vectorial step change of each internal failure event to be one of 0°, 60°, 120°, 180°, 240° and 300°,

identifying the number of internal failures and their corresponding locations based on the determined vectorial step change of the unbalanced current and the normalized phase angle, and

initiating an alarm or a trip signal when the determined total number of internal failures exceeds a first or a second threshold value respectively.

2. The method according to claim 1 , wherein the method further comprises tracking the change of the unbalanced current and continuously comparing the change with a pre-defined threshold value in order to detect the internal failure.

3. The method according to claim 1 , wherein the alarm signal is initiated when the number of internal failures in any of the legs exceeds the first threshold.

4. The method according to claim 1 , wherein the trip signal is initiated when the number of internal failures in any of the legs exceeds the second threshold.

5. The method according to claim 1 , wherein the first and second threshold values are corresponding to a lower and a higher limit of an allowable excessive voltage over the remaining healthy parts of the capacitor bank.

6. An arrangement for detecting a number of internal failures in a capacitor bank connected to a power system, wherein the capacitor bank comprises a plurality of capacitor units that are divided into two Y sections, each phase in each of the Y sections defining a leg, each leg including series and/or parallel-connected capacitor units, and each internal failure event may occur in one or more capacitor elements or units and involve one or more legs, the arrangement comprising,

a first current transformer arranged to measure the phase current in one of the phases,

a second current transformer is arranged at the neutral points of the two Y-sections for measuring an unbalanced current,

a computing unit configured to

calculate the Root Mean Square value, denoted by RMS, of the measured phase current,

calculate the RMS value of the measured unbalanced current,

calculating a per unit value of the unbalanced current based on the calculated RMS value of the measured phase current and the calculated RMS value of the measured unbalanced current, and

calculate the phase angle between the measured phase current and the measured unbalanced current, and

a protective unit configured to

detect the phase angle between the measured phase current and its corresponding measured unbalanced current,

characterized in that the protective unit is further configured to

recognize each internal failure event by tracking the change in the unbalanced current

track and detect a change of unbalanced current between the present calculated per unit value of the unbalanced current and the previous calculated per unit value of the unbalanced current,

determine the vectorial step change of the unbalanced current for each internal failure event,

normalize the phase angle of the determined vectorial step change of each internal failure event to be one of 0°, 60°, 120°, 180°, 240° and 300°,

identify the number of internal failures and their corresponding locations based on the determined vectorial step change of the unbalanced current and the normalized phase angle, and

initiate an alarm or a trip signal if the determined number of internal failures exceeds a first or a second threshold value.

7. The arrangement according to claim 6 , wherein the protective unit is further configured to track the change of the unbalanced current and continuously compare the change with a pre-defined threshold value in order to detect the internal failure.

8. A computer program product for detecting a number of internal failures in a capacitor bank connected to a power system, wherein the capacitor bank comprises a plurality of capacitor units that are divided into two Y sections, each phase in each of the Y sections defining a leg, each leg including series and/or parallel-connected capacitor units, and each internal failure event may occur in one or more capacitor elements or units and involve one or more legs, wherein the computer program product is directly loadable into the internal memory of a computer, comprising software to perform the steps of,

receiving measured phase currents,

calculating the RMS values of the measured phase currents,

receiving measured unbalanced currents between the two Y sections,

calculating the RMS values of the measured unbalanced currents,

calculating the phase angle between the measured phase current and the measured unbalanced current,

calculating a per unit value of the unbalanced current based on the calculated RMS value of the measured phase current and the calculated RMS value of the measured unbalanced current,

recognizing each internal failure event by tracking the change in the unbalanced current,

tracking and detecting a change of the unbalanced current between the present calculated per unit value of the unbalanced current and the previous calculated per unit value of the unbalanced current,

determining the vectorial step change of the unbalanced current for each internal failure event,

normalizing the phase angle of the determined vectorial step change of each internal failure event to be one of 0°, 60°, 120°, 180°, 240° and 300°,

determining the number of internal failures and their corresponding locations based on the determined vectorial step change of the unbalanced current and the normalized phase angle, and

initiating an alarm or a trip signal when the determined total number of internal elements failures exceeds a first or a second threshold value.

9. A computer readable medium, having a program recorded thereon, where the program is to make a computer perform steps, when said program is run on the computer, for detecting a number of internal failures in a capacitor bank connected to a power system, wherein the capacitor bank comprises a plurality of capacitor units that are divided into two Y sections, each phase in each of the Y sections defining a leg, each leg including series and/or parallel-connected capacitor units, and each internal failure event may occur in one or more capacitor elements or units and involve one or more legs, comprising the steps of,

receiving measured phase currents,

calculating the RMS values of the measured phase currents,

receiving measured unbalanced currents between the two Y sections,

calculating the RMS values of the measured unbalanced currents,

calculating the phase angle between the measured phase current and the measured unbalanced current,

calculating a per unit value of the unbalanced current based on the calculated RMS value of the measured phase current and the calculated RMS value of the measured unbalanced current,

recognizing each internal failure event by tracking the change in the unbalanced current,

tracking and detecting a change of the unbalanced current between the present calculated per unit value of the unbalanced current and the previous calculated per unit value of the unbalanced current,

determining the vectorial step change of the unbalanced current for each internal failure event,

normalizing the phase angle of the determined vectorial step change of each internal failure event to be one of 0°, 60°, 120°, 180°, 240° and 300°,

determining the number of internal failures and their corresponding locations based on the determined vectorial step change of the unbalanced current and the normalized phase angle, and

initiating an alarm or a trip signal when the determined total number of internal elements failures exceeds a first or a second threshold value.