hhhansen
Electrical
- Jan 14, 2004
- 61
I have some technical questions regarding single phase power transformers which I am not able to answer.
Please refer to attached PDF file. (Sorry for page 1 being up-side-down).
The electricity company of mine has installed two billing meters on a railway transformer. The main meter is connected to one of the metering cores on current transformer installed in L1. The control meter is connected to one of the metering cores on current transformer installed in L2. The exact same voltage (110 V) is passed to both meters from the capacitive voltage transformers.
The problem is fundamentally that the meters do not register the same amount of energy. MWh registration for main meter is consequently larger than MWh for control meter. In figures the differences is approximately 3% referred to MWh/year and this is far beyond the acceptable limit since billing meters are class 0,2 and current transformers are class 0,2s. I have set-up the measurement systems shown on page 1.
The theory of mine is that a differential current exists between L1 and L2 and that this current is passed to earth through the capacitive stray capacitances to earth, as indicated by the capacitive equivalent diagram (note that this is only my suggestion for a capacitive leakage current diagram).
In order to verify the theory I measure the differential current with a 5 A (RMS) precision current probe and passes the signal to channel 1 on a FLUKE435 power analyser. On the transformer there are two earthing connections and each current is measured by the same types of current probes. The signals are passed to channel 2 and 3 on FLUKE435..
On page 2 samples of measurements are presented.
- Looking first on 2) you will find the two earth current observed. The current probes are placed on the earthing wire with arrow corresponding to current from the tank and towards earth. What I don´t understand is why they are 180° out of phase ?.
- On 3) you see the sum of the earth current shown on figure 2. The peak value of the earthing current is approximately 500 mA and lots of fluctuations are observed.
- On 1) you see the result for the differential current (blue curve). The peak value is 10 mA on secondary and on 1) this is converted to the primary side by multiplication of 300/5 = 60. A. The red curve shows the difference between earthing current in case earthing current 2 is to be shifted 180° before adding it to Ie1 - However I am very much in doubt of this!.
If graphic 3) indicate the "true" earthing current due to earth leakage capacitance, I find it difficult to explain what happens with the differential current, which is physically observed by means of my recordings.
To eliminate any sources of errors I have considered removing just one of the earthing connection temporary while additional measurements are performed. Is this a good idea ?
I would appreciate very much some comments on this issue. Thank you.
Please refer to attached PDF file. (Sorry for page 1 being up-side-down).
The electricity company of mine has installed two billing meters on a railway transformer. The main meter is connected to one of the metering cores on current transformer installed in L1. The control meter is connected to one of the metering cores on current transformer installed in L2. The exact same voltage (110 V) is passed to both meters from the capacitive voltage transformers.
The problem is fundamentally that the meters do not register the same amount of energy. MWh registration for main meter is consequently larger than MWh for control meter. In figures the differences is approximately 3% referred to MWh/year and this is far beyond the acceptable limit since billing meters are class 0,2 and current transformers are class 0,2s. I have set-up the measurement systems shown on page 1.
The theory of mine is that a differential current exists between L1 and L2 and that this current is passed to earth through the capacitive stray capacitances to earth, as indicated by the capacitive equivalent diagram (note that this is only my suggestion for a capacitive leakage current diagram).
In order to verify the theory I measure the differential current with a 5 A (RMS) precision current probe and passes the signal to channel 1 on a FLUKE435 power analyser. On the transformer there are two earthing connections and each current is measured by the same types of current probes. The signals are passed to channel 2 and 3 on FLUKE435..
On page 2 samples of measurements are presented.
- Looking first on 2) you will find the two earth current observed. The current probes are placed on the earthing wire with arrow corresponding to current from the tank and towards earth. What I don´t understand is why they are 180° out of phase ?.
- On 3) you see the sum of the earth current shown on figure 2. The peak value of the earthing current is approximately 500 mA and lots of fluctuations are observed.
- On 1) you see the result for the differential current (blue curve). The peak value is 10 mA on secondary and on 1) this is converted to the primary side by multiplication of 300/5 = 60. A. The red curve shows the difference between earthing current in case earthing current 2 is to be shifted 180° before adding it to Ie1 - However I am very much in doubt of this!.
If graphic 3) indicate the "true" earthing current due to earth leakage capacitance, I find it difficult to explain what happens with the differential current, which is physically observed by means of my recordings.
To eliminate any sources of errors I have considered removing just one of the earthing connection temporary while additional measurements are performed. Is this a good idea ?
I would appreciate very much some comments on this issue. Thank you.
You might get better mileage out of testing your meter and instrument transformer error, and checking the secondary circuits, than pursuing further investigation into capacitive charging currents.