Voltage Drop Test in HV Cables

[jmgibb]

I have to test the actual voltage drop in an 11kv cable, the cable is a 185mm2/cu/3core/xlpe/pvc, 1500mtrs long.
Has anyone carried out such a test and what equipment did you use??

 

[busbar]

jm--could you expand on what is meant by ‘actual voltage drop’? If it had adequate compliance voltage [estimted from published cable data], it may be possible to use a low-voltage circuit-breaker test set. In the simplest form, it would be a singe-phase test.

OTOH, measurement of voltage change via met-lab grade PTs on the ends of the cable would accommodate actual load power factor and dynamic characteristics. Can you give some more details on what is expected from the test? At the extreme, it could get as complex as full GPS-synched ø-ø and ø-g readings at the cable ends. Research physicists can come up with some requests for screwy electrical measurements when there’s enough money in the budget.

 

[jmgibb]

The cable is supplying an 11kV 1200kw motor

The 'Actual Voltage Drop' is defined as the end to end drop in voltage under start and full running load conditions.

I have contacted cable manufacturers in UK,Saudi and Kuwait, all tell me that they don't actually carry out a VD test to determine values for thier published cable data but rather rely on calculation.

I have considered making use of a suitable PT/recorder at the remote end of the cable during operation but am looking for other, less drastic, methods

 

[electricpete]

It seems like you have limited number of options:

#1 - Do calculation. This can be a little involved but there are well-established methods.

#2 - Measure the phase-to-ground voltage during operration on one or all phases using PT's at both ends. As busbar points out, this will only tell you the magnitude of voltage drop, but not the phase of the impedance (that would require comparing phase across the cable).

 

[busbar]

jmgibb—I am not aware of an on-site acceptance test like you describe. In circuit-breaker tests, the procedure offsets the voltage drop in the device under test. In the field, typically one pole at a time is tested. In the described case, two cables would concurrently be loaded at any point in the test sequence.

If by some rare chance the arithmetic is correct, ~85 amperes at ~24 volts would emulate a motor running state. Roughly six times those figures would mimic a starting condition. A full-tilt three-phase test would invariably be more complex. It’s conceivable that some error would be introduced in being unable to extrapolate actual X/R ratios.

 

[jbartos]

Suggestion to the original posting: Please, would you elaborate what magnitude of voltage you intend to measure/test:
1. The magnitude of voltage at the end of cable as a difference between |Vcablesendingend|-|Vcablereceivingend|=V/D1; or
2. The magnitude of voltage across the cable as |Vcablesendingend - Vcablereceivingend|=V/D2.
Generally, V/D1 is different (smaller) from V/D2.

 

[electricpete]

Can you clarify why you want this info?

If for purposes of determining how the bus voltage will be affected (affect on starting and remaining motors), calculation is generally used and should be accurate enough.

 

[jmgibb]

I know that calculation is normally how Volt Drop in a cable is determined, However my client has written this ' Actual Test ' requirement into the test specification.

If anyone has carried out sutch a test, without using PT's and recorders at each end of the cable, I should be most grateful for some advice.

Regards
J

 

[busbar]

For an ‘ Actual Test ' it is unlikely that an acceptable procedure could be found that does not present multiple and very serious risks to life and property.

 

[jbartos]

Suggestion: It appears that the Client would like to see V/D1 test results. It can be implemented either by using PTs or high voltage probes associated with meters. Nowadays, the recorders may be replaced by digital cameras reading meter dials, e.g. CD Rom Mavica300 (by Sony).

 

[busbar]

Edit: For an ‘ Actual Test ' it is unlikely that a acceptable procedure could be found that is of reasonable cost and does not present multiple and very serious risks to life and property.