5. Grounding practices
It is the intent of this clause to provide some insight into the conditions that affect the measurement of It is the intent of this clause to provide some insight into the conditions that affect the measurement of impulse waves. A complete analysis of all ground conditions cannot be
impulse waves. A complete analysis of all ground conditions cannot be given because each test setup is given because each test setup is dif- dif-ferent. Once the philosophy of grounding is understood, compromises can be made to
ferent. Once the philosophy of grounding is understood, compromises can be made to assure the most accu-assure the most accu-rate and safest
rate and safest measurement.measurement.
The currents ßowing in the impulse circuit generally are fairly large and have high rates of change (d
The currents ßowing in the impulse circuit generally are fairly large and have high rates of change (d ii / d/ dt t ).).
Consequently
Consequently, a voltage drop exists between points , a voltage drop exists between points connected by a conductor through which connected by a conductor through which an impulse cur-an impulse cur-rent ßows. Because of this, it is difÞcult to hold two diffecur-rent points at the same potential or, stated another rent ßows. Because of this, it is difÞcult to hold two different points at the same potential or, stated another way, to have two different points at ground potential.
way, to have two different points at ground potential.
The difference in voltage between two points will depend upon the length of the interconnecting lead and the The difference in voltage between two points will depend upon the length of the interconnecting lead and the rate of change of
rate of change of the current ßowing in the the current ßowing in the lead. The voltage difference can be substantial. For example, if alead. The voltage difference can be substantial. For example, if a current changing at a rate of 1000 A/
current changing at a rate of 1000 A/mms ßows through a wire 10 ft (3 m) long, the two ends of the wire wills ßows through a wire 10 ft (3 m) long, the two ends of the wire will differ in voltage by 3000 V to 4000 V
differ in voltage by 3000 V to 4000 V. This is . This is not at all unusual not at all unusual for the ordinary impulse circuit. Because for the ordinary impulse circuit. Because of of this, impulse circuits are carefully arranged. This is particularly true of the circuits used for front-of-wave this, impulse circuits are carefully arranged. This is particularly true of the circuits used for front-of-wave testing.
testing.
The following are two prime considerations when grounding practices are established:
The following are two prime considerations when grounding practices are established:
aa)) SSaaffeetty ty to po peerrssoonnnneell b)
b) AcAccucuraracy cy of of memeasasururememenentsts
For safe operation, all the devices in the vicinity of the operator should
For safe operation, all the devices in the vicinity of the operator should be at the same voltage. If the be at the same voltage. If the devicesdevices have unlike voltages, there is the danger of the operator coming in contact with two pieces of equipment at have unlike voltages, there is the danger of the operator coming in contact with two pieces of equipment at different volta
different voltages. For accurate measurement, the ges. For accurate measurement, the measuring system should be connected directly across measuring system should be connected directly across thethe two points to be
two points to be measured such as the leads measured such as the leads of a voltmeter. In some cases this would electrically elevate theof a voltmeter. In some cases this would electrically elevate the chassis of the oscilloscope with respect to other apparatus in the vicinity since the transformer under test chassis of the oscilloscope with respect to other apparatus in the vicinity since the transformer under test might be located some distance from the
might be located some distance from the oscilloscope. FulÞlling these two considerations is sometimes difÞ-oscilloscope. FulÞlling these two considerations is sometimes difÞ-cult and some compromises are made. This is
cult and some compromises are made. This is illustrated by considering severaillustrated by considering several circuits.l circuits.
In Þgure 45, the
In Þgure 45, the voltage measured by the divider is between points A and B. The voltage measured by the divider is between points A and B. The main current paths are indi-main current paths are indi-cated by the heavy lines. On the fronts of full and
cated by the heavy lines. On the fronts of full and chopped waves, the voltage drop between B and C is usuallychopped waves, the voltage drop between B and C is usually negligible, and the capacitive current to the control
negligible, and the capacitive current to the control room shield is also room shield is also small. On the small. On the fronts of front-of-waves,fronts of front-of-waves, the drop across BC is dependent on the capacitive current that ßows through the transformer and the the drop across BC is dependent on the capacitive current that ßows through the transformer and the induc-tance of the lead BC. The capacitive current for large kilovoltampere, low-voltage windings may produce a tance of the lead BC. The capacitive current for large kilovoltampere, low-voltage windings may produce a voltage drop across the lead inductance that will be
voltage drop across the lead inductance that will be almost 25% of the total voltage measured by almost 25% of the total voltage measured by the voltagethe voltage divider
divider. To elimi. To eliminate the voltage drop BC, the nate the voltage drop BC, the divider should be connected to point C as divider should be connected to point C as shown in Þgure 46,shown in Þgure 46, and the return lead from the
and the return lead from the transformer should be run directly to the bottom transformer should be run directly to the bottom end of the impulse generator.end of the impulse generator.
Figure 44ÑTransf
Figure 44ÑTransformer switching impulse test ormer switching impulse test reportreport Name of
Name of Manufacturer _______________________________________Manufacturer _____________________________________________________________________________________________
Purchaser_____________________________________________________________________________
Purchaser_____________________________________________________________________________
Date of
Date of TestTest___________________ PurchaserÕs Order No. ___________________ PurchaserÕs Order No. ______________ MfrÕs Ref.____________________________ MfrÕs Ref.______________
Serial
Serial Number(s) _________________________________________________________________Number(s) _____________________________________________________________________________
H wi
H windndining _g ___________________ __ kVkVA A WWinindiding ng ______________________ __ kVkVAA Y wiY windndining _g ______________________ k_ kVVAA ___________V
___________V ____________ ____________ V V ____________ V____________ V ___________
___________ BIL BIL ____________ ____________ BIL BIL ____________ ____________ BILBIL Ta
Tap p connection: connection: H H winding winding ________ ________ v v X X winding winding ___________ ___________ v v Y Y winding winding ___________ ___________ vv
Test witnessed by___________________________________________ Date
Test witnessed by___________________________________________ Date ______________________________________________
I hereby certify that this is a
I hereby certify that this is a true report based on factory test made in accordance with IEEE true report based on factory test made in accordance with IEEE Std C57.12.00-Std C57.12.00-1993 and IEEE Std C57.12.90-C57.12.00-1993.
1993 and IEEE Std C57.12.90-1993.
Signed _________________________________ Date
Signed _________________________________ Date _______________ Approved ________________________________ Approved _________________
Terminal
With this arrangement, the divider is connected to read the voltage from A to C. However, the stray With this arrangement, the divider is connected to read the voltage from A to C. However, the stray capaci-tive current ßowing from the generator and the high voltage leads to the control room and building ground tive current ßowing from the generator and the high voltage leads to the control room and building ground and from transformer tank to ground will ßow back to the generator through the lead BC and the building and from transformer tank to ground will ßow back to the generator through the lead BC and the building ßoor. The potential difference between the control room and oscilloscope will depend upon the magnitude ßoor. The potential difference between the control room and oscilloscope will depend upon the magnitude and rate-of-change of the current. However, it is common practice to ground the oscilloscope to the control and rate-of-change of the current. However, it is common practice to ground the oscilloscope to the control room for personnel safety. This forces current to ßow from C through the sheath of the measuring cables to room for personnel safety. This forces current to ßow from C through the sheath of the measuring cables to the control room ground and back to B and causes disturbances on the oscillograms of both the voltage and the control room ground and back to B and causes disturbances on the oscillograms of both the voltage and current. To minimize this effect, lead BC should be as short as possible. In severe cases a multiplicity of current. To minimize this effect, lead BC should be as short as possible. In severe cases a multiplicity of leads or wide foil may be run from B to C. In special cases a double shielded control room may be used leads or wide foil may be run from B to C. In special cases a double shielded control room may be used [B31]
[B31]..
The method of Þgure 46 is especially useful when the generator is some distance from the transformer The method of Þgure 46 is especially useful when the generator is some distance from the transformer pro-viding the bottom end of the generator has sufÞcient insulation to the ground plane and the voltage drop viding the bottom end of the generator has sufÞcient insulation to the ground plane and the voltage drop between point C and
between point C and the oscilloscope can be kept small.the oscilloscope can be kept small.
For generators that are not insulated sufÞciently at the bottom end and therefore must be grounded to the For generators that are not insulated sufÞciently at the bottom end and therefore must be grounded to the building at their bases, the method in
building at their bases, the method in Þgure 47 is used. Þgure 47 is used. In this method, it In this method, it is particularly helpful to run severalis particularly helpful to run several connections from B to the tank (as
connections from B to the tank (as indicated by BC and BD) indicated by BC and BD) and have many leads or a wide foil from B and have many leads or a wide foil from B backback to the bottom end
to the bottom end of the generator to of the generator to reduce the voltage drops between these reduce the voltage drops between these points. Howeverpoints. However, the , the measuredmeasured voltage will be in error by the magnitude of the voltage drop between the tank and point B. The preferred voltage will be in error by the magnitude of the voltage drop between the tank and point B. The preferred grounding method shown in Þgure 46
grounding method shown in Þgure 46 eliminates the measurement of the voltage drop in lead BD.eliminates the measurement of the voltage drop in lead BD.
The location of the resistance shunt for current measurement is also selected with consideration of the The location of the resistance shunt for current measurement is also selected with consideration of the ground problem. In Þgure 45 with the resistance shunt located at the transformer tank, the cable sheath is ground problem. In Þgure 45 with the resistance shunt located at the transformer tank, the cable sheath is raised above ground by the voltage drop in lead BC. Current will ßow from C through the cable sheath and raised above ground by the voltage drop in lead BC. Current will ßow from C through the cable sheath and back to B causing disturbance on the current wave. If the shield is allowed to ßoat at
back to B causing disturbance on the current wave. If the shield is allowed to ßoat at point C, sufÞcient clear-point C, sufÞcient clear-ance from shield to inner conductor is
ance from shield to inner conductor is provided.provided.
Small disturbances may appear on current oscillograms, which are due to the voltage drops in the ground Small disturbances may appear on current oscillograms, which are due to the voltage drops in the ground leads spitting to nonconnected metal. For instance, if a piece of metal was ßoating electrically near the leads spitting to nonconnected metal. For instance, if a piece of metal was ßoating electrically near the ground lead it would be possible for the lead to ßash to the metal. The disturbance indicated on the scope ground lead it would be possible for the lead to ßash to the metal. The disturbance indicated on the scope would be a function of the capacitance-to-ground of the metal. A large capacitance would cause a greater would be a function of the capacitance-to-ground of the metal. A large capacitance would cause a greater
Figure 45ÑA grounding method Figure 45ÑA grounding method
disturbance since more energy would be required to charge up the capacitance. If the ßoating metal was disturbance since more energy would be required to charge up the capacitance. If the ßoating metal was located near the measuring cables, the disturbance on
located near the measuring cables, the disturbance on the oscillogram would be even greater.the oscillogram would be even greater.
When front-of-wav
When front-of-waves are applied, the best es are applied, the best procedure is to locate the chopping gap procedure is to locate the chopping gap directly on the bushing of directly on the bushing of the transformer under test because of the voltage drops that develop when the capacitive current is ßowing.
the transformer under test because of the voltage drops that develop when the capacitive current is ßowing.
For the low-voltage large capacitance windings, the voltage determined by the gap spacing can be more For the low-voltage large capacitance windings, the voltage determined by the gap spacing can be more accurate than the oscillogram record when it is
accurate than the oscillogram record when it is not possible to obtain a not possible to obtain a well-grounded circuit. As pointed outwell-grounded circuit. As pointed out in the discussion, the voltage drop across lead BC in
in the discussion, the voltage drop across lead BC in Þgure 45 might be 25% of Þgure 45 might be 25% of the total voltage measured. If the total voltage measured. If the gap were connected
the gap were connected between A and B, full between A and B, full front-of-wavfront-of-wave voltage would not be e voltage would not be applied to the transformer.applied to the transformer.
Figure 46ÑPreferred grounding method Figure 46ÑPreferred grounding method
Figure 47ÑGrounding with grounded generator Figure 47ÑGrounding with grounded generator
Another method employed is shown in Þgure 48. In this method the
Another method employed is shown in Þgure 48. In this method the shunt can be located close to the groundshunt can be located close to the ground mat or ßoor of the test area, and the exposed lead from the transformer to the shunt can be kept short thus mat or ßoor of the test area, and the exposed lead from the transformer to the shunt can be kept short thus minimizing electrostatic pickup. On the other hand, this method collects all the current ßowing out of the minimizing electrostatic pickup. On the other hand, this method collects all the current ßowing out of the transformer and generally will result in a higher initial inrush current
transformer and generally will result in a higher initial inrush current than the previously described methods.than the previously described methods.
Methods of dealing with high initial current are presented in 2.8 and 2.9.
Methods of dealing with high initial current are presented in 2.8 and 2.9.