Download Method for Static and Dynamic Resistance Measurements of HV

Method for Static and Dynamic Resistance
Measurements of HV Circuit Breaker
Zoran Stanisic
Megger Sweden AB
Stockholm, Sweden
[email protected]
Abstract— S/DRM testing methods usually use long, heavy cables
to connect high current source from ground to the circuit
breaker. Alternative is to move current source close to the circuit
breaker by using heavy batteries, transformers, dc/dc etc. In new
proposed method batteries are replaced with ultra capacitors and
constant current charger, resulting in a few hundred grams of
powerful current source, capable to generate few hundred
amperes into the breaker contacts, or any other power contacts.
Measuring voltage drop across the circuit breaker and the
current through it continuously monitors discharging of ultra
capacitors, giving a high accurate resistance value of the
measured object.
2-wire measurement is the simplest way to assess
resistance. It is generally used when the contact resistance,
series lead resistance or parallel leakage resistances has no
effect on the quality of the measurement. Most multi-meters are
based on 2-wire measurement, which provide sufficient
accuracy in resistance range 10 Ω -10 MΩ.
Keywords-component; contact resistance; circuit breaker; ultra
capacitor
Four-wire testing is the most accurate method when
measuring circuits below 10 Ω as this method eliminates errors
due to lead and contact resistances. This test method is
associated with low resistance ohmmeter and micro ohmmeter.
Four-wire measurements use two current and two sensing leads
as shown in Figure 1. This arrangement allows for cancellation
of voltage drop across current cables thus ensuring accurate
measurement of the resistance. Additional accuracy
enhancement is achieved by using high current source, usually
above 10 A.
I.
INTRODUCTION
Industries that consume electrical power always have
energy losses due to different reasons. Part of these losses
comes from unwanted heating of current conductive parts as a
result of increase in resistance of the conductor itself or the
resistances in contacts. The resistances of high current
components are usually very low (10-1000 μΩ); nevertheless
their increase above acceptable values may lead to serious
problems, since operation of electrical equipment depends on
the controlled flow of current within the design parameters of
the given piece of equipment.
Three-wire technique is used for measurements of very
high resistances (above 10 MΩ) and is typically applied when
measuring insulation of high voltage equipment. The third wire
acts as a guard preventing leakage currents across the surface to
influence measurement.
As examples of elements where resistance must be kept
under control are rail bonds, ground bonds, circuit breaker
contacts, switches, transformer windings and tap changer
contacts, bus bar connections, battery strap connections, motor
windings, squirrel cage bars, bus bar with cable joints and bond
connections to ground beds.
II.
MEASUREMENT TECHNIQUES
Depending of desired accuracy and absolute value of
resistance there are three methods of measuring – 2-wire, 3wire and 4-wire measurements [1].
Figure 1.
Example of 4-wire measurement
III.
EXISTING APPROACHES FOR LOW RESISTANCE
MEASUREMENT
Different methods can be used to measure low contact
resistance of power circuit breakers, isolating switches, and
also of the bus bars and welded joints. Most of these include
heavy weight current source and are based on four contacts
connection to achieve high accuracy and eliminate power
cables resistance. One of the simplest methods is based on
rectified AC current source [2].
Figure 2. Diagram of microohmeter based on rectifed AC.
Figure 4. Voltage and current waveforms using rectified AC current source.
Using this method, high pulsating current passing through
the test object (15 μΩ) is phase shifted with the voltage across
it, which indicate existence of not negligible inductance in the
circuit as shown in Figure 4. . Here resistance measurement
becomes impedance determination by dividing RMS values of
voltage and current. This method can be acceptable if inductive
component in the test circuit can be disregarded and if there is
no need for high accuracy.
In order to avoid the difficulties in resistance measurement
caused by inductive component of the circuit, a DC current
source based micro ohmmeters are widely used. Battery or
DC/DC converter is used as a high current source, able of
generating more than 100 A through the test object.
Battery solution can be heavy and is not attractive because
of that as well as the possibility of battery degradation caused
by accidental short circuiting.
It is possible to measure only resistive component of the
test object by using current pulse injection, a method proposed
in [3] and depicted in Figure 3. . Electrolytic capacitor bank is
charged and then discharged through the test object. PC based
data acquisition is used to record current through and voltage
across the contacts. When the current derivative is zero, voltage
and current value are divided, giving a resistance value.
Current and voltage drop can take higher or lower value,
depending on voltage level that capacitor bank was charged up
and the contact resistance.
It is obvious that noise level of the voltage drop across the
contacts must be filtered in order to achieve high accuracy (see
Figure 5. ). This makes determination of the resistance more
complex and less accurate, especially if current transformer is a
part of the test circuit. Core saturation may interfere with the
voltage measurement, causing resistance calculation very
difficult.
Figure 5. Voltage and current waveforms using discharge of large capacitor
bank.
Figure 3. Low resistance measurement with large capacitor bank.
The concept with DC/DC converter depicted in Figure 6. is
highly promising and giving a high current capability, low
weight and good control possibilities. Current is slowly ramped
up, then kept constant during the resistance measurement and
afterwards ramped down, offering a robust and safe operation.
Still though, there is one problem: EMI.
Switching noise is high and its intensity depends on
generated current. It is evident from Figure 7. that switching
noise has much higher intensity than voltage drop across the
test object. With proper DC/DC topology and filtering the
voltage drop, influence of the switching noise can be
minimized but not eliminated completely.
Figure 6. Microommeter based on switching current source.
10-100A/s, depending on equivalent serial resistance, thus
eliminating the influence of inductance of the circuit.
Battery is not used to generate high current trough the
contacts, instead of that it is used to supply DC/DC converter,
which will then charge ultra capacitor with constant current.
During the resistance measurement, DC/DC is turned off to
eliminate high frequency switching ripple. Only linear
analogue parts are then supplied with the battery, resulting in
ripple free and high accurate, low resistance measurement.
The proposed low resistance meter (Figure 8. ) is based on a
large value capacitor with ultra-low internal resistance, a
current switch, a charger for the capacitor plus control and
measurement circuitry allowing for a high DC current (250 A
to be generated [5]. Compromise between capacitance and
equivalent serial resistance (ESR) must be taken to fulfill
desired operational conditions and specification. Typical
capacitance, ESR value for ultra capacitors used in this
application are 100-600F and 0.5-3 mΩ.
Charging method with constant current provide full control
over charging time and voltage across the capacitor.
By charging ultra capacitor using different voltage values
allows for generation of different currents through the testing
object.
Figure 7. Voltage and current waveforms using switching current source.
Figure 8. Low resistance meter based on large capacitor.
Following descriptions of low resistance measurement
solutions and methods, it becomes apparent that two
parameters are of highest importance, namely low weight and
high accuracy. Possibility to design a portable and accurate
micro ohmmeter is possible when combining all the advantages
of aforementioned methods.
IV.
LOW RESISTANCE METER WITH ULTRA CAPACITORS
Present advances in capacitor technology allows for an ultra
capacitor with capacitance in the range of few hundred Farads
to be used as a current source, thus storing high amount of
energy. When discharging the ultra capacitor, high capacitance
value will then dominate the time constant of the test circuit,
which means that the slope of discharged current is in range of
Measurement of the resistance takes place during a short
(1 s) interval where a transient recorder is employed. The
measurement and the operation of the switch are synchronized
using an operation control (in the transient recorder).
A mains powered supply or battery powers the ohmmeter.
When powered, the control circuitry senses the voltage level of
the capacitor. When the capacitor voltage is below a certain
value the capacitor is charged (using a DC/DC converter giving
a constant current output). Above a preset voltage level the
charger stops charging and signals to the user that the unit is
fully charged.
The measurement is initiated by operating the transient
recorder, which sends a trig signal to the current source. The
current source activates its switch control during a preset time
and the current flows through the resistance, which shall be
measured.
The current is switched on/off using power transistors. The
transient recorder measures both voltage and current and
calculates the resistance or optionally uses an analog dividing
circuit.
During charging and discharging the capacitor and the
surrounding circuitry may become hot. For this reason the
control circuitry is provided which supervises the temperature
and prevents from using the unit while too hot.
V.
STATIC AND DYNAMIC RESISTANCE MEASUREMENT
One application area where low resistance measurements
are frequently employed is assessment of main contact
resistance of high voltage circuit breakers. In addition there are
two specific cases, namely, measurement of static and dynamic
resistance.
An ultracap based current source prototype was build with
350F ultracapacitor, charged to different voltage levels no
higher then 2.7V, giving a possibility to generate currents up to
300Apeak.
Figure 9. Voltage and current waveform using an ultra capacitor
During current generation and measurement the charging
circuitry is disconnected. A typical unit may use a relay to
disconnect the load from the circuitry during charging, to allow
other measurements to be made on the unknown resistance. For
example it could be a high voltage circuit breaker contact,
which has opened during or after the resistance measurement
and it may be desired to use another instrument for contact
timing or for measuring a parallel grading capacitor.
Combining an uncontrolled discharging of ultracapacitor
with dc/dc converter (Figure 10), it is possible to regulate
generated current through the resistor. Doing this, it is possible
to simplify resistance recording by measuring voltage drop
across the resistor only. For example, if output current is
regulated to 100A, then unknown resistance equals to voltage
drop divided by 100. In this case, time for generating constant
current is limited and depends on measured resistance. Care
must be taken of filtering high frequency components to avoid
EMI and influence to measured results. Polyphase dc/dc
converter is good candidate to use in this application, resulting
in efficiency higher than 90% and very low current ripple.
In case of static resistance the current and voltage supplied
by ultra capacitor decay (see Figure 11. ); however providing
that resistance is linear it is possible to simply average its value
over measurement time. Using 12-bit A/D converter the
accuracy of measurements on calibrated current shunts were in
within accuracy limits (±0.5%) and the ripple seen in voltage as
well as resistance graph is due to digitization of analog signal.
Similarly this technique can be applied when testing circuit
breaker contacts in operation. This test is called dynamic
resistance measurement and it reveals the condition of arcing
contacts or other moving electrical contact surfaces. Current is
injected into the contacts few milliseconds before command for
moving is given. When contacts begin to move, voltage drop
and current though them are continuously measured, divided
and displayed as dynamic resistance. By doing so, erosion of
the contact surface as well as shortening of arcing contacts can
be early detected, preventing damage in form of wrong timing
information, high temperature developing or in worst case
explosion.
Figure 11. Traces of measured current and voltage and calculated resistance
during static resistance measurement. Voltage is multiplied by factor 5 for
better illustration.
Figure 10. Low resistance meter based on combination of large capacitor and
polyphase dc/dc power supply.
Rcb = (t / C(lnUmax – lnUc)) – R
Figure 12. depicts an example of dynamic resistance
measurement on high voltage circuit breaker where transition
from static resistance value to open contact is shown.
Where R represent known resistance of cables and static
contacts.
CONCLUSIONS
In this paper several methods for low resistance
measurement were presented and compared. Laboratory
prototype with ultra capacitor solution was build and
experimental result was shown, demonstrating benefits in form
of high accuracy, low weight and extremely high portability.
Figure 12. Example of dynamic resistance measurement on main contacts ogf
high voltage circuit breaker.
This solution has possibility to be used in high and low
power applications. Charging ultra capacitor on different
voltage levels, discharging current is indirectly predicted.
There are possibilities for series/parallel connection of ultra
capacitor based micro-ohm meters, increasing test current to
very high levels. Accuracy of both static and dynamic
resistance measurements depends for the most part on
instrumentation used for voltage and current measurement.
Also here a high current source is used, previously heavy
car batteries or similar, but the large capacitors have solved the
weight and portability issues.
REFERENCES
If only estimation of the dynamic resistance is needed for
calculating position of the circuit breaker contacts during
operation, one simple way to do that is to calculate capacitance
of ultracapacitor during charging with constant current and use
this value to estimate total resistance of contacts, cables and
circuit breaker contacts during discharging of ultracapacitor.
Voltage across the capacitor and discharging time are
continuously measured and resistance of the circuit breaker
contacts is calculated as:
[1]
[2]
[3]
[4]
[5]
A Guide To Low Resistance Testing. Megger Ltd.
Programma MOM200A datasheet. www.megger.com
M. Runde et al. “Condition Assessment of Contacts in Gas-Insulated
Substations”, IEEE Trans. on Power Delivery, vol.19, no.2, April 2004
pp609-617
Programma Mjölner 200 datasheet, www.megger.com
(WO/2009/131530) RESISTANCE MEASUREMENT IN HIGH
POWER APPARATUS ENVIRONMENTS