What To Check When Commissioning Variable Speed Drives (on photo: Control techniques drives at Michelin; by emersonindustrial.com)
The purpose of commissioning VSDs
The main purpose of commissioning VSDs is to ensure that:
The AC converter and motor have been correctly installed and meet the wiring and safety standards such as IEC 60364 or Australian standard AS 3000.
The power and motor cables are correctly sized, installed and terminated.
All power cable shields have been correctly earthed at both ends, to the PE terminal at the converter, at the motor and at the DB or MCC.
The control cables have been installed according to the control system design.
All control cable shields have been correctly earthed at one end only, preferably at the process control system end (‘cleaner’ earth).
There are no faults on the cables prior to energization for the first time.
Selecting the correct application settings
Once all the basic checks have been completed and the commissioning test sheet completed, the VSD is ready for energization.
It is recommended that, when energizing the converter for the first time, that the motor cables should be disconnected until all the basic parameter settings have been installed into the converter.
This will avoid problems with starting the motor in the wrong direction, starting with an acceleration time which is too fast, etc. There is no danger in running a PWM converter with the output side completely open circuit.
Danfoss variable frequency drive setting/operation
Once all the initial settings and on-load checks have been completed, the motor cable can be insulation tested and connected for the final on-load commissioning tests.
Selecting the correct parameter settings
A variable speed drive will only perform correctly if the basic parameters have been correctly set to suit the particular application.
The following are the basic parameters that must be checked before the VSD is connected to a mechanical load:
The correct base voltage must be selected for the supply voltage and to suit the electric motor connected to the output. In Australia, this standard voltage is usually 415 V, 3-phase. This will ensure that the correct output volts/Hz ratio is presented to the motor.
The correct base frequency must be selected for the supply voltage and to suit the electric motor connected to the output. In Australia, this standard frequency is usually 50 Hz. This will ensure that the correct output volts/Hz ratio is presented to the motor.
The connections to the cooling fan should be checked to ensure that the correct tap on the transformer has been selected.
Thereafter, the remaining parameters settings can be selected as follows:
1. Maximum speed
Maximum speed, usually set to 50 Hz, but often set to a higher speed to suit the application. Ensure that the maximum speed does not take the drive beyond the loadability limit.
2. Minimum speed
Minimum speed, usually 0 Hz for a pump or fan drive, but often set at a higher speed to suit constant torque applications. Ensure that the minimum speed does not take the drive below the loadability limit.
3. Rated current of the motor
Rated current of the motor, this depends on the size of the motor relative to the rating of the converter. The current rating of the converter should always be equal to or higher than the motor rating. For adequate protection of the motor, the correct current rating should be chosen.
4. Current limit
Current limit, determines the starting torque of the motor. If a high breakaway torque is expected, a setting of up to 150% will provide the highest starting torque.
5. Acceleration time
Acceleration time, determines the ramp-up time from zero to maximum speed. This should be chosen in relation to the inertia of the mechanical load and the type of application. For example, in a pumping application, the acceleration time should be slow enough to prevent water hammer in the pipes.
6. Deceleration time
Deceleration time, determines the ramp-down time from maximum speed to zero. This setting is only applicable if the ‘ramp to stop’ option is selected. Other alternatives are usually ‘coast to stop’ and ‘DC braking’.
On high inertia loads, this should not be set too short. If the deceleration time is below the natural rundown time of the load, the DC voltage will rise to a high level and could result in unexpected tripping on ‘over-voltage’.
The deceleration time can only be shorter than the natural rundown time if a dynamic braking resistor has been fitted.
7. Starting torque boost
Starting torque boost, can be selected if the load exhibits a high breakaway torque. This feature should be used cautiously to prevent over-fluxing of the motor at low speeds. Too high a setting can result in motor over-heating.
Only sufficient torque boost should be selected to ensure that the VSD exceeds the breakaway torque of the load during starting.
Final words //
There are also many other settings commonly required on modern digital VSDs.
The above are the most important and must be checked before starting. The remaining parameters usually have a ‘default’ setting which will probably be adequate for most applications. However, these should be checked and adjusted for optimum operation.
Reference // Practical Variable Speed Drives and Power Electronics - Malcolm Barnes (Get hardcopy from Amazon)
Maintenance and testing of the overcurrent protective devices (on photo: SIEMENS’s new 3VA2 molded case ciruit breaker up to 630 A, ETU, 400/690 V, up to 150 kA)
Electrical system reliability
When designing electrical distribution systems, required maintenance and testing of the overcurrent protective devices is a very important consideration.
The electrical system reliability, component and circuit protection, and overall safety are directly related to the reliability and performance of the overcurrent protective device and can depend upon whether the required testing and maintenance are performed as prescribed for the overcurrent protective device utilized.
The required maintenance and testing of the system can depend upon the type of overcurrent protective device selected.
Circuit Breakers //
Many engineers and owners view molded case circuit breaker systems as “easy”…just install it, reset the devices if needed and walk away. However, periodic testing and maintenance of circuit breakers is extremely important to the system reliability and protection.
Schneider Electric’s 3-phase molded case circuit breaker type NSX 630A
NFPA 70B
NFPA 70B – Recommended Practice for Electrical Equipment Maintenance indicates that testing and maintenance of molded case circuit breakers should be completed every 6 months to 3 years, depending upon the conditions of use.
This includes typical maintenance such as:
Tightening of connections,
Checking for signs of overheating, and
Checking for any structural defects or cracks.
Manual operation of the circuit breaker is typically recommended to be completed once per year. Testing of molded case circuit breakers to assure proper overcurrent protection and operation is also recommended during this period.
This includes removing the circuit breaker and verifying the protection and operation for overloads (typically 300%) with the manufacturer’s overcurrent trip data. Additional molded case circuit breaker (MCCB) testing of insulation resistance, individual pole resistance, rated hold-in, and instantaneous operation are recommended by NEMA and may require special testing equipment.
It is important to realize that if a deficiency is discovered during testing and maintenance, the only solution is to replace a molded case circuit breaker because adjustments or repairs cannot be made to this type of device. In addition, replacement is typically recommended after the molded case circuit breaker has interrupted a short-circuit current near its marked interrupted rating. This process results in additional expenses and may involve delays in finding a replacement device.
Per NFPA 70B, testing and maintenance of low-voltage power circuit breakers is even more expansive and can be required after tripping on an overcurrent condition. It is important to realize that the maintenance and testing of these devices can only be completed by a qualified person.
Often special testing companies are used for this purpose or the device must be sent back to the manufacturer, requiring spare devices during this period.
The question is, how often is this completed?
In commercial installations, the answer is probably never. This lack of maintenance and testing can adversely affect the reliability and protection capabilities during overcurrent conditions in the electrical distribution system.
Fuses
NFPA 70B recommends checking fuse continuity during scheduled maintenance, but testing to assure proper operation and protection against overcurrent conditions is not required.
Fusible switches and fuse blocks require maintenance, such as tightening of connections and checking for signs of overheating as recommended per NFPA 70B.
A fused 3-phase safety switch serves as the PV service disconnect at a site employing a supply side connection
Resetting Overcurrent Protective Devices
As mentioned previously, circuit breakers are sometimes selected over fuses because circuit breakers can be reset where fuses have to be replaced. The most time consuming activity that results from the operation of the overcurrent protective device is typically investigating the cause of the overcurrent condition.
A known overload condition is the only situation that permits the immediate resetting or replacement of overcurrent protective devices per OSHA. If the cause for the operation of an overcurrent protective device is not known, the cause must be investigated.
Thus, having a device that can be easily reset, such as a circuit breaker, possibly into a fault condition, could be a safety hazard and a violation of OSHA regulations.
Because a fuse requires replacement by a qualified person, it is less likely to violate OSHA. Also, when an opened fuse is replaced with a new fuse in the circuit, the circuit is protected by a new factory calibrated device.
Generally, overload conditions occur on branch-circuit devices. Typically this is on lighting and appliance circuits feed from circuit breaker panelboards, where resetting of circuit breakers may be possible. Motor circuits also are subject to overload considerations.
However, typically the device that operates is the overload relay, which can be easily reset after an overload situation. The motor branch-circuit device (fuse or circuit breaker) operates, as indicated in NEC® 430.52, for protection of short-circuits and ground-fault conditions. Thus, if this device opens, it should not be reset or replaced without investigating the circuit since it most likely was a short-circuit condition.
Can’t see this video? Click here to watch it on Youtube.
Overcurrent conditions in feeders and mains are typically the result of short-circuits and are infrequent. Because they are most likely short-circuits, the circuit should be investigated first before resetting or replacing as well.
Also, if a feeder or main is protected by a circuit breaker that has opened, the circuit breaker should be examined and tested to be sure it is suitable to be placed back in service.
Reference // Electrical Plan Review – Overcurrent Protection and Devices, Short-Circuit Calculations, Component Protection, Selective Coordination, and Other Considerations – COPPER BUSSMANN
Procedure for Transformer Lightning Impulse Test (photo credit: ABB)
Purpose of the test
The purpose of the impulse voltage test is to secure that the transformer insultations withstand the lightning overvoltages which may occur in service.
Testing equipment
Impulse generator
Figure 1 – Basic circuit diagram of the impulse generator
Where:
C1 – Impulse capacitor
Rc – Charging resistor
Rs – Series resistor
Ra – Low-ohmic discharging resistor for switching impulse,
Rb – High-ohmic discharging resistor for switching impulse
F1…Fn – Main spark-gaps,
Fal…Fan – auxiliary spark-gaps
The impulse generator design is based on the Marx circuit. The basic circuit diagram is shown on Figure 1 above. The impulse capacitors Cs (12 capacitors of 750 nF) are charged in parallel through the charging resistors Rc (45 kΩ) (highest permissible charging voltage 200 kV).
When the charging voltage has reached the requider value, breakdown of the spark-gap F1 is initiated by an external triggering pulse. When F1 breaks down, the potential of the following stage (points B and C) rises. Because the series resistor Rs is of low ohmic value compared with the discharging resistor Rb (4.5 kΩ) and the charging resistor Rc, and since the low-ohmic resistor.
Ra is separated from the circuit by the auxiliary spark-gap Fa1, the potential difference across the spark-gab F2 rises considerably and the breakdown of F2 is is initiated. Thus the spark-gaps are caused to break down in sequence.
Concequently the capacitors are discharged in series-connection. The high ohmic discharge resistors Rb are dimensioned for switching impulses and the low-ohmic resistors Ra for lightning impulses. The resistors Ra are connected in parallel with the resistors Rb, when the auxiliary spark-gaps break down, with a time dalay of a few hundred nanoseconds.
This arrangement is necessary in order to secure the functioning of the generator.
The required voltage is obtained by selecting a suitable number of seriesconnected stages and by adjusting the charging voltage. In order to obtain the necessary disscharge energy parallel or series-parallall connections of the generator can be used. In these cases some of the capacitors are connected in parallel during the discharge.
Max. test voltage amplitudes: 2.1 MV lightning impulse. 1.6 MV switching impulse.
Figure 2 – Equivalent diagram of the impulse test circuit
Where:
Cr – Resulting impulse capacitance
Rsr – Resulting series resistance
Rar – Resulting discharge resistance
Lr Lp – Stray inductances
Ci – Input capacitance of transformer
Li – Transformer inductance
C1 – Capacitance of voltage divider
F1 – Spark gaps of impulse generator
F2 – Calibration sphere gap
R2 – Protective resistor.
The required impulse shape is obtained by selecting the series and discharge resistors of the generator suitably.
The front time can be calculated approximately from the equation:
T1 ≈ 2,5 · Rsr · (Ci + C1)(formulae 1)
and the time to half value from the equation:
T2 ≈ k · √(Li · Cr)(formulae 2)
The factor k depends on the quantities Rsr, Rar, Li and Cr. In practice the testing circuit is dimensioned according to experience.
Voltage measuring circuit
The impulse shape and the peak value of the impulse voltage are measured by means of an oscilloscope and a peak voltmeter which are connected to the voltage divider (Figure 3). The measuring range can be changed by shortcircuiting part of the high voltage capacitors or changing the low voltage capacitor of the divider.
Figure 3 – The impulse voltage measuring circuit
Where:
E – Damped capacitive voltage divider
W – Measuring cable (= wave impedance = Rp)
P1 – Oscilloscope
P2 – Peak voltmeter
Rp – Terminal resistance of the measuring cable
R1 – Damping resistor of voltage divider
C1 – High voltage capacitor of voltage divider
C2 – Low voltage capacitor of divider.
The measuring circuit is checked in accordance with the standards (formulae 2) and (formulae 3). If necessary the sphere-gap calibration of the measuring circuit can be performed in connection with the testing according to the standard (figure 4 below).
Transformer testing and fault detection connections
The lightning impulse test is normally applied to all windings. The impulse testsequency is applied successively to each of the line terminals of the tested winding. The other line terminals and the neutral terminal are earthed (singleterminal test, Figure 4a and 4b).
Figure 4 – Transformer impulse testing and fault detection connections
Where:
a, b – 1-terminal testing
c – 3-terminal testing
d – 2-terminal testing
e – test with transferred voltages
f – neutranl terminal testing
When testing low voltage windings of high power the time to half-value obtained is often too short. However, the time to half-value can be increased by connecting suitable resistors (Ra in Figure 4b) between the adjacent terminals and earth.
According to the standard IEC 76-3 the resistances of the resistors must be selected so that the voltages at the adjacent terminals do not exceed 75 % of the test voltage and the resistance does not exceed 500 Ω.
A delta-connected winding (and star-connected winding, unless the neutral is available) is also tested with an impulse test-sequence applied to the line terminals of the tested winding connected together, while the other windings are earthed (three-terminal test, Figure 4c).
For delta-connected windings the single and three-terminal testings can be combined by applying the impulse to two line terminals at a time, while the other line terminals are earthed (two-terminal testing, Figure 4d). In this case two phases are simultaneously tested in a single-terminal connection and one phase in a test connection corresponding to three-terminal testing.
The two- and three-terminal testings are not included in the standard, but they can be done if it is so agreed.
When the low voltage winding cannot in service be subjected to lighting overvoltages from the low voltage system (e.g. step-up transformers, tertiary windings) the low voltage winding may (by agreement between customer and manufackturer) be impulse tested simultaneously with the impulse tests on the high voltage winding with surges transferred from the high voltage winding to the low voltage winding (Figure 4e, test with transferred voltages).
According to IEC 76-3 the line terminals of the low voltage winding are connected to earth through resistances of such value (resistances Ra in Figure 4e) that the amplitude of transferred impulse voltage between line terminal and earth or between different line terminals or across a phase winding will be as high as possible but not exceeding the rated impulse withstand voltage.
The resistance shall not exceed 5000 Ω. The neutral terminal is normally tested indirectly by connecting a high-ohmic resistor between the neutral and earth (voltage divider Ra, Ru) and by appluying the impulse (Figure 4d) to the line terminals connected together.
The impulse test of a neutral terminal is performed only if requested by the customer.
For fault detection in single-terminal and two-terminal tests the neutral of star-connected windings are earthed via a low-ohmic resistor (Ru). The current flowing through the detection resistor during the test is rocorded by means of an oscilloscope. Evidence of insultaion failure arising from the test would be given significant discrepacies between the calibration impulse application and the full voltage applications in recorded current wave-shapes.
Certain types of faults give rise to discrepancies in the recorded voltage wave-shapes as well.
For fault detection in three-terminal tests and tests on the neutral terminal the adjacent winding is earthed through a low-ohmic resistor. The fault detection is then based on recording the capacitive current which is transferred to the adjacent winding.
Lightning Impulse Test on Transformer (400KV/15KV, 160MVA)
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When DC voltage is applied to an insulation, the electric field stress gives rise to current conduction and electrical polarization. Consider an elementary circuit as shown in Figure 1 below, which shows a DC voltage source, a switch, and an insulation specimen.
When the switch is closed, the insulation becomes electrified and a very high current flows at the instant the switch is closed.
However, this current immediately drops in value, and then decreases at a slower rate until it reaches a nearly constant value.
The current drawn by the insulation may be analyzed into several components as follows:
The capacitance charging current is high as the DC voltage is applied and can be calculated by the formula:
Figure 1 – Electrical circuit of insulation under DC voltage test
C represents charging current
RA represents absorption current
RL represents volumetric leakage current (dielectric loss)
where:
ie is the capacitance charging current
E is the voltage in kilovolts
R is the resistance in megohms
C is the capacitance in microfarads
t is the time in seconds
e is Napierian logarithmic base
The charging current is a function of time and will decrease as the time of the application of voltage increases. It is the initial charging current when voltage is applied and therefore not of any value for test evaluation.
Test readings should not be taken until this current has decreased to a sufficiently low value.
The dielectric absorption current is also high as the test voltage is applied and decreases as the voltage application time increases, but at a slower rate than the capacitance charging current. This current is not as high as the capacitance charging current.
The absorption current can be divided into two currents called reversible and irreversible charging currents. This reversible charging current can be calculated by the formula:
ia = VCDT−n
where:
ia is the dielectric absorption current
V is the test voltage in kilovolts
C is the capacitance in microfarads
D is the proportionately constant
T is the time in seconds
n is a constant
The irreversible charging current is of the same general form as the reversible charging current, but is much smaller in magnitude. The irreversible charging current is lost in the insulation and thus is not recoverable.
Again, sufficient time should be allowed before recording test data so that the revers- ible absorption current has decreased to a low value.
The surface leakage current is due to the conduction on the surface of the insulation where the conductor emerges and points of ground potential.
This current is not desired in the test results and should therefore be eliminated by carefully cleaning the surface of the conductor to eliminate the leakage paths, or should be captured and guarded out of the meter reading.
The partial discharge current, also known as corona current, is caused by overstressing of air at sharp corners of the conductor due to high test voltage. This current is not desirable and should be eliminated by the use of stress control shielding at such points during tests.
This current does not occur at lower voltages (below 4000 volts), such as insulation resistance test voltages.
The volumetric leakage current that flows through the insulation volume itself is of primary importance. This is the current that is used to evaluate the conditions of the insulation system under test. Sufficient time should be allowed for the volumetric current to stabilize before test readings are recorded.
The total current, consisting of various leakage currents as described above, is shown in Figure 2.
Figure 2 – Various leakage currents due to the application of DC high voltage to an insulation system
The remainder of the twelve factory tests are briefly summarized below. The details of the test set connections and formulas of some of the listed tests are already described in separatly published articles, and for the rest you are directed to ANSI/IEEE Standard C57.12.90 for these details.
This list is not complete, there are few tests missing, not mentioned here, like Turn ratio test or Measurement of voltage ratio and check of phase displacement, but you can find them also separatly published at EEP (use Search).
The tests measures the no-load losses at specified excitation voltage and a specified frequency. Sine-wave voltages are used unless a different waveform is inherent in the operation of the transformer.
The recommended method is the average-voltage voltmeter method, employing two parallel-connected voltmeters. One voltmeter is an average-responding but RMS calibrated voltmeter and the other voltmeter is a true RMS-responding voltmeter.
The test voltage is adjusted to the specified value as read by the average-responding voltmeters. The readings of both voltmeters are used to correct the no-load losses to a sine-wave basis.
This current is measured in the winding used to excite the transformer with the other windings open-circuited. It is generally expressed in percent of the rated current of the winding. No-load excitation current is not sinusoidal and contains, as we have seen, odd harmonics (predominantly third harmonic current).
The ammeter used to record the no-load excitation current is an RMS meter which reads the square root of the sum of the squares of the harmonic currents.
The transformer must be in a specific state before the load losses and impedance voltage are measured. The temperature of the insulating liquid must be stabilized and the difference between the top and bottom oil temperatures shall be less than 5°C.
The winding temperatures must be measured (using a resistance method) before and after the test and the average taken as the true temperature. The difference in the winding temperature before and after the test must not exceed 5°C.
The two test methods for measuring load losses and impedance voltage are:
Wattmeter-voltmeter-ammeter method and
Impedance bridge method.
Circuit for the impedance and load-loss measurement
These tests generally apply a reduced voltage to one set of windings with the other set of windings short-circuited. For three-winding transformers, these tests are repeated for each combination of windings taken two at a time.
These tests consist of applied-voltage tests and induced-voltage tests.
Applied-voltage tests apply a high voltage to all bushings of a winding, one winding at a time, with the other windings grounded. A 60 Hz voltage is increased gradually over 15 s and held for 40 s and reduced to zero over 5 s.
Induced-voltage tests apply a high voltage across a winding with the other windings open-circuited in order to test the quality of the turn-to-turn insulation. In order to prevent core saturation at the higher excitation voltage, the frequency of the induced-voltage test is increased (typically around 120 Hz). The induced voltage is applied for 7200 cycles or 60 s, whichever is shorter.
The switching impulse test applies a switching impulse wave between each high-voltage line terminal and ground.
The test series consists of one reduced voltage wave (50%– 70% of specified test level) followed by two full-voltage waves. Either positive or negative polarity waves, or both, may be used. A voltage oscillogram is taken for each applied wave. The test is successful if there is no sudden collapse of voltage. Successive oscillograms may differ because of the influence of core saturation.
The test sequence consists of one reduced full wave, two chopped waves, and two full waves. Tap connections are made with the minimum effective turns in the winding under tests and regulating transformers are set to the maximum buck position. Oscillograms are taken of each wave.
The general technique for interpreting the results is to look for differences in the shapes of the reduced full wave and the two full waves, which indicate turn-to-turn insulation failure.
Transformer impulse testing and fault detection connections
Additional test criteria are found in IEEE Std. C57.98-1993. The impulse tests probably have the highest likelihood failures among all of the factory tests that are typically performed.
This test detects radio-frequency (0.85–1.15 MHz) noise generated from partial discharges within voids in the insulation. An applied voltage is gradually increased until partial discharge starts to occur, which is the inception voltage. The voltage is then decreased until the partial discharge stops, which is the extinction voltage.
The extinction voltage must be less than the operating voltage of the transformer; otherwise, once partial discharge starts in the field (due to some voltage transient), it would continue indefinitely and possibly cause damage or failure.
Insulation power factor is the ratio of the power dissipated in the insulation in watts to the apparent power (volt-amperes) under a sinusoidal voltage. The applied 60 Hz voltage of this test is generally lower than the operating voltage of the trans- former. The Doble Test Set is designed specifically to carry out this test.
Portable versions are used to measure the insulation power factor of transformers in the field. This test usually must be done by a trained technician. The test results are temperature-corrected to a reference temperature of 20°C.
This test applies a high-voltage DC voltage to one winding at a time with the other windings grounded. The leakage current is measured and the insulation resistance is calculated using Ohm’s law.
A Insulation resistance test set is designed specifically to carry out this test, and its meter is calibrated in megohms in order that the calculation may be avoided. The Megger as well as other manufacturers has a portable instrument that can easily carried around in the field.
The noise measurement test is carried out while the transformer is energized at rated voltage with all of the cooling equipment running. Room geometry can greatly affect the measurements, so it is preferable that the transformer be inside an anechoic chamber. However, if such a chamber is not available, no acoustically reflecting surface may be within 3 m of the measuring microphone other than the floor or ground.
The recording microphones are positioned in 1 m intervals around the perimeter of the transformer, with no fewer than four (4) microphone positions for small transformers..
Noise measurement zone and layout of measuring points of Power transformer 242 kV / 15.65 kV, 112 MVA (credit: Aleksandra Petrovic, Ljubomir Lukic, Milan Kolarevic and Dusica Lukic at University of Kragujevac)
Sound power levels are measured over a specified band of frequencies. The sound power levels are converted into decibels (dB).
The transformer is energized at rated voltage in order to generate core losses. The windings are connected to a loading transformer that simultaneously circulates rated currents through all of the windings in order to develop load losses.
Naturally, the excitation voltage and the applied circulating currents are electrically 90° apart to minimize the KW requirements for this test. Nonetheless, a large power transformer can consume up to 1 MW of total losses and the heat run test is an expensive test to perform.
Therefore, in order to reduce the total expense, heat run tests are normally performed on only one transformer on a purchase order for multiple transformers, unless the customer chooses to pay for testing additional units.
The short-circuit test is generally reserved for a sample transformer to verify the design of a core and coil assembly unless the customer specifies that a short-circuit test be performed on transformers that are purchased.
The customer should be cognizant of the ever-present risk of damaging the transformer during short-circuit tests.
A low-voltage impulse (LVI) current waveform is applied to the transformer before and after the applications of short-circuit test. The ‘‘before’’ and ‘‘after’’ oscillograms of the LVI currents are compared for significant changes in waveshape that could indicate mechanical damage to the windings.
This is the simple list of basic terms you can often hear when testing and measurements of electrical installation (in general) is being performed. While expirienced electrical engineers will find this list short, I hope beginners will catch the essence and continue exploring this field of electrical engineering.
Feel free to suggest me an expression (along with description) you think it should be listed, it will be my pleasure to add it to the list and to move away from number 13
Active accessible conductive part is the conductive part of an electrical installation or appliance such as the housing, part of a housing etc. which can be touched by a human body. Such an accessible part is free of mains voltage except under fault conditions.
Electric shock is the pathophysic effect of an electric current flowing through a human or animal body. Very dangerous, have eyes on your back while testing.
It’s very important to know what to do if electric shock occurs.
Electric shock – Emergency resuscitation procedures (photo credit: uksafetystore.com)
Earthing electrode is a conductive part, or a group of conductive parts, which are placed into earth and thus assure a good and permanent contact with ground.
Nominal voltage (Un) is the voltage which electrical installations or components of electrical installations, such as appliances, loads etc. are rated at. Some installation characteristics also refer to nominal voltage (e.g. power).
Fault voltage (Uf) is the voltage that appears between the active accessible conductive parts and the passive ones or ideal ground in the case of a fault on appliances connected to the mains installation (connected appliance).
The figure below represents the Fault voltage (Uf) and division of the voltage into the Contact voltage (Uc) and voltage drop on floor/shoes resistance (Us).
Figure 1 – Presentation of the voltages Uf, Uc and Us in case of a fault on an electric load
Where:
ZB – Impedance of human body
RS – Floor and shoes resistance
RE – Earth Resistance of active accessible conductive parts
If – Fault current
Uc – Contact voltage
Us – Voltage drop on floor/shoes resistance
Uf – Fault voltage
Uf = Uc + Us = If × RE (floor material is placed to ideal ground)
Contact voltage (Uc) is the voltage to which a human body is exposed when touching an active accessible conductive part. The body is standing on the floor or is in contact with passive accessible conductive part.
Measuring: Contact voltage is measured between the earthing electrode and two measurement electrodes connected together and placed 1m away from tested earthing electrode.
Voltage apportion across the Earth Resistance – voltage funnel
There are certain situations where it is desirable for a room to be totally isolated from the Protective Earth conductor (e.g. for conducting special tests in a laboratory etc.). These rooms are regarded as an electrically safe area and the walls and floor should be made of non-conductive materials.
The arrangement of any electrical equipment in those rooms should be of such a manner that:
It is not possible for two live conductors , with different potentials , to be touched simultaneously in the case of a basic insulation fault.
It is not possible for any combination of active and passive accessible conductive parts to be touched simultaneously.
A protection conductor PE that could drive a dangerous fault voltage down to the ground potential is not allowed in non-conductive rooms. Non-conductive walls and floors protect the operator in case of a basic insulation fault.
The resistance of non-conductive walls and floors shall be measured with an Insulation Resistance tester using the procedure described below. Special measurement electrodes described below are to be used.
Figure 1 – Measurement electrode
The measurement is to be carried out between the measurement electrode and the protection conductor PE, which is only accessible outside of the tested non-conductive room. To create a better electrical contact, a wet patch (270 mm × 270 mm) shall be placed between the measurement electrode and the surface under test.
A force of 750N (floor measurement) or 250N (wall measurement) shall be applied to the electrode during the measurement.
The value of test voltage shall be:
500 V – where the nominal mains voltage with respect to ground is lower than 500 V
1000 V – where the nominal mains voltage with respect to ground is higher than 500 V
The value of the measured and corrected test result must be higher than:
50 kW – where the nominal mains voltage with respect to ground is lower than 500V
100 kW – where the nominal mains voltage with respect to ground is higher than 500 V
Two important notes //
It is advisable that the measurement to be carried out using both polarities of test voltage (reversed test terminals) and the average of both results be taken.
Wait until the test result is stabilized before taking the reading.
Figure 2 – Resistance of walls and floor measurement using METREL’s Eurotest, Instaltest or Earth-Insulationtester
Reference // Measurements on electric installations in theory and practice – METREL (Download guide)
Yes, PLC programs are never final, it is always possible to make corrections and subsequent adaptations to new system according the customer requirements. Even during commissioning, program changes are often necessary. The commissioning of a system can be divided into four steps:
Each sensor, switch and button is connected to a specific input and each actuator to an output. During engineering process addresses and wires must not be mixed up. Also, the sensors and actuator placing should be checked (that they are where they have to be in the automated system).
During checking procedure, the outputs are set in a test mode. The actuators must then meet the specified requirements (functions). If changes are made, then the documentation (allocation list, drawings, etc) must also be updated to respond to reality.
PLC hardware (photo credit: BR Automation)
1.1 Testing inputs and outputs
Input devices, e.g. switches, can be manipulated to give the open and closed contact conditions and the corresponding LED on the input module observed. It should be illuminated when the input is closed and not illuminated when it is open.
Failure of an LED to illuminate could be because the input device is not correctly operating, there are incorrect wiring connections to the input module, the input device is not correctly powered or the LED or input module is defective. For output devices that can be safely started, push buttons might have been installed so that each output can be tested.
Prior to commissioning, all available off-line and virtual PLC program testing tools should be used intensively to find program faults. For example, such test tool is in STEP 7 as subprogram S7-PLCSIM. It simulates the work of a PLC (virtual PLC) and allows the user written PLC program to be tested.
Following this, the program is transferred to a central processing unit in the virtual PLC. The entire program is executed without using the real PLC.
The user has to simulate the input signal changes and verify how the outputs react to it. Some PLCs offer simulation in a real PLC: the entire program is executed in a PLC without the real inputs and outputs being connected. Processing of the PLC outputs thus only takes place in the PLC image table. The physical PLC I/Os are not updated to/from the PLC I/O images.
Therefore this eliminates the risk of damaging machines or system parts.
After this, the individual user program parts and system functions are tested: manual operation, setting, individual monitoring programs etc. and finally the interaction of the program parts with the help of the main program.
The system can and should be commissioned step-by-step. Important aspects of commissioning and fault detection are the test functions of the programming system, such as the single-step mode or the setting of stop points. The single-step mode in particular is of importance, whereby the program in the PLC memory is executed line-by-line or step-by-step. In this way, any program faults which may occur in the program can be immediately localized.
User programs can almost always be improved after the first test run. It is important that any corrections or modifications are made not just in the PLC user program, but are also taken into account in the documentation.
4. Commissioning of the entire system
This already occurs in part during the testing and optimization phase. Once the final status of the PLC user program and the documentation is established, all the controller functions (in accordance with the automation task) need to be executed step-by-step again.
If no faults occur by the entire system commissioning, then the system is ready to be handed over to the customer.
PLC panel during commissioning (photo credit: megawatts.com.sg)
One month after initial energization and annually thereafter
Gauge readings, ambient temperature, and kvA load should be measured and recorded. Any abnormal reading suggests that further diagnostic testing or inspection should be done. If pressure/vacuum gauge and/or fluid level gauge readings suggest a possible tank leak, perform a pressure test according to instructions (15 Pre-energization Tests and Checklist).
Tank leaks must be repaired immediately to prevent serious damage to the transformer and danger to life.
Transformer winding and liquid temperature measuring (photo credit: acontact.com)
Check the cooling fans (if any) by setting the fan “auto/manual” control switch to the “manual” position. The fans should rotate at full speed within approximately five seconds. The fans should rotate smoothly with minimal vibration.
Control wiring should be checked to insure that wire insulation is in good condition. The control cabinet and associated conduit should be inspected to ensure that weather seals are intact.
Control power supply voltage should be checked and compared to the voltage stated on the wiring diagram.
Transformer control wiring box to check (photo credit: canammachinery.com)
Sample the insulating fluid as described below. The dielectric strength of the insulating fluid should measure at least 26 kV.
Sampling of Insulating Fluid //
Transformers are filled with insulating fluid, which provides electrical insulation within the transformer tank and transfers heat generated in the coils to the tank wall and radiators.
The fluid is either:
Conventional transformer oil (mineral oil),
Envirotemp® FR3 fluid, or
Silicone fluid.
Periodically check the transformer for proper fluid level by reading the fluid level gauge. Add fluid if necessary. When adding fluid, add only the same type fluid that is in the transformer.
It is also recommended that a fluid sample be drawn annually and tested for dielectric strength. Samples should be drawn from the bottom of the tank. Use proper sampling procedures to prevent erroneous test results. Dielectric strength should measure 26 kv minimum.
Bushing and surge arrester insulators should be clean. If the surfaces are excessively dirty, they should be cleaned while the transformer is not energized.
Arcing horns on transformer bushings – for lightning protection (photo credit: Wikipedia)
One month after initial energization and annually thereafter
If the transformer is energized and under load, measure bushing terminal temperatures using an infrared scanner. Excessive bushing terminal temperature indicates a loose or dirty connection. If the transformer is not energized, use a torque measuring device to make sure terminal connections are tight.
Infrared (IR) scan of a transformer bushings (photo credit: npm-services.com)
Visually check all gaskets for cracking or other signs of deterioration. Replace as necessary.
When replacing a gasket carefully clean mating surfaces to remove any rust, dirt, transformer fluid, old gasket material, or other contamination that might prevent a good seal. Use an appropriate gasket cement when installing new gaskets.
Do not reuse old gaskets. Six months after replacing a gasket, check and retighten if necessary.
Transformer seal gaskets to check for for cracking or other signs of deterioration (photo credit: powermag.com)
Reference // Instructions Installation, Operation, and Maintenance of Medium Power Substation Transformers – Howard Industries // Substation Transformer Division
Large ground systems, such as those found in substations and power stations, are an important part of the protection of the electricity supply network. They ensure that fault current will enable protective devices to operate correctly. A substation must have a low ground resistance to reduce excessive voltages developing during a fault which could endanger safety of nearby people or damage equipment.
When installing a ground system the resistivity of the surrounding soil should be measured. Inaccurate resistivity tests can lead to unnecessary costs in the design of the system.
Checking the earth resistance of a ground system at a substation using a Megger ground tester
After installation it is vital to check that the electrical grounding system meets the design criteria and should be measured periodically to ensure corrosion or changes in the soil’s resistivity do not have an adverse effect. Ground networks may not appear faulty until a fault occurs and a dangerous situation arises.
To obtain a sufficiently low value of ground resistance, ground systems may consist of an earth mat covering a large area or many interconnected rods.
Suitable test techniques must be used for large systems to ensure that valid readings are obtained. This is unlike a small single ground rod (for example, a lightning protection system or residential ground) which can be simple to test.
Testing Challenges in Large Ground Systems
Securing valid measurements when testing large ground systems requires that proper techniques and instrumentation be used. The nature of substation and power station grounding systems and related conditions make testing far more complex than on a simple ground rod.
Following are the three key challenges in testing substation ground systems //
1 // The physically large area of a substation/power station ground system results in a large “resistance area” and, consequently, long distances to the test probes; ideally, the current test probe should be placed 10 times the maximum distance on the ground system (e.g., 3000 ft for a 300 ft2 ground grid) to find the “flat” portion of the characteristic resistance curve.
2 // The large “resistance area” typically gives ground resistance values of less than 0.5 Ω. Test instrument resolution is critical if small variances in readings are to be observed.
If the test instrument does not have suitable resolution, instrument errors can overwhelm the results.
3 //Large electrical networks contain noise consisting of the frequency of the power utility and its harmonics, plus high frequency noise from switching, etc., and induced signals from other sources. The ground tester must retrieve and analyze a small test signal in a much larger test environment.
Most ground testers only inject a single frequency (usually 128 Hz) which is adequate in most situations because it avoids harmonics of standard line frequencies. Unfortunately, it is often not adequate in substations. This type of interference can cause significant measurement errors.
Addressing the Testing Challenges in Large Ground Systems
In the ideal world, testing a large ground system would be conducted in complete accordance with the Fall-of Potential Method. Unfortunately, the large “resistance areas” found in large ground systems may make it unfeasible or even impossible to carry out this test.
Measuring earth resistance with fall of potential method (photo credit: eblogbd.com)
As noted above, setting the current test probe 10 times the maximum distance of the ground system can require leads to be many thousands of feet. In these situations, the Slope Method can be used effectively because it does not require the user to find the “flat” portion of the curve or to know the electrical center as a point from which to measure.
Readings are taken at 20 percent, 40 percent and 60 percent of the current probe distance and fit into a mathematical model of the resistance characteristic.
The other challenges faced in testing large ground systems relate to the capabilities of the test instrument. Improved technology has made it possible for instruments to be designed that address problems created by the characteristics and conditions found in and around large ground systems.
For the Slope Method to provide meaningful results, accurate measurement of the variations at different points is critical. Since large ground systems typically have resistance values of less than 0.5 Ω, the differences can be quite small. An instrument with 1 mΩ measurement resolution can indicate the small differences between low readings.
Potential probe locations for using the slope method (figure credit: Whitham D. Reeve)
Noise is a major problem in testing large ground systems, and must be addressed to ensure accurate results. To be effective, the test instrument must be designed to overcome the effects of significant noise in the test environment.
Among the technical capabilities that can help offset the noise problem are:
A variable test frequency (rather than a single, fixed test frequency) which can help remove any stray noise that could affect the reading.
A high peak-to-peak interference suppression level.
A sophisticated filter system to reject more noise.
Various current settings to improve the signal-to-noise ratio when necessary.
Reference // Getting Down To Earth – MEGGER (Download guide)
The following schemes show how to connect a Megger insulation tester to various types of electrical equipment. The schemes also show in principle how equipment must be disconnected from other circuits before the instrument is connected.
These illustrations are typical and will serve as guides for testing insulation resistance of practically all types of apparatus and conductors. Before proceeding with tests, read the part on Preparation of apparatus for test at the bottom of this article.
REMEMBER! The Megger insulation resistance tester measures whatever resistance is connected between its terminals. This may include series or parallel leakage paths through insulation or over its surface.
Connections for testing the insulation resistance of a motor, starting equipment and connecting lines, in parallel. Note that the starter switch is in the “on” position for the test. It is always preferable to disconnect the component parts and test them separately in order to determine where weaknesses exist.
Figure 1 – Testing the insulation resistance of a motor
With the brushes raised as indicated, the brush rigging and field coils can be tested separately from the armature. Likewise the armature can be tested by itself. With the brushes lowered, the test will be that of brush rigging, field coils and armature combined.
Figure 2 – Testing the insulation resistance of DC generators and motors
Connections for testing to ground each circuit separately, working from the distribution panel.
Figure 3a – Testing the insulation resistance of wiring installation Figure 3b – Testing the insulation resistance of main power board
Connections at the main power board, from which point the entire system can be tested to ground at one time, providing all switches in the distribution panel are closed.
4. Appliances, Meters, Instruments and other electrical apparatus
Connections for testing an appliance. The test is made between the conductor (the heating unit, motor, etc.) and exposed metal parts. The apparatus must be disconnected from any source of power and placed on some insulating material.
Figure 4 – Testing the insulation resistance of appliances and meters
Connections for testing the insulation resistance of a power cable. When testing cable, it is usually best to disconnect at both ends in order to test the cable by itself, and to avoid error due to leakage across or through switchboards or panelboards.
Figure 6a – Testing the insulation resistance of power cable Figure 6b – Testing the insulation resistance of power cable
Connections for testing insulation resistance of a transformer high voltage winding and bushings, and the high tension disconnect switch, in parallel, with reference to the low voltage winding and ground. Note that the low voltage winding is grounded for this test.
Figure 7 – Testing the insulation resistance of power transformer
With this connection, the insulation resistance will be that of the generator stator winding and connecting cable combined. To test either the stator winding or the cable itself, the cable must be disconnected at the machine.
Figure 8 – Testing the insulation resistance of AC generators
Preparation of apparatus to test //
1. Take out of Service
Shut down the apparatus.
Open switches.
De-energize.
Disconnect from other equipment and circuits, including neutral and protective (workmen’s temporary) ground connections.
2. Make Sure Just What is Included in the Test
Inspect the installation very carefully to determine just what equipment is connected and will be included in the test, especially if it is difficult or expensive to disconnect associated apparatus and circuits.
Pay particular attention to conductors that lead away from the installation. This is very important, because the more equipment that is included in a test, the lower the reading will be, and the true insulation resistance of the apparatus in question may be masked by that of the associated equipment.
It is always possible, of course, that the insulation resistance of the complete installation (without disconnecting everything) will be satisfactorily high, especially for a spot check. or, it may be higher than the range of the Megger instrument in use, in which case nothing would be gained by separating the components, because the insulation resistance of each part would be still higher.
For an initial test, it may be necessary to separate the component parts, even though labor and expense are involved, and test each one separately. Also make a test of all the components connected together. With this information on record, it may not be necessary to separate the components on future tests unless unaccountably low readings are observed.
3. Discharge of Capacitance
It is very important that capacitance be discharged, both before and after an insulation resistance test. It should be discharged for a period about four times as long as test voltage was applied in a previous test.
Megger instruments are frequently equipped with discharge circuits for this purpose. If a discharge function is not provided, a discharge stick should be used. Leave high capacitive apparatus (i.e., capacitors, large windings, etc.) short circuited until ready to re-energize.
4. Current leakage at Switches
When apparatus is shut down for the insulation resistance test, make sure that the readings are not affected by leakage over or through switches or fuse blocks, etc. Such leakage may mask the true insulation resistance of the apparatus under test, or, what may be more serious, current from an energized line may leak into the apparatus and cause inconsistent readings, particularly if the live line is DC.
However, such leakage usually can be detected by watching the pointer of the Megger instrument at the moment the test leads are connected to the apparatus and before the instrument is operated.
Before making these observations, be sure that all capacitance is discharged by short circuiting or grounding the apparatus.
CAUTION: Never connect a Megger insulation tester to energized lines or equipment. Never use the tester or any of its leads or accessories for any purpose not described in this article.
Observe all rules for safety when taking equipment out of service. Block out disconnect switches. Test for foreign or induced voltages. Apply workmen’s grounds.
Remember that when working around high voltage equipment there is always a possibility of voltages being induced in apparatus under test or lines to which it is connected, because of proximity to energized high voltage equipment.
Therefore, rather than removing a workmen’s ground in order to make a test, it is more advisable to disconnect the apparatus, such as a transformer or circuit breaker, from the exposed bus or line, leaving the latter grounded. Use rubber gloves when connecting the test leads to the apparatus and while operating the Megger instrument.
Apparatus Under Test Must Not Be Live!
If neutral or other ground connections have to be disconnected, make sure they are not carrying current at the time, and that when disconnected no other equipment will lack necessary protection.
Pay particular attention to conductors that lead away from the circuit being tested and make sure they have been properly disconnected from any source of voltage.
Shock Hazard from Test Voltage
Observe the voltage rating of the Megger instrument and regard it with appropriate caution. Large electrical equipment and cables usually have sufficient capacitance to store up a dangerous amount of energy from the test current. Make sure this capacitance is discharged after the test and before handling the test leads.
Explosion and Fire Hazard
So far as is known, there is no fire hazard in the normal use of a Megger or any other insulation tester. There is, however, a hazard when testing equipment located in inflammable or explosive atmospheres.
A tutorial on insulation testing. I cover the different test methods, an analogy on how insulation testing works, safety and the use of insulation test equipment. In this video:
What is insulation testing / what is megger testing / what is PAT (Portable Appliance Testing) testing?
Electrical safety when working in a DB board / breaker box or around high energy circuits.
Insulation test types: Proof test, Spot test, Time Resistance testing, Step Level Voltage test, Polarization Index (PI) test, Dielectric Absorption Ratio (DAR) test.
Leakage currents: conductive leakage current, capacitive charging leakage current and polarization absorption leakage current in a dielectric.
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