Electrical Equipment and Systems Tests and Trials

TEST INSTRUMENTS

During substation equipment maintenance, it is important to have a more accurate diagnosis of the condition of the installed equipment. Electrical testing provides insight into the equipment’s status by evaluating its current condition and identifying any anomalies that could potentially render the equipment unavailable. Below are explanations of the main equipment and the tests commonly performed in electrical systems.

Applied Voltage Test Instrument (Hipot)

The Hipot (High Potential) test instrument is used to verify the insulation strength of electrical equipment.

Under normal conditions, any electrical device will produce a minimal amount of leakage current, depending on the voltage class and the dielectric strength of the material. This phenomenon is a natural property of materials and is observed during their manufacturing process.

However, due to issues such as moisture absorption, accumulation of dirt, and other contaminants, leakage current can become excessive. This situation can lead to equipment malfunction, potential damage, and even pose an electric shock hazard to individuals who come into contact with the faulty equipment.

The test involves applying a high voltage to the equipment for one minute, during which the dielectric insulation must not break down. If an insulation failure occurs during the test, the Hipot device should detect the leakage current and immediately shut down the test. In this case, the equipment is considered to have failed.

Occasionally, a situation may arise where the equipment under test has insulation failure, but the Hipot does not shut down, continuing to apply high voltage and ultimately damaging the equipment.

The instrument comes with one cable for voltage application and another for return, in case the equipment under test cannot withstand the applied voltage. Typically, the Hipot is used for testing high-voltage cables.

The Hipot test is carried out using a very simple connection setup: the Hipot device, powered by an external energy source, is electrically connected to the cable under test and its shielding, as applicable. The device then applies a voltage pulse to the cable, and based on the cable’s response, analyses are performed to determine whether the tested cable is suitable for installation or meets product validation standards.

The first consideration regarding the device is the power supply voltage for the Hipot, as well as the safety mechanisms it includes due to the high voltages involved.

Several warnings are provided concerning potential accidents and equipment damage. Since the device operates with high voltage levels and stored energy (which is inherent to its function), the Hipot requires careful operation, with heightened attention to all possible safety measures.

During use, the operator must be attentive to all necessary safety precautions to ensure personal protection.

It is important to emphasize that the operator must always maintain the greatest possible distance from the energized components of the system during testing (such as cables, connectors, and the Hipot device itself). Additionally, it is recommended that the cables under test be arranged to run freely in the air, without any contact with other points of potential voltage or grounding.

The main safety feature identified by the team in the analyzed equipment is the system that ensures the equipment panel is always at zero electrical potential (i.e., grounded). This measure is essential to prevent accidental human contact with energized parts of the Hipot, thereby avoiding electrical shock.

Another critical point requiring close attention during Hipot operation is the level of voltage applied. Due to the various reference types commonly used (for example, phase-to-phase, phase-to-neutral, etc.), discrepancies between the intended and actual applied test voltages are common. Therefore, it is essential to routinely verify the electrical parameters being used in order to avoid undesirable incidents or even measurement errors by the equipment or the operator.

According to technical specifications, a cable with insulation failure, when subjected to a Hipot test, will exhibit a rising leakage current. This increase is only interrupted when the current reaches the cutoff value of the equipment (a value that, in some cases, is preset by the operator). Hence, it is crucial to configure all the parameters involved in the procedure before conducting the test.

Main Applications of the Equipment

The primary users of the Hipot equipment include cable and wire manufacturers, electric utility companies, telecommunications firms, and field engineering professionals.

Pre-Test Procedures

Grounding and Safety.

To ensure a successful and incident-free test, it is essential that all safety standards and procedures are fully followed. Therefore, a qualified and safety-conscious operator must be selected to handle the testing equipment (Hipot), as well as the cables, connection wires, and other materials involved in the test process.

Additionally, proper grounding of the system is crucial to prevent accidents involving unwanted voltages that may arise in the test environment. As such, the first step after verifying the safety of both the equipment and the operator is to perform and confirm the grounding of the equipment and its chassis. Ensuring a correct connection to a reliable grounding system is vital for both safety and the reliability of the upcoming tests.

Power Supply Connections in the System

Next, the Hipot’s power supply must be connected, once again verifying the system’s grounding. After this, the power source and high-voltage cable should be connected to the appropriate terminals. These final connections must also be properly grounded using suitable cables.

At this stage,the terminal guard becomes particularly important. Its main function is to protect the Hipot device’s chassis, internal circuits, and all related connections. The terminal must be solidly grounded to ensure zero potential at the required points.

Making the connections in the correct order and arrangement is essential to ensure full protection during testing, as well as to obtain reliable readings of leakage current, insulation resistance of the tested material, and other measurements. To achieve this, the operator must always follow the manufacturer’s instructions for both the Hipot equipment and the cable being tested, ensuring that no physical or operational limits are exceeded.

Connection of the Cables Under Test

The connections for the cable to be tested are detailed in the user manual, depending on the type of cable used—taking into account factors such as the presence of shielding and the construction type (single-phase or three-phase).

Finally, the equipment’s potentiometer must be adjusted to an appropriate trip current value. The manufacturer initially recommends a maximum value of 5 mA.

Test Adjustments and Procedures

Voltage Level Adjustment

When powering on the equipment, ensure the voltage settings are properly configured (initially set to the “start” position). Once any discrepancies in the settings or reference values are corrected, the HV (High Voltage) indicator light should turn on, indicating that the equipment is ready and enabled to supply voltage for testing.

Next, adjust the voltage level to the desired value (according to current standards and the test requester’s specifications). Always remember that, since this involves high-voltage testing, any adjustment must be made gradually to avoid sudden electrical variations, which could trigger protective mechanisms or cause the equipment to shut down improperly.

At this point, the operator should be able to read the leakage current from the cable or material under test. It is then the engineer’s or operator’s responsibility to verify whether the leakage current is within acceptable limits. If not, corrective actions must be taken concerning the test material.

Selecting the Appropriate Voltage Level

The Hipot equipment manufacturer typically provides a reference list of standard voltage levels according to various international standards, including: VDE (Verband der Elektrotechnik, Elektronik Und Informationstechnik, IEC (International Electrotechnical Comission), IPCEA (Insulated Power Cable Engineers Association) and AEIC (Association of Edison Illuminating Companies).

Measurements and Test Results

Once the initial test parameters are defined and the equipment has been properly configured, the test can be carried out. Before proceeding, recheck all electrical connections between the test cable and the equipment terminals. If anything does not comply with the manufacturer’s manual or relevant standards, adjustments must be made before continuing.

Measurements obtained during the test must be analyzed carefully to avoid errors due to incorrect readings or misinterpretation. It is essential that the test is executed properly, as repeating a high-voltage test multiple times is not advisable—even though it is considered non-destructive—because the voltage applied often exceeds normal operating levels, potentially affecting the cable’s lifespan.

By following all procedures outlined in this section, the test should yield reliable and valuable results for assessing cable lifespan, validating installations, and other applications.

Micro-ohmmeter

A micro-ohmmeter is used to accurately measure low contact resistance in circuit breakers and disconnect switches. It can also be used to measure the ohmic resistance of transformer windings. The test current typically ranges from 1 mA to 100 A. During testing, a current is applied through the equipment under test, resulting in a voltage drop. According to Ohm’s first law, dividing the measured voltage by the applied current gives the electrical resistance.

Turns Ratio Tester (TRT)

The TRT is an instrument used to accurately measure the turns ratio of a transformer. Since a transformer is a magnetic device operating on a fixed ratio between windings, measuring this ratio allows assessment of the winding condition in terms of transformation ratio and continuity.

The instrument measures the turns ratio, phase shift, and polarity in power transformers, potential transformers (PT), and current transformers (CT).

The device is supplied with four cables:

 – H1 and H2: used to energize the high-voltage winding

 – X1 and X2: used to measure current in the low-voltage winding

When conducting the measurement, it is important to identify the primary and secondary winding configuration of the transformer under test, as well as its operating voltage on both sides.

TRT devices may be either digital (electronic TRT) or analog (manual crank TRT).

Transformer Oil Analysis

Chromatographic Analysis

Chromatographic analysis of gases determines the concentration of gases dissolved in the insulating mineral oil. The formation of these gases inside the equipment may indicate certain problems, such as poor contact between internal components, energy leakage between windings, stress from high short-circuit currents, or prolonged operation under heavy loads.

Physicochemical Analysis

In addition to providing electrical insulation, mineral oils in transformers also serve a cooling function. This fluid transfers the heat generated in the magnetic circuits of the windings and in the magnetic core itself to the transformer’s casing through convective currents, which then dissipates the heat into the environment. Since paper is also used as an insulating material, the oil is responsible for insulating the windings from each other, from the magnetic circuit, and from the casing.

The physicochemical analysis evaluates the insulation capacity and the aging condition of the mineral oil. The results are compared against predefined values set by standards. Measurements outside the specified limits indicate the need for thermo-vacuum treatment, oil replacement, or mineral oil regeneration.

Electrical Testing of Insulating Materials

The development of techniques and instruments for testing the insulation of electrical equipment has been a concern since the early stages of electric power generation for lighting and power applications. Initially, insulation resistance was measured using direct current (DC); this method remains widely used to this day, even after the emergence of alternating current (AC). In the early 20th century, the first instrument specifically designed for insulation testing was introduced in England under the name “megger.” This instrument has been improved over the years and adapted to the evolution of electrical systems, with higher voltage scales, and remains one of the most widely used tools for measuring insulation resistance in electrical equipment.

As electrical machines increased in size, new testing techniques were developed to address certain limitations not fully covered by DC tests. Thanks to these advances, a wide range of testing methods is now available to assess the insulation condition of electrical equipment.

a) DC Tests

. High-voltage tests with step voltage increments.

. Fixed high-voltage tests.

. Insulation resistance tests.

. Determination of polarization and absorption indices.

b) AC Tests

. High-voltage tests with industrial frequency.

. High-voltage tests using resonant transformers.

. High-voltage tests at very low frequency (0.1 Hz).

. Dielectric loss and power factor tests.

. Corona discharge tests (average and peak values).

. Impulse tests with high frequency.

Modern electronics and software techniques are being applied in the effort to detect small signals in the presence of noise. The development of compact and lightweight generators capable of producing exponentially damped sinusoidal waves is also under study. This approach enables the generation of high-voltage, low-frequency signals from relatively small and lightweight sources. Another issue currently being researched involves determining the remaining service life of insulation. Studies aimed at answering these questions are being carried out using intelligent systems that analyze the parameters obtained from insulation testing.

2.1 TESTS WITH DIRECT CURRENT

The equivalent dielectric circuit of an electrical device, composed of the winding conductors, insulating material, and the iron structure, can be considered, for our purposes, as a parallel-plate capacitor.

When a DC voltage is applied to the winding of a machine, as shown in Figure 1 (a), a transient electric current, “it”, can be observed. Studies on the subject have shown that this current is composed of three basic components:
b) capacitive current (ic);
c) absorption current (ia); and
d) ionic or conduction current (ii).

Figure 1 – a) Testing a dielectric with DC voltage.

Figure 1 – b) Capacitive current;

Figura 1 – c) Absorption current;

Figura 1 – d) Conduction current.

This way we have:

2.1.1 Capacitive Current

The capacitive current originates from the capacitive effect created by the conductors, insulating material, and metallic structure of the machine. This current is expressed by the equation:

Where E is the open-circuit voltage of the source in volts; R is the ohmic resistance of the circuit, including the internal resistance of the source, in ohms; C is the capacitance of the equipment in farads; t is the time variable in seconds; and e is the base of natural logarithms. The capacitance, C, depends on the characteristics of the equipment, the size and shape of the enclosure, the thickness of the insulating material, and the dielectric constant.

In practice, the time constant, RC, is usually very small, so the capacitive current disappears quickly most of the time and is often neglected. However, there may be cases where the capacitance is so large and the charging time so long that the current affects the test results. The time required to capacitively charge the equipment can be determined by the formula:

Where ln denotes the natural logarithm.

2.1.2 Absorption Current

This current is due to the polarization of the dielectric molecules. The molecules are electrically neutral as a whole, meaning they are formed by an equal number of positive and negative charges. If the centers of positive and negative charges coincide at the same point, the molecules are called polar or permanent dipoles. Conversely, if the centers of charge coincide, the molecules are called non-polar.

A dipole consists of two equal charges, one positive and one negative, located at a certain distance apart; the dipole is quantitatively defined by its dipole moment, which is the product of one of the charges and the distance between the charge centers. This number measures the tendency of dipoles to orient themselves when subjected to the action of an electric field. Non-polar molecules, when subjected to an electric field, modify their atomic structure so that positive charges move in the direction of the field, and negative charges move in the opposite direction, forming temporary dipoles. When the field ceases, they immediately return to the position that held them in place. Generally, polar molecules have a high dielectric constant (water 81) compared to non-polar molecules (mica 3 to 6). In industrial dielectrics, one of the predominant polarizations occurs at the interfaces of heterogeneous materials, especially if the mechanical resistance is very different and there is a large accumulation of dirt.

The polarization process is relatively slow; non-polar molecules orient themselves faster than polar molecules, since forming oriented dipoles requires a much smaller electronic displacement. The time needed to polarize all the molecules depends fundamentally on the dielectric’s characteristics, i.e., the amount and tendency for polarization of the polar molecules present. In practice, the time required to consider an insulator polarized (for example, a motor under normal operating conditions) is ten minutes.

The energy applied to the dielectric is partly transformed into heat due to intermolecular friction during the accommodation of the dipoles; another part is absorbed and stored in the dielectric because of dipole alignment; this energy appears as residual voltage between the capacitor plates after the voltage source is disconnected. For safety reasons, both for people and equipment, this energy must be immediately discharged after the test by short-circuiting the windings to the core for a time never less than four times the duration the voltage was applied.

The absorption current can be expressed by the following formula:

If the time constant is small, the capacitive current will be negligible after a few seconds. On the other hand, if the conduction current is very small, the current read on the microammeter may be mistaken for the absorption current. If the conduction current is not negligible, as is the case in most situations, it can be estimated as the current that would flow through the microammeter once the dielectric is fully charged, that is, when the capacitive and absorption currents are so small that the current on the microammeter remains practically constant, which usually occurs after 8 to 10 minutes of applied voltage. The conduction current can also be calculated using the following equation:

Where i₁, i₃.₁₆, and i₁₀ are the currents read on the microammeter at times of 1, (3.16) and 10 time units, respectively; if the chosen time unit is 1 minute, the times would be 1, (3.16) and 10 minutes, respectively. Once the conduction current iᵢ is calculated using the above formula, the absorption current iₐ will be determined by subtracting the conduction current iᵢ from the current read on the microammeter.

Plotting the current values obtained at times 1, (3.16) and 10 time units on a time-current coordinate graph in logarithmic scale will result in a straight line.

The constant n can be calculated using the following formula:

The n value ranges between zero and one and defines the absorption rate of the dielectric and its impurities for each particular case. Since the absorption current is an exponential function, n will determine the slope of the line.

2.1.3 Conduction Current

The conduction current represents the true leakage current that defines the insulation resistance of the dielectric. This current has two basic components:

a) Surface component, which flows over the dielectric surface; and
b) Volume component, which flows through the thickness of the insulating material.

The surface current results from ionizations formed by the dissolution of countless environmental particles deposited on the surface of the insulator, such as oil, grease, carbon dust from brushes, and other materials originating from the manufacturing process. This current can provide an idea of the real insulation conditions, and for this reason, it should, whenever possible, be measured separately.

The conduction current iᵢ is determined by Ohm’s Law:

Where Rᵢ​ is the ohmic insulation resistance, and E is the voltage applied to the dielectric.

Figure 2 – Example of a DC Test.

Theoretically, the insulation resistance should remain constant for any voltage value within its rated class. If this occurs, we can state that the insulation is holding up and that the dielectric could be destroyed if the voltage continues to be applied.

2.2 INSULATION MEASUREMENTS, THE MEGGER

Insulation resistance can be determined using Ohm’s law, as seen in the previous section, by applying a direct current voltage and measuring the current that flows through the galvanometer. The insulation resistance is expressed in megohms (10⁶ ohms) due to the high values typically involved.

One of the first instruments designed specifically for insulation measurements was the “megger,” a registered trademark of James Biddle.

The megger was introduced in England in 1904 and in America in 1910; over the years, it has been recognized as one of the most efficient instruments for evaluating the insulation of electrical equipment. The megger evolved alongside industrial development so that, as electrical equipment increased in power and voltage, new instruments with higher voltages and scales were introduced. Today, there are instruments with ranges up to 500,000 MΩ and 100,000 V, compared to the original models which were limited to 500 MΩ and 500 V.

The current importance of the megger as an insulation evaluation instrument is based on a better understanding of dielectric polarization phenomena and improvements in DC testing techniques.

2.2.1 Principle of Operation

The megger is fundamentally an ohmmeter with a permanent magnet and crossed coils, designed so that the readings are accurate and independent of the supply voltage. Figure 3. The ohmmeter essentially consists of two coils mounted on the same moving system, along with the pointer, which is free to rotate within a permanent magnetic field. The system is attached to a spring and pivots on a ruby bearing.

Deflecting coil A is connected in series with the damping resistor R’ and the test resistance, which is connected to the “Line” and “Earth” terminals. Coil B is connected in series with the control resistor R. Coils A and B are mounted on the moving system at a specific angle to each other and are connected in such a way that they produce opposing torques. The pointer, however, will move in such a direction that both torques are balanced.

Figure 3 – Principle of operation of the megger: A, main coil; B, holding coil; R, R’ damping resistors.

When the insulation under test is perfect, or when no resistance is placed between the “line” and “Earth” terminals, no current will flow through coil A. Coil B, however, will receive current and tend to position itself perpendicular to the poles, indicating an “inf” (infinite) reading on the scale. When an infinite value resistance is connected between the “line” and “Earth” terminals, a current will flow through the deflecting coil A, producing a torque opposite to that of coil B, causing the mechanism to rotate until the two torques balance. Thus, coil B acts like a retaining spring.

The control coil is divided into two parts, forming an unstable system. The main part B develops a torque proportional to the permanent magnet field. The other part B’ is mounted outside the permanent magnet field and connected in series with the main coil B, but in opposition, so that external magnetic fields mutually neutralize their effects. In other words, any external field attempting to move the mechanism from the infinite position will produce equal and opposite torques; the instrument is immune to this type of error.

If the “line” and “Earth” terminals are short-circuited, the pointer simply moves to the zero position on the scale. The damping resistor R’ protects coil A from excessive current. When the short-circuit is removed, the pointer returns to the “inf” position on the scale.

In manual meggers, the instrument’s accuracy is not affected by variations in crank speed or partial loss of magnetism in the permanent magnet, since both coils are affected equally.

Figure 3 shows how the line terminal is shielded by a metal ring to prevent errors caused by leakage currents at the other terminals; this current is diverted and carried to the source without passing through the deflecting coil (Figure 4).

Figure 4 – External view of a manual megger.

2.2.2 Calibration Verification

The procedure described is general and should be validated for most types of instruments available on the market:

a) Measure the ohmic resistance of coil A using a Wheatstone bridge between the “line” and “Earth” terminals;

b) Select a decade box or rheostat with a resistance value never less than 10% of the resistance found in item a;

c) Connect a 100 MΩ decade box with minimum insulation equal to or greater than the megger voltage to the “line” and “Earth” terminals;

d) Select the megger voltage scale to be calibrated. Make sure not to apply a voltage to the decade box higher than its insulation rating;

e) Energize the megger and record the reading value on the calibration sheet;

f) Connect the rheostat or decade box from item b to the “line” and “guard” terminals;

g) Adjust the decade box connected to the “line” and “Earth” terminals to 1 MΩ;

h) Energize the megger and adjust the rheostat so that the pointer indicates the reading found for 10 MΩ. Maintaining the rheostat in this position, the pointer will always indicate the resistance connected to the “line” and “Earth” terminals multiplied by 10. The resistance is expressed by the following formula:

Where Rm is the resistance indicated by the instrument; Rx is the resistance connected to the “line” and “Earth” terminals; Ra is the measured resistance of coil B; Ry is the resistance adjusted on the rheostat.

i) Gradually increase the resistance of the 1 MΩ decade box up to 100 MΩ. The maximum scale value that can be adjusted will be 100 × 100 = 10,000 MΩ;

j) Repeat the procedure for all the megger scales.

2.2.3 Preparations for the Test

a) Transport the equipment to the test location in a horizontal position and on an elastic base to avoid vibrations and shocks;

b) Place the instrument next to the equipment under test, on a firm and horizontal base. Level the instrument by adjusting the base screws so that the bubble in the level is perfectly centered;

c) Connect the cables to the instrument terminals “line,” “Earth,” and “guard”; handle carefully to avoid excessive bending or contamination with grease or other substances that could damage the cable insulation; strictly avoid stepping on or placing weights on the cables or instrument;

d) Adjustment of infinity, “inf.” suspend the cable terminals so they are isolated from each other; select the test voltage and the lowest scale; turn on the motor or manually rotate the crank at normal speed. If the megger is well adjusted, the pointer will slowly move to the “inf” position; if there is any deviation, it should be adjusted. Check all scales in ascending order at the voltage at which the test will be performed;

e) Zero check, after checking the “inf”, position and with normal test voltage applied, quickly short the “line” and “Earth” terminals and observe if the pointer suddenly moves to zero on the scale; if not, check for broken or poorly connected cables;

f) Connect the cable clamps to the equipment according to the corresponding test. Select the appropriate test voltage and the lowest scale on the instrument. Gradually select higher scales to obtain the most accurate readings possible;

g) Take the insulation readings at the respective times according to the specific procedure for each test;

h) Turn off the megger and set the function switch to the discharge position, “discharge,” to discharge the capacitive and absorption energies. The time needed for total discharge of the dielectric is estimated as four times the time it was energized; for example, if the test lasted 1 minute, the total discharge time will be 4 minutes. The discharge can also be performed using a discharge stick by short-circuiting the instrument terminals to the casing through a resistor to limit the current value.

If no megger is available, the insulation resistance can be measured using the circuit shown in Figure 6. A highly sensitive DC voltmeter, at least 100 ohms per volt, is connected in series with the machine winding through a diode.

Figure 6 – Measurement of the insulation resistance of a rotating machine when a megger is not available.

The following sequence is recommended:

1) Select the voltmeter scale at 500 or 600 V;

2) With cables L1 and L2 disconnected from the machine, close switch CH1; adjust the transformer voltage so that the voltmeter indicates the desired test voltage U1;

3) Connect cable L1 to the machine winding, short-circuiting all winding ends; connect L2 to the machine frame;

4) Open switch CH1; as the dielectric charges, the voltage on the voltmeter decreases until it stabilizes; read and record the voltage U2 at the specified intervals.

The insulation resistance is calculated by the formula:

Where Ri is the insulation resistance of the machine; Rv is the internal resistance of the voltmeter; U1 is the voltage on the voltmeter with switch CH1 closed; U2 is the voltage on the voltmeter after a certain period (1 minute or ten minutes), with switch CH2 open.

2.3 FACTORS THAT INFLUENCE INSULATION RESISTANCE

There are some factors that can significantly influence insulation resistance and should be taken into account for a correct interpretation of the tests.

2.3.1 Influence of Surface Condition

Foreign conductive materials, such as carbon dust, when deposited on the surface of insulators and non-insulated surfaces like connectors, slip rings, etc., reduce the surface insulation resistance. On the other hand, non-conductive materials can become conductive when mixed with oils and greases. This is especially important in machines with slip rings due to the large amount of exposed conductive material. For this reason, the dielectric must be perfectly clean before performing the tests.

2.3.2 Influence of Surface Moisture

Regardless of cleanliness, when the dielectric is at a temperature below the dew point, a film of condensed moisture will form on the surface; this moisture will be absorbed by the insulating materials due to their hygroscopic nature. Condensation is more aggressive if the surface is dirty. In this case, the insulation resistance will be very low.

2.3.3 Influence of Temperature

Insulation resistance varies greatly with temperature. In rotating machines, it can be considered that for every 5ºC increase in temperature, the insulation resistance is reduced by half; for example, if a motor has an insulation resistance of 50 MΩ at 20ºC, it will be about 25 MΩ at 25ºC.

To compare insulation values over the equipment’s lifetime, test results must be corrected to the same temperature. There are tables that provide correction factors for different types of equipment.

Figure 7 – Verification of the resistivity of a typical insulator with temperature.

2.3.4 Influence of Voltage Level

The measurement of the insulation resistance of an electrical equipment in satisfactory operating conditions should increase over time, since capacitive and absorption currents decrease, as explained in sections 2.1.1 and 2.1.2.

The insulation resistance is not influenced by the test voltage level when the insulation is in good condition. However, insulation tests are normally performed with voltages ranging from 500 to 5000 V. During the test, suggested voltage levels for each particular equipment will be recommended.

2.4 POLARIZATION AND ABSORPTION INDICES

Figure 8 shows the classic drying curve of a rotating machine; the equipment was placed in an oven at 25ºC and set to a permanent final temperature of 75ºC. Insulation resistance measurements were taken every 4 hours, with readings recorded at 1 and 10 minutes after the application of each voltage. On the x-axis, the time in hours that the equipment remained in the oven is shown, and on the y-axis, the insulation resistances read at 1 and 10 minutes.

It can be observed that as the temperature rises from 25 to 75ºC, the insulation resistance decreases (paragraph 2.4.3); on the other hand, as moisture is expelled, the insulation resistance increases. After approximately 100 hours in the oven, the insulation resistance stopped increasing and remained constant, indicating that the equipment was dry.

The polarization index of electrical equipment is defined as the ratio of the insulation resistances measured at 10 minutes to those measured at 1 minute. The absorption index is defined as the ratio of insulation readings taken at 1 minute to those taken at 30 seconds.

Figure 8 – Classic drying curve of a machine winding.

If the polarization indices were calculated over the drying period, we could observe that during the first hours the polarization index decreases, reaching a critical value approximately 20 hours after the drying process begins. This can be explained as a consequence of the expansion of water molecules and the increase, due to temperature, in their ability to dissolve impurities and form ions. On the other hand, as the degree of drying increases, both the polarization and absorption indices also rise; it can be seen that the polarization index reaches its maximum value when the machine is completely dry. This makes the polarization index an effective method for assessing the moisture level in electrical equipment. Table 14.8.

Table 14.8

2.5 DC HIGH VOLTAGE INSULATION TEST

High voltage AC tests are normally considered destructive and are systematically applied during manufacturing processes, as well as due to contractual requirements or quality control within the company. These tests are mostly performed to ensure that a given insulator can withstand a specified voltage level for a defined period. Because of this, high voltage tests are considered “go/no-go” tests. However, in DC testing, an experienced operator can often predict a fault before it occurs. For this reason, DC tests are widely used in maintenance.

Figure 9 briefly shows the diagram of equipment for DC high voltage testing. The equipment is connected to the terminal of a high voltage cable; the leakage current through the porcelain surface is carried by one of the parallel cables directly to the transformer terminal without passing through the meter.

Figure 9 – Simplified diagram of a DC testing instrument.

One of the most commonly used testing methods consists of applying the voltage in progressive stepped increments. The objective is to determine the leakage current as a function of time for each different voltage level. In a clean, dry insulator free of air bubbles, the ionic current increases linearly with voltage according to Ohm’s law. The absorption current is calculated by the principle of superposition and is expressed by the formula:

Where ia is the absorption current in amperes; Ek is the voltage of each step in volts; C is the capacitance of the system in farads; D is the proportionality factor; k is the voltage step considered; t is the duration of a test in minutes; N is the time interval between each voltage step, related to the absorption rate as defined in section 2.1.2.

2.6 MEASUREMENT OF DIELECTRIC LOSSES

Alternating voltage subjects the molecules of the dielectric to a series of stresses and displacements proportional to the frequency. Since materials are not perfectly elastic, due to viscosity or intermolecular friction, the energy applied to the dielectric during expansion does not correspond to the energy returned during compression; the difference in this energy is converted into heat and constitutes what is called, by analogy with magnetism, dielectric hysteresis or dielectric losses.

In addition to hysteresis losses, there are Joule losses due to conduction currents, measured in DC tests; it is not a major error to consider hysteresis losses as total dielectric losses. These losses can be mathematically expressed by the formula:

Where Kc is a typical constant of the material (specific losses in W/cm³ per period and per kV/mm of electric field); f is the frequency in Hz; and E is the average electric field of the dielectric in kV/mm.

A dielectric can be represented, for calculation purposes, by an ideal capacitor in parallel with a resistance of such value that V²/R represents the losses in the dielectric. Since an ideal capacitor has no losses, the constant Kc provides an excellent index for measuring the quality of insulating materials. However, in practice, measuring the tangent delta (tgδ) as shown in Figure 10 is more interesting, since this measurement does not depend on the volume of the dielectric under test.

In practical field tests, the angle δ is very small, so that tgδ = sinδ.

The tgδ represents the power factor of the dielectric.

The power factor is very sensitive to variations in moisture in the dielectric, which can be explained by the high power factor of water compared to other materials (Table 2.1). This makes the measurement of the power factor an excellent indicator for assessing the condition of dielectrics with respect to the presence of foreign materials such as water, dust, grease, etc.

Figure 10 – Phasor diagram of dielectric losses

Table 2.1

Temperature considerably influences the dielectric characteristics of insulators. Figure 11 shows the variation of dielectric losses and dielectric constant as a function of temperature and frequency for polar and non-polar polymers. Polar polymers exhibit critical temperature regions in which losses and dielectric constant increase sharply.

To compare power factor values across different tests during the equipment’s lifetime—and thus detect changes in the dielectric—it is necessary that the tests are referenced to the same temperature. Correction tables exist to standardize measurements to 20ºC for the most important equipment. The criterion to be applied for temperature determination will be discussed later, in the specific application of the test for each type of equipment.

Ice has a power factor equal to 1; therefore, power factor tests should never be performed below the freezing point of water.

One of the first instruments developed for field testing of dielectric losses was introduced in 1929 by Franck C. Doble. Figure 12 illustrates the operating principle of a power factor field meter by the company Doble Engineering Company. Output “A” is used for instrument adjustment. With the amplifier connected at position B, the meter reading depends on the voltage across terminals Rs, that is, the product of the current I flowing through the dielectric by the fixed resistance Rs. The instrument scale is set such that when the test voltage applied to the dielectric is 2500 V, the pointer directly indicates the value in millivolt-amperes (Figure 13).

When the amplifier is placed at position C, the measured voltage results from the voltages across Rs and Ra, which are opposed with respect to the amplifier. Partial balancing can be achieved by adjusting resistor Ra to minimize the reading. Complete balancing would only be possible if the dielectric quality were equivalent to that of the instrument (an air capacitor). According to the vector diagram, the voltage Vc is:

With the test voltage set to 2500 V, the instrument directly indicates the losses in milliwatts (mW).

The instrument is also equipped with a selector switch that allows the meter to be placed in three positions: “ground”, “guard”, and “UST” (Ungrounded Specimen Test). See Figure 13.

Figure 14 shows, in a simplified manner, the connection of the meter with respect to the Hv (high voltage) and Lv (low voltage) cables; the test circuit represents the dielectric of a transformer with two windings.

 – In position b), the meter measures the dielectric losses of the primary winding insulation to ground and to the secondary winding;

 – In position c), the losses in the insulation of the primary winding to ground;

 – In position d), the losses in the insulation between the primary and secondary windings.

The difference between the losses obtained in tests a) and b) should be equal to the losses measured in c).

Figure 11 – Typical Variations of a Dielectric as a Function of Temperature and Frequency.

Figure 12 – Block Diagram of the Power Factor Meter from “Doble Engineering Company, Model MEU – 2500 V”.

Figure 13 – Front View of the Power Factor Meter – Doble Type MEU – 2500 V.

Figure 14 – Connection Diagram of the Mode Selector Switch of the Doble MEU – 2500. a) Electric Circuit of a Transformer with Two Windings; b) (Ground) – Measurement of losses in CH + CHB; c) (Guard) – Measurement of losses in CH; d) (UST) – Measurement of losses in CHB.

2.7 HIGH-FREQUENCY IMPULSE TESTS

One of the main challenges in measuring inter-turn insulation in the windings of rotating machines is undoubtedly the level of current at industrial frequency required to induce the necessary test voltage. This is due to the high magnetic circuit reluctance. When the rotor is in place, the current needed to induce even just the nominal voltage between turns is essentially equivalent to the inrush current during machine startup—extremely high and unsuitable for insulation testing. Without the rotor, the current can reach unpredictable levels.

Figure 15- Simplified diagram of a high-frequency impulse instrument (Model 6925C from Eletrônica INC).

Figure 15 shows the simplified diagram of a piece of equipment designed by the company Eletrônica for testing inter-turn insulation of windings in rotating machines. Transformer T1 gradually raises the test voltage to the desired level. Capacitors C3 and C4 are charged with the peak voltage from T1 through diodes D1 and D2 and the coils under test, L1 and L2. The circuit is synchronized so that the negative half-cycle of the AC triggers ST1, discharging capacitor C3 through L1 and C4 through L2. During the discharge phase, each capacitance resonates with its respective load coil, producing an AC current that is detected by current sensors T6 and T7. The resonance condition is provided by the impulse resonator module PR1. Coil L1 is identified as the reference coil, and L2 as the test coil. Assuming that coils L1 and L2 are identical, and C3 equals C4, the resonant currents produced by C3–L1 and C4–L2 should be identical.

The waveforms of the resonant currents, captured by sensors T6 and T7, are displayed on an oscilloscope. If the insulation of the test coil is different or presents any issue during the test, the resonant currents in C4–L2 will differ from those in C3–L1. When this happens, two different current waveforms will appear on the oscilloscope screen, as shown in Figure 17. The operator will immediately recognize that the coil under test has a problem.

Figure 16 – Examples of test connections: a) Test of the winding of a three-phase motor with two groups of windings in parallel. b) Test of the armature of a direct current (DC) machine.

Figure 17 – Examples of waveform types that may appear on the oscilloscope screen and their respective diagnoses.

2.8 AC HIGH VOLTAGE TESTS

These tests are defined by standards as decisive indicators that a given piece of equipment can withstand a specified voltage and is therefore free from manufacturing defects. They are referred to as “pass-fail” tests. The test voltage is typically twice the rated voltage. The voltage is gradually increased from zero to the specified value smoothly and continuously, within a time not exceeding 10 seconds, and it is maintained for 1 minute. The voltage is also reduced smoothly and continuously to avoid inducing overvoltages.

2.9 AC HIGH VOLTAGE TESTS VERSUS DC TESTS

Although AC tests are standardized for new equipment testing, DC tests are generally preferred for maintenance purposes because:

1. They provide a better indication of insulation conditions compared to AC tests.

2. They are controllable, meaning they can anticipate the likelihood of failures before they occur, unlike AC tests which are more of a go/no-go evaluation.

3. The voltage source is relatively lightweight and portable, especially when compared to the bulky and heavy AC equipment. This issue is being mitigated through the use of resonant transformers and low-frequency (0.1 Hz) testing.

A limitation of DC testing is that, in equipment insulated with different superimposed materials, such as the final fiberglass layers over mica layers in rotating machines, the distribution of the electric field across each material is directly proportional to their respective resistivities. However, in AC tests, the electric field is governed by the dielectric constant of the materials.

It has also been observed that in cables showing early stages of deterioration due to water ingress, the application of DC voltage tends to accelerate treeing effects as a result of corona discharge activity.