About the Aged Degradation of the Materials Used for Medium-Voltage Distributors

Abstract

The medium-voltage components in the ignition installations for gasoline engines contain electroinsulating materials that lose their properties over time. The purpose of this paper is to measure and analyze the insulation resistance, dielectric absorption ratio and polarization index of the insulation of materials (three types of materials) used for medium-voltage distributors, for several operating periods, in automotive ignition installations. Experiments were conducted with old (operation tens of thousands of km, some with surfaces that have been cleaned) and new medium-voltage distributors, and a megohmmeter was used to measure, over time, the insulation resistance between the central terminal and the output terminals at different test voltages. The insulation resistance of the distributors depends on the use: in the old ones, they have values of tens of GΩ (e.g., up to 100 GΩ) and, in the new ones, of the order of TΩ (e.g., 4–7 TΩ). The more distributors are used, for the same distributor, there are greater differences between the measurements made between terminals and the average values (87% for used distributors, respectively, 2% for new ones). For new or less used distributors, higher values were obtained for the dielectric absorption ratio (1.26–1.27; for used ones, 0.91–0.95) and polarization index (1.15–1.25; for used ones, 0.96–1.15). The results show the importance of the volume insulation resistance of the electroinsulating material compared to the surface resistance and the insignificant improvement when cleaning the internal and external surfaces of the medium-voltage distributors.

1. Introduction

The world’s automotive manufacturing industry is one of the world’s largest industries and plays an important role in the development of countries due to its intensive structure and its high employment rate. Changes in the automotive industry push the boundaries of possible solutions based on advanced electrical and electronics solutions, automotive maintenance and repair solutions. The automobile industry is crucial to the global economy and is considered to be one of the most important sectors for the global economy, society and individual lifestyle [1,2,3,4].

To ensure proper operating conditions of internal combustion engines, more well-developed and known tools must be developed, as well as new research is being carried out to advance our knowledge of information. The main problems with component production in the automotive industry are low component quality, high failure rates and subsequent complaints. Problem-solving techniques affect operational efficiency, reduce material waste, improve production coherence, upgrade performance, enhance production cycle time and increase financial resources [5,6].

Various industries need to better understand their materials to shorten design cycles, improve inspection, process monitoring and quality assurance. Each material has unique electrical characteristics depending on its dielectric characteristics. The dielectric material measurement can provide crucial information about the design parameters for automotive applications. For example, cable insulation loss is related to its dielectric properties [7,8,9].

Solid dielectric materials are used in all types of circuits and equipment to isolate parts that transport one current to another when operated at different voltages. Good dielectrics must have low dielectric loss, high mechanical resistance, no gas inclusion or humidity and resistance to thermal and chemical degradation. Solid dielectrics have a higher breakdown resistance than liquids and gases. When the decomposition occurs, solids are permanently damaged, while gas and liquids partially restore their dielectric resistance after the electric field is removed. Decomposition mechanisms are complex phenomena in solids, depending on the time of application, and can be classified as follows: (a) intrinsic or ionic decomposition, (b) electromechanical failure, (c) the failure due to tree and tracking, (d) thermal breakdown, (e) electrochemical decomposition, and (f) internal discharges cause failure [10,11,12,13].

In dielectric materials, conductivity and valence bands are separated by large energy gaps, so there are no conducting electrons. The interaction between charge and dielectric in an external electric field is described by polarization. When the electric field changes over time, the load distribution changes, and this is called the displacement current. The total current density passed through the material can then be expressed as the total amount of the conductor and the moving current density [14,15,16,17].

Due to the dielectric degradation of the insulation material, the electrical devices are affected by the effects of overvoltage, internal defects, overtemperature and other elements, and high-current degradation is of great importance to operational stability. The selection of the best dielectric material depends on the operating conditions and requirements of the particular application. Technology advancements have created increasing demands for insulation materials and systems that are reliable in many environmental and operational conditions [18,19].

The breakdown voltage of solid insulation material decreases with the aging of dielectric materials. The process of deterioration is caused by the acceleration of partial discharges and the accumulation of heat in cavities and micro-holes in solid insulation materials [20,21,22,23,24]. The study of the selection, behavior and performance of insulating materials is important for the design of electrical equipment, machines and devices. In addition to conductors, insulators are the backbone of all electrical systems and are the weakest link in the system. Thus, choosing a specific application insulation material requires greater care and attention to achieve the desired performance under the worst working conditions [25,26,27,28,29,30].

Insulation is one of the most important parts of the gasoline engine ignition system, and the thickness of insulation increases the operating voltage. The insulation materials must meet the following requirements: greater dielectric resistance, longer and more standard service life, greater heat resistance, mechanical flexibility and humidity and moisture resistance. The insulation type depends on the operating voltage, temperature, type and protection level [14,27]. The impulse cutting voltage can be defined as the maximum value of the impulse cutting voltage that causes any material, such as solid insulation, to have a hole in the wave tail and a voltage value in the front of the wave. The thickness of the dielectric material clearly affects the decomposition voltage. Short-term electrical strength depends more on thickness than on sample area [20,31,32].

All petrol engines use spark plugs because of the nature of gasoline as a fuel. In a combustion system, the mixture of air and fuel in the combustion chamber of the internal combustion engine is burnt by a combustion system. This process uses an electric field induced in a line by a magnet or generator to create dozens of kVs to generate sparks in spark plugs. A time circuit is used to distribute the medium voltage between the engine cylinders. The spark must be timed to occur at a precise point in relation to the position of the piston when it reaches the top dead center on the compression stroke of the engine. The resulting current flow passes through a conductor and ends in the spark plug inside the combustion chamber. The voltage required for the ignition plug to start burning is 20–30 kV [33]. For medium compression ratios, a voltage of up to 20 kV (to a special type, up to 40 kV) may be necessary. The scope of the ignition system must convert the normal 12 V DC battery voltage to about 8–25 kV AC and deliver this voltage to the right cylinder at the right time. Ignition systems can have more advanced systems controlled by electronics (ignition module). The basic operation of most ignition systems is very similar. Since the production of a high voltage is carried out by magnetic induction, a primary winding (small number of turns) is turned on and off, causing a medium voltage to be induced in a second winding (large number of turns). The coil ignition system consists of various components which actual design and construction primarily depend on the engine to be used. When the medium voltage produced by the ignition system is applied between the central electrode and the ground electrode of the plug, the insulation between the electrodes breaks down, the current flows in the discharge phenomenon and an electric spark is generated. The electric spark is formed through the spark plug exactly at the moment when the mixture of air and fuel has been thoroughly compressed in the combustion chamber. This results in a controlled explosion that provides the power to turn the engine [32,34].

Although ignition systems have certainly evolved over time—in particular, with more and more electronic components—they still bear the hallmarks of the original coil ignition systems that were introduced more than 100 years ago. Petrol engines require an ignition source to cause an explosion of the mixture of air and fuel within the cylinder. There are four different fire systems, each with advantages and disadvantages, but all have the same basic goal: to effectively fire a quick explosion that burns all the fuel in the cylinder [16]. Electronic ignition is now used in almost all spark ignition vehicles, because conventional mechanical systems have more disadvantages: mechanical problems with contact breakers (limited life and difficult to adjust), current flow in the primary circuit is limited to 4 A, laws require strict emission limits and weaker mixtures require a medium voltage to ensure successful ignition, even at medium speed [34,35].

The purpose of the article is to analyze the measurements made on some medium-voltage distributors (three types of materials: Bakelite, epoxy resin and thermoplastic polyester resin) in order to analyze the influence of the aging of the dielectric material when a medium voltage is used. The measurements of insulation resistance, dielectric absorption index and polarization index (the most common measurements that can be performed on dielectric materials) were made with a high-performance megohmmeter.

The paper is divided into seven parts. The second section describes the main types of ignition systems for gasoline engines. The third section presents the electric model of medium-voltage distributors, and the fourth section shows the materials and methods. The experimental measurements of medium-voltage distributors and the discussion are presented in Section 5 and Section 6. The conclusions on the degradation of medium-voltage distributor materials are given in the seventh section.

2. Types of Ignition Systems Used for Petrol Engines

Petrol engines require an ignition source to start the explosion of the mixture of air and fuel in the cylinder. There are four main different ignition systems, each with advantages and disadvantages, but they all have the same basic objective: to efficiently trigger a quick explosion to burn all the fuel in the cylinder. The magneto-inductive system is the simplest and lightest, but it is also inefficient and inconsistent. This system is mainly used in two-stroke engines with a piston for small machines such as go-karts, light tools, engine outboards, etc.

Inductive discharge systems consist of a magnet on the flywheel connected by a fixed electrical coil to one or two spark plugs, often through a transformer to increase the voltage and/or an interrupter to control the time. When the magnet passes through the coil, a medium voltage builds up until a spark is generated. This allows the system to operate without batteries. The simplicity of the system makes it very lightweight and reliable and ideal for small engines. One problem with inductive discharge systems is that the voltage and size of the flame that it generates depend on the speed of the magnet (and the engine). At medium rotation per minute (RPM), the explosion has more power and wastes much energy, thereby wasting fuel. Since the magnet passes through the coil in each rotation, the system generates a fire at each fire plug in each rotation unless there is a complicated interrupter. This is ideal for a two-stroke engine, but for a four-stroke engine, it produces additional losses [34].

The conventional distributor ignition system, also known as the Kettering ignition, was named after its inventor Charles Kettering and has been in use for more than 100 years (Figure 1). The main function of a conventional ignition distributor is to connect the medium-voltage output of the ignition coil (one or more) to the right spark plug in the correct firing sequence (Figure 1). It is very popular in multicylinder engines in cars, trucks, boats and aircraft. Over time, contact breakers have been associated with mechanical type, inductive pulse generators, Hall generators or optical pulse generators [33].

The system uses a battery and an ignition coil to produce energy in a magnetic field, which can be rapidly discharged into sparks. The medium voltage of the ignition coil is directed by a distributor to the spark plug at the right time. The Kettering system is much more than a magneto system when used on four-stroke engines, because it does not generate “waste shocks”. The generated energizer is, also, much more consistent, because the current limiter ensures that each energizer distributes the same amount of energy. However, due to the energy consumption of this resistor, the system is not as efficient as other systems, especially at low RPMs. The Kettering system also requires a battery and a battery charging circuit, medium-voltage conductors, which adds a lot of weight and costs, making it impossible to use for very small and light engines. The system also requires a distributor connected to the engine’s camshaft, making the system more complex and costly.

The ignition coil is a type of medium-voltage transformer which secondary winding is designed with more turns than the primary winding. When the magnetic field collapses, it will therefore induce a medium voltage into the secondary winding rather than into the primary (Figure 2). The primary winding of an ignition cable usually contains between 150 and 300 turns of wire; the secondary winding typically contains between 15,000 and 30,000 turns of wire or about 100 times more than the primary winding [16].

Properties of medium-voltage distributor caps (Figure 2) used for ignition systems [10]:

-

Insulating material resistant to a medium voltage: phenol formaldehyde resin (Bakelite), epoxy resin and thermoplastic polyester resin;

-

Laquered inside and outside for further protection against condensed water and chemical effects;

-

System-specific connector area;

-

System-specific bracket, screw or spring clip attachment;

-

With/without screening cover made of conductive plastic to the return interference to ground and maintain electromagnetic compatibility according to the manufacturer’s requirements.

The capacitive discharge system (CDI) uses a capacitor, switch and transformer to store energy, then boost and release it through a spark plug (Figure 3). The capacitor is charged by a battery or a generator, then discharged through a transformer by a switch or a thyristor (SCR) [34].

One of the most attractive features of this system is the ability to create extremely medium voltages, but if used on a real engine, it cannot achieve maximum voltage. In addition, there are other specifications, such as the total spark energy dictated by the coil and, usually, greater than the maximum voltage. CDI systems are also considered to be less reliable than Kettering systems because they use capacitors, thyristors and transistors and have historically been less reliable than mechanical contactors and distributors. The medium-voltage distributor is similar to a more conventional system (mechanic contact breakers, inductive pulse generators, Hall generators or optical pulse generators) [33,36].

Distributor-free ignition has all the features of programmed ignition systems using a special type of ignition coil, which is delivered to the spark plug without the need for a mechanical medium-voltage distributor (without a distributor cap and a rotating rotor). Coil-on-Plug (COP) and Coil-Near-Plug (CNP) ignition are very similar to the distributor ignition system but reverse the distribution and coil order. In COP systems, the coil is placed directly next to the plug, while, in CNP systems, the transformer is placed a little further away and is connected to the plug through short medium-voltage conductors. Electronic distributors are composed mainly of MOSFETs and are controlled by electronic control modules (ECMs)/electronic control units (ECUs).

The benefits of these systems are medium-scale efficiency and very consistent and predictable flashes. They also allow ECM/ECU to operate variable ignition timings to maximize efficiency while preventing knocking and pining. Computer control also allows advanced engine diagnostics, such as selective deactivation of cylinders. The disadvantages of these systems are complicated energy and control systems. This limits its use to large engines and other applications that require medium efficiency. The COP and CNP systems are also less reliable because they are exposed to medium temperatures and high vibrations, leading to several failures, such as the fracture of flammable coils [34].

3. Electrical Model for Medium-Voltage Distributors

Figure 4 shows the electrical model for a medium-voltage ignition distributor that is not connected to the medium-voltage circuit. X marked the central terminal that connects to the induction coil (HV), and 1, 2, 3 and 4 marked the peripheral terminals that connect to the medium-voltage conductors that connect to the spark plugs through medium-voltage conductors. These impedances: Z_X1, Z_X2, Z_X3 and Z_X4, when they are a new medium-voltage distributor, have values of hundreds of GΩ to values of TΩ. With the duration of operation (usually expressed in the distance traveled by the vehicle), these impedances can decrease hundreds or even thousands of times, which worsens the quality of the spark obtained from the spark plugs. The decrease in the impedance value is due to the aging of the material, respectively, the dirt and moisture deposited on the outer surface of the distributor and/or the graphite dust and dirt deposited on the inner surface of the distributor. The most important effect is the aging of the electroinsulating material from which the distributor is built.

Figure 5 shows the electrical model (a simple model) for the medium-voltage ignition distributor that is connected in the circuit, with the connections made through the medium-voltage conductors (Z_C1, Z_C2, Z_C3 and Z_C4 with small values) to the spark plugs (Z_GS1, Z_GS2, Z_GS3 and Z_GS4). The rotary mechanical switch (consisting of a graphite pellet and an electroinsulating element in the shape of the letter L in rotation) inside closes the contacts S₁, S₂, S₃ and S₄ (connected, in turn, each) and, in turn, short circuits the impedances Z_X1, Z_X2, Z_X3 and Z_X4, which determines the application of a high voltage, in turn, on the spark plugs Z_GS1, Z_GS2, Z_GS3 and Z_GS4. In this way, sparks appear one electric discharge at a time at well-established times and durations. The voltage generated by the induction coil is high enough to cause an electric discharge between the electrodes of the spark plug.

If, due to some constructive defects (pores, inhomogeneous material), or if bypasses appear on the material of the medium-voltage distributor, then an important current will flow through that area (Z_X1, Z_X2, Z_X3 or Z_X4), and when a spark plug from another circuit is connected, the spark energy will be greatly reduced [33,34].

The impedances (the exception is the impedances Z_C1, Z_C2, Z_C3 and Z_C4) from Figure 4 and Figure 5 are made up of a very high insulation resistance R_is (of the order of GΩ or TΩs, depending on the type and aging) and a very small capacitance C (from pF to hundreds of pF, depending on the type). Each impedance consists of two components that are connected in parallel—Figure 6.

Medium-voltage conductors serve to transmit voltage pulses from the induction coil to the distributor and spark plugs. The medium-voltage conductors must meet the following conditions: withstand, without high losses, the high voltage; have a capacity per unit length as low as possible (below 100 pF/m); resist the action of acids, fuels and lubricants; to be flexible and to function in good conditions at temperatures of the order of 80–100 °C. To mitigate radio interference, medium-voltage conductors have an electrical resistance embedded along their length with a value of 2 kΩ/m. The core of the conductor consists of a bundle of wires twisted and enclosed in a plastic covering with a maximum percentage of ferrite. Above this cover is wound the conductor made of iron and nickel alloy (diameter 0.11 mm). On the outside, it is protected in a 6 mm thick neoprene cover (electrically insulating rubber resistant to high temperatures and chemical aggression). At the ends, it is equipped with protective rubber sleeves and contact shoes (for the connectors of the distributor cap and for the connectors of the spark plugs). The medium-voltage conductor insulation test is performed at a temperature of 100 °C at effective voltages of 15 to 35 kV and a frequency of 50 Hz [16,33].

Spark plugs serve to produce the electric spark necessary to ignite the fuel mixture in the cylinders of the internal combustion engine. The casing is made of steel and is provided on the upper part with a hexagonal profile for tightening and on the lower part with a thread which length must be equal to the thickness of the threaded wall of the cylinder head. The insulator in the central part of the spark plug, operating under extremely difficult conditions, must meet the following conditions: withstand compression stresses, expansion of the central electrode, mechanical stresses, mounting and dismounting from the engine and pressure variations in the combustion chamber (0.95–70 daN/cm²). It must have a high thermal conductivity, withstand temperature variations from −25 °C (in winter) to 2400 °C (during the engine cycle) and withstand voltages of the order of 25–30 kV in the case of tests. It must also resist, as well as possible, the chemical action of combustion products at high temperatures. Due to deposits and aging of the insulator, the insulation resistance in the cold state can decrease from 1 to 5 GΩ for new spark plugs and to 1.0–0.5 MΩ for very used spark plugs, the capacitance between the electrodes being of the order of pFs. The insulator is made of steatite, which is a magnesium silicate, and aluminum oxides. The outer surface is glazed to improve the insulating quality and reduce the degree of impurity deposits [34,35].

4. Materials and Methods

This study used a medium-voltage distributors cap (five pieces) of a four-cylinder engine (1289 cm³) used in a Kettering ignition system. Experience with medium-voltage distributors ranges from very old (tens of thousands of kilometers) to new. Experimental measurements (of three materials: Bakelite, epoxy resin and thermoplastic polyester resin) were made using a performance megohmmeter [36], using different test voltages (1, 2.5 and 5 kV), and measured between the distributor’s central terminals (seen with the induction coil; this terminal was marked with x) and the medium-voltage conductor terminals (1, 2, 3 and 4). Each measurement (x-1, x-2, x-3 and x-4) was carried out in one hour for the resistance of insulation (R_is), the dielectric absorption ratio (D_ar) and the polarization index (P_i). The distribution types A (approximately 50,000 km), B (approximately 20,000 km) and D (approximately 25,000 km) are made of phenol formaldehyde resin (Bakelite); the distribution types C (approximately 5000 km) of epoxy resin and the distribution types E (new) of thermoplastic polyester resin.

In order to design and operate electrical equipment and installations, certain safety measures and normal operations must be observed, which are always based on DC high-voltage (hundreds or thousands of V) insulation resistance measurements using megohmmeters. The isolation resistance is not constant over time [10,14,15]. Usually, the insulation resistance of electric insulation material changes over time, as shown in Figure 7.

When a high voltage to the electroinsulating materials, initially, the current can be tens or hundreds of μA; after which, it decreases non-linearly in time to tens or units of μA (Figure 8). The current passing through the electroinsulating material consists of conduction or leakage current (initially, it has a value of 0; after which, it increases over time—a few seconds—to a stabilized value of tens of μA); capacitive charging current (initially, it has a value of hundreds of μA; after which, it decreases over time—a few seconds—to 0) and absorption current (initially, it has a value of tens of μA; after which, it decreases over time—a few tens of seconds—at a stabilized value of μA). The sum of the three currents determines the total current through the electroinsulating material. Megohmmeters apply a DC high voltage on the electrical insulating material and measure a very small current of the order of μA or nA and then apply Ohm’s law to measure the insulation resistance.

Other important factors in determining the condition (quality) of the insulation are the dielectric absorption ratio (D_ar) and the polarization index (P_i) [6,25].

$$D_{ar}=\frac{R_{after 1 \mathrm{m}\mathrm{i}\mathrm{n}}}{R_{after 30 \mathrm{s}}}$$

(1)

$$P_i=\frac{R_{after x \mathrm{m}\mathrm{i}\mathrm{n}}}{R_{after 1 \mathrm{m}\mathrm{i}\mathrm{n}}}$$

(2)

In Equation (2) usually, x = 10 min. In principle, P_i < 1.5 signifies bad insulation, P_i > 2 signifies good insulation, D_ar < 1.1 signifies bad insulation and D_ar > 1.25 signifies good insulation (these values are indicative). The values of P_i and D_ar are used for estimating the quality of the insulation. This is achieved by plotting the insulation resistance depending on time and calculating the P_i and D_ar values.

For medium-quality insulation materials, resistance should be increased during measurement voltage (Figure 9).

Distributor types A (approximate operation 50,000 km), B (approximate operation 20,000 km) and D (approximate operation 25,000 km) are made of phenol formaldehyde resin (Bakelite), distributor type C (approximate operation 5000 km) is made of epoxy resin and the type E distributor (new) is made of thermoplastic polyester resin.

The measurements made on medium-voltage distributors were made over a long period of time at ambient temperatures of 22–24 °C and humidity 45–55%. The measurements between two terminals made with the megohmmeter were made in a timespan of 1 h. The making of another group of measurements was made on another day (min. 24 h) for resting the material.

5. Experimental Measurements with Medium-Voltage Distributors

In the following experiments, the insulation resistance (R_is) of five different medium-voltage distributors was tested using the UNILAP ISO 5 kV, LEM (test voltages of 1 kV, 2.5 kV or 5 kV, measures up to 30 TΩ, P_i and D_ar) megohmmeter [36]. The aspects of the measurements are presented in Figure 10. The experimental measurements in each graph (Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17, Figure 18, Figure 19, Figure 20, Figure 21, Figure 22 and Figure 23) were made in one hour (U_t is the test voltage). Measurements were made on distributors after they were used in ignition installations (Figure 10, Figure 11, Figure 12, Figure 13, Figure 15, Figure 16, Figure 18, Figure 20 and Figure 23), and in some measurements, distributors were used that were thoroughly cleaned (after cleaning) with alcohol on the inner and outer surfaces; after which, they were left to dry 24 h (Figure 14, Figure 17, Figure 19, Figure 21 and Figure 22). For the type D distributor, two successive chemical cleanings (with industrial alcohol) on the surfaces were carried out (Figure 21 and Figure 22).

6. Discussions

As a result of the experimental measurements, it was found that the insulation resistances between the terminals can be very different (tens or even hundreds of times, as it depends especially on the age) at different distributors (from new to old). Insulation resistances change value during the experiments. For used distributors, depending on the service life and material of the medium-voltage distributor, the insulation resistances can be tens to hundreds of GΩ (for old ones) to TΩ (on new ones)—Figure 11a, Figure 12a, Figure 13a, Figure 14a, Figure 15a, Figure 16a, Figure 17a, Figure 18a, Figure 19a, Figure 20a, Figure 21a, Figure 22a and Figure 23a. A longer operation of medium-voltage distributor uses leads to a decrease the insulation resistance by tens or hundreds of times (compared to the insulation resistance of new or newer distributors). At new medium-voltage distributors, dielectric absorption ratios and polarization indices have the highest values. Medium-voltage distributors new or newest with quality materials have an insulation resistance that increases over time (Figure 11a, Figure 12a, Figure 13a, Figure 14a, Figure 15a, Figure 16a, Figure 17a, Figure 18a, Figure 19a, Figure 20a, Figure 21a, Figure 22a and Figure 23a).

For the same medium-voltage distributor, the insulation resistances can differ a lot (they can be 1.5–3 times different; Figure 11, Figure 12, Figure 14 and Figure 18a).

In

Table 1. Isolation resistance R_is (GΩ)—average values (calculated over the entire measurement period of 60 min).

MVD	Status	Ut (kV)	Risx-1 (GΩ)	Risx-2 (GΩ)	Risx-3 (GΩ)	Risx-4 (GΩ)	Risavg (GΩ)	Max. Deviations (%)
Type A	used	1	42.79	91	26.29	29.79	47.76	+90.53
Type A	used	2.5	41.5	32	20.85	49.5	35.96	−42
Type A	used	5	42	33.57	23.27	26.06	31.23	+31.28
type A	cleaned	5	43.93	37	18.17	27.92	31.76	+38.31
type B	used	2.5	104.71	118.36	111.64	117.29	113	+4.74
type B	used	5	99.29	102.57	96.71	90.86	97.36	−6.67
type B	cleaned	5	75.71	74.43	72.71	76.21	74.77	−2.75
Type C	used	5	4120	4920	7000	5760	5450	+28.44
type C	cleaned	5	4910	4900	4410	4670	4730	−6.76
type D	used	5	91.86	88.79	91.57	90.71	90.73	−2.13
type D	cleaned 1	5	90.07	86.14	92.07	81.86	87.54	−6.48
type D	cleaned 2	5	81.71	75.43	77.57	73.64	77.09	+6
type E	new	5	7260	7970	7780	7950	7740	−6.2

,

Table 2. Isolation resistance R_is (GΩ)—last values after 60 min.

MVD	Status	Ut (kV)	Risx-1 (GΩ)	Risx-2 (GΩ)	Risx-3 (GΩ)	Risx-4 (GΩ)	Risavg (GΩ)	Max. Deviations (%)
Type A	used	1	43	83	23.2	28.6	44.45	+87.72
Type A	used	2.5	41	32	21.1	48	35.52	−40.59
Type A	used	5	42	34	22.3	26.5	31.2	+34.6
type A	cleaned	5	44	38	18	28.1	32.02	−43.78
type B	used	2.5	104	119	112	117	113	−7.96
type B	used	5	97	104	96	91	97	7.21
type B	cleaned	5	74	77	72	77	75	−4
Type C	used	5	4400	5200	7300	5600	5625	+29.77
type C	cleaned	5	5100	5100	4800	4900	4975	−3.51
type D	used	5	94	91	92	90	91.75	+2.45
type D	cleaned 1	5	89	85	92	84	87.5	+5.14
type D	cleaned 2	5	79	74	77	76	76.5	+3.26
type E	new	5	8100	8400	8100	8300	8225	+2.12

,

Table 3. Dielectric absorption ratio D_ar (-).

MVD	Status	Ut (kV)	Darx-1 (-)	Darx-2 (-)	Darx-3 (-)	Darx-4 (-)	Daravg (-)
Type A	used	1	1.00	0.92	0.86	0.87	0.91
Type A	used	2.5	0.87	1.00	1.01	0.93	0.95
Type A	used	5	1.00	1.03	1.01	1.00	1.01
type A	cleaned	5	1.02	1.03	1.01	1.01	1.02
type B	used	2.5	1.02	1.04	0.99	0.99	1.01
type B	used	5	0.94	0.91	0.9	0.9	0.91
type B	cleaned	5	0.89	1.01	0.88	0.91	0.93
Type C type C	used	5	1.46	1.87	1.11	0.64	1.27
	cleaned	5	1.25	1.07	1.15	1.17	1.16
type D	used	5	1.00	0.94	0.91	0.9	0.94
type D	cleaned 1	5	0.94	0.92	0.91	0.89	0.91
type D	cleaned 2	5	0.92	0.94	0.91	1.04	0.95
type E	new	5	1.26	1.24	1.26	1.3	1.26

and

Table 4. Polarization index P_i (-)—average values.

MVD	Status	Ut (kV)	Pix-1 (-)	Pix-2 (-)	Pix-3 (-)	Pix-4 (-)	Pixavg (-)
Type A	used	1	1.02	0.92	1.01	0.99	0.99
Type A	used	2.5	1	1	1	0.98	1
Type A	used	5	1	1.02	1	1.04	1.01
type A	cleaned	5	1	1.07	1.02	1.03	1.03
type B	used	2.5	0.98	1.02	1	1	1
type B	used	5	0.97	1.02	1.01	1.01	1.01
type B	cleaned	5	0.99	1.06	1.01	1.01	1.01
Type C	used	5	1.23	1.21	1.14	1.1	1.17
type C	cleaned	5	1.12	1.11	1.19	1.17	1.15
type D	used	5	1.04	1.06	1.02	1	1.03
type D	cleaned 1	5	1.01	1	1.03	1.06	1.03
type D	cleaned 2	5	0.97	0.99	1.02	1.03	1
type E	new	5	1.42	1.22	1.18	1.18	1.25

, MVD is a medium-voltage distributor type.

In Table 1 and Table 2, in the last column, the maximum deviations for R_isx-j, j = 1–4 compared to the average values (for the same medium-voltage distributor) were determined (the values in the module matter, not the fact that they are with + or −).

Insulation resistances have values up to 100 GΩ for used distributors and values over 4 TΩ for newer or new distributors (Table 1). At the same time, it was found that, for the same medium-voltage distributor, important differences appear among the insulation resistance measurements between the terminals (e.g., for type A, 26 GΩ and 91 GΩ, the maximum deviations from the average isolation resistance of +90.53%; for type C, 4.12 TΩ and 7 TΩ, the maximum deviations from the average isolation resistance +28.44%).

Table 2 shows the insulation resistances after 1 h of measurements. In general, the insulation resistances have the same order of magnitude as the average values of the insulation resistances and the maximum deviations from Table 1. For the distributors used (A, B and D types), R_is can be higher or lower (e.g., type A: 83 GΩ compared to 91 GΩ, 43 GΩ compared to 42.79 GΩ, type D: 90 GΩ compared to 90.71 GΩ and 94 GΩ compared to 91.86 GΩ) at the end of the 1 h measurement (Table 1) compared to the average values (Table 2). These indicate an uneven degradation of the material of the distributors between the terminals. For new or newer distributors (type E and C), R_is at the end of the measurements are always higher than the average values (e.g., type C: 4.4 TΩ compared to 4.12 TΩ and type E: 8.3 TΩ compared to 7.95 TΩ).

Table 3 shows the D_ar (dielectric absorption ratio) for the experiments carried out in Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17, Figure 18, Figure 19, Figure 20, Figure 21, Figure 22 and Figure 23.

For the same distributor, D_ar can be modified (e.g., type D: D_ar between 0.9 and 1), especially for used distributors. The average values for D_ar for used distributors are between 0.91 and 1.02 and, for the newest or new ones, between 1.26 and 1.27 (Table 3). For the same distributor, the Pi can be modified (e.g., type A: Pi between 0.92 and 1.02), especially for used distributors. The average values for Pi for used distributors are between 0.99 and 1.03 and, for the newest or new ones, between 1.17 and 1.25 (Table 4).

From one circuit to another, the same medium-voltage distributor can have a four to five times insulation resistance (min and max). It has been found that a high-value isolating resistance may have a small polarization index that decreases over time, indicating the low quality of the material the distributor manufactures. At different test voltages (1, 2.5 or 5 kV), different values and evolutions of the insulation resistance and polarization indices can be obtained (e.g., for type B: 42.79 GΩ for 1 kV, 41.5 GΩ for 2.5 kV and 42 GΩ for 5 kV). Through experiments, it has been determined that a lower insulation resistance can have a higher polarization index, indicating a better distributor insulation quality.

In practice, an increased distribution insulation test voltage results in lower insulation resistance values for low-quality insulation (mainly due to age). For low-quality insulation, the polarization indexes can change from one test voltage to another and have a decreasing evolution over time. For ordinary quality insulations, the insulation resistance and polarization index have the same values over time. With uniform use of the distributor, the insulation resistances have similar evolutions. For the distributors used, polarization indices have almost constant evolution over time. For new distributors, the resistance to insulation has values in the order of TΩ (e.g., for type C and E: 4 to 7 TΩ), which increase over time, the resistance to insulation can differ from each other and the polarization index can evolve with increasing or remaining almost constant over time. As the test voltage increases, insulation resistance, dielectric absorption ratios and polarization indices may decrease in low-quality insulation. As an average value, the dielectric absorption ratio is slightly lower than the polarization index for all measurements.

Chemically cleaning the internal and external surfaces of distributors can slightly improve the insulation resistance (with 10–15%) in some cases, and in most cases, the insulation resistance does not change or can decrease (with 20–30%) relative to the insulation resistance of unwashed distributors (Table 1 and Table 2). This indicates that the volume insulation resistance of the distribution material is much more important than the surface (internal and external) insulation resistance of the distribution material. Over time, the volume of the dielectric material is affected, not so much the surface of the dielectric material. Therefore, chemical cleaning does not improve the resistance to insulation, the dielectric absorption ratio or polarization indices. Chemical cleaning can produce similar values and evolutions in insulation resistance. When distributors are attracted, dielectric absorption rates and polarization indices have insignificant changes (increasing or decreasing).

Variable DC high-voltage test megohmmeters (the megohmmeter used in experiments measures up to 30 TΩ [36]) have the advantage of being able to measure some of the most important parameters that can be used to check dielectric materials (including medium-voltage distributors): insulation resistance, dielectric absorption ratio and polarization index. The values obtained from the measurements are comparable with those from References [13,24,27,30,37,38]. A disadvantage of using this method is that it requires time when performing a measurement and pause times between measurements.

Other dielectric material properties are permittivity, relative dielectric constant, resistivity and the relative loss (loss tangent), which can be achieved with various measuring devices, at high voltage: Schering measuring bridges, LCR meters and impedance analyzers. It is important to note that the dielectric material properties are not constant and can change with the frequency, temperature, orientation, mixture, pressure and molecular structure of the material [7,10,12,20,22].

7. Conclusions

As a result of the experimental analysis on the medium-voltage distributors used for petrol engines, it was found that:

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for used distributors, depending on the duration of use and the material, the insulation resistances can be from tens to hundreds of GΩ (up to 100 GΩ for oldest ones) to TΩ (4–7 TΩ, for new ones);

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a longer duration of use of the distributors leads to the decrease of the insulation resistance by tens or hundreds of times, and the resistances between the distribution terminals can be very different (e.g., for type A, 26 GΩ and 91 GΩ, maximum deviations from the average isolation resistance of +90.53%);

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an insulation resistance with a higher value can have a polarization index that is small (e.g., type A: P_i between 0.92 and 1.02) and decreases and a lower dielectric absorption ratio (e.g., type D: D_ar between 0.9 and 1), which indicates the low quality of the insulation;

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a lower insulation resistance can have a higher and increasing polarization index and a higher dielectric absorption ratio, which indicates a better insulation quality (Figure 14a,b);

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for a lower-quality insulation, the polarization indices and dielectric absorption ratios can change from one test voltage to another; they can have a decreasing evolution over time (Figure 13b and Figure 22b);

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by chemical cleaning of the distribution surfaces, the isolation resistances, dielectric absorption ratios and polarization indices will not be improved (e.g., for type B: before cleaning, R_isavg = 97 GΩ; after cleaning, R_isavg = 75 GΩ; e.g., for type C: before cleaning, R_isavg = 5.625 TΩ; after cleaning, R_isavg = 4.975 TΩ);

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over time, the volume of electrical insulating material is affected, not so much the surface of the electrical insulating material.

Future research will be carried out on the medium-voltage assembly used in the COP and CNP ignitions that are frequently used today in petrol engine ignition.