Designing Refiner Motors to Withstand Switching Voltage Transients

During the past 25 years, the capacity of typical single-disc thermal mechancial pulp refiner motors has increased by a factor of three (see Figure 1). With pulp output concentrated in fewer machines, the reliability associated with larger motors becomes increasingly significant.

Stator winding insulation systems are a critical component affecting over-all motor reliability. Paralleling the development of TMP pulping technology since the 1970s, has been the introduction and continuous improvement of high-voltage (up to 15 kV) resin vacuum pressure-impregnated (VPI) stator insulation systems. The use of improved materials along with a more comprehensive understanding of the physics related to the aging and failure mechanisms of dielectric systems have resulted in stator insulation systems that achieve the high reliability expected of larger refiner motors.

In the past, insulation systems for refiner motors have been primarily designed and tested based on line-frequency voltages. Some equipment, such as switchgear and transformers, was designed for occasional lightning and switching surges, and impulse testing used to verify an impulse withstand level or basic impulse level (BIL) for the equipment. In the past 20 years, triggered by the introduction of vacuum circuit breakers, rotating machine designs have taken into account switching surges as a design parameter.

In the 1950s, turn-to-turn insulation was based on line-frequency voltages of the order of 100 or 200 volts RMS per turn. Then it was realized that transient and surge voltages caused severe turn-to-turn voltage stress, and the first issue of IEEE 522 [1] recommended a two per unit voltage capability with a 0.2 microsecond rise time based on line-to-ground peak voltage. Most manufacturers selected their turn-to-turn insulation to meet this two per unit factor. Investigations by utilities [2,3,4] in the mid-1980s established that operation of both vacuum and air-magnetic circuit breakers introduced multiple voltage surges or spikes that had a peak magnitude at the motor terminal box as high as 4.6 per unit (line-to-ground peak voltage), with a rise time to peak as low as 0.1 microseconds.

Subsequently, IEEE Guide 522 was revised to include a 3.5 per unit test value instead of a two per unit level for those applications that could be termed as high surge applications, Fig. 2. NEMA standard MG 1-1993, Part 20.87.5 [5] recognizes the higher impulse withstand envelope of IEEE 522, but the motor buyer must specify the increased capability at the time of purchase. These switching transients are not continuous and usually occur on circuit breaker opening and closing operations. There are still some motor manufacturers that have not changed their design practices to use the 3.5 per unit value in place of the previous two per unit recommendation, and therefore their turn-to-turn insulation could be at risk under some breaker operating conditions.

Some vacuum breaker manufacturers realized early in their development of the vacuum interruptors that re-strikes could create voltage spikes and they supplied, with their switchgear assemblies, suitable surge suppressors or arrestors to limit the peak transient voltages [6]. Every feeder breaker designated for motor switching was equipped with these suppressors. Some manufacturers of vacuum interrupters have introduced different metals into their contacts to reduce restrike transients, but this can have an adverse effect on the interrupting capability of the contact design. Careful design of the breaker mechanism with sufficient wipe and contact separation speed can also improve the transient switching performance of vacuum breakers. There are a number of technical papers describing the re-strike phenomenon [7,8,9].

Transient Effects
The transient voltages have a rise time of 0.1 to 0.2 microseconds to crest value. This fast rate of rise is equivalent to a frequency between 1.25 and 2.5 MHz. The transient is a traveling wave phenomenon that impinges on the motor stator winding, stressing the turn-to-turn insulation of the first turn. As the transient travels along the turns of the first coil, it loses some of its energy due to capacitive leakage to the grounded stator and capacitive coupling between turns. At the end of the first coil there is a change in the surge impedance due to the shape of the series connection to the second coil. This results in a minor voltage reflection, increasing the voltage stress on the last turn of this first coil.

As the transient passes to each of the remaining coils, the crest voltage and the rate of rise of that crest decrease. Therefore, transient-related insulation failures usually occur as a turn-to-turn fault on the first or terminal coils in a machine. Transients with slower rise times (>5 m-msec, corresponding to a frequency of 50 kHz) distribute themselves over the winding more gradually, resulting in lower turn-to-turn voltage stress. The voltage stress distribution in the machine from terminal to neutral connection will vary from machine to machine depending on the surge impedance of the winding. Using the IEEE 522 surge factor of 3.5 per unit, the required turn insulation capability can be selected on the basis of the following formula:

Vt 3.5(Vp-p/sr3) (sr2)/T

where:
Vt = turn-to-turn voltage capability in crest volts;
Vp-p = phase-to-phase RMS voltage at system frequency;
sr =square root
T = number of turns per coil.

As an example, the minimum required turn-to-turn voltage capability for a motor on a 13.8-kV system with a five-turn coil would be 7.9 kV crest per turn. The turn-to-turn insulation thickness will probably be in the range of 1.0-mm (40-mil), so the average voltage stress is approximately 7.9 kV peak volts per mm (200 peak volts per mil). Ground wall insulation, conductor to ground average stress, is in the range of 2.8 kV peak volts per mm (70 peak volts per mil).

In general, the turn-to-turn insulation is stressed higher than the ground wall insulation, and therefore the turn-to-turn insulation should receive special attention both from a design and a manufacturing standpoint.

Void-Free System
The distribution of electric stress with more than one dielectric in series is inversely proportional to their respective dielectric constants.

In a system with two dielectrics, Fig. 3, the voltage distribution follows the relationship:

V1 = [K2S1 / (S1K2+S2K1)] V

where:
V1 = voltage across insulation with dielectric constant K1;
V2 = voltage across insulation with dielectric constant K2;
V = total voltage across system;
S1 = thickness of insulation with dielectric constant K1;
S2 = thickness of insulation with dielectric constant K2.

For example, assume a turn-to-turn insulation that has a thickness of 1.0 mm (40 mils), and a 0.025-mm- (1 mil-) thick air gap or void occurs in the insulation. Based on an insulation with a dielectric constant of 3.5 and with the dielectric constant for air as 1, the voltage across the air gap will be 8 per cent of the total voltage applied. With a 7.9-kV voltage stress, the air gap or void will have 633 peak volts or 447 volts RMS. This stress in the air is too high and the air breaks down with a partial discharge. The partial discharge activity will cause deterioration of the insulation with time, and ultimately there will be a failure of the insulation system.

Electrically, the insulation system can be represented by parallel or series connections of a resistor and a capacitor. Figure 5 is a parallel representation, one branch having a void in the solid insulation; it also shows a vector diagram, where the angle d is known as the loss angle. Poor insulation systems will have a large loss angle because the IR vector (leakage current) will be large. The tangent of this angle is referred to as the dissipation factor (DF) or tan delta, which is often considered as a measure of the quality of the insulation system. Poorly compacted or void-filled insulations will have a high DF. For rotating machines with VPI insulation systems, the DF for a full phase winding at rated voltage should be less than 4 per cent, or the tangent of the loss angle must be less than 0.04.

Paschen's Curve
The breakdown characteristics of air can be approximated by a modified Paschen's curve, Fig. 5. If the voltage stress across the air gap or void in the system is greater than the breakdown strength of the air, as was the case in our example, then partial discharges will occur and the system will deteriorate with time. Note that there is a minimum voltage at which breakdown can occur. Gaps smaller than 6 mm (0.24 mils) will require higher voltages to break them down. It also indicates that as long as the voltages are below 280 volts RMS, partial discharge would not occur and very small air gaps would not be critical. The curves are based on standard atmospheric pressure and using electrodes with a diameter of 12.7 mm (1/2 inch). A decrease in electrode diameter will lower the breakdown value. High frequencies in the megahertz range will also lower the breakdown voltage.

Areas of Concern
The cross section of a large high-voltage coil is shown in Fig. 6. In VPI systems, there have been some manufacturing problems in ensuring that the impregnating resin penetrates between turns to eliminate air voids in this area. As well, there is concern in filling the triangular space between turns at the location marked "A" in Fig. 6 [10].

Some manufacturers use a resin with a very low viscosity to ensure that there is complete penetration. The disadvantage to this low-viscosity approach is the tendency for the resin to flow out of larger void areas during the time between the impregnation of the resin and the baking process to cure the resin. One of the ways to overcome these difficulties is to use resin rich mica tapes for the turn insulation. The tapes are applied to the strand bundle as the coil bobbin or loop is formed. After spreading, the slot sections of the coil are placed in a press and heat and pressure are applied. The heat and pressure causes the resin, which is now already in the turn-to-turn location, to flow and displace any air during the curing process.

As the coil section is held within irons during the pressing procedure, the turn package is rectangular, with a flat surface along the sides of the coil and the air spaces at "A" have been eliminated. The ground wall insulation is applied over these pressed turns and the conventional VPI process is followed for the remainder of the insulation system. The major benefit of the pressed turn is minimizing the chance of air voids in a critical area of the system. On high-voltage coils the VPI resin must work its way from outside the coil, through the thick ground wall insulation, and with the pressed turn system the VPI resin does not need to penetrate beyond the turns to the strands.

The electrical stress is highest at the surface of the conductor. Dielectric field plots have indicated that the electrical stress at the corners of the coil, next to the copper, can be double the stress along the side of the coils. Figure 8 is a diagram showing the equipotential lines at the corner of a stator coil. Note that the lines are closest together next to the copper. Therefore, voids or deficiencies in the insulation next to the strands and turns are critical, and damaging partial discharges will occur first in these areas. For this reason, the use of corona-resistant enamels as a strand insulation can delay the propagation of a fatal carbon track through the insulation and increase the life and reliability of the system.

The use of non-film-backed tapes, the elimination of corona-sensitive materials and the use of fillers to give corona-resistant characteristics to tapes and resins also improves life. Figure 8 is a curve of voltage endurance showing the extended life that can be achieved with the use of corona-resistant materials.

Using Capacitors
Provided the turn-to-turn insulation has been designed to meet the 3.5 per unit surge factor, capacitors, arrestors and resistors would not be required to protect the stator winding turn insulation. However, capacitors and arrestors have been applied for many years and they will protect against power system surges due to system switching operations and lightning strikes on the supply system. They do slope the wave front of surges and minimize the magnitude. This distributes the stress in the winding and will reduce transient partial discharge activity. They are good insurance.

The addition of series resistors with the capacitors to critically damp the motor, cable and supply system so that multiple restrikes on vacuum breakers do not occur, has some disadvantages. The resistors reduce the amount of wave sloping produced by the surge capacitors. The selection of the resistors is dependent on the surge impedance of each specific cable run from the switchgear to the motor terminals, as well as the impedance of the source bus and its connected loads, such as other motors. If additional motors are added to the bus, or conversely a motor is removed, then the resistors lose some of their effectiveness and should be replaced. Non-inductive resistors are complex to design, particularly considering that they must be capable of withstanding any initial single surge voltage, terminal to terminal. Non-inductive resistors are usually counter wound so that electrical clearances are limited.

Conclusions
New motors are available that are capable of withstanding potential re-strikes produced by vacuum interrupters. Older motors, designed to the two per unit surge value, should have arrestors and capacitors, and it may be practical to add partial discharge monitors to the machine so that there would be an advanced indication of deterioration due to transient partial discharges caused by breaker operations [11].Adding resistors in series with the surge capacitors is adding complexity to the installation and additional insulation paths to ground.

These factors could be adversely affecting reliability relative to a motor designed initially to meet the 3.5 per unit turn-to-turn levels described in IEEE 522 along with the use of advanced corona-resistant materials.

Purchasers of large refiner motors should specify a stator insulation system that is capable of meeting the higher IEEE surge withstand envelope.

Furthermore, all stator coils should be impulse tested individually after installation in the stator but prior to connection.