The Hoisting Machines as Source of Higher Harmonics in Underground Mines

Abstract

The chapter describes the structure of drive systems used in hoisting machinery of underground mines. These devices, due to their special application and power, are characterized by special traits. The most commonly used systems with DC reciprocating motors are described. The so-called complex (multi-pulse) converters are used in those solutions. An extra attention was paid to the impact of these systems on the power supply network, especially the generation of higher harmonics. The results of measurements made under real conditions are presented. Measurements were made before and after the installation of new solution in connection with the modernization of systems due to the need to increase the efficiency of these machines.

1. Introduction

To analyze higher harmonics as one of the elements of the quality of electricity supply, it is necessary to clarify what their sources are and how it is possible to describe waveforms containing higher harmonics.

Higher harmonics—such a term is used to describe waveforms with frequencies that are integer multiples of the fundamental frequency. If we assume that the frequency of the fundamental waveform is 50 Hz, then the harmonics will have frequencies as given in Table 1.

Harmonic order (h)	Harmonic frequency [Hz}
2	100
3	150
4	200
5	250
…	…
n	n*50

Table 1.

Summary of harmonic orders and their frequencies.

Sources of higher harmonics are loads with nonlinear current-voltage characteristics. Such a load, connected to the power grid, causes a current flow in the network with a distorted waveform. The flow of non-sinusoidal current causes voltage drops on the network reactances, which are not sinusoidal waveforms, although the source voltage has a sinusoidal waveform.

Analysis of distorted waveforms is possible by using Fourier series [1, 2, 3, 4, 5, 6, 7]. A periodic, distorted time waveform of voltage or current can be decomposed into sinusoidal waveforms of different frequencies. Eq. (1) is the Fourier series for the function f(t) [8]:

$$f\left(t\right)=\frac{a_0}{2}+\sum _{h=1}^{\infty }{a}_{\left(h\right)}cos\left(\mathit{h\omega t}\right)+\sum _{h=1}^{\infty }{b}_{\left(h\right)}sin\left(\mathit{h\omega t}\right)$$E1

Where:

$$a_0=\frac{1}{\pi }{\int }_0^{2\pi }f\left(t\right)\mathit{dt}$$E2

$$a_{\left(h\right)}=\frac{1}{\pi }{\int }_0^{2\pi }f\left(t\right)cos\left(\mathit{h\omega t}\right)\mathit{dt}$$E3

$$b_{\left(h\right)}=\frac{1}{\pi }{\int }_0^{2\pi }f\left(t\right)sin\left(\mathit{h\omega t}\right)\mathit{dt}$$E4

In another form, the Fourier series can be written:

$$f\left(t\right)=\frac{a_0}{2}+\sum _{h=1}^{\infty }{A}_{\left(h\right)}sin\left(\mathit{h\omega t}+{\phi }_h\right)$$E5

The components of the sum in Eq. (5) are called harmonics of the periodic waveform f(t).

Distorted waveforms are a very big problem in power grids. Assessing the degree of distortion and defining acceptable distortion levels is a major challenge. The most well-known indicator for determining the level of distortion is total harmonic distortion (THD). It will be described in detail in the following section.

2. The power grids of mines

The subject described here concerns phenomena related to higher harmonics in the electrical networks of underground mines in Poland. According to data published by the State Mining Authority, there are currently 36 underground mines in Poland [9]. These include coal mines, ore mines, and salt mines (Table 2).

Underground mining facilities extracting	Number of mines
Coal	20
Coal (mines in decommissioning)	8
Coal (mines under construction)	1
Copper ores	3
Zinc and lead ores (mines in decommissioning)	1
Salt	1
Gypsum and anhydrite	1
Brine for therapeutic purposes	1

Table 2.

Number of mines in Poland [9].

Each of the aforementioned types of mines has different characteristics of work, and consequently, the electrical networks of these plants are subject to different disturbances. However, it is possible to find some common features that are present in all mining plants, as they are subject to the same regulations. These include regulations on crucial equipment, the operation of which affects the safety of crews and equipment.

The power supply of underground mines is carried out in accordance with the principle of its reliability; that is, each mine must have a power supply from two independent sources of electricity supply. In most mines, the power supply is provided by double overhead or cable lines with a rated voltage of 110 or 220 kV. Due to the fact that the decommissioning of mines in Poland has been progressing in recent years, some mines have a medium-voltage power supply in addition to the meddium voltage (MV) level. However, MV power supply is a backup power supply, due to the impossibility of powering all technological equipment, which power exceeds a dozen megawatts. The use of a power supply from the medium-voltage level as a backup power supply is in accordance with current regulations, which specify that the second power supply must have ensure parameters that it is possible to run devices that guarantee the safety of people, equipment, and the environment. This should be understood in such a way that the backup power supply is sufficient, in terms of parameters, for the operation of equipment used to evacuate the crew and ensure the operation of the main dewatering pumps and the main ventilation fans of the mine.

Like any industrial plant, in a mine, special attention is paid to the correct operation of equipment because only in this case it is possible to ensure adequate economic performance and safety of workers. According to Ref. [10], in every mining, there are so-called basic facilities, that is, those which correct operation guarantees the safety of the crew and equipment. Equipment that are elements of these facilities should have two independent sources of power supply. Based on Article 29 [10], basic facilities include:

1. shafts with equipment;

2. hoisting machines in shafts;

   1. stations:

   2. main fans,

   3. de-methanation together with a network of pipelines;

3. headquarters of the company-wide telephone communications system, dispatching centers of the traffic dispatcher’s systems, mining geophysics stations, and trunk telecommunications networks;

4. main air compressor stations, together with a network of shaft pipelines;

5. main drainage facilities and systems;

6. main depots for fuels, oils, and lubricants, as well as permanent fuel-filling chambers for transport vehicles;

7. main facilities for producing and transporting backfill and sealing mixtures;

8. stationary air-conditioning equipment with a nominal cooling capacity of more than 1 MW;

9. transport equipment, the means of transport of which move on a track with an inclination of more than 45^o, in mine workings;

10. high-voltage and medium-voltage electrical equipment, installations, and networks supplying the facilities, machinery, and equipment referred to in items.

The permissible duration of interruption of power supply to these devices is determined by the Mine Site Operation Manager, and each time results from the specific conditions in the mine. For example, if the main ventilation fan is damaged and the standby fan cannot be started, underground work is halted, electrical equipment in the relevant methane fields (if applicable) is switched off, and the evacuation of the crew begins. The special, detailed regulations for basic equipment testify to a very complex problem when it comes to powering the mine and individual equipment. It is also related to the fact that special attention is paid to the problems of interference in the networks of mining plants and the consequences it can have on the safety of the working crew. In each underground mine, the surface and underground parts of the power grid can be distinguished. In each case, the network layouts of the surface and underground parts are different.

Figure 1 shows a block, simplified diagram of the power grid of a mining plant. The dashed line indicates the backup power supply. There may be situations where a mining plant has three power supply points.

Figure 1.

The example diagram of a power grid in a mine.

From the medium-voltage switchgear in the surface section, the most important primary facilities are supplied: hoisting machines and fan stations. Two types of network layouts are used in mine networks: IT and TN. The insulated network is used to supply power to equipment operating underground. TN systems are used in networks on the surface. This is important for further consideration because when supplying three-phase equipment from a network with an isolated zero point, there is no zero component, that is, harmonics, the order of which is an integer multiple of 3 in the expansion of the function into Fourier series. One of the criteria for evaluating multi-pulse power systems for hoisting machinery is the level of their negative impact on the power supply network. In particular, it refers to the distortion of voltage due to the flow of distorted current. The legislation [11] defines numerical measures for dimensioning this impact. The basic coefficients used to assess voltage distortion are the total harmonic distortion (THD) and the values of individual harmonics [12, 13, 14, 15].

The evaluation is carried out on the medium-voltage side (on the primary side of the converter transformer). Regulations [11] strictly define permissible levels of harmonics and the value of the voltage THD.

3. The sources of higher harmonics in networks of mining plants

In the power grids of mines, the sources of higher harmonics are primarily power electronic converters used to supply the drives of various equipment. Such equipment includes:

1. Hoisting machines,

2. Fans,

3. Drives for conveyors and pumps.

AC/DC converters, AC/DC multi-pulse systems, AC/DC/AC intermediate frequency converters, and soft-starts are used in the mentioned equipment. The hoisting machine in any mine is the largest load and the largest source of interference. The power of hoisting machines ranges from several hundred kilowatts to several megawatts. In Poland, the most common machines range from 1.5 to 7 MW. In comparison, the powers of the main fan motors range from 900 kW to 1.5 MW. In addition, the fan operates in a continuous mode in contrast to the exhaust machine, as will be discussed later in the chapter. Other receivers pose a potential threat, but the magnitude of this threat is determined by the number of these devices, which varies from mine to mine, and the simultaneity of operation.

3.1 The hoisting machine

A hoisting machine transports ore, materials, and crew from underground mines to the surface. It is usually the only transport route from underground workings. That is why it is so important for these machines to work properly.

The hoisting machine is an electromechanical system that includes:

• Koepe pulley or drum,

• hoisting vessels (cage, skip),

• ropes,

• motor,

• converter together with a transformer,

• machine control system,

• braking system,

• signaling and shaft communication.

Figure 2 shows a diagram of a winding machine with two types of linkage: a Koepe wheel (Figure 2a) and a drum (Figure 2b).

Figure 2.

Block diagram of the hoisting machine. (a) with Koepe pulley, (b) with drum.

In the machine with the Koepe pulley, the support rope is wound through a special type of wheel. The wheel is equipped with special grooves with a lining to increase the coefficient of friction. For proper operation of the system, it is necessary to use an equalizing (tail) rope to bring the system into full balance. Failure to use an equalizing rope would result in the need for a much more powerful motor, and the entire system would be subject to the occurrence of slippage of the rope relative to the drive wheel. The use of an equalizing rope means that the size of the drive motor is selected only because of the weight of the load being carried in the hoisting vessel. This mass determines the static overweight that occurs in this kinematic system.

The second type of machine, shown in Figure 2b, is a drum machine. In this case, the rope is wound on a drum.

Regardless of the type of executive system implementing the displacement of vessels in the shaft, the following are used to drive the rope drive:

• separately excited DC motors (Figure 3a),

• squirrel-cage induction motors,

• slow-rotating synchronous motors (Figure 3b).

Figure 3.

The winding machine with a DC motor (a) and a synchronous motor (b). [SIEMAG TECBERG POLSKA].

You can still find working systems of hoisting machines with slip-ring motors. The speed of these motors is controlled through attached resistances in the rotor circuit. For economic reasons, such solutions are no longer used.

3.2 The operation of the winding machine: Driving diagram

The operation of a hoisting machine is cyclic. The duty cycle of a hoisting machine consists of three stages: acceleration, travel with fixed speed, and deceleration. The length of the driving cycle, and thus the lengths of its individual parts, depends on the depth of the shaft, driving speed, acceleration, and deceleration, and on the mode in which the machine is used, that is, whether it is to transport ore, materials, or people. Figure 4 shows an example of a seven-period driving diagram. This is a driving diagram for a system with two vessels (skips) for transporting ore from mining level. The diagram is seven-period because it still takes into account additional stages, where the vessel moves in special guidance systems so that precise travel to the skip filling or emptying point is possible. It differs slightly from the diagram for a machine used to transport people. For the transportation of people, the vessel cannot move faster than 12 m/s. (respectively 43.2 km/h). This restriction does not apply to machines transporting materials or excavated material.

Figure 4.

The hoisting diagram.

The time of the entire cycle ranges from tens of seconds. For a machine in which the vessels move at a speed of 12 m/s, the acceleration and deceleration is 1.0 m/s², and the travel distance is equal to 500 m the duration of one cycle is about 80 seconds (including the time required for loading and unloading).

Therefore, this type of high-power device is very troublesome with regard to the formation of disturbances in the power grid. The load is cyclically variable, depending on the load—that is, on the mass of excavated material poured into the vessel or the number of people in the cage. The timing of the start in the excavated material conveying equipment depends on the quality of the excavated material and affects the start and end of the entire cycle.

It is not possible to synchronize the operation of the machine with other equipment in the mine, since it is not possible to precisely determine the moment of start and end of the cycle for operating hoisting machines.

3.3 The hoisting machine with the separately excited DC motor

Most Polish mines (more than 90%) use a slow-speed DC motor (gearless solution). Due to its simplicity and linear dependencies, the mathematical description has been available in the literature for years [16]. More relevant is such a power supply for a reciprocating motor that allows smooth speed control from 0 to its rated value. Several decades ago, this was reached through the so-called Leonard circuit. This is an electromechanical system that provides smooth speed control over the entire control range. In the 1970s, static thyristor inverters were introduced. Power supply to the motor was provided by two controlled rectifier systems (Figure 5): PEC1—the system supplying the motor armature (also known as the main circuit of the winding machine), and PEC_E—the rectifier supplying the motor excitation.

Figure 5.

Basic power supply diagram of separately excited DC motor; MV—Middle voltage source, tr—Main transformer (form armature), TrE—Excitation transformer, PEC1—Power electronic converter, PEC_E—Power electronic converter for excitation.

The direction of rotation of the machine can be changed by inversing the direction of current flow in the excitation circuit or in the armature circuit. Due to the cost of four-quadrant converters, which depended on their power, the change of direction of rotation by changing the direction of current flow in the excitation was used. This operation is subject to very strict control, due to the fact that a decrease in motor excitation current causes an increase in speed. In a hoisting machine, speed control is achieved by changing the armature voltage. The system is equipped with two interrelated control loops: voltage regulation and excitation current regulation. When the direction of rotation is changed, if the system has not managed to reach zero speed, with a reduction of the excitation current. The armature supply voltage is also reduced.

This problem does not occur in machines equipped with a four-quadrant converter in the armature circuit. The direction of rotation is altered by changing the direction of current flow in the armature. The excitation circuit is supplied from a single-quadrant rectifier, so there is no danger of de-energizing the machine.

A serious problem, from the point of view of the impact on the power grid, is the rectifier of the armature circuit. Figure 6 shows the supply voltage waveforms and the phase current waveform for a 6-pulse bridge. Based on Ref. [8], the THD_I harmonic content factor for a three-phase thyristor bridge converter is determined by Eq. (6). The equation does not take into account the effect of commutation and the variable component of the load current:

Figure 6.

The waveform of the supply voltage and phase current of a 6-pulse bridge rectifier.

$${\mathit{THD}}_I=\sqrt{{\left(\frac{I}{I_1}\right)}^2-1}\approx 0,31$$E6

where:

I—root mean square (RMS) value of the phase current,

I₁—RMS value of the fundamental component of the phase current.

Figure 7 shows the current harmonic spectrum of a 6-pulse bridge system—the results of simulation studies. According to Eq. (7), there are characteristic harmonics in the current waveforms depending on the number of pulses of the converter:

Figure 7.

The harmonic current spectrum of a three-phase 6-pulse bridge system.

$$\mathrm{h}=\text{6m}\pm 1$$E7

where:

m = 1,2,…n.

h – harmonic order.

In such system, the harmonics fifth and seventh are the most significant for the distortion of the power grid waveforms. Their value in relation to the fundamental harmonic is 20 and 14%, respectively.

The current harmonic content factor is almost 0.29.

The impact of a high-power 6-pulse system is a problem in terms of the proper operation of the power grid. Therefore, complex converters are used to power high-power drives. DC converters containing two thyristor bridges connected in series are used in hoisting machines. This eliminates connecting thyristors in series to achieve higher output voltages. Figure 8 shows a schematic of the power supply to the armature circuit of a DC hoist motor through a compound converter.

Figure 8.

Diagram of the 12-pulse circuit.

Two bridge converters are connected in series on the output voltage side. The voltage supplying the armature circuit of the hoisting motor is the sum of the output voltages of the individual bridges. Each of the bridges is powered by a converter transformer. The transformer can be of a three-winding design. Two transformers are used in hoisting machine drives for safety reasons. Such a solution allows the drive to operate in emergency conditions. Damage to one of the converters or transformers allows the motor to operate with limited parameters but does not completely eliminate the drive. It ensures safe evacuation of people during the failure of certain elements of the hoisting machine. The converter transformers are powered from a medium-voltage line, and their connection groups shift the phase-to-phase voltages of the secondary sides of the transformers by 30^o with respect to each other (Figure 9).

Figure 9.

The configuration of converter transformers.

The RMS value of the harmonic h of the source phase current is [8]:

$$I_{\mathit{hz}}=2I_h\sqrt{\frac{1+{\left(-1\right)}^{m}cos\left(h∆\alpha \right)}{2}}$$E8

where:

$$\mathrm{\Delta \alpha }=\left({\mathrm{\alpha }}_1-{\mathrm{\alpha }}_2\right),$$E9

$$I_h=\frac{\sqrt{6}{I}_d}{\mathit{h\pi }}$$E10

For m = 2 k-1, where k = 1,2,3,…. and for α₁ = α₂, the I_hz of relation (8) is 0. Thus, a converter supplied by transformers with different connection groups and controlled in such a way that the angles of the thyristors’ switching delays are equal will produce harmonics of the source current:

$$\mathrm{h}=12\mathrm{m}\pm 1$$E11

That is, this converter is a 12-pulse system, and the control implemented to ensure the equality of the driving angles of the two converters is common.

A characteristic of these converters is that there is an alternating component in the voltage on the DC side with a frequency 12 times higher than the frequency of the supply voltage. It is therefore lower than when using a single hexapulse rectifier. Theoretically, in this system, the lowest of the higher harmonics is the 11th harmonic. The use of common control means that there are no harmonics with strictly defined frequencies in the phase currents of the supply line. Only the so-called characteristic harmonics occur.

There is still a second type of control of complex converters. This is sequential control. It consists of the operation of one of the bridges with a full overdrive, that is, with the maximum conduction angle of the thyristors. At this time, the other converter operates with a variable switching angle of the thyristors. This type of control causes the entire converter to operate as a 12-pulse converter only for short periods of time. In addition, its operation is perceived by the grid as a 6-pulse system, resulting in an increased impact of the system on the power grid. Therefore, common control is used in most cases.

Figure 10 shows the harmonic spectrum of the phase current of a 12-pulse converter. The THD of the current of this converter has decreased compared to the 6-plus converter to about 6%.

Figure 10.

Harmonic current spectrum of a three-phase 12-pulse system.

4. The systems with an increased number of pulses and less impact on the power grid

In order to further eliminate higher harmonics generated by the converter system, the number of pulses of the system can be increased. It is possible to build DC motor power systems with a number of pulses greater than 12. By adding more 6-pulse converters and appropriately configuring power transformers, systems with a number of pulses of 18, 24, and more are built. Such solutions are used, for example, in traction systems. However, in the mine, where unification of solutions is required, and, above all, their simplicity and economic efficiency, such systems are exceptions. Figure 5 shows diagrams of multi-pulse systems formed by a suitable combination of basic—6-pulse systems (Figure 11).

Figure 11.

The configurations of multi-pulse systems: (a) 6-pulse, (b) 12-pulse, (c) 18-pulse, (d) 24-pulse.

For systems with different numbers of pulses, the characteristic harmonics are defined as follows [5]:

• for a 6-pulse system: h = 6 m ± 1,

• for a 12-pulse system: h = 12 m ± 1 (for common control),

• for an 18-pulse system: h = 18 m ± 1,

• or a 24-pulse system: h = 24 m ± 1,

where m = 1, 2, 3, ….

In addition to the characteristic harmonics for a given type of compound converter, there are also harmonics of orders other than those specified, but their value relative to the basic harmonic is small.

5. The higher harmonics in real 12-pulse systems

The investment cost of building a hoisting machine is significant in the overall cost of operating a mine. However, it is not as high as the cost of a longwall complex used for mining. The product life for a hoisting machine is estimated to be 20–30 years. However, it happens that they work much longer. Hoisting machines operating in Polish mines undergo modernization processes after about 40 years of operation. Due to the current situation of the coal sector, the cost of modernization is reduced each time, and the problems of the impact of the hoisting machine drive system on the mine’s power grid are secondary problems. This often leads to major problems related to the proper operation of other equipment in these networks. As an example, the results of measurement work carried out at two mines where hoisting machine upgrades were carried out will be presented.

5.1 The modernization of the hoisting machines

The purpose of modernization, as in many cases, was to increase the mining capability of a mining plant. The hoisting machine is one of the most important components of the process line. The correct production process depends on the performance of the hoisting machine. In this case, in order to ensure that the hoisting machine was not the weakest component, a major modernization, or rather a replacement with a new one, was necessary.

5.1.1 The modernization number 1: The conditions before modernization

The parameters of the system in operation before the upgrade are shown in Table 3.

Lp	Parameter	Wartość
1	Motor type	PW-101
2	Power	1450 kW
3	Supply voltage	800 V
4	Rated current	2000 A
5	Excitation current	144 A
6	Rotational speed	61 obr/min
7	Linear speed	16 m/s
8	Mass in the skip	7,5 Mg
9	Depth (drive way)	489 m

Table 3.

Parameters of the machine before modernization.

The machine was built as a two-bulb, two-stroke machine with a 5 m diameter Koepe wheel. The 1450 kW slow-speed, reciprocating motor was powered by thyristor converters. Figure 12 shows an overview diagram of the system.

Figure 12.

Diagram of the drive before the upgrade.

The power supply to the hoisting motor was implemented through two converter transformers of 1 MVA, each with a voltage of 6/0.4 kV and connection groups Yyn0 and Dyn5. This was a series connection of two six-pulse systems. This resulted in a twelve-pulse effect on the power grid. The change in the direction of rotation of the motor occurred as a result of changing the direction of current flow in the excitation circuit. To make this possible, the excitation circuit converter was bidirectional, i.e., constructed from two reciprocating bridges controlled accordingly. Thanks to this design, the entire drive was able to operate at full assumed load and full draw speed. If one of the main circuit converters failed, it was possible to operate at half speed and full load. The system then operated as a six-pulse system. The switching of the main circuit was provided by a special switching circuit, the so-called switcher.

5.1.2 The modernization number 1: The conditions after modernization

After the upgrade, a DC hoisting motor powered by a system of static converters in the armature and excitation circuits was used. Due to the need for special operations when changing ropes and due to the increase in the weight that can be transported by skip, the rated power of the hoisting motor was increased by 850 kW. The inverse of the rotational speed is changed by the direction of current in the armature circuit. This is a major difference from the system before the upgrade. Now, a non-reversing converter is used in the excitation circuit. A diagram of the solution is shown in Figure 13, and the parameters of new machine are summarized in the Table 4.

Figure 13.

The diagram of the power circuit of the hoisting machine after modernization.

Lp	Parameter	Wartość
1	Motor type	PW-124
2	Power	2200 kW
3	Supply voltage	853 V
4	Rated current	2810 A
5	Excitation current	225 A
6	Rotational speed	54,57 obr/min
7	Linear speed	12 m/s
8	Mass in the skip	12,5 Mg
9	Depth (drive way)	489 m

Table 4.

The parameters of the machine after modernization.

The armature circuit is powered through four converters in a series-parallel arrangement. This ensures that the required current is achieved (parallel connection of two converters in each branch). Each pair of parallel-connected converters is supplied from a separate converter transformer with the appropriate connection group. The series connection of pairs of converters provides an adequate level of voltage supply to the motor, as well as implements the 12-pulse effect of the drive on the power grid – reduces the magnitude of low-order harmonics, especially fifth and seventh. In the event of a converter failure, the system can operate as 6-pulse. The speed of the drive is then reduced to half the rated speed, which is the result of halving the supply voltage.

Prior to the upgrade, the hoisting engine was already powered by static converters that interacted with the mine plant’s network. In such cases, in order to determine whether the new system would work properly without generating additional interference, it was necessary to take measurements of the operating machine and the new hoisting machine after its installation.

Figures 14 and 15 show the locations of the measuring instruments. For the measurement, Fluke 1760 power quality index analyzers were used. These are class A analyzers and have the appropriate parameters for measuring the aforementioned quantities, according to [11]. The first of the measurement points was selected at the mine’s substation, directly behind the 110 kV/6 kV transformer. Thanks to this location, it was possible to determine the parameters of the quality of power supply at the main connection point of the mine.

Figure 14.

The first of the measurement points. 110 kV/6 kV substation.

Figure 15.

The second measuring point. Power supply of the winding machine.

The second measurement point was located in the medium-voltage 6 kV switchgear in the field from which the winding machine switchgear is supplied. This made it possible to observe the “behavior” of the hoisting machine during the entire duty cycle. No other mine consumers are supplied with this switchgear, so there is no danger of introducing interference from other mine equipment.

From the point of view of the mine’s overall power infrastructure, it was important to determine whether the upgraded system has increased the level of introduced disturbances of higher harmonics or voltage variations. In addition to comparing the level of individual voltage harmonics, the level of THD was also analyzed, as well as the Plt coefficient and voltage variations. Total Harmonic Distortion (THD) is defined as the ratio of the RMS value of the sum of all harmonic components (up to the harmonic order 40 in Poland) to the RMS value of the fundamental harmonic [6, 7]:

$${\mathit{THD}}_U=\sqrt{\sum _{h=2}^{40}{\left[\frac{U_h}{U_1}\right]}^2}$$E12

where

U_h—RMS value of the h-order harmonic of the voltage,

U₁—RMS value of the fundamental harmonic voltage.

The analysis of the system’s impact on the power grid included the determination of the levels of individual harmonics and the THD_U voltage distortion factor. Table 5 summarizes the voltage THD_U for three phases before and after the upgrade. The permissible level specified in [10] was not exceeded in both measurements. However, note an increase in the coefficient after the upgrade compared to the value when the previous system was in operation. This is an increase of about 50%. Therefore from the point of view of the regulations, the system is working properly, but the occurrence of negative phenomena associated with the increase in the level of harmonics in the system cannot be omitted. Table 5 shows the values of individual harmonics as well. A comparison of the voltage waveforms supplying the winding machine before and after moderrnization is shown in Figures 16 and 17. Figures 18 and 19 show the harmonic spectrum of voltage for the same cases.

Size; normalized; (harm. row)	Values; Permissible [%]	CP95 [%]; Before modernization			CP95 [%]; After modernization
	According to [3]	UL1	UL2	UL3	UL1	UL2	UL3
THDU	8	2.21	2.25	2.18	3.42	3.45	3.48
3	5	0.21	0.25	0.18	0.50	0.26	0.48
5	6	0.30	0.29	0.25	0.46	0.27	0.26
7	5	0.57	0.63	0.60	0.48	0.44	0.43
11	3.5	1.26	1.44	1.37	1.47	1.53	1.55
13	3	1.16	0.87	1.05	1.06	1.04	1.08
23	1.5	0.62	0.74	0.68	0.95	0.94	1.00
25	1.5	0.67	0.57	0.57	0.85	0.84	0.89

Table 5.

THD coefficients and characteristic harmonic levels before and after modernization.

Figure 16.

The course of the voltage supplying the winding machine before the upgrade.

Figure 17.

The waveform of the voltage supplying the winding machine after the modernization.

Figure 18.

The harmonic spectrum of the voltage supplying the hoisting machine before the modernization.

Figure 19.

The harmonic spectrum of the voltage supplying the hoisting machine after the modernization.

Table 5 lists the values of the characteristic harmonics of this system, i.e., 11, 13, 23, 25. The presence of harmonics 5 and 7 may indicate the presence of other disturbances (such as voltage changes) or may be the result of irregularities in the control system of the converters. These harmonics were also present before the system was upgraded. Analyzing the values in Table 5, it can be assumed that in the system after the modernization, the cause of these harmonics was partially eliminated, as their level decreased by about 25%. However, an increase in the characteristic harmonics of the system was noted and analyzed.

5.1.3 The modernization number 2

The modernization of the hoisting machine has been carried out at one of the coal mines. The hoisting machine is located on the tower. It has operated in the Pung system since modernization. The block diagram of the system is shown in Figure 20.

Figure 20.

The diagram of a twin-engine winding machine operating in the Pung system.

The hoisting machine consists of two reciprocating motors (M1 and M2) running on a common shaft on which the Koepe wheel is mounted. The main circuit of the hoisting machine consists of series-connected armature motors and DC sides of converters (P1 and P2). This configuration eliminates the interaction of the motors on the mechanical side. The excitations of both motors are supplied from bidirectional bridge rectifiers (P3 and P4). The system can be controlled sequentially or jointly. In the first phase of machine operation, the system was configured to operate with sequence control. This caused interference at other points in the mine’s power grid due to the operation of the hoisting machine (Figure 21).

Figure 21.

The power supply voltage waveforms measured at 110 kV substation.

The supply voltage waveforms shown in Figure 22 are distorted. Commutation kinks are clearly visible, occurring at moments when subsequent semiconductor elements take over conduction. The shape of the circuit’s supply current significantly deviates from a sinusoidal waveform. The supply voltage with the shape shown in Figure 22 may pose a threat to the correct operation of the converter due to the difficult synchronization of switching of semiconductor elements.

Figure 22.

The course of voltages supplying the winding machine.

The spectrum of harmonics indicates the operation of a 6-pulse system. The largest harmonic values in relation to the fundamental harmonic are reached by harmonics fifth and seventh (Figure 23). The presented waveforms of phase-to-phase voltages occurring at the mine’s power supply point speak for the absence of significant distortion due to equipment operation (Figure 21). This means that distortions negatively affect equipment operating inside the network at the medium-voltage level of 6 kV. The distance between devices can also negatively affect the amplification of a certain type of disturbance. In this case, the distances between devices are several kilometers (Figure 24).

Figure 23.

Harmonic spectrum of the phase voltages.

Figure 24.

The mutual location of equipment in the power grid.

6. Conclusions

Modernization of mining shaft hoists is among the largest investments in the structures of mining plants. Therefore, it is important to prepare the entire project properly, with special attention to the power supply side of the hoisting machine. As an electrical device with a high degree of complexity, the hoisting machine requires a power supply with appropriate parameters. On the other hand, it is one of the largest devices in the mine network that is a source of disturbance, and is therefore a real threat to other consumers of the grid and safety systems.

In the case presented in Section 4.1, the preliminary analysis became the benchmark for determining the level of impact of the newly created circuit. That biggest concern was the change in power circuit configuration. In Poland, more than 90% of hoisting machines using a DC reciprocating motor operate in a system with reversion through the excitation circuit. There was concern that the use of high-power four-quadrant converters in the armature circuit would cause interference in the mine network to increase beyond acceptable values. The tests carried out did not confirm this thesis. Therefore, it should be concluded that the use of a system with reversion through the main (armature) circuit is justified from a technical point of view because it does not cause an increase in the levels of higher voltage harmonics above the limits. The advantage of this system is the increase in the speed of the hoisting machine, which can be reflected in the efficiency of the hoisting machine. This situation appears due to the differences in the time constants of the armature and excitation circuits.

The conclusions that emerged from the measurements could not be considered as a basis for discontinuing the monitoring of power quality indicators in the network at this mine. It should be borne in mind that the level of interference increased compared to the previous system. This means that the phenomena that are the results of the increase in the level of interference in this network were anticipated. In a mine network, this is important as long as it can affect protection systems, which is a potential danger to the crew. Shortly after the completion of this stage of measurements, a ferroresonance phenomenon occurred in the network, which contributed to significant property damage.

Currently, technical possibilities are being searched for to build hoisting machines in a way that significantly reduces their negative impact on the power grid. One such solution is the use of slow-speed synchronous motors powered by intermediate frequency converters with active input rectifiers.

Another possibility is the use of active higher harmonic filters in already operating winding machines with DC helical motors. The choice of a method to reduce the negative impact of the drive on the power grid depends on many factors and should be analyzed in each case.