New Optical System for Long Distance Control of Electrical Energy Flows

Abstract

The problems that arise during the transmission of control commands for key elements and data on their execution via communication systems over long distances at energy facilities are considered. The necessity of a new approach to solving this problem using a fiber-optic communication line (FOCL) is substantiated. A new design of analog FOCL has been developed for transmitting control commands for key elements of substations and switching data. A new method for forming an optical signal in a laser transmitting module using direct current modulation is proposed. The advantages of using a new method for generating commands for control signals and switching data using a sequence of command codes compared with digital signals currently used in control and monitoring systems are substantiated. The main parameters of the developed analog FOCL have been calculated. An experimental prototype of an analog FOCL was assembled, and research was conducted. The results of calculations and experimental data are compared. The directions for further modernization of the developed FOCL are determined to increase the distance for transmitting an optical signal up to 600 km without the use of servers, the maintenance of which in the power transmission line system in some cases is challenging, expensive, and not always possible.

1. Introduction

One of the urgent tasks of modern energy is to increase the reliability of various systems through which electrical power is supplied to consumers [1,2,3]. The reliability of the operation of electrical energy transmission systems depends on many parameters and various factors [2,3,4,5,6]. One of these factors is the coordinated operation of key elements in the distribution systems of electrical energy between consumers or various units (for example, electric motors at a mobile object with a nuclear power plant and compressors at fuel transfer stations) [7,8,9,10,11,12,13]. Key elements also play an important role in the redistribution of electric energy flows in the energy systems of countries with several time zones (for example, USA, Russian Federation, China, Australia, and others) [14,15,16]. Therefore, more attention is paid to solving problems related to managing and controlling key elements (switches at various transformer substations or high-voltage switches) [17,18,19,20,21].

Fiber optic communication lines (FOCL) are used in substations and switching systems to ensure the reliability of various relays, panel switches, and other equipment [14,15,16,17,21,22,23,24,25,26]. The use of FOCL is the only solution that can significantly reduce the effect of electromagnetic interference on the reliability of the control and monitoring system for electrical energy flows. The developed designs of FOCL make it possible to ensure the transmission of information with control signals over distances of up to 250 km without any problems without optical amplifiers. The use of optical amplifiers in the power transmission line zone, where the FOCL is located, is impossible due to a change in the polarization of laser radiation in the optical amplifier from sparks and lightning. A change in polarization will lead to the transformation of the optical signal with information during optical amplification (the command encoding will change, and another switch will be performed at the substation). It should be noted that the FOCL operates in standby mode. In this mode, the FOCL keeps a low level. Intense electromagnetic radiation with a discharge spark can create a slight surge in the FOCL, amplified by an optical amplifier and identified as a control signal [27,28,29,30,31,32].

Therefore, to avoid creating such complex problems when transmitting information with control signals to substations or switching systems located at more than 250 km from the control center, a server station is used. In the Russian Federation, ROSSETI PJSC deploys server stations every 200–215 km of the route with FOCL (depending on various conditions). The server station is located on the territory of the automated substation at a small distance from the switching systems in a separate room. In this room, it is necessary to provide a certain temperature regime and conditions for the operation of the server and auxiliary equipment. The server must operate in continuous mode (provide constant transmission of information about the position of key elements on relays and panel switches). Therefore, the server room also hosts a backup set of equipment, which is also maintained in working order. Furthermore, for the same reason, there was a complete rejection of the use of optical amplifiers, which in a certain period cannot ensure uninterrupted transmission of information over FOCL.

It is necessary to conduct preventive maintenance in the area where the server station is located to prevent failures in the operation of the equipment. This requires a substantial amount of money, especially in winter and during the rainy season (huge problems occur in mountainous areas). For example, for several regions of the Russian Federation, especially in the northern regions and some regions of Siberia, these tasks are solved with great difficulty, especially during severe frosts and snowstorms. The departure of PJSC Rosseti employees for more than 1000 km (round trip) becomes a major problem. In addition, it is a highly expensive and sometimes dangerous undertaking. Similar problems can be attributed to several territories in the USA, Canada, Brazil, Kazakhstan, China, Argentina, Chile, and others. Considering that roads were laid to automated substations during their installation, which are in different conditions (roads also require preventive repairs and maintenance), these problems are solved with great difficulty.

This command-and-control system’s pain point is when a server substation needs to be installed on the power line route. If the distance between the control center and the automated substation is more than 300 km, it is necessary to install a server substation for stable information transfer. The roads are in a bad state (access to the server substation is extremely difficult). In most cases, you must walk part of the way from the car to the server station on foot with equipment through challenging terrain. This is a difficult job in the presence of snow, heavy precipitation, etc. In addition, the impact of negative weather factors on such a server station is much higher than if it is located on the territory of an automated substation. Vandalism is possible from people (theft of equipment) or wild animals. Now the problem of failure in the operation of such server stations is being solved with great effort (there is no other choice yet). Therefore, reducing the number of such “pain” points in automated substations’ control and monitoring system is a highly urgent task.

In addition, it should be noted that the size of the automated remote substation does not affect the preventive maintenance schedule of the server station and the operating conditions of the equipment on it. Service standards are uniform (considering the geographical region’s characteristics). Failure to comply with these standards leads to power outages for consumers and accidents, which negatively affect the operation of various consumer equipment [6,7,9,11,13,16,18,19,24,33,34]. Often, the costs of operating small, automated substations exceed revenues, and they are forced to cover them from the entire company’s work. Some small problems also make it challenging to operate automated substations using servers. On the other hand, it is extremely difficult to perform this operation without a server station. Therefore, the search for new solutions to this complex problem is highly relevant to the energy systems of many leading countries.

There is a substantial amount of research trying to solve this complex problem, and the previous authors’ work is not the only possible solution. Our work proposes a fundamentally new approach to solving the problem noted in comparison with other studies. In the research of other scientists, the main emphasis is on developing a system of strong points using digital FOCL [35,36]. In essence, the tasks are reduced to the creation of shift zones to reduce the departure distance of the maintenance team for both the automatic substation and maintenance work at the server station. In the PJSC Rosseti (Russian Federation) system, these developments were not widely used; in reality, their effectiveness was low. The second main direction in the research of other scientists relates to the possibility of introducing new equipment, for example, electro-optical modulators made based on new materials. These materials are more resistant to electromagnetic radiation and temperature instability [37,38,39]. Furthermore, optical systems with new electro-optical modulators are planned to be integrated into the power transmission line structure with the rejection of server stations. Modern technologies currently allow the production of optical materials that are less sensitive to a strong electric field than quartz glass and its various modifications [40,41,42,43,44,45]. In the case of using these materials in communication systems, it is necessary to develop new designs of multiplexers, demultiplexers, and electro-optical modulators, as well as several components for the operation of these elements as part of FOCL. Leading companies in the world (Thorlabs inc., Emcore, Hamamatsu Photonics) are in no hurry to develop and manufacture these devices. Therefore, it is necessary to look for another approach to solve this problem in the development of FOCL. Therefore, it is necessary to look for another approach to solve this problem in the development of FOCL.

2. The Concept of an Analog Fiber-Optic Communication Line and a New Method of Information Transmission

The developed FOCL design proposes not to use direct modulation of laser radiation for current in the transmitting laser module to form analog optical signals with information. This will make it possible to exclude from the design of the FOCL the electro-optical modulators and various elements associated with it, which are expensive and have a high-temperature dependence. It creates additional problems in their operation. For the transmission of modulated laser radiation via FOCL, we propose to use a sinusoidal signal with a subcarrier frequency of 200 MHz. For a control command that switches key elements, a new encoding method based on the formation of a sequence of command codes is used.

The control command is a set of four numbers separated by a hyphen. The first number consists of two, three, or four characters and is, for example, the number of a relay or panel switch. The second number consists of two or three digits and is the number of the key element (its position changes). The third number (two digits) corresponds to the initial position of the key element. The fourth number (two digits) corresponds to the final position of the key element after switching. In this case, it is necessary to steadily transmit four numbers in a certain sequence over the FOCL and indicate the start of the command countdown and its completion. It is proposed that a certain value of the pulse amplitude in relative units (0.950, 0.955, 0.960, …, 0.995) corresponds to the numbers: 0, 1, 2, …, 9. The amplitude value of 0.94 corresponds to a hyphen (-), i.e. is separating one number from another. In addition, the amplitude value of 1.0 in relative units corresponds to the start of the set of all four numbers and its end in the sequence of command codes. Figure 1 shows, for example, the encoding of the command 1231-47-58-29 using the sequence of command codes proposed by us.

In the Russian Federation, one feature is associated with the transfer of control commands. The territory is extensive; it hosts many power plants of various types, so control commands, in some cases, must be transmitted over long distances (up to 500 km).

In addition, in such a situation, the FOCL will be placed on the upper part of the power line to exclude the influence of many technogenic factors on it, especially in the northern regions, steppes, and deserts, as well as in the middle highlands. The same feature is present in the energy systems of the USA, Canada, China, Australia, and several other countries. Under such conditions, the optical signal amplification on the path is excluded due to the great difficulties in its implementation in a strong electromagnetic field and changing climatic conditions (difficulties in ensuring a thermally stable operating mode of the optical amplifier).

Using the developed sequence of command codes makes it possible to transmit information over distances of 500 km via optical fiber. This possibility is due to the fact that laser radiation with a power of P_in ≈ 100–110 MBT (20.0–20.4 dBm) can be used to transmit these signals. Modern FOCL designs use laser radiation with a power of up to 15 mW (11.8 dBm) to transmit digital signals over various distances. In the near future, it is planned to implement operation of FOCL with a laser power of up to 20 mW. A further increase in power for FOCL, in which digital signals with a high bit-sequence density and channel spectral division multiplexing are transmitted, is limited by the factors number. These factors relate to both technical (the effect of radiation at high power from one channel on the signal in another channel, etc.) and physical phenomena (Kerr nonlinearity, etc.). The stimulated Mandelstam–Brillouin scattering (SMBS) in the case of a pulse duration in a bit sequence much shorter than the photon lifetime has a threshold of more than 20 mW. The Stimulated Raman scattering (SRS) has a threshold of about 600 mW. In this situation, Kerr nonlinearity exerts a significant influence on the information transfer process with an increase in the laser radiation power. The edges of the pulses are distorted (the pulses in the bit sequence are expanded). This expansion of pulses and the incursion of chromatic dispersion leads to the imposition of pulses on each other and the formation of bit errors during signal registration.

In addition, for digital signals, the signal-to-noise ratio at the photodetector must be at least 20 (in dBm). This provides a BER error probability of the order of 0.3 × 10⁻⁶. In the case of overlapping pulse fronts, this signal-to-noise ratio is extremely difficult to provide.

The signal-to-noise ratio on the photodetector for the sequence of command codes should be no worse than 8 (in times). This is ensured by registering the signal on the photodetector in a narrow band, because pulses with a duration of 1 to 3 ms are used. It provides an error probability erfc (BER) which amounts to 10⁻⁵ [46]. Such a BER value is sufficient to clearly identify a value of pulse amplitude.

It should be noted that with a duration of pulses with information of 1 ms (number) and 2 ms (hyphen), and 3 ms (start and end of the command countdown) with an interval between pulses of 3 ms, the effect of chromatic dispersion will not lead to the superposition of their fronts. The incursion of chromatic dispersion in a single-mode fiber at λ = 1550 nm over a distance L = 550 km is ≈2.3 ns in each pulse edge. For pulses with a duration of 1 ms, these changes are insignificant, and it will not change the width of the signal spectrum.

In contrast to a digital bit sequence, with such pulse durations and the interval between them, the effect of Kerr nonlinearity will be less. In order to introduce an error into signal transmission, this non-linearity must change the upper level of the pulse. This requires more laser power than when transmitting digital signals to distort the fronts. In fact, it is necessary to start the collapse of impulse. In this case, the information will be difficult to identify. The signal-to-noise ratio will decrease (BER will increase). The spectrum of the recorded signal is broadened.

The stimulated Mandelstam–Brillouin scattering will also affect the signal-to-noise ratio and the spectrum of the recorded signal. The threshold for the occurrence of this radiation, in contrast to the case of digital signal transmission, will be higher due to the use of a special fiber and a sequence of command codes. It is difficult to estimate the threshold value of SMBS bias occurrence for the pulse train and special fiber used by us. On the one hand, the spectral width of the pulse is much less than Δν_B. This is considered in the classical theory as continuous pumping. On the other hand, the interval between pulses is 3 ms (that is three times longer than the pulse duration). There is no continuity. In addition, with the signals we use, it is impossible to make assumptions about the random connection of the spectrum of a sequence of zeros and ones, as is accomplished with the transmission of digital signals. In addition, in the signal we use, the ratio between the pulse amplitudes changes only in the upper part of the sequence. Zero is not at the noise level. Therefore, we will evaluate the influence of SMBS bias experimentally by the appearance of nonlinear distortions in the amplitude characteristic of FOCL.

The Stimulated Raman scattering (SRS) for the FOCL design developed by us and the sequence of command codes used has a threshold of about 500–550 mW. Therefore, the SRS and the effects associated with it will not have an impact on the process of changing the amplitude FOCL characteristic.

The proposed method for transmitting control commands is a fundamentally new solution in data transmission using analog signals via FOCL. The sequence of command codes developed by us (Figure 1) makes it possible to use in FOCL for data transmission, which involves various sources of laser radiation with direct modulation of the pump current. This makes it possible to ensure a high stability of laser radiation in terms of power, as well as a change in the depth of modulation over a wide range.

It should be noted that a further increase in P_in is inexpedient. This is well illustrated by the following formula:

$$P \left(\mathrm{dBm}\right) = 10·{\log }_{10}\left(P \left(\mathrm{mW}\right) / 1 \mathrm{mW}\right),$$

(1)

An analysis of Formula (1) shows that an increase in P_in to 150 mW leads to an increase in power to 21.76 dBm. The signal transmission distance increases slightly. This increases the likelihood of various non-linear distortions and scattering. This will lead to changes in the transmitted pulses shape, leading to errors in decoding the governance command.

The use of pulses with a duration of 1, 2, and 3 ms in the developed sequence of command codes, as well as a time interval between pulses of 3 ms, makes it possible to use a subcarrier frequency F_s in the range from 0.1 to 200 MHz for their transmission over FOCL. This makes it possible to provide a long transmission time (“drift”) of the FOCL when transmitting a signal over long distances L. In addition, using such values of subcarrier frequencies F_s makes it possible to record an optical signal on a photodetector in the band from 0.1 to 1 MHz with a small signal-to-noise ratio (S/N), which significantly increases the energy balance of FOCL.

A feature of the system we proposed for transmitting control commands via FOCL is that the pulses in the sequence of command codes (Figure 1) are the envelope for the subcarrier frequency signal, which is fed to the laser transmitting module only at the moments of receipt of these pulses. This makes these signals more resistant to various effects on the FOCL than digital ones. When transmitting digital signals over FOCL, the subcarrier signal is an envelope for a bit sequence consisting of zeros and ones.

The key element switching confirmation command is similarly formed from only three numbers, which are also separated by a hyphen. For example, the operator will receive the confirmation command 1231-47-29 on the central computer in response to the previously sent control command (Figure 1). Figure 2 shows this confirmation command.

To transmit this command, an analog FOCL is also used (its design is identical to the analog FOCL for transmitting control commands) with a transmitting laser module with direct modulation using a subcarrier frequency in the range of 0.1–200 MHz. These two independent FOCLs are placed in one shielded cable on the top of the transmission line. You cannot place two fibers to transmit two commands in one reflective layer. Because the power of laser radiation is large, one channel will negatively impact the other communication channel. It will result in a failure in the transmission of information.

3. The Design of an Analog Fiber-Optic Communication Line and the Calculation of Its Parameters

Figure 3 shows a block diagram of the analog FOCL developed by us for transmitting control commands and confirmation signals about the completed switching.

In the developed design of the FOCL, in contrast to the classical schemes for transmitting analog signals, the subcarrier frequency F_s is used. This information transmission feature is because low-frequency signals (sequence of command codes) are transmitted over long distances via FOCL. Laser radiation always contains flicker noise, which is also low frequency. The conducted studies have shown this phenomenon in laser radiation with a power of 150 mW or more (the noise level is about (−62 dBm) or more).

These noises can form on the upper part of a rectangular pulse during direct modulation of laser radiation and affect the signal-to-noise ratio during the registration of an optical signal. The conducted studies have shown that the use of a subcarrier frequency signal F_s with a higher frequency than the transmitted signals, for which rectangular pulses are the envelope, makes it possible to make the effect of noise flicker on the registration of the modulated optical signal insignificant. To achieve this, also in the developed design of the FOCL, an electronic key 8 is used. The subcarrier signal enters the laser 1 only when supplying rectangular pulses with information to its power element.

Such formation of an optical information signal makes it possible to place a tunable LC filter 6 after the photodetector 3 (Figure 3), which cuts out the subcarrier signal. The information processing device 11 after the ADC 7 receives only rectangular pulses with information. Control device 10 is designed to convert information from sensors into command codes. This device ensures that they arrive at the control input of the electronic key 6 and the laser power supply 4 in the specified mode. After processing device 11, the information enters either the central computer 12 or the switching signal generation device 13, which generates commands for switching key elements. After switching the key element, information about its position through 14 is sent to the processing device 15, which converts the received data into the required format for transferring it to 10. In device 10, a command code is generated about the switching performed, which is sent to 8 and 4 to transmit it via FOCL to the central computer 12.

Difficulties arise with registering an optical signal transmitted over long distances and significantly decreases in power. Currently, various models of photodetectors for recording analog signals are produced (for example, PDA400 (company “Thorlabs Inc.” (New York, NY, USA)) or the sensitive InGaAs photodiode PDINCH300 (Company “Emcore”(Lion, France)) in the range of changes in the detected power ΔP_r of laser radiation, which is ≈90–95 dB. Different limits determine different models of photodetector modules ΔP_r (for example, from 10 dBm to −80 dBm or from 0 dBm to −95 dBm).

Because it is necessary to transmit information over distances of more than 500 km, selecting a receiver with a lower value of the registration range of minus 95 dBm is necessary. At such information transmission distances, additional noise is formed in the optical fiber in addition to photodetector noise and others. The experience of operating FOCL in normal mode shows that all these noises do not exceed 2–3 dBm. In addition, 2 dBm is lost when laser radiation is injected into the optical fiber. In this case, for the FOCL developed by us, the value of ΔP_r is more than 110 dBm. The value of L, when all the factors are considered, can be estimated using the following formula:

L = ΔP_r/α₁₅₅₀ > 550 km,

(2)

where α₁₅₅₀ is the power loss in the optical fiber at λ = 1550 nm (standard losses α₁₅₅₀ ≈ 0.195 dB/km).

The result obtained shows the possibility of transmitting an optical signal and its stable registration at distances L ≈ 550 km. When transmitting over such long distances, there must be a margin for losses (in case of repair and other situations).

Because information is transmitted over FOCL over more than 200 km, it is necessary to calculate the most important parameters of the developed FOCL. These are the rise time of the optical system τ_s, the time of the signal transmitted through the optical fiber τ₀, and the energy balance a_e. These parameters show the capabilities of the developed FOCL design for transmitting an analog signal over a distance of L.

The following data are used to calculate the developed FOCL. Information is transmitted at a wavelength λ = 1550 nm. The optical power P_t of the transmitting laser module 1 (Emcore) is 150 mW, and the modulation depth is 70%. For a given P_t, the width of the laser emission spectrum is ΔF₁ = 600 MHz. The subcarrier frequency is F_s = 100 MHz. A specialized single-mode optical fiber of the G.652 standard with a shifted zero dispersion (triangular profile) M = 0.3 ps/(nm∙km) is used to transmit information. A photodetector module is used to receive an optical signal (with the following parameters: bandwidth ΔF₂ = 1 GHz, NEP = 10⁻¹⁴ W∙Hz^1/2; optical fiber length L = 550 km).

When using high-power laser radiation to transmit command codes, there is a problem with determining the line width Δλ. As the value of P_t increases, the value of Δλ also changes (the manufacturer of the laser transmitters only specifies the maximum permissible value). Therefore, we conducted additional studies of the change in the value of Δλ from P_t (Figure 4).

The obtained results showed that the value of Δλ slightly changes when the value of P_t changes to 160 mW. Furthermore, the value of Δλ changes nonlinearly to 0.227 nm at a power P_t = 250 mW. For P_t = 150 mW, the linewidth is Δλ = 0.112 nm. The following formulas are used to calculate the values of τ_s and τ₀:

$${\tau }_0 = \frac{B}{F_s},$$

(3)

where B = 0.35 is a factor that considers the nature of the linear analog signal.

$${\tau }_s = \sqrt{{{\tau }_1}^2 + {{\tau }_2}^2 + {{\tau }_3}^2},$$

(4)

where τ₁ is the rise time of the transmitter, τ₂ is the rise time of the receiver, and τ₃ is the rise time of the optical fiber are defined as follows:

$${\tau }_1 = \frac{B}{\Delta F_1},$$

(5)

$${\tau }_2 = \frac{B}{\Delta F_2},$$

(6)

$${\tau }_3 = \frac{B}{\Delta F_3},$$

(7)

All values in Relations (5) and (6) are determined by the manufacturers of the receiving and transmitting modules. For calculations, it is necessary to determine ΔF₃ from the following relationship:

$$\Delta F_3 = 0.35/\left(M·\Delta \lambda ·L\right),$$

(8)

As a result of calculations, the following values were obtained: τ₀ = 3.5 ns, τ₁ = 0.58 ns, τ₂ = 0.35 ns, τ₃ = 0.018 ns, τ_s = 0.678 ns. An analysis of the obtained time values showed that the relation τ₀ > τ_s is satisfied in the developed FOCL design. Information about the switching command will be transmitted over 550 km.

The following formulas are used to calculate the energy balance a_e:

$$a_e = a_1 - \left(a_2 + N·a_3 + a_4 + a_5\right),$$

(9)

where a₁ is the loss margin in the optical fiber, a₂ is the attenuation of the optical signal over the entire length of the fiber, a₃ is the attenuation at various connections (welding is used), N is the number of connections, a₄ is the loss at the modulation depth in the transmitting optical signal, and a₅ is the attenuation on classic connections (connectors).

Let us determine for the developed FOCL all the values included in Equation (9):

$$a_1 = P_{in} - P_R,$$

(10)

where $P_R$ is the minimum optical power recorded on the photodetector module.

$$P_R = 10\log \left(\frac{\mathrm{NEP}}{{10}^{-3}}\right) + 5\log \left(\Delta F_t\right) + 0.5·\mathrm{SNR}\left(\mathrm{dB}\right),$$

(11)

The following values are used to calculate the P_R value. The optical signal is recorded with a signal-to-noise ratio (SNR) equal to 8 (in times) with an information transmission bandwidth ΔF_t = 100 kHz.

$$a_2 = {\alpha }_{1550}·L, a_3 = 0.05 \mathrm{dB},N = 99,a_4 = 1.5 \mathrm{dB}, a_5 = 0.24 \mathrm{dB},$$

(12)

As a result of calculations, the following values were obtained: $P_R$ = −84.55 dBm, $a_1$ = 104.55 dB, $a_2$ = 97.50 dB, and $a_e$ = 0.36 dB.

The obtained result shows that the developed FOCL can stably transmit information over distances up to 500 km. The further transmission of information is not recommended because the value of a_e is slightly greater than 0 dB with a recommended margin of at least 1 dB. The distance L = 500 km is critical.

4. Results of Experimental Investigations and Discussion

The dynamic range is the main characteristic of FOCL for the transmission of analog signals. In the conditions of the research laboratory of Bonch-Bruevich Saint Petersburg State University of Telecommunication, with the support of company LLC “T8” and company PJSC ROSSETI, the laboratory layout was assembled for research of FOCL developed by us (the block diagram of laboratory layout is shown in Figure 3). A during developing analog FOCLs with using laser radiation a power of more than 15 mW, modeling of line characteristics is not very promising work due to the difficulty of considering all non-linear distortions and scattering that affect the amplitude and shape of the transmitted signal. It is more reliable to measure everything. Without the results of experimental research, companies do not accept the development for implementation.

In Figure 5 shows the results of measuring the output power Pout from a change in the laser power P_in, which enters the optical fiber for various values of L.

The conducted studies of the change in the dynamic range of the developed design of the analog FOCL showed that stable transmission of control commands to key elements and control of their position over distances of up to 500 km is ensured. The presence of a nonlinear section on the amplitude characteristic in the lower part is explained by the fact that the amplitude of the recorded signal has become comparable to the intrinsic noise of the photodetector module. Moreover, with an increase in L, the number of welded joints increases (the construction length of a solid fiber is 5000 m), which increases the overall loss of the optical signal in the fiber. Therefore, there are changes in the nonlinearity of the amplitude response with increasing L.

Nonlinear distortions in the amplitude characteristic in the upper part can be explained by the influence of SMBS bias and Kerr nonlinearity on the transmitted signal parameters. As a result of experiments, we found that the parameters of laser transmitting modules do not affect the formation of this nonlinearity. During the research, various options for the rectangular pulse formation were used (using a semiconductor laser tunable in power and equipment of company LLC “T8” using an amplifier). Changes in the amplitude characteristics measured for two cases turned out to be insignificant.

As an example, Figure 6 shows the results of a study of the transmission of rectangular pulses with a duration of 1 ms (digit) over the developed analog FOCL with a pulse interval of 3 ms for various values of L.

As an example, Figure 7 shows the results of studying the transmission of rectangular pulses with a duration of 1 ms (digit) over the developed analog FOCL with a duty cycle of 100 MHz with an interval between pulses of 3 ms for various values of L.

As an example, Figure 8 shows the results of the registration of rectangular pulses transmitted over the developed analog FOCL with a duration of 1 ms (digit) with a duty cycle of 100 MHz with an interval between pulses of 3 ms for various values of L.

An analysis of the results obtained shows stable operation of the developed analog FOCL when transmitting information in the form of a sequence of command codes at a power of P_in = 100 mW over various distances.

The absence of significant distortions in the fronts and upper part of the recorded pulses transmitted over the FOCL developed by us shows that for this laser radiation power, the influence of the SMBS bias and Kerr nonlinearity is insignificant.

The investigations conducted made it possible to establish the insignificant effect of changing the subcarrier frequency F_s in the range from 1 to 200 MHz on the amplitude and shape of recorded analog signal transmitted over the FOCL.

5. Conclusions

An analysis of the obtained experimental and calculated data confirms the adequacy of our proposed developments in the implementation of the design of an analog FOCL for transmitting control commands for key elements in various power systems and for obtaining reliable information about the switching performed (the position of panel switches, etc.). The experiments and calculations confirmed the possibility of reliable transmission of control commands and other information over distances of 500 km. It is at least 5–6 times more than in operating control systems operating on other physical principles.

It should be noted that the developed analog FOCL is easily integrated into the power transmission lines in operation. For its placement and functioning, it is not necessary to make any changes and improvements to the structures of power lines installed on the routes. In addition, using the developed FOCL makes it possible to exclude two server stations from the control and monitoring system. The service life of the FOCL developed by us is more than 20 years, and the service life of the server station in the PJSC ROSSETI (Russian Federation) system is ten years at maximum. For example, preventive maintenance on the developed FOCL is performed as needed (possibly never before the expiration of its service life). At the server station in the northern regions of the Russian Federation in winter, preventive maintenance is performed at least once a week and, if necessary, more often (due to snow drifts, icicles, etc.). Such exploitation requires time and significant human and financial resources. Using our development significantly reduces these costs in all areas. These conclusions are made by the authors of this article on the basis of many years of cooperation with company PJSC ROSSETI (formerly with company RJS “Unified Energy Systems of Russia (UES Russia)” until 2008). Specialists of the company PJSC ROSSETI are involved in the development of this project. This project is planned to be implemented in practice in the divisions of this company. At the request of the article authors, an expert specialist group of PJSC ROSSETI performed (at the moment) an economic assessment of operational efficiency for one developed FOCL within 20 years in a power transmission line system compared to existing equipment. Economic efficiency is from 5 to 20 million US dollars. Such a spread is determined by climatic conditions and inflationary processes over the entire period of FOCL operation. The risk of work in the power transmission line system in difficult weather conditions is not included in this amount.

Increasing the range of information transmission (more than 500 km) without the use of optical amplifiers, which are extremely difficult to integrate into existing power transmission lines, is a challenging issue. Its solution requires further research. This is because the classical methods of increasing L (the choice of a photodetector with a lower NEP is associated with the production of such elements, and the choice of an optical fiber with a lower attenuation coefficient α₁₅₅₀) are either technically difficult to implement or sharply increase the cost of the FOCL design (fiber with a core made of pure quartz α₁₅₅₀ = 0.171 dB/km). When using this type of optical fiber, the transmission distance of control commands and necessary information is increased up to 600 km.

It should be noted that the analog FOCLs developed by us are lines for unidirectional data transmission (one way). This eliminates the possibility of unauthorized connection to the FOCL by unknown persons without its identification. When using server stations, these connections are possible. Such circumstances further increase the reliability of the operation of the FOCL developed by us for the management and control of the operation of key elements in power systems.

Considering all of these factors, as well as the circumstances in which the experimental FOCL layout developed by us is assembled using equipment and components from leading world companies, the following statement can be made. The FOCL developed by us can be implemented in the energy systems of the USA, Canada, China, and other countries with minor modifications.