4. Arbitrary splitting ratio coupler based on a 4�4 MMI coupler

If the length is doubled to LMMI ¼ 2L<sup>1</sup> ¼ 3Lπ=2, a new 4�4 MMI coupler is formed and its

In this chapter, we use our proposed modified effective index method (MEIM) for designing MMI structures based on silicon waveguides as shown in [29]. The principle of this MEIM is to use the beat length L<sup>π</sup> of the MMI coupler as the invariant. Here, the beat length of an MMI

the fundamental and first order modes, respectively. We shall find a matching value of the cladding index for the effective index method that forces the beat length L<sup>π</sup> in the equivalent 2D model to be equal to the beat length in an accurate 3D model. By this way, we find an optimal effective cladding index. For our silicon waveguide structure, in this chapter, we found that equivalent effective indices of the core waveguide and the cladding waveguide are

In order to optimally design the MMI coupler, we use the beam propagation method (BPM). We showed that the width of the MMI is optimized to be WMMI = 6 μm for compact and high performance device. Figure 1 shows numerical simulations at optimal MMI length of LMMI ¼ 141:7 μm for signal at input port 1 and at input port 2. The simulations show that a

1 � j 001 þ j 0 1 � j 1 þ j 0 0 1 þ j 1 � j 0 1 þ j 001 � j

� �, where <sup>β</sup><sup>0</sup> and <sup>β</sup><sup>1</sup> are the propagation constants of

(6)

<sup>S</sup> <sup>¼</sup> ð Þ <sup>M</sup> <sup>2</sup> <sup>¼</sup> <sup>1</sup>

3. Modified effective index and numerical methods

structure can be defined as L<sup>π</sup> ¼ π= β<sup>0</sup> � β<sup>1</sup>

very low insertion loss of 0.7 dB for both cases [2].

Figure 1. Power splitting ratio scheme based on 4�4 MMI coupler (a) Input 1 and (b) Input 2.

to 2.82 and 2.19, respectively.

2

transfer matrix is

232 Emerging Waveguide Technology

Figure 2 shows the new scheme for achieving arbitrary coupling ratios in only one 4�4 MMI structure. We use two waveguides with different widths Wco<sup>1</sup> ¼ 300nm and Wco<sup>2</sup> ¼ 500nm at the two arms of the structure. The cross-sectional view of the SOI waveguide is shown in Figure 3. Here, the height of the SOI waveguide hco ¼ 220nm, the width of the SOI waveguide varies from 300 nm to 500 nm for single mode operation at wavelength of 1550 nm.

By using the FDM method, the effective refractive index of the SOI waveguide at different waveguide width of 300 nm to 500 nm can be calculated as shown in Figure 4. As an example, Figure 5 shows the mode profile for the waveguide at the width of 300 nm and 500 nm, respectively.

A change in the effective index will induce the change in the phase shift. The phase difference between two waveguide then can be expressed by [30].

$$
\Delta \varphi = \frac{2\pi}{\lambda} \Delta n\_{\text{eff}} L\_{\text{arm}} \tag{7}
$$

Figure 2. Power splitting ratio scheme based on 4�4 MMI coupler.

Figure 3. SOI waveguide structure.

Figure 4. Effective refractive index of the SOI waveguide calculated by the FDM.

where <sup>τ</sup> <sup>¼</sup> sin <sup>Δ</sup><sup>φ</sup>

circuitry [31].

shown in Figure 10.

2 can be expressed by:

2

, and <sup>κ</sup> <sup>¼</sup> cos

Δφ 2

Figure 6. Length of the waveguide Wco2 required to achieve the phase shift from 0 to π.

Out<sup>1</sup> <sup>¼</sup> sin<sup>2</sup> <sup>Δ</sup><sup>φ</sup>

2

from 0 to π or changing the length of the Wco2 waveguide from 0 to 3:51μm.

input port 1 of the coupler in Figure 2, the normalized powers at output port 1 and output port

From Eqs. (7) and (9), the normalized output powers can be calculated and plotted in Figures 7 and 8. It is showed that any power splitting ratio can be achieved by changing the phase shift

Consider the length of the Wco2 waveguide variation, the normalized output powers are shown in Figure 9. The simulation shows that the changes in normalized output powers are very small (nearly 0% in the range of �50 nm to +50 nm). It is feasible for the current CMOS

Finally, we use FDTD method to simulate our proposed structure and then make a comparison with the analytical theory. In our FDTD simulations, we take into account the wavelength dispersion of the silicon waveguide. We employ the design of the MMI coupler presented in the previous section. A continuous light pulse of 15 fs pulse width is launched from the input to investigate the transmission characteristics of the device. The grid size Δx ¼ Δy ¼ 20nm and Δz ¼ 20nm are chosen in our simulations. The FDTD simulations for the whole device are

, and Out<sup>2</sup> <sup>¼</sup> cos2 <sup>Δ</sup><sup>φ</sup>

. Therefore, if an input signal having power presented at

Multimode Waveguides on an SOI Platform for Arbitrary Power Splitting Ratio Couplers

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235

2

(9)

Figure 5. Field profile of the SOI waveguide (a) width of 300 nm and (b) width of 500 nm.

where Δneff is the difference in the effective index, Larm is the length of the waveguide with the width of Wco2. As a result, the length of the waveguide Wco<sup>2</sup> to achieve the phase shift from 0 to π is presented in Figure 6. We see that the short length of 3.5035 μm is required to achieve a phase shift of π.

Due to the phase shift Δφ, the complex amplitudes at the input ports and output ports of the coupler of Figure 2 can be described in terms of cascaded transfer matrices as

$$\mathbf{S} = \frac{1}{\sqrt{2}} \begin{bmatrix} 1 & j \\ j & 1 \end{bmatrix} \begin{bmatrix} e^{i\Lambda\varphi} & 0 \\ 0 & 1 \end{bmatrix} \frac{1}{\sqrt{2}} \begin{bmatrix} 1 & j \\ j & 1 \end{bmatrix} = e^{\frac{\Delta\varphi}{2}} \begin{bmatrix} \pi & \kappa \\ \kappa^\* & -\pi^\* \end{bmatrix} \tag{8}$$

Figure 6. Length of the waveguide Wco2 required to achieve the phase shift from 0 to π.

where Δneff is the difference in the effective index, Larm is the length of the waveguide with the width of Wco2. As a result, the length of the waveguide Wco<sup>2</sup> to achieve the phase shift from 0 to π is presented in Figure 6. We see that the short length of 3.5035 μm is required to achieve a

Due to the phase shift Δφ, the complex amplitudes at the input ports and output ports of the

1 ffiffiffi 2 p

1 j j 1 � �

¼ e j Δφ 2

τ κ <sup>κ</sup><sup>∗</sup> �τ<sup>∗</sup> � �

(8)

coupler of Figure 2 can be described in terms of cascaded transfer matrices as

0 1 " #

� � e<sup>j</sup>Δ<sup>φ</sup> 0

Figure 5. Field profile of the SOI waveguide (a) width of 300 nm and (b) width of 500 nm.

<sup>S</sup> <sup>¼</sup> <sup>1</sup> ffiffiffi 2 p

1 j j 1

Figure 4. Effective refractive index of the SOI waveguide calculated by the FDM.

phase shift of π.

234 Emerging Waveguide Technology

where <sup>τ</sup> <sup>¼</sup> sin <sup>Δ</sup><sup>φ</sup> 2 , and <sup>κ</sup> <sup>¼</sup> cos Δφ 2 . Therefore, if an input signal having power presented at input port 1 of the coupler in Figure 2, the normalized powers at output port 1 and output port 2 can be expressed by:

$$Out\_1 = \sin^2\left(\frac{\Delta\phi}{2}\right), \text{and } Out\_2 = \cos^2\left(\frac{\Delta\phi}{2}\right) \tag{9}$$

From Eqs. (7) and (9), the normalized output powers can be calculated and plotted in Figures 7 and 8. It is showed that any power splitting ratio can be achieved by changing the phase shift from 0 to π or changing the length of the Wco2 waveguide from 0 to 3:51μm.

Consider the length of the Wco2 waveguide variation, the normalized output powers are shown in Figure 9. The simulation shows that the changes in normalized output powers are very small (nearly 0% in the range of �50 nm to +50 nm). It is feasible for the current CMOS circuitry [31].

Finally, we use FDTD method to simulate our proposed structure and then make a comparison with the analytical theory. In our FDTD simulations, we take into account the wavelength dispersion of the silicon waveguide. We employ the design of the MMI coupler presented in the previous section. A continuous light pulse of 15 fs pulse width is launched from the input to investigate the transmission characteristics of the device. The grid size Δx ¼ Δy ¼ 20nm and Δz ¼ 20nm are chosen in our simulations. The FDTD simulations for the whole device are shown in Figure 10.

Figure 9. Normalized output powers at ports 2 and 3 for different Wco2 length variation.

Multimode Waveguides on an SOI Platform for Arbitrary Power Splitting Ratio Couplers

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237

Figure 10. Optical field propagation through the coupler for input signal presented at port 1 and for length of the Wco2,

(a) 1.75 μm and (b) 3.51 μm, and (c) mask design.

Figure 7. Normalized output powers at ports 2 (out1) and 3 (out2) for different phase shifts.

Figure 8. Normalized output powers at ports 2 and 3 for different Wco2 lengths.

Multimode Waveguides on an SOI Platform for Arbitrary Power Splitting Ratio Couplers http://dx.doi.org/10.5772/intechopen.76799 237

Figure 9. Normalized output powers at ports 2 and 3 for different Wco2 length variation.

Figure 7. Normalized output powers at ports 2 (out1) and 3 (out2) for different phase shifts.

236 Emerging Waveguide Technology

Figure 8. Normalized output powers at ports 2 and 3 for different Wco2 lengths.

Figure 10. Optical field propagation through the coupler for input signal presented at port 1 and for length of the Wco2, (a) 1.75 μm and (b) 3.51 μm, and (c) mask design.

References

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Technology Letters. 2008;20:599-601

coupler of arbitrary shape. Applied Optics. 2008;47:38-44

IEEE Photonics Technology Letters. 2002;14:483-485

[1] Payne F. Design principles of ring resonator waveguide devices. Private Communication.

Multimode Waveguides on an SOI Platform for Arbitrary Power Splitting Ratio Couplers

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239

[2] Le T-T. Multimode Interference Structures for Photonic Signal Processing. Germany: LAP

[3] Xia F, Sekaric L, Vlasov YA. Mode conversion losses in silicon-on-insulator photonic wire

[4] Soldano LB, Pennings ECM. Optical multi-mode interference devices based on selfimaging: Principles and applications. IEEE Journal of Lightwave Technology. 1995;13:

[5] Suzuki S, Kazuhiro O, Yoshinori H. Integrated-optic double-ring resonators with a wide free spectral range of 100 GHz. Journal of Lightwave Technology. 1995;13:1766-1771 [6] Jeong S, Matsuo S, Yoshikuni Y, et al. Flat-topped spectral response in a ladder-type

[7] Oguma M, Jinguji K, Kitoh T, et al. Flat-passband interleave filter with 200 GHz channel spacing based on planar lightwave circuit-type lattice structure. Electronics Letters. 2000;

[8] Takiguchi K, Jinguji K, Okamoto K, et al. Variable group-delay dispersion equalizer using lattice-form programmable optical filter on planar. IEEE Journal of Selected Topics in

[9] Yariv A. Critical coupling and its control in optical waveguide-ring resonator systems.

[10] Choi JM, Lee RK, Yariv A. Control of critical coupling in a ring resonator–fiber configuration: Application to wavelength-selective switching, modulation, amplification, and oscil-

[11] Takiguchi K, Jinguji K, Ohmori Y. Variable group-delay dispersion equaliser based on a lattice-formprogrammable optical filter. Electronics Letters. 1995;31:1240-1241

[12] Besse PA, Gini E, Bachmann M, et al. New 22 and 13 multimode interference couplers with free selection of power splitting ratios. IEEE Journal of Lightwave Technology. 1996;

[13] Dai D, He S. Proposal for diminishment of the polarization-dependency in a Si-nanowire multimode interference (MMI) coupler by tapering the MMI section. IEEE Photonics

[14] Dai D, He S. Design of an ultrashort Si-nanowaveguide-based multimode interference

[15] Levy DS, Li YM, Scarmozzino R, et al. A multimode interference-based variable power splitter in GaAs-AlGaAs. IEEE Photonics Technology Letters. 1997;9:1373-1375

interferometric filter. IEICE Transactions on Electronics. 2005;E88-C:1747-1754

based racetrack resonators. Optics Express. 2006;14:3872-3886

Figure 11. FDTD simulations compared with the theoretical analysis at different lengths of the Wco2 waveguide.

Figure 11 shows the FDTD simulations compared with theoretical analysis. The simulations show that the device operation has a good agreement with our prediction by analytical theory.

#### 5. Conclusions

We presented a compact structure with a footprint of 6<sup>150</sup> <sup>μ</sup>m2 based on silicon on insulator waveguides for 22 couplers with free of choice power splitting ratios. The new structure requires only one 44 multimode interference coupler. The wide SOI waveguide is used to achieve the phase shift. By changing the length of the wide waveguide from 0 to 3.51 μm, any power splitting ratios can be achieved. The device operation has been verified by using the FDTD. This coupler can be useful for optical interconnects, microring resonator applications.

#### Author details

Trung-Thanh Le\* and Duy-Tien Le

\*Address all correspondence to: thanh.le@vnu.edu.vn

Vietnam National University (VNU), International School (VNU-IS), Hanoi, Vietnam

### References

Figure 11 shows the FDTD simulations compared with theoretical analysis. The simulations show that the device operation has a good agreement with our prediction by analytical theory.

Figure 11. FDTD simulations compared with the theoretical analysis at different lengths of the Wco2 waveguide.

We presented a compact structure with a footprint of 6<sup>150</sup> <sup>μ</sup>m2 based on silicon on insulator waveguides for 22 couplers with free of choice power splitting ratios. The new structure requires only one 44 multimode interference coupler. The wide SOI waveguide is used to achieve the phase shift. By changing the length of the wide waveguide from 0 to 3.51 μm, any power splitting ratios can be achieved. The device operation has been verified by using the FDTD. This coupler can be useful for optical interconnects, microring resonator applications.

Vietnam National University (VNU), International School (VNU-IS), Hanoi, Vietnam

5. Conclusions

238 Emerging Waveguide Technology

Author details

Trung-Thanh Le\* and Duy-Tien Le

\*Address all correspondence to: thanh.le@vnu.edu.vn


[16] Lai Q, Bachmann M, Hunziker W, et al. Arbitrary ratio power splitters using angled silica on siliconmultimode interference couplers. Electronics Letters. 1996;32:1576-1577

**Chapter 13**

**Provisional chapter**

**Longitudinal Differential Protection of Power Systems**

**Longitudinal Differential Protection of Power Systems** 

This chapter describes using optical waveguide for communication between two relays on the opposite ends of the power systems transmission line (or transmission line). Transmission lines are a very important part of the power system. Because of that, relay protection must be fast and safe. Longitudinal differential protection satisfies these requirements. Pilot wire differential relays are commonly used for the protection of short lines. The existence of the pilot wires is a disadvantage. This protection is limited to lines of a few tens of kilometers. However, if optical protection ground wires (OPGWs) are used, instead of pilot wires, the length of the line ceases to be a limiting factor. The following sections tell more about constructions, assembly and utilization of the optical waveguides in differential protection. Also, the newest algorithms of this protection are listed.

**Keywords:** differential protection, relay, communication, optical protection ground

Due to the transient stability of the power system, faults on the lines near the power plant or large substations must be switched off quickly. Longitudinal differential protection can be

The longitudinal differential protection principle is based on the comparison of the currents located at the beginning and at the end of the line, resulting in a quick, sensitive and simple protection concept that ensures that the faulted line is disconnected from the network. The protected zone is defined by the position of the current transformers from which signals are

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.76621

**Transmission Lines Using Optical Waveguide**

**Transmission Lines Using Optical Waveguide**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.76621

wires, transmission line

brought into the differential relay.

applied for fast and selective protection of lines.

**1. Introduction**

Tomislav Rajić

**Abstract**

Tomislav Rajić


#### **Longitudinal Differential Protection of Power Systems Transmission Lines Using Optical Waveguide Longitudinal Differential Protection of Power Systems Transmission Lines Using Optical Waveguide**

DOI: 10.5772/intechopen.76621

Tomislav Rajić Tomislav Rajić

[16] Lai Q, Bachmann M, Hunziker W, et al. Arbitrary ratio power splitters using angled silica on siliconmultimode interference couplers. Electronics Letters. 1996;32:1576-1577

[17] Truong C-D, Le T-T. Power splitting ratio couplers based on MMI structures with high bandwidth and large tolerance using silicon waveguides. Photonics and Nanostructures -

[18] Le TT, Cahill LW, Elton D. The design of 22 SOI MMI couplers with arbitrary power

[19] Le TT and Cahill LW. The design of multimode interference couplers with arbitrary power splitting ratios on an SOI platform. In LEOS 2008, Newport Beach, California, USA, 9-14

[20] Ke X, Liu L, Wen X, et al. Integrated photonic power divider with arbitrary power ratios.

[21] Piggott AY, Petykiewicz J, Su L, et al. Fabrication-constrained nanophotonic inverse

[22] Cherchi M, Ylinen S, Harjanne M, et al. Unconstrained splitting ratios in compact double-

[23] Bachmann M, Besse PA, Melchior H. General self-imaging properties in N N multimode interference couplers including phase relations. Applied Optics. 1994;33:3905-3911 [24] Le D-T, Le T-T. Coupled resonator induced transparency (CRIT) based on interference effect in 44 MMI coupler. International Journal of Computer Systems. 2017;4:95-98 [25] Heaton JM, Jenkins RM. General matrix theory of self-imaging in multimode interference

[26] Le T-T. Two-channel highly sensitive sensors based on 4 4 multimode interference

[27] Le T-T, Cahill L. The design of 44 multimode interference coupler based microring resonators on an SOI platform. Journal of Telecommunications and Information Technol-

[28] Le DT, Do DT, Nguyen VK, et al. Sharp asymmetric resonance based on 44 multimode interference coupler. International Journal of Applied Engineering Research. 2017;12:

[29] Le T-T. An improved effective index method for planar multimode waveguide design on

[31] Dan-Xia X, Schmid JH, Reed GT, et al. Silicon photonic integration platform—Have we found the sweet spot? IEEE Journal of Selected Topics in Quantum Electronics. 2014;20:

an silicon-on-insulator (SOI) platform. Optica Applicata. 2013;43:271-277

[30] Petrone G, Cammarata G. Modelling and Simulation. Rijeka: InTech Publisher; 2008

(MMI) couplers. IEEE Photonics Technology Letters. 1999;11:212-214

Fundamentals and Applications. 2013;11:217-225

Nov 2008

240 Emerging Waveguide Technology

Optics Letters. 2017;42:855-858

design. Scientific Reports. 2017;7:1786

MMI couplers. Optics Express. 2014;22:9245-9253

couplers. Photonic Sensors. 2017;7:357-364

ogy. 2009:98-102

2239-2242

8100217

coupling ratios. Electronics Letters. 2009;45:1118-1119

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.76621

#### **Abstract**

This chapter describes using optical waveguide for communication between two relays on the opposite ends of the power systems transmission line (or transmission line). Transmission lines are a very important part of the power system. Because of that, relay protection must be fast and safe. Longitudinal differential protection satisfies these requirements. Pilot wire differential relays are commonly used for the protection of short lines. The existence of the pilot wires is a disadvantage. This protection is limited to lines of a few tens of kilometers. However, if optical protection ground wires (OPGWs) are used, instead of pilot wires, the length of the line ceases to be a limiting factor. The following sections tell more about constructions, assembly and utilization of the optical waveguides in differential protection. Also, the newest algorithms of this protection are listed.

**Keywords:** differential protection, relay, communication, optical protection ground wires, transmission line

#### **1. Introduction**

Due to the transient stability of the power system, faults on the lines near the power plant or large substations must be switched off quickly. Longitudinal differential protection can be applied for fast and selective protection of lines.

The longitudinal differential protection principle is based on the comparison of the currents located at the beginning and at the end of the line, resulting in a quick, sensitive and simple protection concept that ensures that the faulted line is disconnected from the network. The protected zone is defined by the position of the current transformers from which signals are brought into the differential relay.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Analogue longitudinal differential protection is used for shorter, single-circuit transmission lines in double-fed networks. Pilot cable line connects secondary current transformers on the opposite sides of the protected line. A great disadvantage is the existence of pilot wire, and such protection is limited to short lines. If optical protection ground wires (OPGWs) are used, the length of the line ceases to be a limiting factor [1–3].

OPGW is a dual functioning cable performing the duties of a ground wire and also providing a patch for the transmission of voice, video or data signals. It is located at the top of the power line tower.

The second section presents a classic approach of longitudinal differential protection of transmission lines. The operating principle is explained [1–3].

The third section talks more about OPGW. It describes two different constructions of the OPGW. The same characteristics of them are mentioned and showed. The elements for connecting OPGW with the tower are enumerated and shown [4–8].

The next section describes relay protection realized with pilot wires [9–11].

The fifth section discusses the use of digital protection. The algorithms mentioned in recent works are listed. A short recapitulation is performed. Of course, all solutions or algorithms are difficult to be implemented without using OPGW [12–23].

Equations (1) and (2) are used for calculating the stabilization and differential currents indi-

Longitudinal Differential Protection of Power Systems Transmission Lines Using Optical…


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243


*stab* = |*I L* ¯ |+|*I R* ¯

*diff* = |*I L* ¯ − *I R* ¯

**Figure 2** shows the tripping characteristic of the differential relay. The minimum tripping current (*Imin*) defines the minimum relay tripping threshold and is set to 20–50% of the rated transformer current. This quantity is defined since in an actual system, in a non-fault condition, there is always a difference between the currents measured on the opposite line ends due

The relay trips if the operating point, defined by the differential and stabilization currents'

OPGW is the high-technology equipment for sending/receiving different kinds of data

The transmission lines are perhaps the most important part of the power system. It connects other power system elements such as power plants and substations. Electrical energy finds

vidually for each phase [1–3]:

with the following explanation:

*I*

**Figure 2.** Differential relay tripping characteristic.

*I*

*IL*—basic harmonic phasor of the left-end phase current. *IR*—basic harmonic phasor of the right-end phase current.

**3. Optical protection ground wires (OPGW)**

through electrical transmission lines.

to the current transformers' imperfection and the charging current.

RMS values, is located within the relay tripping area (**Figure 2**) [1–3].

### **2. Longitudinal differential protection: a classic approach**

**Figure 1** shows the longitudinal differential protection operating principle. If the fault occurs outside the protected zone, the left- and right-end currents have the same direction and approximate intensities, that is, their difference is negligible and the protection does not trip. Should the fault occur within the protected zone, the right-end's current changes its direction, establishing a significant current through the differential relay M, causing its tripping. Differential current is the current difference that tends to initiate operation and stabilization current is the current proportional to thought current that tends to inhibit operation.

**Figure 1.** Longitudinal differential protection of transmission lines—Protection operating principle.

Longitudinal Differential Protection of Power Systems Transmission Lines Using Optical… http://dx.doi.org/10.5772/intechopen.76621 243

**Figure 2.** Differential relay tripping characteristic.

Analogue longitudinal differential protection is used for shorter, single-circuit transmission lines in double-fed networks. Pilot cable line connects secondary current transformers on the opposite sides of the protected line. A great disadvantage is the existence of pilot wire, and such protection is limited to short lines. If optical protection ground wires (OPGWs) are used,

OPGW is a dual functioning cable performing the duties of a ground wire and also providing a patch for the transmission of voice, video or data signals. It is located at the top of the power

The second section presents a classic approach of longitudinal differential protection of trans-

The third section talks more about OPGW. It describes two different constructions of the OPGW. The same characteristics of them are mentioned and showed. The elements for con-

The fifth section discusses the use of digital protection. The algorithms mentioned in recent works are listed. A short recapitulation is performed. Of course, all solutions or algorithms

**Figure 1** shows the longitudinal differential protection operating principle. If the fault occurs outside the protected zone, the left- and right-end currents have the same direction and approximate intensities, that is, their difference is negligible and the protection does not trip. Should the fault occur within the protected zone, the right-end's current changes its direction, establishing a significant current through the differential relay M, causing its tripping. Differential current is the current difference that tends to initiate operation and stabilization current is the current proportional to thought current that tends to inhibit

the length of the line ceases to be a limiting factor [1–3].

mission lines. The operating principle is explained [1–3].

necting OPGW with the tower are enumerated and shown [4–8].

are difficult to be implemented without using OPGW [12–23].

The next section describes relay protection realized with pilot wires [9–11].

**2. Longitudinal differential protection: a classic approach**

**Figure 1.** Longitudinal differential protection of transmission lines—Protection operating principle.

line tower.

242 Emerging Waveguide Technology

operation.

Equations (1) and (2) are used for calculating the stabilization and differential currents individually for each phase [1–3]:

$$I\_{\rm stab} = \left| \underline{I}\_{\rm L} \right| + \left| \underline{I}\_{\rm E} \right| \tag{1}$$

$$I\_{off} = \begin{array}{c} \left| \underline{I\_L} - \underline{I\_R} \right| \end{array} \tag{2}$$

with the following explanation:

*IL*—basic harmonic phasor of the left-end phase current.

*IR*—basic harmonic phasor of the right-end phase current.

**Figure 2** shows the tripping characteristic of the differential relay. The minimum tripping current (*Imin*) defines the minimum relay tripping threshold and is set to 20–50% of the rated transformer current. This quantity is defined since in an actual system, in a non-fault condition, there is always a difference between the currents measured on the opposite line ends due to the current transformers' imperfection and the charging current.

The relay trips if the operating point, defined by the differential and stabilization currents' RMS values, is located within the relay tripping area (**Figure 2**) [1–3].

#### **3. Optical protection ground wires (OPGW)**

OPGW is the high-technology equipment for sending/receiving different kinds of data through electrical transmission lines.

The transmission lines are perhaps the most important part of the power system. It connects other power system elements such as power plants and substations. Electrical energy finds the way from plants to consumption, thanks to these lines. Electrical engineers found out a very easy way for data transmission using OPGW and mentioned lines.

**3.1. OPGW cable constructions**

is multi-loose tube type.

*3.1.1. Central loose tube type*

1300 nm & 1550 nm band [4].

strength and resistance to corrosion.

*3.1.2. Multi-loose tube type*

**Figure 4.** Central loose tube type.

Two types of the OPGWs are discussed. One is the central loose tube type and the other one

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The steel tube is sealed and water resistant. The fibers are positioned in the center of the tube and are surrounded with water blocking gel. The stainless steel tube protects optical fibers from possible damages under abnormal operation conditions or during installation. The aluminum layer could be placed around this tube. Aluminum-clad steel wires create the external protection. This protection involves single or multiple layers of these wires. These aluminum wires provide compact construction. It has a dual function. One provides mechanical protection for sever conditions and the other one controls temperature rise during short-circuit

It could be up to 48 fibers placed inside of the stainless steel tube. For easy identification, these optical wires are colored and are in different shapes. Wires are organized in many layers also. This organization provides high mechanical strength and good sag tension performance.

The core is much smaller. It is about 9 microns. This type of cable is suitable for greater distance than multimode cable. Only one light wave can travel through the fiber. This is the reason for distortion absence. The attenuation parameter for the single-mode fiber is typically 0.35 dB/km at 1310 nm and 0.23 dB/km at 1550 nm. This fiber is optimized for use in the

The elements included in construction are almost the same. Optical fibers surrounded with water blocking gel are placed inside the stainless steel tube. This type has more than one tube. The tubes are helically positioned around the center of the cable. This is why it is called multi-loose tube type. The aluminum alloy is positioned in the external layer to give greater

conditions. **Figure 4** shows the cross-section of the central loose tube type.

Transmission lines have at least three phase conductors. These conductors transmit electrical power. For rated voltage above 110 kV, line towers have one or more earth wires on their top. It could be a metal wire with protective function. This earth wire conducts one part of the fault current and protects people around the tower from dangerous voltage. It serves as a lightning protection also because it is on the highest point of the tower.

OPGW could be used as the earth wire. It has the same protection functions like a metal earth wire (lightning and high over-voltages) but also includes communication that is especially important for relay protection. This fiber optic communication provides reliability in the power system protection and data transmission. OPGW construction and number of layers depend on the requirements (both mechanical and electrical). Originally, fiber is placed in the tube. Several metallic strands are located around the tube.

Nowadays, OPGW finds its place in electrical engineering. Displacing the metal earth wires by the OPGW is quite well represented. Dual function of these wires has been won.

**Figure 3** shows the position of the OPGW on the top of the tower [4–8].

However, OPGW could be used for fast-data signal transmission. These signals could be protection signals, operation system data, signals for line testing and monitoring signals. Instead of this, video material or voice could be also carried out from one end of the line to another. OPGW is a multi-function conductor. The most important thing is the absence of additional investments for the trace. Transmission lines have already existed, so the only investment is in replacing the old wire with the new one.

**Figure 3.** Position of OPGW.

#### **3.1. OPGW cable constructions**

Two types of the OPGWs are discussed. One is the central loose tube type and the other one is multi-loose tube type.

#### *3.1.1. Central loose tube type*

the way from plants to consumption, thanks to these lines. Electrical engineers found out a

Transmission lines have at least three phase conductors. These conductors transmit electrical power. For rated voltage above 110 kV, line towers have one or more earth wires on their top. It could be a metal wire with protective function. This earth wire conducts one part of the fault current and protects people around the tower from dangerous voltage. It serves as a lightning

OPGW could be used as the earth wire. It has the same protection functions like a metal earth wire (lightning and high over-voltages) but also includes communication that is especially important for relay protection. This fiber optic communication provides reliability in the power system protection and data transmission. OPGW construction and number of layers depend on the requirements (both mechanical and electrical). Originally, fiber is placed in the

Nowadays, OPGW finds its place in electrical engineering. Displacing the metal earth wires

However, OPGW could be used for fast-data signal transmission. These signals could be protection signals, operation system data, signals for line testing and monitoring signals. Instead of this, video material or voice could be also carried out from one end of the line to another. OPGW is a multi-function conductor. The most important thing is the absence of additional investments for the trace. Transmission lines have already existed, so the only investment is

by the OPGW is quite well represented. Dual function of these wires has been won.

**Figure 3** shows the position of the OPGW on the top of the tower [4–8].

very easy way for data transmission using OPGW and mentioned lines.

protection also because it is on the highest point of the tower.

244 Emerging Waveguide Technology

tube. Several metallic strands are located around the tube.

in replacing the old wire with the new one.

**Figure 3.** Position of OPGW.

The steel tube is sealed and water resistant. The fibers are positioned in the center of the tube and are surrounded with water blocking gel. The stainless steel tube protects optical fibers from possible damages under abnormal operation conditions or during installation. The aluminum layer could be placed around this tube. Aluminum-clad steel wires create the external protection. This protection involves single or multiple layers of these wires. These aluminum wires provide compact construction. It has a dual function. One provides mechanical protection for sever conditions and the other one controls temperature rise during short-circuit conditions. **Figure 4** shows the cross-section of the central loose tube type.

It could be up to 48 fibers placed inside of the stainless steel tube. For easy identification, these optical wires are colored and are in different shapes. Wires are organized in many layers also. This organization provides high mechanical strength and good sag tension performance.

The core is much smaller. It is about 9 microns. This type of cable is suitable for greater distance than multimode cable. Only one light wave can travel through the fiber. This is the reason for distortion absence. The attenuation parameter for the single-mode fiber is typically 0.35 dB/km at 1310 nm and 0.23 dB/km at 1550 nm. This fiber is optimized for use in the 1300 nm & 1550 nm band [4].

#### *3.1.2. Multi-loose tube type*

The elements included in construction are almost the same. Optical fibers surrounded with water blocking gel are placed inside the stainless steel tube. This type has more than one tube. The tubes are helically positioned around the center of the cable. This is why it is called multi-loose tube type. The aluminum alloy is positioned in the external layer to give greater strength and resistance to corrosion.

**Figure 4.** Central loose tube type.

The number of fibers is up to 144. The multi-loose tube type can meet the requirement of huge cross and large-current capacity.

**Fiber count Diameter (mm) Weight (kg/km) Short circuit capacity (KA<sup>2</sup>**

Longitudinal Differential Protection of Power Systems Transmission Lines Using Optical…

**Fiber count Diameter (mm) Weight (kg/km) Short circuit capacity (KA<sup>2</sup>**

**Fiber count Diameter (mm) Weight (kg/km) Short circuit capacity (KA<sup>2</sup>**

 7.8 243 4.7 9 313 8.4 10.2 394 13.9 10.8 438 17.5

**Table 1.** Some characteristics of the single-layer central tube OPGW cable.

 13 671 42.2 15 825 87.9 16 857 132.2 17 910 186.3

**Table 2.** Some characteristics of the double-layer central tube OPGW cable.

 13.4 543,2 74.8 13.4 587,8 68.9 16.4 675,6 190.1 19.9 750 426.6 21.2 891,4 498.6

**Table 3.** Some characteristics of the multi-loose tube OPGW cable.

**Figure 6.** Double dead-end set passing for OPGW cable.

**s)**

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**s)**

**s)**

This fiber has light traveling in the core in many rays called modes. It is made of glass fibers. The main application for multimode fibers is for short-reach optical transmission systems such as local area network (LAN) application. The attenuation parameter for multimode fiber is typically 0.8 dB/km at 1310 nm [4].

Telecom companies prefer this type of OPGW. Multi-loose tube type is good for video material transmission, internet connection and data for the SCADA system which is most important for electrical engineering. It is secure from accidental cutting due to construction work [4–8].

**Figure 5** shows cross-section of the multi-loose tube type.

**Tables 1**–**3** give some information on cable characteristics [7, 8].

### **3.2. OPGW cables' hardware**

OPGW cables are connected with the tower and there are many elements which are used for that such as tension assembly, suspension assembly and attaching clamps and vibration dampers. These elements provide mechanical strength of OPGW and reduce oscillations. **Figures 6**–**9** show mentioned elements.

Sometimes, the distance between two nearest towers are longer then the length of OPGW. In that case, tension assembly has to be used. Connection with the tower is provided with tower clamps also [6].

An assembly with reinforced suspension clamp and neoprene inner covering, especially designed for OPGW cables, is shown in **Figure 8**.

**Figure 5.** Multi-loose tube type.


**Table 1.** Some characteristics of the single-layer central tube OPGW cable.

The number of fibers is up to 144. The multi-loose tube type can meet the requirement of huge

This fiber has light traveling in the core in many rays called modes. It is made of glass fibers. The main application for multimode fibers is for short-reach optical transmission systems such as local area network (LAN) application. The attenuation parameter for multimode fiber

Telecom companies prefer this type of OPGW. Multi-loose tube type is good for video material transmission, internet connection and data for the SCADA system which is most important for electrical engineering. It is secure from accidental cutting due to construction

OPGW cables are connected with the tower and there are many elements which are used for that such as tension assembly, suspension assembly and attaching clamps and vibration dampers. These elements provide mechanical strength of OPGW and reduce oscillations.

Sometimes, the distance between two nearest towers are longer then the length of OPGW. In that case, tension assembly has to be used. Connection with the tower is provided with tower

An assembly with reinforced suspension clamp and neoprene inner covering, especially

cross and large-current capacity.

246 Emerging Waveguide Technology

is typically 0.8 dB/km at 1310 nm [4].

**3.2. OPGW cables' hardware**

**Figures 6**–**9** show mentioned elements.

designed for OPGW cables, is shown in **Figure 8**.

**Figure 5** shows cross-section of the multi-loose tube type.

**Tables 1**–**3** give some information on cable characteristics [7, 8].

work [4–8].

clamps also [6].

**Figure 5.** Multi-loose tube type.


**Table 2.** Some characteristics of the double-layer central tube OPGW cable.


**Table 3.** Some characteristics of the multi-loose tube OPGW cable.

**Figure 6.** Double dead-end set passing for OPGW cable.

The dampers are used to absorb the cable vibrations as shown in **Figure 9**. The number of dampers is determined by the environmental conditions, the distance between towers, the

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Pilot relays involve pilot wires. That is the main characteristic. It means that there is an interconnecting channel between two differential relays positioned at the opposite ends of the transmission line. Different types are used in practice. The distance between relays is the limiting factor. "Wire pilot", "Carrier-current pilot" and "Microwave pilot" are three different

A wire pilot consists of a two-wire circuit of the telephone-line type. This is the easy solution because these circuits already exist as a part of the local telephone company system. Solution

Beyond 10 miles of distances, a carrier-current pilot usually becomes more economical. The circuit consists of a power line as a conductor for low-voltage, high-frequency currents and

When the number of requiring pilot channels becomes larger than economic capabilities,

It could be a d-c connection also but more elements have to be used. **Figure 11** illustrates pilot wire d-c connection to current transformers. This connection is provided via phase sequence networks and saturating transformers. Phase sequence networks are directly connected to current transformers. Three phase currents flow in and a single-phase voltage flows out of

Impedances of relay circuits are not the same rate as the pilot wire impedance. Insulating

Pilot wire has some requirements and limitations of equipment such as the insulation capability. High voltages may occur. Insulation capability depends on the highest values that could

microwave pilots are used. These are radio systems with ultra-high frequency.

**Figure 10.** Schematic illustration of the opposed-voltage principle of the a-c wire-pilot relaying.

**Figure 10** shows the schematic illustration of the a-c wire-pilot relaying principle.

**4. Differential transmission line protection with pilot relays**

type of OPGW cable and the installation parameters.

with a wire pilot is economical for distances up to 5–10 miles.

types of pilot conductors.

the network [9–11].

the ground wire as the return conductor.

transformers match these impedances.

**Figure 7.** Single dead-end set for OPGW cable.

**Figure 8.** Suspension assembly with twisted chain link for OPGW cable.

**Figure 9.** Damper AMG—Four resonances asymmetric Stockbridge.

The dampers are used to absorb the cable vibrations as shown in **Figure 9**. The number of dampers is determined by the environmental conditions, the distance between towers, the type of OPGW cable and the installation parameters.

### **4. Differential transmission line protection with pilot relays**

Pilot relays involve pilot wires. That is the main characteristic. It means that there is an interconnecting channel between two differential relays positioned at the opposite ends of the transmission line. Different types are used in practice. The distance between relays is the limiting factor. "Wire pilot", "Carrier-current pilot" and "Microwave pilot" are three different types of pilot conductors.

A wire pilot consists of a two-wire circuit of the telephone-line type. This is the easy solution because these circuits already exist as a part of the local telephone company system. Solution with a wire pilot is economical for distances up to 5–10 miles.

Beyond 10 miles of distances, a carrier-current pilot usually becomes more economical. The circuit consists of a power line as a conductor for low-voltage, high-frequency currents and the ground wire as the return conductor.

When the number of requiring pilot channels becomes larger than economic capabilities, microwave pilots are used. These are radio systems with ultra-high frequency.

**Figure 10** shows the schematic illustration of the a-c wire-pilot relaying principle.

It could be a d-c connection also but more elements have to be used. **Figure 11** illustrates pilot wire d-c connection to current transformers. This connection is provided via phase sequence networks and saturating transformers. Phase sequence networks are directly connected to current transformers. Three phase currents flow in and a single-phase voltage flows out of the network [9–11].

Impedances of relay circuits are not the same rate as the pilot wire impedance. Insulating transformers match these impedances.

Pilot wire has some requirements and limitations of equipment such as the insulation capability. High voltages may occur. Insulation capability depends on the highest values that could

**Figure 10.** Schematic illustration of the opposed-voltage principle of the a-c wire-pilot relaying.

**Figure 9.** Damper AMG—Four resonances asymmetric Stockbridge.

**Figure 8.** Suspension assembly with twisted chain link for OPGW cable.

**Figure 7.** Single dead-end set for OPGW cable.

248 Emerging Waveguide Technology

**Figure 11.** Pilot wire simplified arrangement.

occur. The main problem of the pilot wires is its length. It depends on loop resistance. There is a maximum value required. It is 2000 Ω. Shunt capacitance also has a limiting value. It is recommended that this capacitance be less than 1.5 microfarads.

The communication channel can be over a dedicated fiber or over a multiplexed network, as shown in **Figure 12**. The dedicated fiber connection typically deploys LED or laser depending on the fiber's distance. The laser option can typically be applied for up to 100 km. The longer

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In order to guarantee the safe and stable operation of high-voltage transmission lines, differential protection is adopted as the main protection for the benefits of its phase-selection

Many various principles for realization of the differential protection have been published in the recent literatures. The fast communication between relays installed at the opposite ends

There are a number of different relay measuring principles used by current differential relays:

This is most like a classic approach. **Figures 1** and **2** show a basic arrangement. At each terminal, an evaluation of the sum of the local and remote current values is made in order to calculate a differential current. Under normal operating conditions or external faults, the current entering at one end of the protected circuit is practically the same as that leaving at the other end. Hence, the differential current value is practically zero and operation of the protection will not occur. For a fault on the protected power line, the differential current value

fiber lengths may need repeaters [12].

**Figure 12.** Protective relaying communications.

of the line is necessary [12].

• Percentage differential relays

**5.1. Percentage differential relays**

• Charge comparison relays • Power differential relays

• Alpha plane relay

function and immunity to power swings and operation modes.

These principles are briefly described in the following subsections.

Connection with the d-c wire pilot means that a lot of elements have to be used. This is one of the disadvantages. An a-c connection does not have these problems. Besides that, a-c connection also is immune to power swings. The good thing about d-c connection is the existence of these wires because of the telephone companies [10, 11].
