The Impact of the Shunt Reactor(s) Location on Offshore Wind Farm Power Export System on Zero-Missing Phenomenon and Switching Overvoltages—The Case of a Polish Power System

Abstract

The observed development of offshore wind farms has resulted in an increasing presence of long extra-high voltage cables, with lengths ranging from 10 to 200 km. These cable lines are compensated for by a shunt reactor(s). The transient states that accompany switching operations, in this type of line, cause a number of challenges. There are slightly different and more dangerous phenomena than in classic uncompensated lines. One such phenomenon is the zero-missing phenomenon. The most effective methods for mitigating these phenomena are still under investigation and comparison to identify the optimal countermeasures. Applying a single minimization countermeasure often leads to avoiding one phenomenon (e.g., zero-missing phenomenon) while enhancing another (e.g., switching overvoltages). This can challenge designers, as they must consider the trade-offs between these competing objectives. The added value of this article is an analysis of the impact of the compensation level and its distribution (shunt reactor(s) location) in the offshore wind farm power export systems on zero-missing phenomenon and switching overvoltages. These analyses are supported by simulation cases mapping the case of the Polish power system. The results of the presented analysis are very important in light of a dynamic development of offshore wind farms and may be utilized by designers of power export systems, and can mitigate the significant risks occurring in such systems during energization cable lines.

1. Development of the Offshore Wind Farms in Poland

Offshore wind energy will make a significant contribution to achieving the climate change targets adopted by developed countries [1]. Within the European Union, this type of electricity generation is expected to grow massively. The EU Strategy on the Offshore Renewable Energy [2] and the documents published by ENTSO [3] anticipate that by 2050, 300 GW of offshore wind capacity will be developed and integrated into the power grid.

In the coming years, Poland is also expected to see a dynamic development of offshore wind farms. The Polish government’s program for the development of OWFs anticipates the growth of this energy source to a capacity of several gigawatts. This development will be implemented through two phases of the support system: phase one—5.9 GWs; phase two—9.4 GWs [4]. The offshore wind farms (OWFs) that will be built in the first and the second phases of the support system are shown in Table 1 and Table 2. The planned location of these wind farms is shown in Figure 1. It is important to highlight that the OWFs outlined in Table 1 will utilize the power export systems based on HVAC technology. By 2030, the majority of these OWFs will be capable of generating energy.

The key element of the OWF is the power export system shown in Figure 2. This system must have high operational reliability throughout its lifetime. In Polish conditions, it is required to have at least 25 years.

One of the technical challenges, in power derivation systems implemented with HVAC technology (Figure 2), is switching operations. Due to the relatively long length of the export systems, extra high voltage (EHV) cables (220 or 275 kV) compensated with shunt reactors are used. In such systems, switching operations may cause some negative phenomena, including switching overvoltages and zero-miss phenomenon. The influence of the location of shunt reactors on the occurrence and parameters of these phenomena is the subject of this article.

These phenomena are very important for the reliable operation of the power export system, and their mitigation is the key to achieving the appropriate availability of the power export system required by legislative documents [4]. An important element of the analysis was the additional consideration of different types of circuit breakers (CBs), which are among the primary means of mitigating these negative phenomena. This approach resulted in a much broader and more practical treatment of the issue and is crucial for designing and properly analyzing phenomena occurring in the power export systems.

As highlighted in the second point of this paper, the authors do not identify publications that would show standardized procedures to prevent this issue around the world.

From the perspective of the research presented in this paper, the length of the power export system is very important. This length is determined between the planned location of wind farms and the onshore power station (approximately defined as the coastline). In Polish conditions (Figure 1), the OWF power export systems will have lengths ranging from several dozen kilometers to one hundred and several dozen kilometers.

2. The Key Phenomena and Selected Means of Mitigating Transient States of Switching Operations in Compensated EHV Cable Lines

2.1. Switching Overvoltages and Zero-Missing Phenomenon

2.1.1. Switching Overvoltages

Switching overvoltages are a type of internal (system) overvoltages, which is related to switching on and off electrical circuits. Switching long EHV cables causes strong transients due to the presence of inductance and high capacitance in the circuit. The resulting overvoltages and overcurrents of significant magnitude and gradient can be dangerous to the operating power equipment and can lead to damage [5]. The levels of overvoltages occurring in a network are shown in Figure 3.

Switching overvoltages differ from a lightning one primarily in duration and generally lower peak value. It can be assumed that the duration of the switching stroke front time is between 200 and 600 μs, and the time to half-peak is about 3000 μs. It is characteristic that the insulation strength of the equipment against these overvoltages is about 15% lower than the strength under normal overvoltage (1.2/50 μs).

The maximum values of switching overvoltages are in the range of 2 to 4 pu and are determined by transient phenomena and equalization runs occurring in the RLC circuits. Overvoltages according to [7] have been grouped in terms of typical oscillation frequencies and wavefront rise times (Figure 4).

According to the aforementioned standard, overvoltages generated during switching operations on the cable lines with a classic CB can be classified as low-frequency casual overvoltages and high-frequency overvoltages. Thus, they are in a group of particular importance from the point of view of insulation coordination and overvoltage exposures of insulation systems of power equipment.

The resistance to switching overvoltages is one of the key factors determining the reliability of the device’s operation. The characteristics shown in Figure 5 present the transient overvoltage strengths for different power equipment. From the curves shown in the figure, it is clear that the objects most sensitive to the overvoltages are transformers and shunt reactors. The shunt reactors are the basic components of the export cable lines of the OWF. Therefore, the occurrence of overvoltages during switching operations (also the results obtained during simulation studies), in any case, should not exceed the curves shown in Figure 5.

The phenomenon of switching overvoltages is described in detail in [8,9,10,11]. In general, the value of the switching overvoltages depends on the value of the voltage at the time of connecting the cable line, and to avoid switching overvoltages, the connection should be made when the instantaneous value of voltage is zero.

Figure 5

Withstand transient overvoltage capability for power equipment [12].

[Figure omitted. See PDF]

2.1.2. The Zero-Missing Phenomenon

The zero-missing phenomenon (also known as missing current zero, current zero-missing phenomenon, or current zero crossing phenomenon) directly refers to the absence of the zero value of the current for many periods of the fundamental component. This phenomenon is crucial in the context of switching operations by circuit breakers in the power systems. The zero-missing phenomenon is described in detail in [13,14,15,16]. The current zero-missing is a phenomenon that occurs during the energization of inductive and capacitive equipment. In practice, this phenomenon occurs especially during the energization of high-voltage AC cable lines with shunt reactors, which compensate for more than 50% of the reactive power. This phenomenon manifests when the current through does not cross zero for several cycles. In practice, this can lead to significant operational challenges for CBs and pose a significant threat to their operation.

The easiest way to understand the zero-missing phenomenon is to analyze the transient behavior of the circuit shown in Figure 6. This figure shows an inductor connected in parallel with a capacitor. The current flowing in the individual branches of this circuit is shown in Figure 7. The CB is closed when the voltage is zero, which causes a maximum DC component. It should be emphasized that the impedance of the inductor and capacitor is the same (equal to each other) and the value of the resistor is very low. In such a situation, the current of individual objects is in phase opposition and has equal amplitude.

In transient conditions, the current has a high DC component. The value of these components depends on the instantaneous voltage value at the energization point. According to theory, if the inductor (shunt reactor in practice) is connected in a voltage peak the current should be zero. However, if the shunt reactor is connected when the voltage is zero the current should have a peak value. As the current in the inductor must maintain its continuity and it was zero at connection time (t = t₀, before the connection of the inductor), a DC component will appear in the inductor current to maintain its continuity. The DC component is equal to minus the value of the AC component in the connection time (t = t₀) [14].

The DC component maintenance time depends on the damping, which is a function of the resistance in the circuit. In practice, because high voltage devices have very low resistances, if the energization occurs at a voltage zero point, where the current is at its maximum, the DC component can prevent the current from crossing zero for an extended period of time. This can sometimes last several seconds.

The obtained results can be related to the real system. With some simplifying assumptions the shunt reactor can be modeled as an inductor in series with a resistor, and the cable line can be modeled as a capacitor [6]. Results obtained during energization operations of a shunt reactor with an EHV cable line are not very different from the one in Figure 6. This phenomenon is critical for the CBs because they are designed to interrupt a current at zero crossings, and the lack of this phenomenon can lead to equipment damage.

To sum up, it can be stated that to avoid the zero-missing phenomenon the connection should be made when the voltage is at a peak, and to avoid switching overvoltages the connection should be made when the voltage is zero [17]. In general, it is not possible to avoid both situations at the same time. Therefore, in practice, special measures are often used, e.g., a CB with a PIR (pre-insertion resistor) and a CB with a PoW (point-on-wave) switching.

2.2. Selected Measures to Mitigation Transients States

The egree of shunt compensation for overhead lines is typically restricted to 60% to 80% due to risks of line resonance during open-phase conditions. This phenomenon is caused by a capacitive coupling between the disconnected phase and the remaining energized phases. Given that there is no capacitive coupling between insulated cables, the restriction on a low degree of shunt compensation does not apply. Compensation degrees close to 100% can be achieved for the underground cable lines [18]. The phenomena that occur during the switching operations in these types of compensated cable lines are very dangerous and can cause significant damage to the equipment. Therefore, selected measures are often used in practice.

A detailed analysis of the literature on minimizing the risks resulting from transients occurring in the considered compensated cable lines [18,19,20,21,22,23,24,25,26,27,28,29,30] allowed us to identify that there are several key methods used in practice. These methods are as follows: decreasing the amount of compensation directly connected to the cable line, using the CB with the PoW, using the CB with the PiR, and a single phase tripping. As only the healthy phases experience this zero-miss effect, it is possible to postpone the opening time of this phase a few milliseconds until the effect has passed, an automatic sequential switching (first switch off the faulty phase, then the reactor, and after a few hundreds of milliseconds the healthy phases). For weak grids, one utility in Europe has chosen a solution where the cable lines and each shunt reactor have their own circuit breaker [30]. The shunt reactor is connected to the bus bar of the substation and not directly to the cable line. The CB works simultaneously and by doing so, the voltage drop stays within the defined limits and the zero-miss phenomenon will be avoided. A summary of these methods, taking into account their advantages and disadvantages, is presented in Table 3.

It should be noted that in the countermeasures listed in Table 3, there is no method for locating the shunt reactor in the cable lines—e.g., the beginning of the cable lines or the end of the cable. Simulation studies conducted by the authors of this paper have shown that this factor can have a significant impact on the transient states during energization (switching overvoltages and a zero-miss phenomenon). Selected research results in this area are presented in this paper. The simulation studies also included the use of a CB with a PIR and a CB with a PoW switching as a key means of minimizing the risk of switching operations.

2.2.1. The CB with PiR—Operating Concept

Pre-insertion resistors are devices used primarily in a CB to effectively dampen the current surge resulting from the cable lines energization process and the transformer switching on, thereby limiting transient states and minimizing overvoltages. The operation of the PIR switch is shown in Figure 8. The resistor shown in the figure is connected in series to the circuit for the first 10 ms (for 50 Hz) from switching on. After this time, the resistor is bypassed and from that moment the current passes through the metallic connection by a CB interrupter bypassing this resistor. The optimal value of the pre-insertion resistor depends on various factors, including the specific characteristics of the electrical system. Accurate calculations and sometimes detailed simulations of electromagnetic transient states (EMT) are required to determine the optimal resistance value to achieve the desired damping of transient phenomena. In summary, the pre-insertion resistors are particularly useful in systems with capacitive or inductive loads limited electrical transients during switching operations, thereby protecting equipment and ensuring the reliability of power systems.

2.2.2. Operating Concept of a CB with PoW

The operational concept of PoW switching is shown in Figure 9. It involves synchronizing the closing or opening of the CB to a specific point on the AC voltage waveform. The CB contacts should close at the moment when the instantaneous value of the voltage in a given phase is most favorable from the point of view of transient switching conditions. By timing the switching at these optimal points, PoW can reduce inrush currents or switching overvoltages and electrical stresses on the power system.

Switching on the system using PoW requires the use of three 1f CBs with very good operating parameters (time contact spread should not exceed 1 ms). In practice, the time of switching on the system should be adapted to the parameters of the connected system and the parameters of the CB (depending, among other things, on the environmental conditions, especially the temperature). It should be emphasized that only very precise connection gives the desired results.

3. EMT Simulation Model

The objective of the research is to identify the transient conditions that occur during switching operations in the compensated EHV cable lines of the OWF power export system. The research should determine how the location of the shunt reactor in the power export system affects the electromagnetic transient states.

In particular, the characteristics of overvoltages and overcurrents and the zero-miss phenomenon occurring in the function of:

• different compensation variants of the cable lines: with or without shunt reactor, shunt reactor location, level of compensation;

• the type of switching used: the PoW switching, the CB with PiR switching.

To simulate these phenomena the simulation studies were performed using MATLAB Simulink R2022b software, which allows the study of electromagnetic transient states. In particular, the Simscape Electrical Specialized Power System R2022b software is dedicated to this purpose. This tool allows us to simulate electromagnetic transients, and dynamic behavior in the transmission lines, the shunt reactors, and other power equipment. This software also provides the ability to design different types of control systems, for example, PoW switching.

In this software, a test model was developed, consisting of an equivalent power system, a cable line, a CB, and shunt reactors. The length of the modeled cable line was 100 km. This is the length of cable lines that may occur in the power export systems developed in Phase 1 of the Polish OWF (see point 1). The schematic diagram of the model is shown in Figure 10.

Figure 11 shows the view of the model in the MATLAB Simulink software. Basically, the test model consists of reference models of power equipment in the form of:

• Three-phase source—representation of the power system;

• Distributed parameters line—representation of the cable line;

• Reactors—representation of the shunt reactors;

• Three-phase breaker—equipped with auxiliary systems enabling the analysis of simulation variants of switching operations carried out using: a classic CB; a CB with PIR; and a CB with PoW.

4. Results

The research was carried out for various scenarios of the power export system operation and compensation. In particular, the following were considered:

• Different technologies of a CB:

  • ○. classical CB;

  • ○. CB with PiR;

  • ○. CB with PoW;

• Different methods of compensation:

  • ○. different level of compensation;

  • ○. different power of shunt reactors;

  • ○. different locations of shunt reactors.

Table 4 shows the description of the simulated cases. The results of the analyses are presented in Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18 and Table 5. For the considered analysis scenarios, the figures show the results in the form of:

• u₁(t)—voltage waveform at the beginning of the power export system;

• u₂(t)—voltage waveform at the end of the power export system;

• i₁(t)—current waveform at the beginning of the power export system.

The locations of the measurement signals u₁(t), u₂(t), and i₁(t) are shown in Figure 10.

Table 5, on the other hand, presents the results of the analyses in the form of:

• Maximum instantaneous voltage at the beginning of the power export system;

• Maximum instantaneous voltage at the end of the power system;

• Maximum instantaneous current at the end of the power system;

• Risk of the CB switching off during zero-missing phenomenon.

To facilitate analysis of the results, the figures in this section include scenario labels that define the CB used and the compensation levels of the power export system (see Table 4).

5. Discussion

The presented studies have shown that the impact of the shunt reactor(s) location on the OWF power export system on the transient states can be significant. In particular, this concerns the zero-missing phenomenon. However, the influence of the compensation level and its distribution in the export system on the switching overvoltage levels is significantly lower.

The main results of the simulation are shown in Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18 and Table 5. The principle adopted in this article for the presentation of the simulation results was to facilitate the analysis of the effect of the used CB type on the resulting voltage and current waveforms. Therefore, the results for one power export system compensation scenario and three variants of the used CB are presented in each of the figures and Table 5.

Figure 12 shows the curves obtained for the zero-compensation scenario (0% compensation). The results obtained after applying the power export system compensation up to a level of approximately 50% in different distributions are shown in Figure 13, Figure 14 and Figure 15, while the results obtained after applying approximately 100% compensation are shown in Figure 16, Figure 17 and Figure 18. In practice, 100% compensation of the power export system is rather seldom applied, but this level reflects very well the phenomena occurring in the analyzed systems, so the authors decided to present the results for such a level as well. In addition, it is emphasized that the results presented in Figure 12 can be considered baseline results to which the results of other scenarios can be related.

By analyzing the results in Figure 12, it is possible to see how significantly the use of a particular type of CB affects the transient conditions that occur—in Table 5, these are the first three rows.

In the scenarios analyzed in Figure 12, the largest overvoltages occur for the classical CB, significantly lower overvoltages are induced by the CB with PiR, and the lower and shortest overvoltages are induced by the CB with PoW. Due to the lack of compensation, the zero-missing phenomenon does not occur here. Extending this analysis to the other cases in Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18 and the results presented in Table 5, it can generally be concluded that the classical CB causes the highest overvoltages (2.1 p.u.), while the CB with PiR (1.6 p.u.) and the CB with PoW (1.5 p.u.) significantly reduce the switching overvoltages in the system. The situation is different regarding the DC component (Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18), which causes a dangerous zero-miss phenomenon (consistent with the literature, refer to Section 2.1.2). In this case, the CB with PoW causes DC offset in three phases, the classical CB causes the DC component in one phase, while the CB with PiR does not cause the DC component (in this case the CB switching off during zero-missing phenomenon risk indicator is the smallest—see Table 5).

Analyzing the results in Figure 12, Figure 13 and Figure 16 for the case of using different levels of compensation of the power export system located only at the beginning of the system, it is concluded that the level of compensation used has a small impact on the switching overvoltages:

• Using Classic CB, in each case (Figure 12a, Figure 13a and Figure 16a), the overvoltages are almost equal and very high at both ends of the cable line;

• Using CB with PIR (Figure 12b, Figure 13b and Figure 16b), the use of larger compensation values almost eliminates the overvoltages, which are relatively lower anyway;

• Using CB with PoW (Figure 12c, Figure 13c and Figure 16c), the use of different compensation levels has a small effect on the switching overvoltage values when compensation is applied—the higher the compensation level of the power export system, the lower the overvoltage values.

Analyzing the results in Figure 12, Figure 13 and Figure 16 and values in Table 5 for the case of using different levels of compensation of the power export system located only at the beginning of the power export system, it is concluded that the level of compensation used has a significant impact on the zero-miss phenomenon:

• Using Classic CB (Figure 12a, Figure 13a and Figure 16a), there is a higher DC component in only one phase as the compensation increases;

• Using CB with PiR (Figure 12b, Figure 13b and Figure 16b), the increase in the compensation value does not affect the DC component, which is minimized by the CB operating principle;

• Using CB with PoW (Figure 12c, Figure 13c and Figure 16c), the DC component observed in all three phases increases with increasing compensation;

• The zero-miss phenomenon becomes apparent when the power export system exceeds about 50% compensation and reaches its maximum when compensation reaches 100%.

Analyzing the influence of the spatial distribution of the 50% compensation of the power export system on the simulation results (Figure 13, Figure 14 and Figure 15 and Table 5), it can be concluded that:

• Regarding voltage issues, the distribution of the compensation has a small effect on the level of switching overvoltages that occur—a higher level of compensation causes lower overvoltages;

• With regard to the zero-miss phenomenon, the following can be concluded:

• Changing the location of the compensation has a significant effect on the value of the DC component and thus on the zero-miss phenomenon;

• Distributing the compensation equally at both ends of the power export system has a significant effect on reducing the DC component, while placing all the compensation only at the end of the power export system practically eliminates this phenomenon.

Similar conclusions can be reached by analyzing the results for different variants of the distribution of the 100% compensation of the power export system (Figure 16, Figure 17 and Figure 18, Table 5). In the analyzed cases, the distribution of compensation or the use of compensation at the end of the export system also significantly reduced the DC component. It should be noted that the location of the compensation only at the end of the export system gives the lowest DC component. In this case, however, it must be emphasized that low current values (resulting from a highly compensated system) can cause the duration of the zero-fault phenomenon to be longer than when the compensation is closer to 50%. Therefore, in the analyzed case of 100% compensation, the danger for the CB will last for a longer period of time.

6. Conclusions

The paper presents an important issue concerning the effect of the level of compensation and its distribution in the power export system on switching overvoltages and zero-miss phenomena. An important element of the analysis was the additional consideration of different types of CBs, which are among the main measures for mitigating these negative phenomena. This approach resulted in a much broader and more practical treatment of the issue.

The effect of the level of compensation and its location in the power export system is significant for the transient states that occur, particularly related to the zero-miss phenomenon. In general, an increase in the level of compensation used, located at the beginning of the power export system (the most common solution in OWFs), leads to an increase in the probability of occurrence of a zero-miss phenomenon and an increase in its duration—significant risks occur after exceeding 50% compensation—this is in line with publications. On the other hand, the correct location of compensation in the structure of the power export system can significantly mitigate the occurrence of this phenomenon. The most favorable results from the perspective of minimizing the zero-miss phenomenon are obtained when compensation is located at the end of the power export system. This is the most important achievement presented in the minor article (not identified in the literature). It should also be noted that no negative effect on the level of switching overvoltages was observed due to different locations of the power system compensation.

In practice, for distances above several tens of kilometers, the power export system does not use classical CBs. The CB with PiR is a solution very rarely used in Poland. Recently, they have often used the CB in PoW.

Taking into account the results of the analysis in the following article, it is recommended to pay special attention to the location of reactive power compensation used in the power export system, especially when using the CB with PoW. Such CBs are very sensitive to the zero-miss phenomenon, since this phenomenon can occur in all three phases of this circuit. Therefore, when using a CB with PoW in a power export system, it is recommended to locate a large part of the compensation at the end of this power export system, that is, at the offshore OWF station. This will mitigate the possibility of a zero-miss phenomenon, which is one of the most dangerous phenomena for a CB.

The research presented in this article complements the current knowledge very well and raises issues that have important implications for the design of the OWF power export systems. This research has both theoretical and practical importance. Designers of the HVAC export systems being developed in the coming years in the world, including very intensively in Poland, should take into account the conclusions presented in this article.

Due to the importance of the issue and the occurring development of compensated EHV cable lines, the authors will continue research in this area, including the effects on transient states of the detailed selection of switch chambers and capacitor trip devices of the CBs and the use of static synchronous compensators (STATCOM) connected to the busbars of OWF onshore substations. It is expected that a detailed selection of switch chambers and capacitor trip devices can significantly reduce the overvoltages that occur during the energization process [31,32,33,34,35]. On the other hand, STATCOM systems can reduce the voltage level just before the start of the switching process and dump the transient states occurring in the system during energization [36,37].

Author Contributions

Conceptualization, P.R., M.S. (Mateusz Szablicki) and A.H.; Methodology, P.R., M.S. (Mateusz Szablicki), M.N. and A.H.; Software, P.R. and M.N.; Validation, P.R., M.S. (Mateusz Szablicki), M.N., A.H., D.Z. and M.S. (Michał Szewczyk); Formal analysis, P.R., M.S. (Mateusz Szablicki), M.N., A.H., D.Z. and M.S. (Michał Szewczyk); Investigation, P.R., M.S. (Mateusz Szablicki), M.N. and A.H.; Resources, P.R., M.S. (Mateusz Szablicki), M.N., A.H., D.Z. and M.S. (Michał Szewczyk); Writing—original draft, P.R.; Writing—review & editing, P.R., M.S. (Mateusz Szablicki), M.N., A.H., D.Z. and M.S. (Michał Szewczyk); Visualization, P.R., M.S. (Mateusz Szablicki) and M.N.; Supervision, P.R., M.S. (Mateusz Szablicki) and A.H. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data available on request due to confidentiality restrictions.

Conflicts of Interest

Authors Piotr Rzepka, Mateusz Szablicki and Marcin Niedopytalski were employed by the company PSE Innowacje Sp. z o.o. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Location OWFs developed in the Polish economic zone of the Baltic Sea [4].

Figure 2. Structure of the power export system—the case of Polish power system.

Figure 3. The classification of overvoltages occurring in networks and their levels [6].

Figure 4. Classes and shapes of overvoltages, standard voltage shapes, and standard withstand voltage tests according to IEC 60071-1 “Insulation Coordination” standards [7].

Figure 6. Diagram of an inductor in series with a resistor, both in parallel with a capacitor.

Figure 7. Time-domain results of energizing the circuit with an inductor and a shunt reactor: current in the inductor iL(t), in the capacitor iC(t), and their sum iCB(t).

Figure 8. The CB with PIR: (a) The first phase of operation—the circuit being switched on is powered by a resistor; (b) second phase of operation—the circuit is powered directly.

Figure 9. The operational concept of PoW switching.

Figure 10. Schematic diagram showing the power export system, highlighting the part of the system that was modeled in the test model.

Figure 11. Test model view in MATLAB Simulink.

Figure 12. Voltage u1(t) and current i1(t) at the beginning and voltage u2(t) at the end of the power export system, for 0% compensation level at the beginning and 0% compensation level at the end, in case of cable line, energized by: (a) a classical CB; (b) a CB with PIR; (c) a CB with PoW.

Figure 13. Voltage u1(t) and current i1(t) at the beginning and voltage u2(t) at the end of the power export system, for 50% compensation level at the beginning and 0% compensation level at the end, in case of cable line, energized by: (a) a classical CB; (b) a CB with PIR; (c) a CB with PoW.

Figure 14. Voltage u1(t) and current i1(t) at the beginning and voltage u2(t) at the end of the power export system, for 25% compensation level at the beginning and 25% compensation level at the end, in case of cable line, energized by: (a) a classical CB; (b) a CB with PIR; (c) a CB with PoW.

Figure 15. Voltage u1(t) and current i1(t) at the beginning and voltage u2(t) at the end of the power export system, for 0% compensation level at the beginning and 50% compensation level at the end, in case of cable line, energized by: (a) a classical CB; (b) a CB with PIR; (c) a CB with PoW.

Figure 16. Voltage u1(t) and current i1(t) at the beginning and voltage u2(t) at the end of the power export system, for 100% compensation level at the beginning and 0% compensation level at the end, in case of cable line, energized by: (a) a classical CB; (b) a CB with PIR; (c) a CB with PoW.

Figure 17. Voltage u1(t) and current i1(t) at the beginning and voltage u2(t) at the end of the power export system, for 50% compensation level at the beginning and 50% compensation level at the end, in case of cable line, energized by: (a) a classical CB; (b) a CB with PIR; (c) a CB with PoW.

Figure 18. Voltage u1(t) and current i1(t) at the beginning and voltage u2(t) at the end of the power export system, for 0% compensation level at the beginning and 100% compensation level at the end, in case of cable line, energized by: (a) a classical CB; (b) a CB with PIR; (c) a CB with PoW.

Table 1

The offshore wind farms included in Phase I of the government support system [4].

Area No. 1	Name of the Project	Investor	OWF Capacity
1	Baltica 3	PGE Baltica/Orsted	1050 MW
2	Baltica 2	PGE Baltica/Orsted	1500 MW
3	Baltic Power	Orlen/Northland	1200 MW
4	BC-WIND	Ocean Winds	500 MW
5	FEW Baltic II	RWE Renewables	440 MW
6	MFW Bałtyk II	Polenerga/Equinor	720 ÷ 1200 MW
7	MFW Bałtyk III	Polenerga/Equinor	720 ÷ 1200 MW

¹ The location of the offshore wind farm area is shown in Figure 1.

Table 2

The offshore wind farms included in Phase II of the government support system [4].

Area No. 1	Name of the Project	Investor	OWF Capacity
14.E.1	Energa MFW1	Orlen	812 MW
14.E.2	Energa MFW2	Orlen	896 MW
14.E.3	Orlen Neptun 14.E.3	Orlen	1204 MW
14.E.4	Orlen Neptun 14.E.4	Orlen	1204 MW
43.E.1	Baltic 7	PGE/Tauron	990 MW
44.E.1	Baltic 9	PGE	975 MW
45.E.1	Baltica 2+	PGE/Orsted	210 MW
46.E.1	Orlen Neptun 46.E.1	Orlen	966 MW
60.E.1	MFW Baltyk I	Polenergia/Equnior	1560 MW
60.E.2	Baltica 1	PGE Baltica	900 MW
60.E.3	Baltica 1 +	PGE	1185 MW
60.E.4	Baltica 5	PGE/Enea	555 MW

¹ The location of the offshore wind farm area is shown in Figure 1.

Table 3

Countermeasure for minimizing the transient states of switching operation.

Countermeasure	Advantages	Disadvantages
PoW switchingThis technique aims to synchronize the closing of the CB with the voltage waveform to minimize the transient states	Minimize the transient states by switching at proper point of voltage waive	Need to be used along with sequential switching Not a fully reliable solution Limited possibilities of switching operations in the event of point-on-wave controller failure
CB with PIRPre-insertion resistors are devices used primarily in electrical systems to mitigate the effects of inrush currents and switching overvoltages during the energization	Resistance in series with CB damps the DC component and eliminates the zero-missing phenomenon.	Not available as standard design at 220 and 275 kV GIS Resistors switching may create new transients. Expensive additional components, not available as a standard design
Sequential Switching—delay switched of shunt reactorThis involves carefully timing the energization of the inductive and capacitive components to avoid simultaneous energizationEnergize shunt reactor after a few seconds delay from the cable lines energization	Possibility to immediately trip the cable lines if necessary Possibility to immediately trip the shunt reactor if necessary Faults on the cable lines will be taken care of before shunt reactor energization	May cause exceed the reactive power spillage at point of connection for a few seconds Utility approval is required to accept voltage change/reactive power flow at POI for a few seconds Expensive additional equipment, such as a CB on the shunt reactor
Limit degree of line compensation cable lines during line energization	Lowers the DC component of current	Reduce compensation might not fulfill voltage change requirement Very expensive when used as a variable reactor.
Delay opening of healthy phases during faults	In the event of a fault, delaying the opening of CBs for non-faulty phases can help manage the risks associated with zero-missing phenomenon	Complex protection scheme and strategies. Wait for zero crosses to mitigate the risks of CB Significant increase in the risk of destruction in the event of a protection system failure

Table 4

Test scenarios label description.

Figure	Scenario Labels	CB Technology 1	Level of Compensation the Power Export System Cable Line	Distribution of Compensation
				Beginning	End
12a	Q1_0_0	Q1	0%	0%	0%
12b	Q2_0_0	Q2	0%	0%	0%
12c	Q3_0_0	Q3	0%	0%	0%
13a	Q1_50_0	Q1	50%	50%	0%
13b	Q2_50_0	Q2	50%	50%	0%
13c	Q3_50_0	Q3	50%	50%	0%
14a	Q1_25_25	Q1	50%	25%	25%
14b	Q2_25_25	Q2	50%	25%	25%
14c	Q3_25_25	Q3	50%	25%	25%
15a	Q1_0_50	Q1	50%	0%	50%
15b	Q2_0_50	Q2	50%	0%	50%
15c	Q3_0_50	Q3	50%	0%	50%
16a	Q1_100_0	Q1	100%	100%	0%
16b	Q2_100_0	Q2	100%	100%	0%
16c	Q3_100_0	Q3	100%	100%	0%
17a	Q1_50_50	Q1	100%	50%	50%
17b	Q2_50_50	Q2	100%	50%	50%
17c	Q3_50_50	Q3	100%	50%	50%
18a	Q1_0_100	Q1	100%	0%	100%
18b	Q2_0_100	Q2	100%	0%	100%
18c	Q3_0_100	Q3	100%	0%	100%

¹ Q1—Classical CB; Q2—CB with PoW, Q3—CB with PiR.

Table 5

Analysis results for test scenarios.

Figure	Scenario Labels	Maximum Instantaneous Voltage at the Beginning[p.u.]	Maximum Instantaneous Voltage at the End[p.u.]	Maximum Instantaneous Current at the Beginning[p.u.]	Risk of CB Switching off During Zero-Missing Phenomenon 1
12a	Q1_0_0	1.6	2.1	2.7	None
12b	Q2_0_0	1.3	1.4	2.0	None
12c	Q3_0_0	1.2	1.3	1.5	None
13a	Q1_50_0	1.6	2.0	2.5	Medium
13b	Q2_50_0	1.1	1.1	1.5	None
13c	Q3_50_0	1.1	1.3	1.6	Medium
14a	Q1_25_25	1.6	2.0	2.5	Low
14b	Q2_25_25	1.0	1.1	1.5	None
14c	Q3_25_25	1.1	1.2	1.6	Low
15a	Q1_0_50	1.6	2.0	2.5	None
15b	Q2_0_50	1.0	1.1	1.5	None
15c	Q3_0_50	1.1	1.2	1.6	None
16a	Q1_100_0	1.5	1.9	2.5	High
16b	Q2_100_0	1.0	1.1	1.5	Very low
16c	Q3_100_0	1.1	1.2	1.6	Very high
17a	Q1_50_50	1.5	1.8	2.5	High
17b	Q2_50_50	1.0	1.1	1.5	Very low
17c	Q3_50_50	1.1	1.2	1.6	Very high
18a	Q1_0_100	1.4	1.8	2.5	Low
18b	Q2_0_100	1.0	1.0	1.5	Very low
18c	Q3_0_100	1.1	1.1	1.6	Very low

¹ The CB switching off during zero-missing phenomenon risk indicator: none, very low, low, medium, high, very high.

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