1 Introduction

As has been explained in the “AC System Characteristics” and the “AC Network Control Using Conventional Means” chapters, long transmission lines may require reactive power compensation, to keep the voltage within the desired limits. When the line load varies over a large range, the reactive power compensation may need to be switched in and out depending on the power flow. However, when conventional switchgear is used for this purpose, the time taken to switch the reactors may not be acceptable in case of faults in the network. Furthermore, frequent switching will result in the need for more switchgear maintenance.

Controlled variable reactive power control provides a solution to the control of the line voltage and can also provide support to the ac network during disturbances, resulting in increased stability margin. The Chapter “Controllers Using the Saturation of Iron for AC Network Control” in this Green Book has described how, in the 1960s, the nonlinear saturation characteristics of iron was used for variable reactive power control in transmission networks.

This chapter describes the magnetically controlled shunt reactor (MCSR) systems developed in Russia. The need for these controllers arose from the very long transmission lines required because of the large area covered by Russia and other Commonwealth of Independent States (CIS) countries. The power grid also had very significant daily and seasonal variations of power flow. This resulted in the possibility of considerable voltage rise during low load periods because of excessive reactive power generated by the overhead line capacitance. To manage this potential problem, controllable shunt compensation using magnetically controlled shunt reactors (MCSR) was developed to improve voltage control during normal operation and to increase the small disturbance and transient stability performance of the long-distance transmission systems.

This chapter describes the problem to be solved and provides information about the operation of the MCSR, the MCSRs in service at the time of writing, and the description of a typical MCSR installation.

2 The Need for Reactive Power Control

As explained in the “AC System Characteristics” and the “AC Network Control Using Conventional Means” chapters in this Green Book, the conventional shunt reactor is one of the most important long-distance transmission system elements without which normal operation would face considerable technical difficulties (Bernard et al. 1996). Reactors are normally switched on to the transmission grid to avoid overvoltage operation of the power system under light loads. This improves the power factor and reduces the losses associated with large reactive power flows in the system. However, it is not economical to use a multitude of small switched reactors to optimize the system power factor, Therefore, larger reactors are used, which either overcompensate or undercompensate the power system, which results in larger reactive power flows than desirable and, consequently, results in higher than desired power system losses. That is, the power system operation is suboptimal. The reactors of course also have power losses (both no load and load losses when connected) of their own.

As described in the “AC Network Control Using Conventional Means” chapter, the main disadvantages of the conventional switched reactors are potential operational problems due to the lack of fast switching capability. The need to prevent over-voltages results in the reactors having to be in operation regardless of the transmitted power, which reduces the power transfer capability.

The installation of a controllable reactive power device with voltage control capability at an intermediate point of a long transmission line gives the advantage of line sectioning, increasing the transmission line’s power transfer capability. The reactive power consumed by a controllable reactive power device connected to the transmission line can be coordinated with the power flow through the line. The transfer capability is then only limited by the permissible current through the conductors (Belyaev and Smolovik 2003).

A controllable reactive power device can be used as an alternative to a synchronous condenser, or a Thyristor Controlled Series Capacitor (TCSC) (Gama 1999; Gerin-Lajoie et al. 1990).

Information about the alternative FACTS Controllers can be found in this Green Book in the chapters in the following Sections:

Section 2 – Technical Description of FACTS Controllers

Section 3 – Applications of FACTS Controllers

Development of Magnetically Controlled Shunt Reactors in Russia 3

In Russia, the MCSR is used for reactive power shunt compensation in EHV long-distance transmission lines. The MCSR performs the following functions:

  • Control of the line voltage without using circuit breakers in automatic switching systems

  • Decreasing power losses in networks by management of reactive power flows

  • Improving the operational reliability by reducing the number of transformer on-load tap-changer operations

  • Increasing the small signal stability margin

  • Improving power system damping

  • Minimizing the use of synchronous generators as controlled sources of reactive power

The MCSR has been applied at 110, 220, and 330 kV substations with ratings of 25, 100, and 180 Mvar, respectively, within and outside the former USSR power systems.

3 MCSR Operation Principles

The MCSR is a powerful three-phase extension of a magnetic amplifier with inverse-parallel connection of control windings as shown in the left part of Fig. 1. The MCSR has a steel magnetic core and two windings. One of the windings, namely, the power winding is connected to the electrical grid (UHV), while the second winding, the control winding, is connected to a dc voltage source of controlled magnitude (Uc). The power and control windings are inverse-parallel connected and the two cores do not have direct electromagnetic coupling.

Fig. 1
figure 1

Principle diagram of a magnetically controlled electrical reactor phase and typical plots of voltages and currents of a controllable reactor phase (UHV, IHV – voltage and current of the power grid; Uc, Ic – control voltage and current)

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The control winding is fed by a power electronics controlled rectifier providing variable dc magnetization current. In terms of small disturbance stability, the MCSR equivalent time constant Tp is about 3–4 s. However, the time constant can be decreased in special cases by application of magnetic field forcing for a short period down to Tp = 0.1 s.

Figure 1 illustrates three operating levels, i.e., idling (I), rated load (II), and rated overload (III). To increase or decrease the phase current, the dc control voltage UC is changed as shown in the blue line in the third graph, which also shows the system voltage. The controlled current increases as the iron saturates, as can be seen in the second graph. When the desired change in the phase current has been achieved, the control voltage is returned to a very low value, sufficient to maintain the controlled current amplitude at the chosen level. The controlled current remains at the same value as long as UC remains constant. The magnetic fluxes can be seen in the fourth part of Fig. 1.

Each of the phase windings creates its own magnetic flux: the power winding creates an alternating flux of the fundamental frequency, while the control winding produces a constant biasing magnetization flux of controllable magnitude. The constant biasing flux biases the alternating flux to the area of saturation of the magnetization curve, which results in the change of the inductance of the device. Please see the Controllers Using the Saturation of Iron for AC Network Control in this Green Book which provides more details on the saturation characteristics of iron.

The plots of voltage and current variation characterizing this process are shown on the right hand side of Fig. 1. When the terminals of the power winding are connected to the electrical grid, and there is no energy stored in the control loop (Uc, Ic), alternating fluxes of equal value and direction are produced in the split core. The fluxes do not exceed the saturation flux in any section of the magnetic core, and the current in the power winding is practically equal to zero (IHV ≈ 0). That operating condition is the no-load conditions. The graph of the current and voltage variations in this case is presented in Fig. 1 for time interval I.

When energy is applied to or removed from the control loop (Uc Ic > 0 or Uc Ic < 0), the transient process of the increase or decrease of power grid current IHV and control current Ic takes place (time intervals I – II, III – I). The average power of the control loop is about 5% of the rated capacity of the controllable reactor in order to achieve the transition from one stationary mode to another in about two periods of the system voltage frequency.

However, this is necessary only during the transition. In any steady state mode, for instance, in the semiperiodic (nominal) mode or the full-period (maximum) one, the power consumed by the control loop reduces sharply, since it is necessary only to compensate the ohmic losses in the control winding and this power is less than 1% of the rated power.

Advantages of the MCSR compared with an SVC using a coupling transformer:

  • Relatively low cost (approximately 150–200% of a conventional transformer of the same rated power).

  • Small footprint (105% of conventional transformer of the same rated power).

  • Grid connection without additional transformer when line connected.

Drawbacks:

A relatively large time constant (0.1 s) causing a slow response compared to an SVC.

3.1 Mathematical Model

Figure 2 shows the diagram of a magnetically controlled reactor and a possible electric equivalent. The diagram is explained below.

Fig. 2
figure 2

Diagram of a magnetically controlled reactor (left) and a possible electric equivalent (right)

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  • In the equivalent circuit, Lnet. and Lcon. are the inductances of the power and control windings, respectively, with the magnetic system completely saturated;

  • α is the firing angle of the thyristors corresponding to the time interval for which the core is saturated during the half-period of the system voltage, expressed in electrical degrees.

  • The complete range of possible operating modes corresponds to the range of α variations from 0 to π. For example,

    • The firing angle of thyristors of α = 0 corresponds to the no-load conditions of the reactor operation.

    • The angle of α = π/2 corresponds to the mode of semiperiodic saturation (nominal operating conditions).

    • The angle of α = π is the mode of maximum current consumption or full-period saturation.

The equivalent functional scheme is not only a representation that allows the technical performance of a controllable electrical reactor in the power system to be described using the combination of well-known devices. It also reflects the economic potential of controllable reactors. The reactor is equivalent to a transformer which has double-wound windings of comparable capacity and voltage in terms of losses and material consumption. At the same time, the functionality of the reactor corresponds to the widely used thyristor controlled reactor (SVC) connected to the high-voltage grid through a coupling transformer. Thereby, rather than combining a coupling transformer with a reactor and a thyristor switch connected in-series (an SVC), we have only one transformer-type device, in which the inductances of the windings perform the function of a reactor and the controlled saturation of the core acts as the inverse-parallel thyristor pair in the SVC. Thus, instead of three power elements there is one, the cost of which is comparable with the three aforementioned.

The voltage and current of a MCSR is shown in Fig. 3.

Fig. 3
figure 3

Typical plots of voltages and currents of a controllable reactor phase

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The plots presented in Fig. 3 have been obtained by calculations performed in accordance with the circuit diagram of Fig. 2 (left side) using computer software. These graphs can also be reproduced with high accuracy using the equivalent functional scheme in Fig. 2 (right), in which a phase of the controllable reactor is presented as an inverse-parallel thyristor pair with linear inductances connected in-series. In Fig. 3, Vnet is the voltage of power grid and Inet is the reactor current. Correspondingly Vcon and Icon are the voltage and current of the control winding.

3.2 Higher Harmonics Suppression

The design of the magnetic system of MCSR is performed so that the operation with the rated absorption of reactive power is close to so-called half-cycle saturation mode (when the resulting induction of each of the cores is more than the saturation induction of the steel during half of the period), as in this mode, half-cores will be alternately saturated (each for half of the period of the frequency) and hence the current of the MCSR in this operating mode does not contain harmonics (Bryantsev 2010; Dmitriev et al. 2013). Figure 4 presents the current of the power winding and its harmonic composition for the half-cycle mode of reactor operation.

Fig. 4
figure 4

Current in the power winding of the reactor in half-cycle mode (180 MVA, 500 kV reactor, rated current 207 A)

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Intermediate operating conditions of reactive power consumption between no-load and the half-cycle saturation conditions are considered below in Fig. 5.

Fig. 5
figure 5

Current in the power winding of the reactor for the consumption of 40% of rated power

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The power of the reactor is controlled by varying the direct component of magnetic induction in the half-cores by changing the current in the control winding. Consequently, it is necessary to reduce current in the control winding in order to cause the reactor to absorb less than the rated power. As the magnitude of current in the control winding is reduced, the direct component of magnetic induction decreases. The decrease in the direct component of induction will result in a reduction of the part of the period for which each of the half-cores is in the saturated state. Correspondingly, the saturated states of each half-core will alternate with the periods within which they are both not saturated. Therefore, the current in the power winding of the reactor will decrease, and the waveform of the current will be distorted by higher harmonic components.

In Fig. 5, the plot of the power winding current and its harmonic composition are presented for the mode of 40% of rated power consumption. It is evident that the current curve is distorted to a considerable degree. According to Fig. 5, the odd harmonics from the third to the ninth are clearly represented. The total distortion current constitutes 42.3% of the peak value of the first harmonic current, but it makes up 12.8% or 0.13 p.u. with respect to the rated current.

The maximum of the third harmonics corresponds to the power winding current of 80 A (about 40% of the rated power). At that current, the effective value of the third harmonic current totals about 25 A or 12.6% of the rated current of the reactor.

It is obvious that the distortion in the waveform of the power winding current is caused mainly by the third harmonic component. As a rule, in order to compensate the third and other odd triplen harmonics, the design solution is to connect a special (compensation) winding of the reactor with delta connection. The compensation winding (CW) serves two main functions:

  • It reduces the triplen harmonic components;

  • It serves as a supply secondary winding of the reactor, to which converters providing the magnetic biasing of the reactor’s magnetic conductor are connected along with filtering and compensating units (FCU), if they are required.

The influence of the compensation winding on the harmonic composition of current in the power winding can be seen from comparison of Fig. 6a and Fig. 6b, by example of the 40% of rated power consumption, in which the third harmonic component in the power winding current was maximum.

Fig. 6
figure 6

Currents in the power winding and its harmonic composition in the condition of 40% of the rated power consumption. (a) CW is open; (b) CW is closed

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Without the compensation winding, the resulting distortion current constituted 0.13 p.u. (12.8% with respect to the rated current of the reactor), while the presence of the delta-connected winding causes this parameter to decrease to 0.04 p.u. with full compensation of the third and ninth harmonic components. It should be mentioned that the use of only two low-capacity FCUs adjusted to compensate the fifth and seventh harmonic components permits eliminating the distortion of the power winding current almost completely in some operating condition.

The MCSR rated mode is close to the half-cycle saturation condition, in which there is no distortion of the power winding current. In the compensation winding, only odd triplen harmonic currents are prevented. The largest is the third harmonic. It is evident that under the rated conditions, the current in the compensation winding will be low because of the absence of distortion, while the maximum of the compensation winding current takes place when the device carries about 50% of the rated load.

3.3 A Model for Stability Study

The concept of defining a magnetically controlled reactor as a transformer-type device performing the equivalent functions of a semiconductor device was developed as the base of all the developments carried out during the last 10 years and allowed the existing developments both in the area of transformer-building industry and power electronics to be used.

Generally, MCSR control law for power system stability investigations can be expressed as follows:

$$ \left(1+{pT}_R\right){b}_R={b}_{R0}+\left({K}_{0u}+\frac{K_{1u}s}{1+{sT}_{1u}}\right)\Delta {V}_R, $$

Where:

  • bR, bR0 are actual and initial (in previous stable operation) MCSR conductivity, respectively.

  • K0u, K1u are the terminal voltage deviation ΔUp and its derivative control gains.

  • TR is the equivalent time constant of the MCSR control system.

  • T1u is the voltage derivative control loop’s time constant.

4 Magnetically Controlled Shunt Reactor Operation Experience in 110–500 kV Power Grids

Controllable shunt reactors have proved efficient in increasing the reliability of the Unified Power System (UPS) grid of Russia due to its capability to normalize the operating conditions for the transmission lines and power generators (Belyaev et al. 2016; Bryantsev 2010). Operation of long transmission lines of high and extra-high voltage classes showed that for the full utilization of the flow capacity, it is required to control the line reactor’s absorption of reactive power depending on the actual power transmission. The most vivid example was the reduction of the natural power capacity of the 1150 kV overhead line “Ekibastuz-Kokshetau-Kostanai-Chelyabinsk” by more than 50%, due to use of unregulated shunt reactors for reactive power compensation when putting the line in test operation in 1984. Today, a bias-controlled shunt reactor using the extreme saturation of the magnetic circuit sections has become the most widespread option (Bryantsev et al. 2006; Belyaev et al. 2016).

4.1 Overview of the MCSRs in Operation

MCSR implementation began in 1997, when a pilot MCSR prototype of RTU-25000/110-U1 version, as described below, was produced. In 1998, the reactor passed comprehensive tests and subsequently entered trial operation at the VEI STC test site in Togliatti. Afterwards, the reactor was sent to the Northern Electric Networks, Permenergo (Russian Federation), and was installed at the 110 kV substation “Kudymkar.” In September 1999, it was put into operation, together with the existing static shunt capacitors (SSC) which has a capacity of 52 Mvar. It was the first successful experience of the MCSR in commercial operation.

The MCSR (110 kV 25,000 kVA) installed at substation “Kudymkar” Permenergo has been in operation for more than 19 years already (Bryantsev et al. 2006). In fact, a controllable reactive power source (RPS) was implemented, featuring a parallel connection of MCSR and the capacitor bank to provide a smooth regulation of reactive power both in the mode of absorption (within the rated power of the reactor) and in the mode of generation (within the rated power of the capacitors).

To date in Russia and in some other countries (Kazakhstan, Belarus, Lithuania, and Angola), a large number of controlled shunt reactors with a total capacity of more than 8000 Mvar (Fig. 7 and Table 1) have been commissioned. The majority of the MCSR, with a total capacity of more than 6200 Mvar, are installed in Russia.

Fig. 7
figure 7

Total Capacity of MCSR produced, January 2014

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Table 1 MCSR characteristics of different voltage classes
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At the time of writing (2019), none of the equipment listed in Fig. 7 had reported any failures, and the first MCSR has already been in service more than 19 years.

4.2 Benefits of the MCSRs

The 330 kV switch yard of the Ignalina nuclear power plant (NPP, Lithuania) is a major distribution node of the Lithuanian high-voltage power grid, which is part of the Baltic Unified Power System (UPS). Six 330 kV overhead lines (one of which is dimensioned to 750 kV requirements) are connected to switchyard buses, to connect with the power systems of Lithuania, Latvia, and Belarus. The power network around Ignalina is shown in Fig. 8.

Fig. 8
figure 8

Electrical network 330 kV of Baltic republics

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The 750 kV power transmission line (thick black line in the figure) operates at a voltage of 330 kV. The capacitive charging capacity of the adjacent power lines connected to the Ignalina substation is about 280 Mvar

Maintaining acceptable voltage and its stabilization in the nodal points of the power system are critical for ensuring the operational reliability of the equipment. Until 2008, voltage regulation in the 330-kV grid caused some difficulties because of the limited choice of control facilities. Excessive reactive power generated by power lines in the Ignalina area (up to 400 Mvar) made it necessary to limit the voltage levels during the summer and daily minimum. To control the reactive power and voltage at the Ignalina substation, two NPP turbine generators were operated in an under-excited mode consuming up to 280 Mvar. The absorption of reactive power by generators was limited to ensure acceptable power system stability conditions and usually did not exceed 150 Mvar.

In accordance with international agreements, one of the conditions of entry of Lithuania into the European Community was to close the Ignalina NPP, followed by the possible construction of several new units on the site. Thus at least for 10–15 years, the 330 kV switch yard would be without control facilities for the reactive power generated by the above transmission lines at minimum loads, leading to unacceptable rise of operating voltages. Therefore, in accordance with research findings, it was recommended to install a MCSR at the 330 kV busbars at Ignalina substation. The MCSR was installed in August 2008.

4.3 Voltage Control

The primary purpose of the MCSR and the MCSR-based reactive power sources (RPS) is voltage stabilization, reactive power distribution optimization, and reduction of losses in the high voltage grid. At the same time, the problem of potentially increasing power oscillations and decreasing dynamic stability of the power system can be solved.

The accumulated operating experience and research-based recommendations show three feasible options for the installation of the MCSR and the MCSR-based RPS in power systems:

  • As part of extended intersystem transmission lines of 330, 500 kV;

  • At substation (power plant) busbars with a large number of outgoing power transmission lines or lines transmitting power through a very long overhead line

  • In autonomous power systems (or power systems located remotely from high-capacity sources) with a load requiring high voltage quality

It should be noted that most of the MCSR-based RPS are installed in 110 kV grids of the oil and gas producing systems for voltage stabilization, facilitation of motor start operation, and removal of reactive power flows in the grids.

To confirm the need for implementation of the MCSR in the extra-high voltages grids, the operating characteristics of several substations in the 500 kV Central Russia Intersystem Power Grid (IPG) were collected. These showed significant deviation of the voltage levels from the nominal value as shown in Table 2. Table 2 provides information on the range of voltages and reactive power flow to the power facilities located in the territories served by the IPG Center, based on measurements performed in 2013. The ΔUnom column shows the deviation from the nominal operating voltage (in absolute units) at different load conditions. The QΣ column shows the total reactive power flowing to the node (or away from it) of all adjacent power lines in the considered nodes.

Table 2 Nodes in the 500 kV network IPG Center with deviating voltage levels
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This analysis provides important information enabling recommendations for installing MCSR (or MCSR-based RPS) to stabilize the voltage, to prevent excessive reactive power flows in the adjacent grids, and to reduce losses.

Figure 9 shows the deviation of the operating voltage at the nodes from the nominal voltage. Each location marked on the x-axis provides information for four different operating modes, using four color columns, showing the variation of the voltage in different load conditions. The load conditions are winter-max, winter-min, summer-max, and summer-min, with the colors defined in the chart. The hollow bars indicate the absolute value of the reactive power that flows into the node in the corresponding load mode.

Fig. 9
figure 9

Facilities in the 500 kV grid of IPG Center with deviating voltage levels

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This information highlights the relevance of extended implementation of controlled shunt compensation devices in the high-voltage grids of Russia and other countries with well-developed transmission system with a high content of long ac lines.

Figure 10 shows an example of the successful application of a 180 Mvar MCSR on a 500 kV power transmission line installed at the Agadyr substation at the “North-South” transit system of the Republic of Kazakhstan. Figure 11 shows the voltage change at the 500 kV busbar before the commissioning of MCSR. Figure 12 shows the voltage with the MCSR in operation, which demonstrates much smaller daily voltage variations.

Fig. 10
figure 10

North South 500 kV transmission system of the Republic of Kazakhstan

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Fig. 11
figure 11

The chart of voltage change at the 500 kV Agadyr substation before MCSR commissioning

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Fig. 12
figure 12

The chart of voltage change at the 500 kV Agadyr substation after commissioning of the 500 kV, 180 Mvar, MCSR

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After the commissioning of the MCSR, the measurements of the voltage in the period of about 2 weeks, the voltage almost fits into the range of 510–520 kV.

4.4 Power System Damping

Based on the measurement of transients in the large 500 kV power system, and the impact the installed MCSR parameters and their settings discussed in detail in Belyaev et al. (2016), it has been shown that the damping properties of power systems depends mainly on the setting of automatic voltage (excitation) regulators (AVR) of physical and equivalent generators. As a rule, it appeared that the change of the time constant (Tcsr) of the MCSR (using continuous MCSR control law on voltage deviation) within a wide range has little effect on damping performance. Therefore, it was concluded that a fast response of the MCSR for system issues is not required.

Table 3 shows the results of the eigenvalue calculations for the model of a simple transmission system with a long transmission line when a MCSR is installed on power plant high voltage buses. It was assumed that the power generators operate at two different power factors (cos (φ) = 0.992, mode 1, and cos (φ) = 0.9, mode 2).

Table 3 Results of eigenvalue calculations
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The real root shown in Table 3, in the second mode, is larger in absolute value, which illustrates the effect of the conditions of steady-state operation (large value of EMF generator and a smaller transmission angle). A pair of complex roots shows that the parameters of MCSR insignificantly affect the dynamic stability performance – but by increasing the time constant of the reactor, the damping rate is improved. The determining factor is the availability of automatic excitation controls with stabilization of the generator (voltage frequency deviation and voltage frequency derivative).

Dmitriev et al. (2013) shows that the losses in the rotor and stator circuits of power generators in case of power factor (cos (φ)) close to unity is much smaller compared to the operation at nominal power factor. According to Dmitriev et al. (2013), for an electric power plant of 2000 MW the potential savings amount to 30 million rubles ($ one million) per year.

5 Tavricheskaya MCSR, Siberia

Figure 13 shows the MCSR at the Tavricheskaya substation in Siberia.

Fig. 13
figure 13

MCSR at Substation Tavricheskaya, 2005

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The general specifications of the controllable reactor RTU-180000/500 (put into operation in 2005 at the 500 kV substation “Tavricheskaya” in Siberia) as confirmed by factory and field tests is shown in Table 4.

Table 4 The general specifications of the controllable reactor RTU-180000
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