THE METHODS OF REACTIVE POWER COMPENSATION IN THE 25 kV , 50 Hz CONTACT NETWORK Summary

The article analyses theoretical and practical solutions of reactive power compensation in the power grid of 25 kV, 50 Hz of the catenary. The article presents theoretical and practical studies (by providing circuits for connecting capacitor banks, structural circuits diagrams for the automatic control reactive power level, and calculating the parameters of capacitor banks) for the compensation of reactive power.


INTRODUCTION
It is only lately that modern solutions have been applied in the area of reactive power compensation to not only reduce the costs for the use of reactive power but also to improve the quality of the energy system.By compensating the reactive power in the power grid and filtering the harmonics of undesirable currents, higher quality of electric power in terms of voltage is ensured, and the losses of voltage and power are reduced [1,2].The aim of load compensation is to boost the power factor of the system, to support the real power drawn from the system, to regulate the voltage, and to remove current harmonics [3].Reactive power compensation is one of the most effective ways to reduce consumed electric energy and improve power quality.The examples of how reactive power compensation can improve the technical-and-economic indexes of an industrial power grid are as follows: [20].
• reduce cost and generate higher revenue for the customer, • reduce network losses, • avoid penalty charges from utilities for excessive consumption of reactive power, • increase system capacity and save costs on new installations, • improve system power factor, • increase power availability, and • improve voltage regulation in the network.Nowadays, static VAR compensators (SVCs) and static synchronous compensators (STATCOMs) are the most useful devices to control the dynamic reactive power level in the industrial grid [21].This article presents some methods, showing the potential use of dynamic reactive power compensation via the series compensation devices SCB (capacitors banks) located in 25 kV AC catenary the arms of A, B phases and in the line of return current (see Fig. 6a).Having analysed the supply system of alternating current 25 kV AC catenary, the author proposes to locate the series compensation devices SCB (capacitors banks) in the arms of A, B phases and in the line of return current (see Fig. 6a) and methods for dynamic reactive power value automatic control.

COMPENSATION OF REACTIVE POWER BY USING SYCHRONOUS MACHINES
Regulation of reactive power of synchronous generator.The reactive power of synchronous generator is regulated by changing its exciting current [4].Let us say that a synchronous generator is operating connected to the power system, where the capacity of the turbines rotating it is constant, the active power of the generator is P = 3UIcosφ = const.By changing the exciting current of the synchronous generator I f , its EMF is changing E f = C E Ф f n, hence the maximum generator active capacity P max = 3UIE f/ /X is changing.Fig. 1a illustrates three angular characteristics of the generator.The phasor diagrams of the generator armature winding for these three cases have been provided in Fig. 1 c, d, and e.The voltage of the synchronous generator connected to the energy system remains constant by changing the exciting current."U" curves for a synchronous machine (a) and a phasor diagram of a synchronous compensator armature winding (b) are provided in Fig. 2. The variations of I with excitation are known as "U" curves because of their shape.For the same mechanical load, the armature current varies with field excitation over a wide range and therefore causes the power factor also to vary respectively.
Main designation of the diagram in Fig. 1, 2: I f -exciting current; I -load current; U -voltage of power supply; Θ 1 -the synchronous machine angle between U and E f vectors; φ 2 -the angle between I and U vectors; and X -reactance.When over-excited, the motor runs with leading power factor (and supplies vars to the grid) and when under-excited with a lagging power factor (and absorbs VARs from the grid).
In between, the power factor is unity (see Fig. 2).The minimum armature current corresponds to the point of unity power factor (voltage and current in phase).Regulation of reactive power of synchronous condenser.In electrical engineering, a synchronous condenser (sometimes called a synchronous capacitor or synchronous compensator) is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely.Its purpose is not to convert electric power to mechanical power or vice versa but to adjust conditions on the electric power transmission grid.Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage or to improve the power factor.However, synchronous machines have higher energy losses than static capacitor banks [5].Most synchronous condensers connected to electrical grids are rated between 20 MVAR (megavars) and 200 MVAR, and many are hydrogen cooled.There is no explosion hazard as long as the hydrogen concentration is maintained above 70%, typically above 91% [6].

COMPENSATION OF REACTIVE POWER BY USING CAPACITORS
Capacitive compensation.Depending on the method of connecting capacitors with regard to loading (in parallel or in series), shunt or series capacitive compensation is singled out [7].A typical reactive power compensation scheme is used for industrial power system (Fig. 3a).The author of the proposed reactive power compensation scheme for 25 kV, 50 Hz traction system (Fig. 1b) examples of connecting reactive power capacitor banks, phasor diagrams where cosφ 1 = 0.85 (before compensation) and cosφ 2 = 0.97 (after are provided in Fig. 3c.Possible locations of the capacitor bank offered by company ABB have been provided in Fig. 3a).The location is primarily determined by the reason for compensation -A: Direct Compensation.B: Group Compensation.C: Central Compensation at low voltage (LV) side.D: Central Compensation at high voltage (HV) side [8].The power triangle of an installation running at low cost and where the load of the transformer is close to full.The power triangle of the same installation where power factor correction has been applied reduces load on the transformer / releases capacity for additional loads [9].
The technical solutions of company ABB reactive power compensation and the expected financial results have been provided in Fig. 4.
Fig. 4 illustrates phasor diagrams where the power factor cosφ 1, with active power of 60 MW, is increased from 0.85 to 0.97.Calculations of the capacitor bank reactive power and other related technical and financial calculations are provided below.Having analysed the supply system of alternating current 25 kV AC catenary, the author proposed to locate the series compensation devices SCB (capacitors banks) in the arms of A, B phases and in the line of return current (see Fig. 6a).A principal scheme of a reactive power series compensation (a), an equivalent scheme and a diagram of potentials (b), where SCB is located in a single arm, is provided in Fig. 6b).By connecting the capacitor devices to the circuit, the power factor соs φ 2 of the series compensation devices SCB may be increased from 0.88 to 0.98 (see Fig. 6).

AUTOMATIC CONTROL OF THE PARAMETERS OF THE COMPENSATION EQUIPMENT
The schemes of compensation of reactive power equipment connection and automatic control are provided in Fig. 7.For the dynamic reactive power value control of the compensation equipment parameter (C value of the capacity bank), the author proposes to connect an electronic device made of thyristors connected in series and inverse-parallel thyristor valves.By sending respective opening signals to the thyristor gates, the value of the reactive power consumed (in the straightforward direction) and the value of the compensatory reactive power (in the inverse direction) may be controlled [11].The author's proposed use of simplified automatic control schemes of the series compensation devices SCB (capacitors banks), reactor connection, and thyristor connected reactive power compensation (TSC) (capacitive and inductive) have been provided in Fig. 7 [19].In the middle part of the schemes (see Fig. 7), a phasor diagram of the reactive power compensation is provided, where the capacitor bank and the reactor are connected consistently.Control system designing with a reactive control loop was synthesized on the basis of the system -the supply system of alternating current 25 kV AC catenary shown in Fig. 7. To investigate a reactive power compensation mode, it is necessary to perform synthesis of the new control system and control system of the electronic device made of thyristors connected in series and inverse-parallel thyristor valves.Control system is two-loop and consists of two-channel current control systems reactive power control loop and a load control system.It allows for a separate regulation of the active i d and reactive i q current components.Operating conditions of the main circuit diagram have been provided in Fig. 7 by using current sensor CT and control blocks signals.Examples of FACTS for series compensation (schematic).Thyristor-controlled series capacitor (TCSC): a series capacitor bank is shunted by a thyristor-controlled reactor [12].Thyristor-controlled series reactor (TCSR): a series reactor bank is shunted by a thyristor-controlled reactor.Thyristorswitched series capacitor (TSSC): a series capacitor bank is shunted by a thyristor-switched reactor.Thyristor-switched series reactor (TSSR): a series reactor bank is shunted by a thyristor-switched reactor.
Static reactive power compensators.Prior to the invention of the static reactive power compensators, power factor compensation was used to preserve large rotating machines such as synchronous condensers or switched capacitor banks.[13].The thyristor-controlled reactor (TCR) and thyristor-controlled capacitor (TSC) are a set of electrical devices for providing fast-operating reactive power on high-voltage electricity transmission networks [13,14].With the use of phasecontrol thyristors (see.Fig. 8), the thyristor-controlled reactor (TCR) may be variably switched into the circuit and so as to provide a continuously variable MVAR injection (or absorption) to the electrical network [15].In this electrical scheme configuration, coarse voltage control is provided by the capacitors.The thyristor-controlled reactor (TCR) and thyristor-controlled capacitor (TSC) are there to provide smooth control [15].Reactive power compensators should regulate the voltage, power factor and harmonics and stabilize the system [16].Circuit diagram of reactive power static compensator and the schemes of connection and control of reactive power series compensation devices SCB are provided in Fig. 8. Research results.The calculated 25 kV, 50 Hz alternating current catenary feeder system parameters before compensation and after compensation are provided in Table 1.

Table 1
The calculated 25 kV, 50 Hz alternating current catenary system parameters before compensation and after compensation

CONCLUSIONS
1.The new reactive power compensation method in the supply system of alternating current 25 kV AC catenary grid via the series compensation devices SCB (capacitors banks) located in 25 kV, 50 Hz catenary the arms of A, B phases and in the line of return current has been proposed.
The methods of reactive power compensation in… 67.
2. It can be implemented to correct a power factor or compensate static or dynamic reactive power.3. A level of compensated reactive power should be calculated in accordance with load impedance diagrams and parameters of the 25 kV grid side.4. In case of 25 kV, 50 Hz, the average power factor of contact network feeder system is cosφ = 0.8. 5. From the calculations it was established that to increase the power factor cosφ from 0.85 to 0.97 of 25 kV, 50 Hz contact network feeder system power cosφ in the traction substation, 7.4 MVAR reactive power capacitors have to be equipped.6.To increase 25 kV, 50 Hz contact network feeder capacity factor cosφ, synchronous capacitors or capacitor banks may be used.7. To compensate the reactive power of 25 kV, 50 Hz contact network feeder system, synchronous capacitors or capacitor banks may be used.8. Synchronous capacitors are electric machines, operating in a generator mode, and they are more suitable for symmetric electric energy supply systems.9.A 25 kV, 50 Hz contact network supply system is asymmetric, that is, the load of the phases is not equal, hence it is difficult to use synchronous capacitors in a contact network of 25 kV, 50 Hz for the compensation of reactive power.10.Upon installation in a 110/27.5/10kV traction substation 7.4 MVAR reactive power capacitor banks, and using the author's methods for dynamic reactive power value automatic control, cosφ 1 = 0.85 increased to cosφ 2 = 0.97.11.Static reactive power compensators are more expensive than mechanically switched capacitors.12. Static reactive power compensators are used to provide support for fast changes and the mechanically switched capacitors to provide steady-state VARs.13.Optimally designed compensation systems that take into account the requirements of the equipment and systems to be compensated reduce energy costs by complying with the contractually agreed power factor, enable the planned connection of consumers with a high level of load fluctuation, reduce losses in equipment in customer systems such as transformers and cables as well as in the transmission and distribution network of the network operator, stabilize public and industrial networks by reducing total harmonic distortion, and sustainably reduce CO2 emissions.

Fig. 1 .
Fig. 1.Angular characteristics of a synchronous generator (a), currents and electromotive force EMF vector hodographs (b) and armature winding current phasor diagrams (c, d, e), with different values of exciting current

Fig. 2 .
Fig. 2. "U" curves for a synchronous machine (a) and a phasor diagram of a synchronous compensator armature winding (b)

Fig. 6 :
1-electric traction rolling stock ETV; Х с -SCB capacity resistance in the feeder (in the arms of A and B phases); Х Ср -SCB capacity resistance in the line of return current; U 1 -voltage of power supply; U 2 -SCB voltage in the connection point; U 2с -voltage after connection of SCB; I 2 -load current, R s and X s -active and reactive circuit resistance from the power supply to the point of connecting SCB; Х c -resistance of capacity capacitors SCB; φ 2 -the angle between I 2 and U 2 , where SCB is disconnected (Х с = 0) and, where SCB is connected (Х с ≠ 0).a) b) Fig. 6.The author proposes a principal scheme of a reactive power series compensation (a) and an equivalent scheme and a potential diagram (b)

Fig. 8 .
Fig. 8. Circuit diagram of reactive power static compensator (a) and the schemes of connection and control of reactive power series compensation devices SCB (b): I A , I B , I C -reactive power static compensator phases currents; 1-6 -thyristors; 1'-6' -diodes; I d -rectified current; A, B, C -phases [17]

Fig. 9 .
Fig. 9.A diagram illustrating compensation of reactive power (a) and the schemes of capacitor bank (b)