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This article presents a comprehensive review for the dilemma of reactive power flow, while addressing different proposed remedy strategies: conventional and most update solutions. Robust analytical expressions were utilized to exploit the functionality of the proposed solutions and to show clearly the relation between the reactive power and control variables. The article, moreover, proposes a simple, innovative and robust analysis for static performance of the STATCOM. This approach shows clearly the advantages of the STA-TCOM in regulating reactive power and maintaining load voltage level within presumable limits. The approach, furthermore, reveals explicitly via analytical expressions the impact of the STATCOM operation on different aspects of the power system under concern.

Reactive power flow is a salient feature of an electrical power system. Reactive power is mandatory for the operation of different load types either static or rotating [

Reactive power flow, however, has a number of undesirable consequences. It increases the drawn current for the same load level, which in turn increases the losses, maintenance and cost of the power system operation. Moreover, it reduces the power stability margin. It under heavy level of reactive power probably results in voltage instability [

Different reactive power compensating strategies are reported in the literature. These schemes vary from simple techniques such as fixed capacitor/inductor to recent approaches such as Static Synchronous Compensator (STATCOM), Static Series Synchronous Compensator (SSSC) and Unified Power flow controller (UPFC) [

The compensating techniques either conventional or recent suffer from limitations. For example, the reactive output of SVC topologies is proportional to the square of the voltage magnitude. Therefore, the SVC provided reactive power decrease rapidly as voltage decreases, which reduces its stability. STATCOM typically exhibit higher losses and may be more expensive than SVC. Moreover, STATCOM operates under balanced operating conditions; thus its performance is deteriorated under abnormal operating conditions. The UPFC, also as STA- TCOM, could not operate properly under disturbance conditions [

In this article, comprehensive analysis for the phenomenon of reactive power is provided. The different indices for the reactive power are highlighted. Then, examples of reactive power compensating scheme are addressed regarding operational constraints and performance characteristics. A simple and robust analysis for the static performance of the STATCOM is advised. Simple and clear analytical expressions are advised to expose explicitly the relation between the STATCOM and different aspects of power system operation. Finally, the conclusion highlights the main points in the dilemma of reactive power.

This article has a number of objectives:

1) Providing a comprehensive highlighting for conventional and recent techniques for compensating reactive power.

2) Analyzing the static/dynamic performance of different reactive power com- pensator as a simple straightforward approach.

3) Proposing simple and innovative approach for the static performance of the STATCOM, which illustrates via analytical expression the increase in load voltage and the reduction in transmission line losses due to STATCOM operation.

As mentioned before that reactive power is elementary for some load to function; therefore the majority of loads are functioning at lag power factor which is less unity.

Reactive power flow, as mentioned, could result in uneconomic operation of the power system, as it increases the current magnitude and hence copper and reactive losses in the transmission and distribution networks. This is shown

Different techniques are emerged to remedy the problem of reactive power circulation. These approaches range from simple method to most-update approaches. Each approach enjoys merits and suffers from drawbacks. In the following, a concise highlight for these methods is addressed.

A simple compensation technique is inserting fixed capacitor/inductor [

where Q is either inductive or capacitive depending on X. The capacitive reactance powers produced via fixed capacitor elements are shown in

These techniques suffer from serious limitations, such as:

Fixed compensation, which limits their compensation ability;

Inability to respond for load requirements; these techniques fail to provide instantaneous response for load reactive power requirements;

Resonances, the fixed compensator capacitor/inductor values could resonate with transmission line parameters.

SVC gains more advantages than rotating reactive power compensators and fixed reactive elements, such as:

1) High efficiency;

2) Reduced volumetric dimension;

3) Relatively fast responses compared with synchronous condenser;

4) Adaptive operation, SVC could provide continuous compensation compared with fixed reactive elements. SVC could act as source/sink for the reactive power according to the control strategy;

5) Automation, as SVC are mainly solid-state devices, they could be controlled via intelligent systems as microprocessor and microcontroller.

It is worth to mention that SVC suffers from elevated harmonic content in the output voltage and current. This would produce additional problems to the power system.

SVC for shunt operation is composed from two main topologies. These are TCR and TSC, as shown in Figures 5-7. TCR and TSC could act independently generating inductive/capacitive reactive power, or they run as one device producing variable reactive power. In SVC, the reactive passive elements are interfaced to the common coupling point via self-commutated sold-state devices. TCR consists from inductance interfaced via pair of anti-parallel thyristors. The firing angles of the thyristors are controlled in continuous manner. For TCR, there are two modes of conduction: discontinuous current and continuous current. The inductor current is continuous if the firing angle α of the thyristor is greater than π/2 and less than π. For more details, see references [

The reactive power produced by TCR is given by,

_{pu} =1.0 the produced reactive power is 4 pu. A small inductor is sufficient to produce the required reactive power. The range of the firing angles of the thyristor is limited to be 90˚ - 180˚. This is to ensure continuous inductor current conduction.

TSC similarly to TCR is composed from capacitor interfaced to the common coupling point via pair of anti-parallel thyristors (

Here the thyristors are controlled such that current through the thyristor I_{TCR} is function in the thyristor firing angle, α. The RMS value of the current could be obtained by,

Thus, the reactive power could be calculated by,

For more details, see references [

The thyristor RMS current and the capative reactive power are function in thyristor firing angle, α. The reactive powers as a function in thyristor firing angle and the capacitance are shown in

TCR and TSC are usually merged in topology themed SVC (

The characteristics of SVC are shown in

STATCOM is the recent configuration of SVC; it remedies the limitations in the traditional SVC. STATCOM generally utilizes high sufficient frequencies such that STATCOM produces high quality output voltage and current. The main objective of the STATCOM is to produce instantaneous +/- reactive power according to the load requirement and the operating point.

STATCOM consists of mainly a Voltage Source Converter (VSC) attached to large DC capacitor. Fully controlled solid-state devices are used to implement VSC. The basic objective of a VSC is to produce a sinusoidal ac voltage with minimal harmonic distortion from the DC voltage. The DC voltage across the DC capacitor (CDC) of the STATCOM is controlled to be constant. The DC capacitor has the function of establishing an energy balance between the input and output during the dynamic performance of the STATCOM. The size of the capacitor is primarily determined by the ripple input current encountered with the particular converter design [

Static performance of the STATCOM could be obtained from the equivalent circuit shown in _{L} is taken as a reference (

The supply voltage amplitude IV_{s}I = 1.0. The supply angle δ is given by

where P_{L} is load power and the angle θ is given by

where c_{1} and c_{2} are given respectively by,

The load voltage is given by,

The STATCOM modeled current source

The graph of load voltage V_{L} is shown in

The influence of the STATCOM on the transmission line losses are shown in

It is obvious in

The reactive power delivered by STATCOM under different load and power factor levels is illustrated in

A comprehensive overview for the problem of reactive power and different compensating techniques are given in this article. The reactive power is mandatory for operating a power system as different types of loads require reactive power to function. These loads include rotating and staticas induction motor and light loads. However, the circulation of the reactive power influences the operation of different power system components and subsystems such as transmission lines and cables. Moreover, significant voltage drop is produced due to the flow reactive power, which could result in voltage instability.

Different compensation techniques are developed and adopted in the power system. The fixed reactive passive elements produce reactive power that is voltage dependent. Moreover, resonances could be produced due to the interaction of reactive elements with the transmission lines and power system reactances.

Conventional SVC configurations as TCR and TSC emit significant harmonics into common coupling point.

STATCOM enjoys the advantages of fast response, robustness and reliability. STATCOM produces continuous reactive power according to the loading conditions.

The authors are grateful for Shaqra University and College of Engineering for supporting and encouraging this work in particular Prof. Dr. Abdulaziz S. Alsayyari, dean of Engineering College.

AbdElhafez, A.A., Alruways, S.H., Alsaif, Y.A., Althobaiti, M.F., AlOtaibi, A.B. and Alotaibi, N.A. (2017) Reactive Power Problem and Solutions: An Overview. Journal of Power and Energy Engineering, 5, 40-54. https://doi.org/10.4236/jpee.2017.55004