THE REAKTIV POWER COMPENSATION IN NONLINEAR ELECTRICAL LOADS

ABSTRACT

The paper presents and discusses the problems associated with reactive power compensation of nonlinear  loads. Analytical results and simulation studies for selected high power drive used in mine when apply dynamic compensation  together with active harmonics filters are presented. It was  found that application of active power filters allows for  compensation high current harmonics in general, and  suppression of as reactive power flows as well as current  harmonics in the network. On the basis of results conclusions  on efficiency of the active on-line compensation of non-linear  loads of a high power are formulated. Adequate reactive power control  solves power quality problems like flat voltage profile maintenance at all power transmission levels, improvement in power factor, transmission efficiency and system stability.

АННОТАЦИЯ

В статье представлены и рассмотрены проблемы, связанные с компенсацией реактивной мощности нелинейных нагрузок. Представлены аналитические результаты и результаты моделирования для выбранного мощного привода, используемого в шахте, при применении динамической компенсации совместно с активными фильтрами гармоник. Установлено, что применение фильтров активной мощности позволяет компенсировать высокие гармоники тока в целом и подавлять потоки реактивной мощности, а также гармоники тока в сети. На основе результатов сформулированы выводы об эффективности активной оперативной компенсации нелинейных нагрузок большой мощности. Адекватное управление реактивной мощностью решает проблемы качества электроэнергии, такие как поддержание стабильного профиля напряжения на всех уровнях передачи электроэнергии, улучшение коэффициента мощности, эффективности передачи и стабильности системы.

Keywords: Reactive power, compensation, non-linear  load, Voltage Profile, Series compensation.

Ключевые слова: реактивная мощность, компенсация, нелинейная нагрузка, профиль напряжения, последовательная компенсация.

In industrial applications there are high power motor  drives which require speed control. In mining, they include mine host drives, the main ventilation and drainage engines, large air compressors and various pumps. As electric motors are used both DC motors, asynchronous and synchronous. In order to provide variable speed control there are used rectifier systems or systems of high power converters (in which rectifiers are also employed). For drives of a low rated power (up to about 300-400kW) uncontrolled rectifiers (diodes type) are commonly used, while for high power- the controlled rectifiers respectively. As a result voltage drops along impedances of as transmission, distribution and reception systems are also distorted. [1]. Reactive power management has become a most challenging task in power system operation and management. Some of the characteristics of the power systems and their loads can deteriorate the quality of  the supply. An increase in the consumption of reactive power in the network leads to a decrease in the voltage in the network. The voltage drop not only increases the power losses in the network, but also affects the electrical equipment that receives power from this network, especially the operation of electrical machines. It is known that the magnetic driving force and the main magnetic flux created in the motor windings are directly proportional to the voltage value. The requirement of the standard establishes that the change in mains voltage for electrical machines due to the effect of voltage changes on electrical equipment should not exceed ±5% of the total mains voltage. However, the voltage at the receiving end in the transmission end is allowed to vary between the range of  ±5-10% which further restricts the transfer of the reactive power. [2].

REACTIVE POWER COMPENSATION OF NON-LINEAR LOADS

In the case of a linear load RL type, reactive power compensation is performed based on the resolution of the current into two components

                                                                  (1)

where I - rms value of the load current; I_a - active current component (I_a=I⋅cosϕ); I_r – reactive current component (I_r=I⋅sinϕ).

As devices for compensation are used static battery banks  and/or overexcited synchronous motors (rotating compensators). For the non-linear loads current waveform indicates presence of numerous harmonics. Broad spectrum of the current harmonics may result in thermal damage of the capacitors, the insulation of power cables, transformers as well as electrical machines (i.e. capacitive reactance decreases with the increase in frequency therefore, an overload due to high harmonics is a key factor). Thus, for large non-linear loads as an unit to compensate for the negative impact on the network there are used the series connected  passive LC filters and capacitive banks together with protective reactors. The series LC filter of resonant frequency

                                                                     (2)

for given harmonics displays small value of equivalent reactance seen from the terminals of a high power non-linear load. Current (currents) flowing out of this load must be bridged by the filter LC (Fig. 1) without affecting the network itself.

Figure 1.  Equivalent electric circuit with non-linear load, I_hL – current of non-linear load; I_hF - current of passive filter; I_hs - current of supplying network.

In real systems one deals with distorted voltage. However, these deformations are relatively small and their level is determined by the applicable law (THD_U≤2%). In spite of a small voltage waveform deformation the high current harmonics flowing through batteries and passive filters can, in customer installation, reach significant values. Therefore, the only way to limit or to eliminate the high current harmonics is the use of appropriately selected active filters[3]. The main reactive power compensation devices include: synchronous generators, synchronous compensators, static reactive power compensators, capacitor banks. Synchronous generators produce electricity that is supplied to consumers. By influencing the excitations of generators, it is possible to change the generated voltage and thus regulate the transfer of reactive power to the network. Synchronous compensators are synchronous motors that operate without load and perform the function of generating reactive power in overexcited mode. Advances in power electronics made it possible to move from synchronous compensators to static reactive power co-balancing systems, such as the thyristor-switched capacitor bank and the thyristor-controlled reactor. A thyristor-switched capacitor bank allows stepwise regulation of the reactive power that the capacitors supply to the network (figure 2).

Figure 2. Prinsiple scheme of thyristor-switched capacitor bank

A thyristor-controlled reactor can smoothly regulate the reactive power consumed by the inductors. By connecting these devices, it is possible to smoothly regulate the generated and consumed reactive power (figure 3).

Figure 3. Prinsiple scheme of thyristor-controlled reactor

REACTIVE POWER COMPENSATION METHODS

The use of powerful electric motors in the production process leads to a high consumption of reactive power. The reactive power consumed by these motors is compensated in various ways. The reactive power consumed by electric motors is compensated in the following methods:

-Individual compensation;

-Group Compensation;

-Centralized Compensation;

-Combined compensation;

-Automatic compensation.

Individual compensation is achieved by connecting a precisely sized capacitor bank to the terminals of the load whose reactive power is to be compensated. Group compensation is designed to increase the compensation of a group of loads with similar functional characteristics by connecting a common capacitor bank. Centralized compensation is intended in installations with a large number of loads that do not operate simultaneously or are switched on only for a few hours a day. Combined compensation is a compromise between individual and centralized power compensation and takes advantage of both options. In most electrical installations, reactive power consumption is not constant. In such electrical installations, automatic power compensation systems are used. Automatic reactive power compensation systems include:

- current and voltage sensors;

-controller;

- electrical panel with switching and protection devices;

-capacitor batteries.

References:

1. J. Wosik, M. Kalus, A. Kozlowski, B. Miedzinski, M. Habrych “The Efficiency of Reactive Power Compensation of High Power Nonlinear Loads”, ELEKTRONIKA IR ELEKTROTECHNIKA, ISSN 1392-1215, VOL. 19, NO. 7, 2013 
2. M.Fursanov “Determination and analysis of electricity losses in electrical networks of power systems” Minsk – 2006, pp 25-39
3. X. Lin, H. Yang, P. Jun-Min, C. Chen, Y. Gang, Z. Lin-Dan, “Selective compensation strategies for 3-phase sequence cascaded multilevel active power filter using ANF-based sequence decoupling scheme”, Elektronika ir Elektrotechnika (Electronics and Electrical Engineering), no. 2, pp. 15–20, 2010