Title: ANALYSIS OF HARMONICS REDUCTION METHOD SELECTION FOR TRANSFORMER SUBSTATION

The article presents an investigation of the harmonic level and its possible reduction methods for one of the Michelin factory transformer substation. The substation electrical network consists of two transformers (1.6 MVA and 2.0 MVA) supplying a production line composed of several electrical equipment based on DC and AC motors. In order to investigate the harmonics level and its influence on the substation operation the measurements of the current and the load factor of the powered machines as well as the coefficients THDu, THDi, Du and Di have been performed using the 3-phase energy quality analyzer Fluke 435II and the network parameter recorder PEL103. Basing on the measurements data the four harmonics reduction methods (passive filters, active filters, 12-pulse rectifier and the Active Front End system) have been proposed and studied. For this purpose the substation electrical network has been modelled using Emerson Harmonics Estymator software. Furthermore, in order to choose the optimal solution the financial analysis of the potential investments has been performed.


Introduction
Higher harmonics of voltages and currents are among the main disturbances occurring in electric power systems. They appear in power grids due to the constantly increasing number of electrical equipment with nonlinear characteristics and at the same time due to the decreasing tendency of resistive devices share [1]. Even if a unit power of the device installed in the network is small like for example for lighting installations, the considering number of such devices can significantly decrease energy quality in the network by increasing the content of higher harmonics. Then low quality of the energy influences on other equipment present in the network [2]. For the last few decades the knowledge about the problems associated with harmonics has been improved significantly [3][4][5][6][7]. Nevertheless, it allows us only to reduce the threats rather than to completely eliminate them.
This article is focused on the optimal selection of the higher harmonic reduction method for the P50 transformer substation operating in the Michelin tire factory in Olsztyn, in terms of the cost-performance ratio. The harmonic tests in the P50 substation have been carried out due to the substation self-ignition and fire, which took place in 2016. However, it was not a single case concerning only Poland. This problem was also identified in other company factories. It was presumed and then proved that the direct cause of the substations fires had been related to the exceed of the acceptable harmonic levels described in the standards PN-EN 50160: 2010 and PN-EN 61000-4-30: 2011. In the described case the long term higher harmonic presence in the network damaged and decreased the capacitance of certain capacitors used for reactive power compensation what resulted in a sudden reactive power rise and an overheat of the installation. This hypothesis was also confirmed by the thermovision measurements which showed the increased current consumption flowing through the power contactors installed in the capacitor bank (Fig. 1). In order to improve the safety and to guarantee the substation reliability the level of higher harmonics present in the substation network was reduced significantly. For this purpose and in order to choose the best filtration method a number of measurements and tests have been carried out.
The efficiency of the filtration method has been evaluated by measuring certain coefficients like: the ratio of the n th harmonic to the fundamental harmonics for voltage and current (Du, Di) given by equations 1 and 2 as well as the quotient of the harmonic effective value to the effective value of the fundamental harmonic for voltage and current (THDu, THDi) given by equations 3 and 4 [8,9]. (1)

• 100%
(3) (1) • 100% (4) Practically, the upper limit of the summation in equations (3) and (4) is taken as n = 50 or even n = 25 when the risk of resonance for higher harmonics is small. Table 1. presents the limit   values for individual voltage harmonics according to EN50160 and IEC61000 standards [10,11]. The limit values for individual current harmonics has been presented in [12]. According to the standards the limit for THDu is 5% for medium voltage network and 8% for low voltage network. It should be also noted that the coefficients THDu and THDi give only an overall image of the harmonics present in the network what sometimes can lead to misinterpretation of the measurements results especially when the measurements are carried out during partial load of the network (Fig. 2). Therefore, it is then necessary to carry out the complete measurements including the equipment load analysis [1, 13,14].

Research object
The P50 substation consists of two transformers (TR1 and TR2) which powers are 2 MVA and 1.6 MVA respectively. It supplies the extrusion line P01 which serves to mix three components to obtain a product for a tire tread as well as to reuse its own returns. The P01 line

Simulation results
The evaluation of filtration quality for four types of solutions (passive filters, active filters, 12-pulse rectifiers, AFE) has been carried out using the predefined models available in the Emerson Harmonics Estimator simulation software. The measured machines loads have been introduced into the Emerson software individually for each machine as a percentage of nominal machine load. Simulation results have served to predict the potential level of higher harmonics presented at the secondary side of the transformers for different methods of harmonics reduction and to compare the methods with each other.

Passive filters
One of the most economical methods for limiting the negative influence of the electric drives present in the network is the use of passive filters which are individually designed for the particular type of drive and installed on the power supply side [15][16][17]. They are designed taking into account the nominal current of the drive. Their efficiency is high provided that they are installed close to the drive working with its nominal power for which the filter has been designed. The effectiveness of the passive filter (DC choke) depending on the machine load has been presented in Fig. 9. Simulation results have been presented in Fig. 11 and Fig. 12.

Active filters
Active filters reduce the higher harmonics in electrical network by generating those harmonics which are consumed by nonlinear receivers [18][19][20]. For example, if the receiver needs the fifth and seventh harmonics the active filter generates them what results in more sinusoidal shape of the network current.

12-pulse rectifier
Multi-pulse rectifiers have been known for many years as the devices which minimize the higher harmonics generated by drive systems [21,22]. According to the theory the multipulse rectifiers exclude certain harmonics due to the phase shift between the transformer windings [23,24]. Increasing a number of secondary winding phases can be achieved in several ways but the simplest is to use a triangle-star system in which the number of secondary winding phases is 6. After rectification by two parallel rectifier system one can obtain a DC voltage. Simulation results have been presented in Fig. 17 and Fig. 18.

Active Front End
The AFE (Active Front End) technology is a solution which guarantees a very low THD coefficient [25][26][27]. It comprises a fully controlled IGBT transistor input bridge which is used as a supplementary converter called a regenerative drive. It has an energy return function. The inverter design includes then two separate power modulesmotoring drive and regenerative driveconnected together by a common DC bus with a LCL network filter. Unfortunately, the cost of implementation of such solution is very high in a situation where the modernized electrical network consists in majority of DC drives as the AFE converters use only frequency speed control dedicated for AC drives. The one AFE can be also used for several different drives connected by a common DC bus as shown in Fig. 19 but in the studied case an installation of the DC bus has been impossible due to lack of space. Simulation results have been presented in Fig. 21 and Fig. 22.

Discussion of the results
Comparing the simulation results one can observe that the best quality of higher harmonic filtration is realized by the AFE system where all higher harmonics values have been reduced to below 1% ( Fig. 21 and Fig. 22). It is also the more expensive solution valued at The second more expensive filtration method is the use of the 12-pulse rectifiers which also need some equipment replacement. It is valued at 300 000 € because of the need to replace the two of transformers (TR1 and TR2) by the ones with two secondary windings connected in star and triangle. The filtration quality is acceptable for the fifth and the seventh harmonics.
Nevertheless, for the studied case the simulations have revealed also the presence of the eleventh and the thirteenth harmonics what is not favorable for the condition of the network ( Fig. 17 and Fig. 18). The least expensive solution for the higher harmonic filtration is the use of passive filters valued at 44 990 €. The main disadvantage of this method is related to an inability to adapt to the changing network conditions present in the P01 extrusion production line. Moreover, the filtration quality is several times lower than for the other methods ( Fig. 11 and Fig. 12). The last studied method, i.e. active filters ensures effective higher harmonics reduction ( Fig. 14 and Fig. 15) at a relatively low cost of investment (135 000 €).
All of above mentioned methods with their implementation cost and the filtration quality analysis for the fifth and the seventh harmonics have been collected in the Table 2 and compared in the Fig. 23 and the Fig. 24. The filtration quality has been presented in the Table 2 as the THD coefficient values respectively for voltage and current.
Finally, after above mentioned analysis the active filter technology has been chosen for the P01 extrusion line. More precisely, the active filters AccuSine PCS -Power Correction system -Model CE54 -Sizes 300A have been installed in the TR1 and the TR2 networks. More detailed information about the filters can be found in the technical documentation [28]. The active filter implementation has resulted in considerable higher harmonic reduction. Fig. 25, Fig. 26, Fig. 27 and Fig. 28 show the measurements results of the THDu, THDi, Du and Di coefficients captured for the extruder 150 after the filters installation. In comparison to the Fig.   5, Fig. 6, Fig. 7 and Fig. 8 one can observe the THDu reduction from more than 15% to about 5% and the THDi reduction from more than 60% to about 30%. Regarding Du and Di coefficients, particularly important is reduction of the 5 th and the 7 th harmonics which exceeded standard limits before filtration implementation.

Conclusions
In