Researches regarding power quality at the hot rolling mills’ power supply

The study described in the paper was carried out through measurements taken at a feed station in a hot rolling line. The aforementioned measures are appropriate for the purpose of analyzing power quality. The measurements were taken using a three-phase power quality analyzer. Measurements were taken for voltage and current values, as well as for active, reactive, and apparent powers. The analysis of the measurements taken indicates that the power quality is affected by the nonlinearity of the load. As a consequence, the level of reactive power becomes higher, leading to a reduced power factor.


Introduction
Power quality is currently a critical issue, particularly in plants with high power consumers that are occasionally unbalanced [1], [2].These consumers can be found in metallurgical plants like Electric Arc furnaces [3], [4], Laddle furnaces [5], and rolling mills [1].These high power consumers use a lot of electricity and can have an impact on power quality via reactive power, low power factor, and electric current harmonics [6][7][8][9][10].Harmonic currents are caused in large part by the widespread use of power converters, which are found in many electrical and electronic systems, including household appliances.While the power of household appliances is relatively low, harmonic currents cause significant power losses in industrial electrical installations due to the very high power.

Measurements performed at the feed station of the rolling mill
In a metallurgical factory, a series of valuable measurements were conducted to assess power quality.This factory comprises multiple power substations, and the measurements specifically took place at the substation responsible for supplying power to a hot rolling production line.Figure 1 depicts the power supply's single-wire power scheme utilized in the SLH power station.
The SLH power station allocates three supply lines of rolling mill's station to three substations, which subsequently provide power to consumers within the rolling mill's station section.
Each station has at least 2 transformers permanently connected (6KV/0.4KV) to the 6KV lines.At present, at the SLH station, a group of capacitors connects to the rolling mill's station section via the 6 KV bars.
The initial design of the capacitors was designed for a power of Q=5MVAr and a voltage of 35 KV, with the capacitors being connected in a star configuration.
However, subsequent modifications have resulted in the capacitors being connected in a double star configuration.
As of now, the capacitor bank is operational at a voltage of U=6KV.

Figure 1. The SLH power station
For the experimental measurements, a three phase power analyzer was used, and the rms values of the electric currents, THD, active and reactive power, and power factor were recorded.
The rms values of the phase and line voltages, as well as the voltage's THD, were also recorded.Figure 3 depicts all of the variations in line voltages on the same graph.This representation is useful because it shows that the system is relatively balanced, with no significant differences between the three rms voltage values.The RMS values of line voltages phase 3

Figure 3. The line voltage variation for all three phases
For a better visualization of the voltage variation, figure 4 depicts the variation on one of the three phases, phase 1, over a shorter time period, namely 5 minutes, between seconds 600 and 900, or between minutes 10 and 15 from the start of the measurements.
This detail makes it clearer that the system is not perfectly balanced, but the imbalance is minimal.Figure 5 depicts the variations in phase voltage for the three phases.The phase voltages, like the line voltages, are slightly unbalanced.Figure 6 depicts the variations in active and reactive powers during the time range of the measurements.It is obvious that the reactive power has high values, which is to be expected given that the measurements were taken in the absence of SVC.This is also reflected in the variation of the power factor, which is depicted in Figure 7.As a result of the poor energy efficiency, the low power factor is a disadvantage.
Figure 8 illustrates the variation in total active energy consumption and total reactive energy consumption.Reactive energy has very high values, which is reflected in the variation of the power factor.The RMS values of line voltages for all three phases

Modeling and simulations
It was designed as a simulation circuit of the SLH Power station using MATLAB/SIMULINK environment as shown in Figure 9, there are many techniques published in various papers [11], [12].The induction motors are simulated by an RLC load block, for correcting the Power Factor, capacitor banks were introduced in the block diagram shown below.

Conclusions
From the information presented in the paper, it can be concluded that, despite the fact that rolling mills are not the largest energy consumers, their electricity supply still contributes to power quality issues.The presence of highly reactive energy must be avoided, and the power factor must be enhanced.Utilizing components such as an SVC can enhance the power quality by increasing power factor and decreasing reactive power.Research on the enhancement of power quality is the focus of future studies.

Figure 2 a
, b, c depicts the line voltages on the three phases during one hour.

Figure 2 a. The line voltage variation for phase 1 Figure 2 b. The line voltage variation for phase 2 Figure 2 c.
Figure 2 a.The line voltage variation for phase 1

Figure 4 .
Figure 4.The line voltage variation for all three phases for 300 seconds

Figure 11 .
Figure 11.The variation of circuit power factor The results obtained as shown in Fig. 11 are very similar to those measured, power factor values 0.35-0.45.