**2. Analysis of the quality of the electrical power on the bars of 110 kV**

"Analysis of the quality of the electrical power supply of the three-phase general power supply circuit of the user on 110kV" is based on "Study on the quality of power supply including load analysis, problems regarding voltage/current fluctuations, interruptions in the company's power supply, flicker monitoring, asymmetries, harmonic levels on voltage and current, for the general supply on the 110kV side of the company in the Intelligent Electrical Connections Station, without personal workers, with Advanced Metering Infrastructure (AMI)".

The electrical installations (**Figure 1**) of the analyzed industrial user consist of:

• two 110/20 kV 16 MVA substations (one being in reserve) – **Figure 2** and **Table 1**;

#### **Figure 1.**

*110 kV single-wire power supply diagrams of the analyzed company with transformers 1 and 2, 110/20 kV of 16 MVA [copyright © 2020 PLT – Adapted working scheme] .*

*Contributing by Monitoring Energy Efficiency to the Development of Optimization Measures… DOI: http://dx.doi.org/10.5772/intechopen.101801*

#### **Figure 2.** *Real photo - transformer 1 (T1), 110/20 kV of 16 MVA.*


#### **Table 1.** *Characteristics of the transformer in the transformer station.*


The measuring point for the analyzed substation, in which the monitoring equipment was mounted, is represented by the entry point on the 110 kV (**Table 1**) side of the 110/20.5 kV, 16MVA transformer, or the exit point from the 110 kV electrical connection station of the conveyor (the common part with that of the distributor – the IT cells are actually located in the same premises). **Table 2** presents some samples of measurements performed for the analyzed transformer in the electrical connections station.

Thus, an ION7600 Class A type analyzer was installed (**Figure 3**), approved and verified up to date (for monitoring the various electrical quantities that will be presented below), in order to make an energy analysis of the monitored electrical measures for the technological process in the general connections power station, at the supply point inside the company.

The analysis of the data registered in the period 26/06/2021 at 16:20–20/08/2021 at 19:00, for the power supply of the receivers within the company allows for the highlighting of the electrical characteristics of the power supply system.

Measurements performed on the HV bars (High Voltage) in the above mentioned period have been achieved in order to carry out the electric power study

#### **Table 2.**

*Measurement sheet for the transformation station on the first day of the monitoring period 13/03/2021 at 18:00–22/03/2021 at 16:10.*

#### **Figure 3.**

*Real photo – Measure cell for transformer 1 (T1), 110/20 kV of 16 MVA in Consumer's Power Station [copyright © 2020 PLT – Working scheme & photo].*

within the company. The measurements that were recorded are calculated as the average values of the electrical quantities, at an interval of 10 minutes, according to current quality standards IEC 61000–3-4 [1].

The voltage at the supply bars of the receivers shows variations (60.136 … 72.53) kV voltage per phase/(104.254 ÷ 126.17) kV voltage between phases, for TGD (**Figure 4a** and **b**), which must fit within the limits of 60 10% / 110 10% kV

*Contributing by Monitoring Energy Efficiency to the Development of Optimization Measures… DOI: http://dx.doi.org/10.5772/intechopen.101801*

**Figure 4.** *Voltage curve at the supply bar for TGD: a - voltages between phases, b - voltages per phase.*

(54 … 66/100 … 120 kV). Rapid variations in voltage values caused by specific processes within the company (factory) lead to the recording of voltage fluctuations, accompanied by the flicker effect. The 3-phases are relatively symmetrically charged. It has been observed that the values of the supply voltage during the measurements did not exceed the normal (normed) limit values.

The voltages at the supply bars have a shape close to a sinusoid, being characterized by a relatively low total THD distortion factor (**Figure 5**), which fits within the values allowed at the low voltage supply bars (THD admitted = 8%). The total voltage distortion factor - THD U (**Figure 5**) is relatively low (it fits between the values (1 ÷ 2.1%).

**Figure 6** shows the variation of the unbalance factor during recording. It is observed that the values of the negative unbalance factor (*ks = Vunb*):

$$k\_s = \frac{U^-}{U^+} \tag{1}$$

#### **Figure 5.**

*Voltage distortion factor on supply phases a, b, c (THD U) - monitored values.*

#### **Figure 6.**

*Negative voltage unbalance factor at TGD supply bars (ks).*

where *U* is the negative sequence voltage (inverse) and U+ is the positive sequence voltage (direct), does not exceed the permissible values of the negative unbalance factor (<2%) at TGD (power supply point) in the monitored enclosure.

The analysis of the data in **Figure 6** highlights an unbalance factor of about 5%, which indicates the existence of unbalanced and non-linear three-phase users connected in the low voltage network, unevenly, fact which causes different voltage drops on the three phases. It is important that the unbalance factor of the voltages between the phases has values within the accepted limits so that the three-phase motors connected in a triangle in the low voltage network are not affected by the unbalance of the supply voltages. Connecting star motors could lead to a reduction in their operating performance [2].

The variation of the measured values of the negative unbalance factor ks, determined as the ratio between the negative sequence component of the voltage curve U- and the positive sequence component of the voltage curve U+, is indicated in **Figure 6** [3].

The analysis of the data in **Figure 7** highlights the fact that, at the low voltage bars of the receiver supply system, the admitted levels of voltage fluctuations are exceeded in many cases, in the public network (Pst\_admitted = 0.9, and

*Contributing by Monitoring Energy Efficiency to the Development of Optimization Measures… DOI: http://dx.doi.org/10.5772/intechopen.101801*

**Figure 7.**

*Variation in the level of voltage fluctuations at the supply bars of the TGD.*

Plt\_admitted = 1.0) [3–6]. The number of events that lead to values higher than those admitted in the public network is small and is due to specific processes. In addition, in the energy system of other users, the admitted values are different from those in the public network, established according to the effects of the process carried out. The data in **Figure 7** indicate that higher voltage variations occur in phase c, accompanied by a higher level of flicker indicators. It is noted that the curves Pst and Plt in the general connections table (TGD) inside the studied company (factory), have almost the same shape (allure).

Moreover, based on the measurements, it is observed that at the feed bars of the TGD, the flicker level (Pst, Plt) is within the normalized parameters (limit). The disturbances determined at the supply bar are due, first of all, to the variation, in wide limits, of the reactive power necessary for the operation of the furnace. In the case of AC voltage supply, the efficient operation of the oven requires the existence of a low power factor, which requires the adoption of measures to improve it.

The variability specific to the technological process may require adequate compensation of the reactive power. The wide variation of the reactive power absorbed from the mains supply causes rapid voltage variations (voltage fluctuations) at the supply bars (usually within the accepted limits of slow voltage variations, +10%, in mains networks high voltage) accompanied by the phenomenon of flicker in users connected in the same area of use. The complexity of the phenomenon, as well as its propagation in the power supply network, requires experimental determinations to be based on the analysis of the level of disturbances in users in the area and, if necessary, to evaluate measures to reduce the level of disturbances.

The technical study carried out comprises a wide area of use in which it has been identified and analyzed, in particular, the electromagnetic disturbance in the form of voltage fluctuation which causes flicker phenomenon. The determinations in the area followed a large number of parameters and quality indicators, but the detailed analysis focused on the short-term and long-term flicker indicators Pst, measured according to the recommendations of IEC 61000.

Also, the electrical quantities determined in the representative points of the scheme in the area were analyzed, namely at the high voltage supply bar of the analyzed user, at the high voltage supply bar of other users in the area and at the low voltage bar, at which are powered by the affected receptors.

The determined values enable the comparison with the admitted values recommended or imposed by the international CEI standards and the RET&RED performance standard (RET = Electrical transmission networks; RED = Electrical distribution networks).

In order to ensure the quality of electricity, limit values are indicated in the standards and norms in force, technical energy norms for limiting voltage fluctuations, including the flicker effect, in electricity transmission and distribution networks - NTE 012/14/00 and standards international IEC 61000–4-30 ver. III, IEC 61000–3-7: 2008 and IEC 61000–2-2. The national standards NTE 012/14/00, Ord\_12\_2016\_RET and international performance standard IEC 61000–4-30 ver.3, IEC 61000–3-7: 2008 and IEC 61000–2-2 indicate, for the low voltage level, the limits of compatibility in **Table 3**. For the range of medium, high and very high voltages, the planning values of the flicker indicators on short-time (Pst) and longtime (Plt) are indicated in **Table 4**.

For the low voltage level, in any time interval of one week, the long-term flicker indicator must meet the Plt < 1 condition for 95% of the time. The same condition is imposed for the average voltage level [3–6].

Due to the fact that low voltage users are affected by voltage fluctuations (flicker phenomenon) during the operation of the electric arc furnace, experimental determinations have been proposed, in particular, to verify the compliance of the level of disturbances within the accepted limits by international standards. Also, the curves


**Table 3.**

*Compatibility limits for flicker indicators in low voltage networks.*


*\*The values of the flicker indicators were established considering that the transfer factor to the low voltage network is unitary. In real cases, this factor may vary and must be determined for different operating conditions. In this way, the values at the MT and IT levels may be appropriately higher. The values mentioned are for guidance only.*

#### **Table 4.**

*Indicative values of the planning level for flicker indicators, in medium, high and very high voltage networks.*

*Contributing by Monitoring Energy Efficiency to the Development of Optimization Measures… DOI: http://dx.doi.org/10.5772/intechopen.101801*

#### **Figure 8.**

*Electric current curve on the feed bar for TGD. a. Arms [a] – Monitored. b. Arms [a] - instantaneous values.*

regarding the voltage variation in significant points of the scheme and the curves for the variation of the reactive power on the user's power supply line will be analyzed.

As electromagnetic compatibility analyzes are taken into account, in particular values with a probability of 95%, these values will be highlighted in the analysis, which must be compared with the admitted values. Also, as comparison, the values with a probability of 50% (average values) will be highlighted.

The electric currents on the three phases monitoring have quite close values indicating, however, an asymmetric phase charge. The variations of these currents are presented in **Figure 8**.

The total distortion factor of electric current - THD I (**Figure 9**), according to the shape of the graph curves, indicates the sinusoidal shape of the current in the supply circuit. At a first comparison with the data on the level of distortion of the voltage on the bars, there is no direct influence, especially in the case of phase a, in which the harmonic distortion of the electric current is superior to the other two phases. The analysis of **Figure 8a** and **b** also shows a weak imbalance between the loads of the three phases.

It should be noted that the jumps in the variation curves of the harmonic spectrum are due to sudden variations in electric current at the start of various electrical equipment - motors (in-rush current). These values are not specific for the analysis of harmonic distortion of electric current.

The variation of the unbalance factor of electric current, during the recording time, is indicated in **Figure 10**. The negative unbalance factor was determined based on the relation:

$$k\_s = \frac{I^-}{I^+} \tag{2}$$

**Figure 10.** *Variation of the negative current unbalance factor of TGD (ks = Aunb).*

where *I* is the negative (reverse) sequence component of the electric currents, and *I*+ is the positive (direct) sequence component.

The data in **Figure 10** shows that, during the recording, the unbalance factor of the electric current did not exceed the normal values in operation, these being determined especially by the voltage unbalance at the supply bars.

Special attention was paid to monitoring the values of electric currents in the supply circuits to determine both their loads and the level of losses in the user's electrical circuits.

**Figures 11** and **12** show the variation P (active powers) and, respectively, Q (reactive powers) on the IT/MT (at 110 kV) bars, and **Figure 13** shows the variation of the apparent powers transited on the circuit (S), respectively, in **Figure 14** the variation of the three-phase powers comparative. The phase shift between the voltages and the electric currents monitored at the supply bar of the analyzed electrical equipment, in instantaneous values, is presented in **Figure 15**.

*Contributing by Monitoring Energy Efficiency to the Development of Optimization Measures… DOI: http://dx.doi.org/10.5772/intechopen.101801*

**Figure 11.** *P [kW] variation on the IT bars at 110 kV.*

**Figure 12.**

*Q [kVAr] variation on the IT bars at 110 kV.*

**Figure 13.**

*Apparent single-phase powers transited through at 110 kV - S [kVA].*

**Figure 14.**

*Three-phase powers transited at 110 kV. SumP [kW], SumQ [kVAr], SumS [kVA].*

**Figure 15.**

*The phase shift between voltages and electric currents at the supply bar at 110 kV.*

The curves in **Figure 12** indicate that the user absorbs capacitive reactive power over a significant period of time. In comparison with the data in **Figure 14**, it is highlighted that the values of the capacitive power factor are within the limits accepted by the regulations in force. In order to ensure the control of the reactive power in the capacitive zone, and the limitation of some undesired increases of voltage in this interval, concrete solutions for the limitation of the capacitive regime were analyzed.

The PF power factor was determined based on the recorded energy values over a specified time interval:

$$PF = \frac{\mathcal{W}\_{trjtaxat}^{P}}{\mathcal{W}\_{trjtaxat}^{S}} \tag{3}$$

The PF value, although widely used in practical applications, provides correct information on the energy behavior of the consumer only in the case of a constant consumption during the time interval in which the power factor is evaluated.

The variation of the power factor during the recording, and in the time intervals in which the equipment was in operation, is indicated in **Figure 16** for each phase.

*Contributing by Monitoring Energy Efficiency to the Development of Optimization Measures… DOI: http://dx.doi.org/10.5772/intechopen.101801*

**Figure 16.**

*PF variation (power factor) at 110 kV.*

**Figure 17.** *Transformation station loading (load T1).*

As it can be seen from the graph of the variation of the power factor (**Figure 16**), it is highlighted the fact that an important circulation of the reactive power appears in the supply circuit. The recorded values of the harmonic spectrum are affected by the specific operating mode, with frequent starts and stops of motors, as well as other electrical equipment (lighting, etc.). The real values of the harmonic spectrum can only be taken into account for stationary time intervals.

**Figure 17** shows the minimum, average and maximum loads of the transformer during the monitoring period. The average load of the transformer station is about 13%, and the maximum load is 18%. Due to the low level of consumption in most operating cases, the transformer station operates poorly charged, which is an important source of energy loss. There are also times when the load is 6%.

Obs. In the power balance made for the transformer station of consumer, it is found that the transformer load does not exceed 18% of the nominal value (during 13/03/2021 at 18:00–22/03/2021 at 16:10).

**Figure 18** shows the values of the voltage harmonics on the three phases (a, b and c) at 110 kV. **Figure 19** shows the values of the current harmonics on the three phases (a, b and c) at 110 kV (in Cell Measure with Ion7600 Class A).

#### **Figure 18.**

*Voltage harmonics level on phases a, b and c at the supply bars - monitored values at 110 kV. a. Harmonic level [%] phase 1 (voltages). b. Harmonic level [%] phase 2 (voltages). c. Harmonic level [%] phase 3 (voltages).*

The main observations that emerge from the analysis of the electrical measurements performed are:


Under these conditions, the optimization of the power balance aims at the balanced charging of the three phases, the improvement of the power factor compensation, the improvement of the flicker level and the operation with an improved harmonic regime (harmonics of electric current).

*Contributing by Monitoring Energy Efficiency to the Development of Optimization Measures… DOI: http://dx.doi.org/10.5772/intechopen.101801*

#### **Figure 19.**

*The level of electric harmonics on phases a, b and c at the supply bars - monitored values at 110 kV. a. Harmonic electric currents phase1 [%]. b. Harmonic electric currents phase 2 [%]. c. Harmonic electric currents phase3 [%].*

There is a good classification of the voltage level on phases and between phases in the normed parameters (framing in the normed limits).

#### **3. Events recorded during the monitored period**

This chapter presents a series of events recorded during the monitoring period (Dips&Swells) [5, 6]. These events led to production interruptions that caused significant damage to the user through recalibration, cleaning, disposal of inferior products of poor quality, the realization of additional waste and implicitly additional specific costs.

#### **Figure 20.**

*Events noted in the first half of the 110 kV monitoring period in Electrical Station.*

