**6. Conclusion**

The measurements results show that the operation of the analyzed furnace determines interharmonics and harmonics in the phase voltages and harmonics in the currents absorbed from the network.

THD of phase voltages are within compatibility limits, but voltage interharmonics exceed the compatibility limits in all the analyzed situations.

THD of line currents exceed the compatibility limits in all the heating stages. Because ITHD exceed 30%, which indicates a significant harmonic distortion, the probable malfunction of system components would be very high.

THD of line currents are bigger in intermediate state comparatively with the cold state, or comparatively with the end of melting. This situation can be explained by the complex and strongly coupled phenomena (eddy currents, heat transfer, phase transitions) that occur in the intermediate state.

Harmonics can be generated by the interaction of magnetic field (caused by the inductor) and the circulating currents in the furnace charge.

Because the furnace transformer is in / connection, the levels of the triple-N harmonics currents are much smaller on MV Line versus LV Line. These harmonics circulate in the winding of transformer and do not propagate onto the MV network.

On MV Line, 5th and 25th harmonics currents exceed the compatibility limits. The levels of these harmonics are higher on MV Line versus LV Line. Also, THD of line currents and THD of phase voltages are higher on MV Line versus LV Line, in all the analyzed situations. The harmonic components cause increased eddy current losses in furnace transformer, because the losses are proportional to the square of the frequency. These losses can lead to early failure due to overheating and hot spots in the winding.

Shorter transformer lifetime can be very expensive. Equipment such as transformers is usually expected to last for 30 or 40 years and having to replace it in 7 to 10 years can have serious financial consequences.

To reduce the heating effects of harmonic currents created by the operation of analyzed furnace it must replaced the furnace transformer by a transformer with K-factor of an equal or higher value than 4.

Peak factors of line currents are high during the heating stages, and characterizes high transient overcurrents which, when detected by protection devices, can cause nuisance tripping.

The capacitors for power factor correction and the ones from Steinmetz circuit amplify in fact the harmonic problems.

PF is less than unity in all the analyzed situations. But, Steinmetz circuit is efficient only for unity PF, under sinusoidal conditions.

Under nonsinusoidal conditions, any attempt to achieve unity PF does not result in harmonicfree current. Similarly, compensation for current harmonics does not yield unity PF.

For optimizing the operation of analyzed induction furnace, it's imposing the simultaneous adoption of three technical measures: harmonics filtering, reactive power compensation and load balancing. That is the reason to introduce harmonic filters in the primary of furnace transformer to solve the power interface problems. In order to eliminate the unbalance, it is necessary to add another load balancing system in the connection point of the furnace to the power supply network.

### **7. References**

258 Power Quality Harmonics Analysis and Real Measurements Data

Fig. 37. Apparent power per phase 1 in the last stage of the melting process (MV Line)

The measurements results show that the operation of the analyzed furnace determines interharmonics and harmonics in the phase voltages and harmonics in the currents absorbed

THD of phase voltages are within compatibility limits, but voltage interharmonics exceed

THD of line currents exceed the compatibility limits in all the heating stages. Because ITHD exceed 30%, which indicates a significant harmonic distortion, the probable malfunction of

THD of line currents are bigger in intermediate state comparatively with the cold state, or comparatively with the end of melting. This situation can be explained by the complex and strongly coupled phenomena (eddy currents, heat transfer, phase transitions) that occur in

Harmonics can be generated by the interaction of magnetic field (caused by the inductor)

Because the furnace transformer is in / connection, the levels of the triple-N harmonics currents are much smaller on MV Line versus LV Line. These harmonics circulate in the

On MV Line, 5th and 25th harmonics currents exceed the compatibility limits. The levels of these harmonics are higher on MV Line versus LV Line. Also, THD of line currents and THD of phase voltages are higher on MV Line versus LV Line, in all the analyzed situations. The harmonic components cause increased eddy current losses in furnace transformer, because the losses are proportional to the square of the frequency. These losses can lead to

Shorter transformer lifetime can be very expensive. Equipment such as transformers is usually expected to last for 30 or 40 years and having to replace it in 7 to 10 years can have

To reduce the heating effects of harmonic currents created by the operation of analyzed furnace it must replaced the furnace transformer by a transformer with K-factor of an equal

Peak factors of line currents are high during the heating stages, and characterizes high transient

overcurrents which, when detected by protection devices, can cause nuisance tripping.

**6. Conclusion** 

from the network.

the intermediate state.

serious financial consequences.

or higher value than 4.

the compatibility limits in all the analyzed situations.

and the circulating currents in the furnace charge.

winding of transformer and do not propagate onto the MV network.

early failure due to overheating and hot spots in the winding.

system components would be very high.


**11** 

*Spain* 

**Harmonic Distortion in Renewable** 

Miguel García-Gracia, Nabil El Halabi, Adrián Alonso and M.Paz Comech

*University of Zaragoza* 

**Energy Systems: Capacitive Couplings** 

*CIRCE (Centre of Research for Energy Resources and Consumption)* 

Renewable energy systems such as wind farms and solar photovoltaic (PV) installations are being considered as a promising generation sources to cover the continuous augment

With the incoming high penetration of distributed generation (DG), both electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric network (Dugan et al., 2002). This latter issue is an umbrella concept for a multitude of individual types of power system disturbances. A particular issue that falls under this umbrella is the capacitive coupling with grounding systems, which become significant

a. Increase the harmonics and, thus, power (converters) losses in both utility and customer

b. Ground capacitive currents may cause malfunctioning of sensitive load and control

c. The circulation of capacitive currents through power equipments can provoke a

d. Ground potential rise due to capacitive ground currents can represent unsafe

Introducing DG systems in modern distribution networks may magnify the problem of ground capacitive couplings. This is because DG is interfaced with the electric network via

These capacitive couplings are part of the electric circuit consisting of the wind generator, PV arrays, AC filter elements and the grid impedance, and its effect is being appreciated in

most large scale DG plants along the electric network (García-Gracia et al., 2010).

e. Electromagnetic interference in communication systems and metering infrastructure. For these reasons, it has been noticed the importance of modelling renewable energy installations considering capacitive coupling with the grounding system and thereby accurately simulate the DC and AC components of the current waveform measured in the

because of the high-frequency current imposed by power converters. The major reasons for being concerned about capacitive couplings are:

reduction of their lifetime and limits the power capability.

conditions for working along the installation or electric network.

**1. Introduction** 

demand of energy.

equipment.

devices.

electric network.

power electronic devices such as inverters.

*Transactions on Power Delivery*, Vol. 23, Issue 2, (april 2008), pp. 974-984, ISSN 0885- 8977.

