Power Quality Problems Generated by Line Frequency Coreless Induction Furnaces

The increased problems in power networks impose to identify the sources of power quality deterioration. The most important parameters which affect power quality are harmonics, voltage instability and reactive power burden (Arrillaga et al., 2000). They cause low system efficiency, poor power factor, cause disturbance to other consumers and interference in the nearly communication networks (Lattarulo, 2007; De la Rosa, 2006; Muzi, 2008). In induction melting is noticed mainly the efficiency, high heating rate and the reduced oxidation level of the processed material, the improved work conditions and the possibility of an accurate control of the technological processes (Rudnev et al., 2002). Induction heating equipments do not introduce dust and noise emissions in operation, but cause power quality problems in the electric power system (Nuns et al., 1993). Induction-melt furnaces supplies by medium frequency converters generate fixed and variable frequency harmonics. Both current and voltage-fed inverters generate harmonics back into power lines in the process of rectifying AC to DC (EPRI, 1999). Harmonics flowing in the network causing additional losses and decreasing the equipments lifetime. Also, the harmonics can interfere with control, communication or protection equipments (Arrillaga et al., 2000; George & Agarwal, 2008). In addition to the harmonics that are normally expected from different pulse rectifiers, large furnaces operating at a few hundred hertz can generate interharmonics (EPRI, 1999). Interharmonics can overload power system capacitors, introduce noise into transformers, cause lights to flicker, instigate UPS alarms, and trip adjustable-speed drives. High-frequency systems, which operate at greater than 3 kHz are relatively small and limited to special applications. Electromagnetic pollution produced by the operation of these equipments is small. The induction furnaces supplied at line frequency (50 Hz) are of high capacity and represent great power consumers. Being single-phase loads, these furnaces introduce unbalances that lead to the increasing of power and active energy losses in the network. In case of channel furnaces it was found the presence of harmonics in the current absorbed from the power supply network. These harmonics can be determined by the non-sinusoidal supply voltages or the load’s nonlinearity, owed to the saturation of the magnetic circuit (Nuns et al., 1993). This chapter presents a study about power quality problems introduced by the operation of line frequency coreless induction furnaces. The specialty literature does not offer detailed information regarding the harmonic distortion in the case of these furnaces.


Measured signals in electrical installation of the induction furnace
The measurements have been made both in the secondary (Low Voltage Line -LV Line) and in the primary (Medium Voltage Line -MV Line) of the furnace transformer, using the CA8334 three-phase power quality analyzer. CA8334 gave an instantaneous image of the main characteristics of power quality for the analyzed induction furnace. The main parameters measured by the CA8334 analyser were: TRMS AC phase voltages and TRMS AC line currents; peak voltage and current; active, reactive and apparent power per phase; harmonics for voltages and currents up to the 50th order (CA8334, technical handbook, 2007). CA8334 analyser provide numerous calculated values and processing functions in compliance with EMC standards in use (EN 50160, IEC 61000-4-15, IEC 61000-4-30, IEC 61000-4-7, IEC 61000-3-4). The most significant moments during the induction melting process of the cast-iron charge were considered: -cold state of the charge (after 15 minutes from the beginning of the heating process); -intermediate state (after 5 hours and 40 minutes from the beginning of the heating process); -the end of the melting (after 8 hours from the beginning of the heating process). Further are presented the waveforms and harmonic spectra of the phase voltages and line currents measured during the heating of the charge (Iagăr et al, 2009).  In the first heating stage, the electromagnetic disturbances of the phase voltages on LV Line and on MV Line are very small. The 5 th harmonic does not exceed the compatibility limit, but the voltage interharmonics exceed the compatibility limits (IEC 61000-3-4, 1998;IEC/TR 61000-3-6, 2005). On MV Line the current I 2 was impossible to be measured because the CA8334 three-phase power quality analyser was connected to the watt-hour meter input. The watt-hour meter had three voltages (U 12 , U 23 , U 31 ) and two currents (I 1 and I 3 ). Waveform distortion of the currents in cold state is large ( fig. 4, 5). At the beginning of the cast-iron heating the 3 rd , 5 th , 7 th , 9 th , 11 th , 13 th , 15 th harmonics and even harmonics (2 nd , 4 th , 6 th , 8 th ) are present in the currents on the LV Line. The 5 th and 15 th harmonics exceed the compatibility limits (IEC 61000-3-4, 1998). In the cold state the 2 nd , 3 rd , 5 th , 7 th , 9 th , 11 th , 13 th and 15 th harmonics are present in the currents absorbed from the MV Line. The 5 th harmonic exceeds the compatibility limits (IEC/TR 61000-3-6, 2005). In the intermediate state, part of the charge is heated above the Curie temperature and becomes paramagnetic, and the rest of the charge still has ferromagnetic properties. The furnace charge is partially melted.  In the intermediate state, harmonic spectra of the currents absorbed from the LV Line present the 3 rd , 5 th , 7 th , 11 th , 13 th , 15 th , 17 th , 25 th harmonics and even harmonics (2 nd , 4 th , 8 th ). The 5 th , 15 th , 17 th and 25 th harmonics exceed the compatibility limits (IEC 61000-3-4, 1998). On MV Line, harmonic spectra of the currents present the 3 rd , 5 th , 7 th , 9 th , 11 th , 13 th , 15 th , 17 th , 25 th harmonics and even harmonics (2 nd , 4 th , 6 th , 8 th ). The 5 th and 25 th harmonics exceed the compatibility limits (IEC/TR 61000-3-6, 2005). After 8 hours from the beginning of the heating process the furnace charge is totally melted, being paramagnetic.  Waveform distortion of the currents at the end of the melting process is smaller than in cold state, or intermediate state. On LV Line, harmonic spectra of the currents show the presence of 3 rd , 5 th , 7 th , 9 th , 11 th , 13 th , 15 th , 17 th , 25 th harmonics and even harmonics (2 nd , 4 th , 6 th ). The 5 th , 15 th and 25 th harmonics exceed the compatibility limits (IEC 61000-3-4, 1998). On MV Line, harmonic spectra of the currents show the presence of 3 rd , 4 th , 5 th , 7 th , 9 th , 11 th , 13 th harmonics at the end of the melting. The 5 th harmonic exceeds the compatibility limits (IEC/TR 61000-3-6, 2005).

The values computed by the CA8334 analyser
The values computed by the CA8334 analyser are: total harmonic distortion of voltages and currents, distortion factor of voltages and currents, K factor for current, voltage and current unbalance, power factor and displacement factor, extreme and average values for voltage and current, peak factors for current and voltage (CA8334, technical handbook, 2007). Mathematical formulae used to compute the total harmonic distortion (THD) of voltages and currents are: In the above relation I represents the line current, i represents the phase (i = 1, 2, 3) and n represents the order of harmonics. A K factor of 1 indicates a linear load (no harmonics); a higher K factor indicates the greater harmonic heating effects. The unbalanced three-phase systems of voltages (or currents) can be reduce into three balanced systems: the positive (+), negative (-) and zero (0) sequence components. The positive voltage True RMS and the negative voltage True RMS are given by the relations: where 123 ,, VVV represent the phase voltages (using simplified complex) and The positive current True RMS and the negative current True RMS are given by the relations: where 123 ,, III represent the line currents (using simplified complex).
Voltage and current unbalances (unb) are: www.intechopen.com Power factor (PF) and displacement factor (DPF) are computed by relations: In the relations (14), (15): Vpp is the PEAK+ of the phase voltage; Vpm is the PEAK-of the phase voltage; Ipp is the PEAK+ of the line current; Ipm is the PEAK-of the line current; i represents the phase (i = 1, 2, 3); N represents the number of the samples per period (between two consecutive zeros). Peak values (PEAK+/PEAK-) for voltage (or current) represent the maximum/minimum values of the voltage (or current) for all the samples between two consecutive zeros. For a sinusoidal signal, the peak factor is equal to 2 (1.41). For a non-sinusoidal signal, the peak factor can be either greater than or less than 2 . In the latter case, the peak factor signals divergent peak values with respect to the RMS value.  Distortion factor of phase voltages is very small during the heating process of cast-iron charge. In all situations, distortion factor of phase voltages is smaller than total harmonic distortion. Peak factors of line currents are between 1.48 and 1.88. This indicates that the analyzed furnace is a non-linear load. A high peak factor characterizes high transient overcurrents which, when detected by protection devices, can cause nuisance tripping.

Recorded parameters in the electrical installation of the induction furnace
The recorded parameters in the electrical installation of analyzed furnace are: RMS values of phase voltages and currents, total harmonic distortion of phase voltages and currents, power factor and displacement factor per phase 1, active power, reactive power and apparent power per phase 1. Fig.17-21 show the recorded parameters on MV Line, in the first stage of the heating. In the recording period (11:20-12:18), the furnace charge was ferromagnetic. In the recorded period of the cold state, power factor (PF) per phase 1 and displacement factor (DPF) per phase 1 are less than unity; in the time period 12:00 -12:18 PF is less than neutral value (0.92). PF is smaller than DPF because PF includes fundamental reactive power and harmonic power, while DPF only includes the fundamental reactive power caused by a phase shift between voltage and fundamental current.     In the last stage of the melting process, THD of phase voltages are within compatibility limits, being smaller comparatively with the cold state or the intermediate state. The difference between the power factor and the displacement factor is small in the last stage of the melting process ( fig.34). This indicates a decrease of harmonic disturbances and reactive power consumption ( fig.36), comparatively with the cold state or the intermediate state.
In the time period 18:07 -18:12, the values of reactive power per phase 1 increase; consequently, the power factor and the displacement factor per phase 1 decrease. Recorded values of active power per phase 1 are close to the apparent power values.

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 I THD 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, 5 th and 25 th 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.