Case Study - Induction Furnace Application

Introduction
Indianapolis Casting Company (ICC), a foundry located in Indianapolis, Indiana, contacted Northeast Power Systems with the desire to obtain an 8064 kvar 13.8 kV single step capacitor bank. ICC was looking for a quick delivery of three weeks due to power quality problems associated with the startup of a multi-million dollar 10 MW induction furnace. The induction furnace manufacturer and ICC speculated the power quality problems were caused by waveform notching or harmonics created by the new induction furnace’s power electronic circuitry.  The proposed capacitor bank was being installed to attenuate the line notching and higher order harmonics.

Figure 1 - Power Quality Problems Associated With Induction Furnaces Can Be Corrected With Harmonic Filter Banks and Shunt Capacitor Banks

Due to the economic need to get the furnace operational, the project was highly visible and involved engineers from Northeast Power Systems, Inc. (NEPSI), the electric utility, the induction furnace manufacturer, ICC, and engineers hired by ICC to oversee the project.

ICC’s Power System
ICC is served from two separate Indianapolis Power and Light (IP&L) 132kV transmission lines as shown in Figure 2. The voltage of each line is dropped to 13.8kV through two 24/32/40 MVA transformers (named North and South Bus Transformers).  The new Induction furnace is connected to the North Bus through the North Bus Transformer).  The North Bus transformer has an impedance of 9.73%.  The North and South bus are tied together through a normally open tie. 

The new induction furnace is rated at 10.0MW and receives power from the North Bus through a three winding 11.2 MVA transformer with an impedance of 5.5%. In addition to the new induction furnace, the North bus also feeds a large machine shop, other electric furnaces, an IP&L distribution circuit, and the proposed 8064 kvar 13.8kV single step fixed capacitor bank.

During the commissioning period of the induction furnace, electrical problems involving insulation failures and nuisance drive miss-operations occurred in the machine shop.  These problems were very costly to ICC, which led to the eventual purchase of the 8050 kvar fixed capacitor bank.

Figure 2 – ICC Power System Showing Typical
Power Flow and System Voltages

Proposed Capacitor Bank
The proposed capacitor bank consists of twenty-one individually fused 600 kvar, 9.96kV double bushing capacitors connected in an ungrounded-wye configuration.  This equates to 8064 kvar at a system voltage of 13.8kV. Due to space constraints and the requirement for a quick and simple installation, a metal-enclosed bank was chosen (shown if Figure 3).  The capacitor bank is equipped with an integral 500 MVA 15kV ABB vacuum circuit breaker.  This allowed for direct connection to ICC’s 13.8kV North Bus. Over-current protection is provided by an MDP over-current relay that has both phase and ground fault relays.  Blown fuse detection is provided by a relay that will trip the bank off line if two or more capacitor fuses operate.

Figure 3 – Picture of 8050 kvar metal-enclosed capacitor bank (center). To left is a 1500 kvar
metal-enclosed bank and to the right is a synchronizing switch enclosure for the
8050 kvar Bank. All items above were manufactured by NEPSI.

Concerns With Installation of Capacitor Bank
During NEPSI’s capacitor bank design process, there were three primary areas of concerns relating to the installation and energization of the proposed capacitor bank. The primary areas of concern were as follows:

1.       North Bus operating voltage range

2.       Harmonic distortion and resonance

3.       Reduction in notch depth and ringing attenuation

As part of the design process, NEPSI’s power systems engineers performed a power system evaluation to address the above concerns. A brief description of the evaluation and the results are presented in the remainder of this document.

Power System Evaluation Results
Voltage range concerns were analyzed with a load flow program, while the notching/ringing problem and harmonic concerns were addressed with a harmonic analysis program.

Load Flow Calculations
Several load flow calculations were made to determine the operating voltage of the 13.8 kV North Bus at ICC for various operating conditions. The calculations were also made to determine the optimal position for the fixed tap, which exist on the North Bus 132 kV/13.8 kV transformer bank. A summary of the results are provided in Table-1 below.

IP&L indicated that their system was designed to operate in a range of +/- 5% on the primary and/or secondary of the ICC North transformer. The actual voltage as measured for the entire year of 1998 ranged from 0.97 to 1.04 PU with a normal operating voltage range of 1.0 to 1.035 PU. The tap position was selected based on the normal operating range of 1.0 to 1.035 PU.

The loading used in the load flow calculations was based on load reports that were provided by ICC for the first week of 1999. The loadings were based on maximum and minimum load conditions. It was assumed that the maximum load conditions were all coincident in time. The minimum load condition was assumed as 0 MW and 0 MVARS. By selecting these two load conditions, a conservative maximum and minimum voltage in the system was calculated.

Cases-1 shows the operating voltage of the system for the existing system conditions with the proposed capacitor bank on line at 8.064 MVARs. As can be seen in Table 1, the voltage at the 13.8 kV North Bus climbs to 1.089 PU or 15.02 kV. This voltage is above what would be recommended for a safe operating voltage under ANSI standard C84 for equipment rated with a nominal voltage of 13.8 kV. It would result in pre-mature breakdown of insulation and possible failure to sensitive electronic equipment, which is fed from the North Bus. The other two cases under Case Name-1 shows the voltage to be within the safe operating range for both ICC and IP&L.

Cases-2 shows the same cases as cases-1 but with the proposed capacitor bank switched off. This condition would reflect what would exist on the system with no capacitors. As can be seen in table 1, the voltage still may climb to 1.062 PU giving the fact that the load on the North bus is reduced to zero load. IP&L indicated that the voltage for 1998 reached a maximum of 1.04 PU. The voltages in our simulation did not exactly match because the loading at ICC was assumed to be zero, but in fact had some magnitude, even during low load periods, that would lead to some amount of voltage drop. The results shown in Table 1 however, do indicate that the voltage can climb to an un-acceptable voltage.

Cases-3 shows the operation of the system with the 8.064 MVAR capacitor bank on-line with the taps moved up two positions to 135.3 kV. Under this operating scenario, the operating voltage of 0.941 at full load at the North Bus is to low for a nominal voltage delivered. The maximum operation voltage, however, at the North bus reaches a safe maximum of 1.043 PU.

Cases-4 simulations were made to determine if moving the taps only one position would show a more favorable result. For a nominal operating voltage on the utility, the voltage on the North bus is 0.968 PU under maximum load conditions with the capacitor bank on. Under this scenario, the voltage is slightly low. It should however be noted that IP&L indicated that the normal voltage range is 1.0 PU to 1.035 PU. If the average voltage in the system is 1.018 PU then this voltage under maximum load conditions is tolerable at 0.988 PU.

Based on the forging results, NEPSI recommended that IP&L move their transformer tap on the North transformer from the existing tap of 128.7 kV to 132 KV. The maximum voltage may reach as high as 1.069 per unit when the capacitor banks are on and there is no load on the North Bus. Based on this result NEPSI also recommended that the capacitor bank be modified with a supervisory relay, which monitors the bus voltage and opens the capacitor breaker if the system voltage exceeds 1.05 PU.

Harmonic Analysis
A harmonic analysis utilizing a harmonic analysis program was performed on the ICC system to evaluate the likelihood of harmonic resonance when the capacitor bank is energized. Impedance scans were developed to show how the system impedance varies with frequency for different system configurations. The system model included the new capacitor bank, distribution capacitors, 132kV and 13.8kV system impedances, and ICC motor loads and capacitors.

A total of 8 different impedance scans representing different system configurations were made, and are listed in Table 2.  The impedance scan for Case 2A, shown in figure 4, shows that when the 8064 kvar capacitor bank is energized, a resonance near the 5^th  (5.4^th harmonic) exist.  This resonance is of only slight concern, since measurements indicate very low levels of 5^th harmonic current.  If 5^th harmonic currents become high enough to cause significant amounts of voltage distortion, it may be necessary to remove three capacitors from the new capacitor bank (yielding 6910 kvar and moving the resonant point to the 5.9^th harmonic).   

Figure 4 – Capacitor Energized with IP&L Distribution
Circuit Connected to the South Bus

Line Notching and Ringing Analysis
The presence of distinct line notching and ringing from the induction furnace resulted in discussions on how to best filter the ringing that is associated with it. The ringing, more than likely, results in double zero voltage crossings, which cause sensitive electronic equipment to miss-operate (i.e. UPS, AC & DC drives). This section discusses the best method and the worst method in reducing the line notch associated with the induction furnace. By reducing the line notch depth, the likely hood of a zero voltage crossing is less likely to occur.

The oscillation within the ring, or “ringing”, is caused by the switching action of the SCR cells. This phenomenon is difficult to predict but is usually not a problem if there is significant damping (resistance) in the system. The ringing is caused by the ac line inductance and distributed capacitance, which being in parallel, will have a natural frequency response when excited by a step change in voltage. This ringing can, if not properly damped, add significantly (as much as the notch depth) to the total notch depth.

The prediction or calculation of the ringing is very difficult and requires an electromagnetic transients program (EMTP).  The notch depth however, can easily be calculated (ignoring the ringing). By showing a reduction in the line notch depth, a corresponding reduction in the ringing will occur.

Figure 5 – Impedance Scans for
Notch Ring Calculations

Case 2 – System with Capacitor Bank Energized
Case 5 – System with Filter Bank Energized
Case-1 Existing System Without Capacitor Bank or Harmonic Filter Bank

The notch in the voltage waveform results from two phases that are successively shorted to each other as the current commutates from cell to cell in the rectifier. This line notching occurs twelve times per cycle for a twelve-pulse rectifier. Four of these notches are twice as deep as the other eight. The deep notches occur when the phase-to-phase short occurs across the same phase that is measured. The other eight are due to the interaction of the phases as they commutate.

The notch depth can be calculated analytically by using the voltage divider principle, since the voltage notch is the result of a phase-to-phase fault through the system impedance. By using the impedance model as developed in the harmonic scans, the voltage notch depth can be calculated. Figure 5 shows three harmonic impedance scans. The scans show the impedance of the ICC system looking from the North 13.8 kV bus. The per unit impedance at the 70^th harmonic is recorded for each scan. This impedance will be used as the source impedance for the voltage divider to determine the notch depth for the various system configurations.

The notch depth for the existing system is shown in the calculation below. It was calculated by using the plot shown above and the impedance of the furnace transformer.

X_s:  Source Impedance at 70^th harmonic (From Plot):  1.082 Per Unit

X_t:  Furnace Isolation Transformer Impedance at 70^th Harmonic: 70 x (10/11.2) x 0.055 = 3.44 Per Unit

Notch Depth % :    

The same calculations can be made for the other two cases shown in the plot above. The results of all three are shown in Table 3 below.

As can be seen from the results above the capacitor bank by itself is the most effective means of reducing the notch depth and the ringing associated with the notch. The harmonic filter is not effective because the filter becomes inductive beyond its tuning point.

Conclusions and Recommendations
To avoid excessive voltage during light loads, NEPSI recommended that the utility move the transformer tap on the North transformer from the existing tap of 128.7 kV to 132 KV. The maximum voltage may reach as high as 1.069 per unit under this operating condition when the capacitor bank is energized and there is no load on the North Bus. Based on this result, NEPSI recommended that the capacitor bank be modified with a supervisory relay which monitors the bus voltage and opens the capacitor breaker if the voltage exceeds 1.05 PU.

The energization of the proposed 8064 kvar capacitor bank will create a resonance near the 5.4^th harmonic that is of only slight concern since there is only small amounts of 5^th harmonic current injected from ICC.  If the injection currents come of such magnitude to cause high voltage distortion, three capacitors may be removed from the bank to shift the resonance point to near the 5.9^th harmonic.

Line Notch analysis indicates the capacitor bank will reduce the line notching from 23.9% (existing system with IPL distribution circuit connected to the North Bus) to 0.52%.  If the capacitor bank is changed to a filter bank, the ability of the capacitor bank to reduce line notching will be greatly diminished to 19.6%. 

Based on the forgoing, NEPSI recommended the capacitor bank be installed and energized.  NEPSI also recommended that both voltage and harmonic measurements be taken during the initial startup to check for adverse system conditions and to verify the results of this report.