An Alternative Solution for SCR-Controlled Transformer-Coupled SiC Heating Elements
A better solution combines the best aspects of both methods. With the advent of digital SCR technology, Control Concepts Inc. began developing a new hybrid firing mode: Zero Cross Transformer Mode, or ZCT Mode.
This innovation was thoroughly tested with an O’Brien & Gere/Denton TSI 48-inch rotary-hearth furnace in a real-world application. The performance of ZCT Mode was measurably superior to phase-angle and zero-firing modes, as evidenced by reviewing the characteristics of these two traditional methods.
Phase-Angle Control Mode
In phase-angle control, the SCR is gated every half cycle by delaying the gating of the SCR sometime after crossing zero line voltage. Power is controlled on a sub-cycle basis (Fig. 1).
Applying voltage every half cycle allows the voltage to be gradually increased to the transformer primary. This gradual turn-on (typically one second or more) eliminates the possibility of forcing the transformer core into saturation. The phase-angle waveform is distorted (chopped), however, so as the demand (% of full-scale-rated current output of the SCR) decreases, the power factor will continue to decrease while the harmonic distortion increases. What is the resulting impact?
Power factor is the simple ratio of real power delivered at the load – watts (KW) –to the apparent power – volt-amps (KVA) –of the supply circuit. It is given a dimensionless number ranging from 0 to 1. The lower the power factor, the higher the current required to deliver a given amount of power to the load. These higher currents require larger distribution-system conductors and infrastructure, incurring higher initial system deployment costs. Higher currents also produce ongoing energy waste in the form of resistive heat loss and persistence of the unusable reactive power (VARs).
Additionally, electric utility providers must provide higher VAs to commercial operations with low power factor and will typically assess substantial penalties in order to recuperate their higher cost of service delivery.
When a resistive load is constant and the conduction angle is full (180 electrical degrees), the power factor is at unity (KVA = KW). In the event the conduction angle is less than 180 degrees, a degradation of the power factor occurs. When using phase-angle control, portions of the sine wave are cut. In this situation a full sine wave of voltage is supplied to the SCR, but only a portion of the voltage is applied to the load. Using Ohm’s Law, the required KVA is a factor of the line voltage times the load current. However, the KW is determined by the load voltage times the load current. The apparent power factor is KW ÷ KVA. So if the applied voltage is 480 V and the load current 100 A, the KVA = 48 kVA. If the load voltage is 300 V and the load current is 100 A, however, the KW = 30 kW. The power factor is:
When power factor penalties are applied by the utility, it is typically for a power factor less than 0.9. The utility provider will usually assess these penalties as part of the demand charge on the power bill. Cost savings are real but dependent on the utility provider and the rate structure negotiated.
Harmonic Distortion Considerations
Harmonics are the higher-frequency components of a waveform fundamental frequency (typically 50/60 Hz) caused primarily by any distortion of the pure sine wave. High harmonic content in the line can result in large induced currents. These currents can be destructive, causing overheating of supply transformers, overloading of neutral conductors and producing interference that adversely affects other sensitive electronic equipment.
Considering the disadvantages, resulting additional costs and potential power-factor penalties are incurred with phase-angle control. It may seem more intuitive to use a zero-voltage-fired SCR unit and eliminate the transformer altogether.
Inherently, zero-voltage-firing (zero-fired) mode (Fig. 3) offers several key advantages when compared with phase-angle mode (Fig. 1). As shown, the sine wave remains virtually undistorted in zero-fired mode, and switching occurs at zero voltage. The resulting power factor is near unity and harmonic content is low.
Most zero-fired controllers have a variable time base that is made up of two periods – an ON period and an OFF period. The unit is on for a number of AC cycles and off for a number of cycles. Most furnace applications are very dynamic, and predetermining an off period is nearly impossible. A transformer is limited in the amount of voltage it can support, and a typical zero-voltage-fired SCR power controller – not being able to limit the amount of voltage in the AC cycle – will cause core saturation.
Silicon carbide is a variable-resistance element that changes resistance with time and temperature. As power is applied and the heating cycle begins, the resistance change can be as much as 70%. Additionally, there is as much as a 4:1 change in resistance over the life of the element. To maintain a required load power, the required load voltage is twice what is needed when the elements are new. Therefore, to maintain a constant power over a 4:1 resistance change, a 2:1 voltage change is required. Power being a product of both current and voltage, the current requirement for new elements is twice that of the aged elements. The required power control system must be sized for the worst-case conditions of both current and voltage.
A New Solution
Instantly applying full AC voltage to the primary of the transformer can lead to core saturation and fuse blowing. Since there is no way of limiting the voltage in zero-voltage-fired mode, the possibility of core saturation is very high. Therefore, a true zero-voltage-fired unit is ineffective. We also know that while phase-angle firing can eliminate saturation, it causes power-factor and harmonic issues. The ideal solution, therefore, is to combine both features into one firing mode – ZCT Mode.
The key challenge driving this innovation is prevalent throughout industry – power quality improvement. The goal was clear: Develop a cost-effective solution to address power-factor and harmonics issues without sacrificing performance, quality and reliability.
A digital power controller makes it possible to soft start to set the core flux and then switch to zero-voltage fired for a period of time (Fig. 4). An intelligent unit can remember how voltage was applied on the last half cycle. Therefore, we can expect the unit to turn on in the phase-angle mode on the first half cycle of any on period. Furthermore, we can set the number of cycles the ramp time will last (8-20 cycles). This adjustable cycle rate will allow operation in various types of transformers since not all transformer cores are the same (Fig. 4).
O’Brien & Gere/Denton TSI installed a rotary-hearth furnace utilizing ZTC Mode in a real-world production facility where power factor was an issue requiring attention (Fig. 5). The furnace design included a 68-kW multi-tap transformer and six secondary, connected SiC heating elements operating at temperatures from 1500-2350°F (816-1288°C). During commissioning, the furnace was tested for uniformity and conformance to the specifications and met all requirements.
Advantages of ZCT Mode
• The combination of soft-start phase-angle and zero cross-firing modes results in a 0.92 power factor or greater with 50% or greater command signal.
• Because this is a firmware feature of a digital power controller, no additional hardware costs are needed to make a change to ZCT Mode.
• Limiting the number of cycles of phase-angle control will also decrease the overall generation of destructive current harmonics in the system.
• The ability to set the number of phase-angle cycles allows for operation of a wide range of transformer designs.
• This solution is energy-efficient and minimizes operation costs.
This new mode of SCR control will help reduce power-factor penalties and harmonic interference. The level of savings will be dependent on the site and the rate structure negotiated with the local utility provider. Operating at a power factor of 0.92 or higher will definitely reduce the potential for power-factor penalty charges by the utility provider. Reducing the amount of phase-angle effect will also reduce the amount of destructive harmonics that are generated. IH
For more information: Contact Dan Bender, director of sales, Control Concepts, Inc., 18760 Lake Drive East, Chanhassen, MN, 55317; tel: 952-345-6205; fax: 952-474-6070; e-mail: firstname.lastname@example.org; web: www.ccipower.com OR Pete Komrowski, product manager, O’Brien & Gere/Denton TSI, 7600 Morgan Road, Liverpool, NY 13221; tel: 315-956-6037; e-mail: email@example.com; web: www.dentontsi.com