Digital SCR (silicon controlled rectifier) power control technology was successfully incorporated into two new furnaces in an expansion of the thermal processing department for a supplier to the glass coatings industry, resulting in saving over $2000 per month in real utility power costs. Digital SCR power controllers have the flexibility to control power in the synchronous (zero-cross) mode firing into a transformer coupled resistive heater load. Traditional design dictates phase-angle firing of the power controller to avoid the pitfalls of transformer core saturation.
Signature Vacuum Systems Inc. was contracted to build and install two vacuum furnaces for the customer's thermal-processing department as part of a capacity expansion project. To maximize energy savings for the customer, Signature Vacuum Systems opted to use synchronous firing SCR power controllers to control the furnace heat input. The controller selected was Spang Power Electronics 853, three-phase, digital ac power controller (Fig.1).
A factor in selecting the Spang 853 digital SCR power controller is the unit's ability to fire the SCRs directly into transformers in either phase-angle or synchronous firing mode. This provided an installation that could be operated initially in phase-angle mode (the traditional method of control), and later reprogrammed to zero-cross mode (the desired method of control). This flexibility facilitated the comparison of process quality and power cost levels.
The customer was accustomed to their existing phase angle-fired systems. Using the dual firing mode capability of the 853 controller, the furnace control system was designed to permit the operator to switch between synchronous and phase-angle firing "on the fly." This is accomplished via a Modbus serial link between the operator interface panel and the power controller. This flexibility gives the furnace operator the ability to monitor the balance of voltage and current in phase-angle mode (for short-circuit detection), and then switch to synchronous firing for maximum operating economy.
The specific zone ratings for the installed furnaces are listed below:
- Input voltage: 480 VAC
- Output voltage: 45 VAC
- Output current: 2,566 AAC
- Power rating: 200 kW
The two furnaces each contain two ~200-kW zones for a total of 800 kW. During operation, the furnaces represent about 50% of the total facility power consumption. The installation used identical power systems for each zone. In addition, each zone included an 853 digital SCR power controller and an output transformer feeding the resistive heating elements.
Digital SCR power control
The 853 digital SCR power controller is a state-of-the-art three-phase ac power controller built around digital microprocessor technology. The processor power and flexibility allowed the furnace supplier to:
- Integrate the power control into the automated furnace control system
- Use software to select operating modes
- Connect to the instrument communication network
- Optimize the power controller parameters for the application
- Provide improved process reliability and troubleshooting
In this application, the power controllers included:
- Three-phase 350 AAC units
- Integral local digital controls
- Modbus connectivity to an automation direct PLC
- Fuseless design (eliminate I2T fusing)
- Ability to accept 4-20 mA analog input for temperature control
The benefits analysis was based on the direct comparison of phase-angle to zero-cross process heater control. Key to quantifying the benefits from the use of the digital power controllers are accurate process measurements. A Square D Powerlogic Series 3000 circuit monitor was installed on the input feed to the facility to measure the power parameters. Initially, all zones were controlled in the phase angle-fired mode and measurements recorded for the total process. Each zone was sequentially reconfigured to zero-cross mode and measurements of the same parameters taken at each stage. A summary of the data is provided in Table 1. The data illustrates that the kW requirements remained constant while the KVA demand was substantially reduced. A corresponding increase in power factor (PF) occurred at each stage.
The cost impact of this change in control mode depended on the power costs actually charged to the customer. The power consumption rate schedule applied to the facility by the utility company is shown in Table 2. The energy cost analysis In Table 3 is based on one month of continuous operation at a base load of 900 kW. This analysis assumes all four zones running in either phase-angle or zero-cross mode.
Other process and power consumption benefits were measured as a result of the transition from phase angle to zero cross control. Total harmonic distortion (THD) was reduced on the system. Table 4 shows the THD for both voltage and current as each zone was changed from phase-angle to zero-cross control mode.
When firing SCR power controllers in zero cross mode, consideration must be given to minimizing the power peaks, or power pulsing, that are characteristic of this power control method. (Power pulsing is sometimes detected in the flickering of overhead lighting.) To minimize power pulsing in a multiple-zone installation, the 853 digital SCR power controller provides the functionality of node synchronization. Multiple units may be configured to fire at staggered intervals to avoid simultaneous turn-on and, thereby, minimize the power peaks. For example, if two furnace zones are operating at 50% each, then each zone will be ON for one cycle and then OFF for one cycle. Node synchronization causes the two zones to alternate the ON period as illustrated in Fig. 2. The resultant power draw from the distribution system is constant, and peak power fluctuation is zero. Applying this technique to multiple SCR power controllers operating in zero-cross mode minimizes power fluctuation.
To validate the tangible benefits from the use of digital power control technology, the customer isolated the process, conducted comparative process measurements and calculated the cost benefits from the use of the digital SCR power controllers. This resulted in over $2000 per month savings in power costs. The pay back on the additional investment was achieved through savings in the first electric bill.
Measurable benefits of installing four 853, three-phase, digital ac power controllers include:
- 5% reduction in monthly utility power costs
- 17.5% improvement in power factor
- 66% reduction in total voltage harmonic distortion
- 53% reduction in total current harmonic distortion
- Reduction in radio frequency interference (RFI)
- Virtual elimination of peak power fluctuations