Applications

Some Rules to Follow to Achieve Power Quality


Figure 1: Example of software-controlled waveform inputs that can be set up using the Preen AFV-P Series. These and other variants such as dropouts can be programmed to run in desired sequences for testing and analysis.
 

    Today’s electronic devices mostly run on highly accurate and regulated DC. However, they first rely on AC, mostly from the public grid or other sources, which is then converted to DC. The public grid is commonly referred to as the mains supply. Those sources can often introduce errors and disruptions, creating challenges for equipment power supplies. This article will look at traditional power supply technology drawbacks.

Conventional technology can fall short in guarding against the errors that may be introduced or arise from the design of the equipment itself. In this article we will also consider new approaches that are more accurate and reliable, using solid-state power sources. Greater accuracy and reliability in detecting and diagnosing grid-introduced errors enables equipment under test to be brought into compliance with the equipment’s own design specs as well as industry standards.

Using a programmable power supply (PPS) makes it possible to reliably and accurately supply the power in the form (voltage, current, frequency) required. As well, PPS can introduce, detect, measure, and record errors that could affect performance.  Equipment under development (EUD). requires different forms of AC and/or DC power regarding voltage, frequency, and current. In use, however, it mostly relies on the public power grid, as do the instruments used for development.

Rectification Issues

Mechanical motor generators have traditionally been used to convert voltage and frequency, but doing so can frequently introduce errors that affect the performance of the equipment under development (EUD). In addition, military equipment often has custom voltage and frequency needs, which in turn means special requirements for the power supply used in development. It will be more difficult to use traditional methods to provide such custom requirements. 

Mechanical motor generators also have size, weight, and power (SWaP) disadvantages and are now largely being replaced by solid-state devices. In addition to being programmable to meet a large range of requirements, PPS can be supplied with built-in instrumentation. This makes it possible for them to introduce known errors from the AC source supplied to the EUD while monitoring both input and output.

What is expected from a PPS today is, first, stable and adjustable voltage and frequency conversion in the form of a clean output that is protected from any grid-introduced errors. Second, they must be able to accurately measure (and possibly record) both input and output—from both the grid and the supply to the EUD. Third, they must be able to introduce variations in voltage and frequency as well as errors such as dips and dropouts. The ability to measure the effects is of course also required.

Finally, actually the whole purpose of the process, they must be able to help the developer find the source of such errors and correct them. Successful development depends on a clean AC supply as well as the ability to introduce known errors and observe the results. And to achieve this a well-designed, capable, and reliable programmable power supply is a must. In support of such testing and verification, the International Electrotechnical Commission (IEC) has published a set of standards for AC fault detection, testing, and compliance that can be incorporated into a PPS. These standards are described below.

IEC Test Standards for AC Fault Protection

IEC 61000-4-11 is the standard for testing immunity (or susceptibility) to voltage variations like dips, dropouts, sags, and interruptions. These are the most common form of disturbances in the mains supply. Such faults are considered the most common causes of poor mains supply quality, and IEC 61000-4-11 provides a comprehensive set of tests (See Figure 1).  

IEC 61000-4-14 is directed at detecting the effects of disturbances in the mains supply voltage and is often referred to as the “basic test standard.” Randomly varying heavy loads like arc furnaces, rolling mills or large motors with varying loads commonly cause voltage fluctuations. The fluctuations result in voltage drops until the supply can compensate. A common symptom is transient flicker in a lighting system.

IEC 61000-4-28 addresses the immunity of devices to variations in the frequency of their AC power sources. It is also considered a “basic test standard” and specifies basic tests specified to help meet generic standards. Since the frequency of AC mains supplies is usually 50Hz to 60Hz, their effects on equipment are fairly simple to calculate. However, the problems they can cause should not be underestimated.

The fourth standard that needs to be covered by a PPS is the Aircraft Electric Power Characteristics standard, MIL-STF-704 Revision F. Although it is an American military standard, MIL-STF-704 has been adopted by a number of countries around the world including China. The new Preen PPSs address all these issues.

Delivering Clean Power

As mentioned above, traditionally motor generators have been used to convert the frequency and variable transformers to change the voltage. Next, rectifiers are used to produce DC if needed. But solid-state designs are gaining momentum. Some programmable power supplies are transformer-based, while others are transformerless and thus able to include added capabilities. The goal is an electrical power source converted from the electrical grid system to stable, clean power. If there is noise on the grid system or the voltage or frequency fluctuate, the power source will rectify from AC to DC and rebuild to AC or DC form with regulated and low noise output. 

The problems with the motor generator approach are that their output depends on the grid and that they have issues with SWaP. The motors and transformers make them large, bulky, and noisy. And they are expensive and require regular maintenance. In addition, the output transformer can only be adjusted for voltage, so control systems using the AC frequency as a timing reference can enter error states. They can also fall out of sync with other elements in systems controlled by more accurate DC timing sources.

AC motors increase and decrease rotational speed according to frequency, which can affect their load-handling ability. And, if the electrical loads need simultaneous adjustment of frequency and voltage, using the rotary approach will require the M-G set and an additional variable transformer, adding to the SWaP and maintenance issues. Solid-state frequency and voltage solutions are replacing the outmoded motor generator approach to conversion.

Solid-state conversion eliminates transformers by using insulated gate bipolar transistors (IGBTs). The IGBTs first convert the AC to DC and next reconvert it to AC. This method enables AC output matched to the voltage and frequency needed by the project at hand. Sometimes, if variable voltage is required, a variable output transformer will be used in the PPS.

For example, supporting solid-state design approaches with a variety of product series, the company’s AFC series is suitable for the Certified Bureau. In addition, the AFC series will work in the production and R&D of various industries including home appliances, electronics, medical equipment, and lighting. The AFV+ series is a high-power programmable AC power source that delivers power with THD ± 0.5% and up to 2000kVA.

Ideally suited to simulating different regions’ voltage and frequency conditions, it features programmable functions to easily simulate single or continuous output changes (Figure 2). These and additional optional features make it suitable for R&D design verification, quality assurance, and production checks.The solid-state approach has become much more economical recently, so its many advantages are now within reach. The absence of moving parts improves SWaP, lowers maintenance costs, decreases temperature and electrical noise, and raises overall efficiency.

Figure 2: The AFV+ series employs a 7" touchscreen to provide intuitive and easy-to-use control and display. Users can quickly access output settings and measurements, including voltage, current, frequency, real power, apparent power, PF and sum of each phase’s parameters. Complex sequences and system configurations can also be done via the touch screen.



There are advantages and disadvantages associated with having an output transformer, which can be an option for a PPS. A PPS designed without an output transformer can provide a wider frequency range and more power line disturbance testing features (e.g. transient test or DC offset test…etc.). However, having no galvanic isolation on the output can result in damage to the devices if there is a malfunction on the test unit or if reverse current goes back to the power supplies. A PPS design that includes an output transformer is more robust with regard to inductive load, while those without an output transformer more closely match the needs of transient testing, wider frequency applications, or harmonic testing. The biggest advantage of a PPS, however, is its programming capabilities.

Setting up a Programmable Power Supply

A programmable power supply can be set up using the IEC test standards described above. Once set up, the PPS can test, record, diagnose, and certify the equipment under development as well as test and do fault analysis on that equipment and the supply being designed into it. The objective is to make sure that it performs reliably and is fault-free in the field. To do this, the PPS itself must be certified as IEC-compliant and have the ability (preferably built-in) to set up and test in compliance with the IEC standards. In this example, the AFV+ supports applications requiring galvanic isolation protection.

The AFV+ Series is designed to simulate different regions’ voltage and frequency conditions and can cover applications for home appliances, motors, medical equipment, lighting, and EMC laboratories. Features included and which should be sought in any PPS include STEP and RAMP programmable functions to simulate single or continuous output changes. Three-phase independent adjustment, optional remote sensing, and optional phase angle adjustment all provide convenient control to simulate different kinds of line disturbance plus dropouts.

These features are designed for test applications of R&D design verification, quality assurance, and production checks. The AFV+ Series has a standard RS-232/RS-485/RS-422 interface card as well as an Ethernet interface for easy setup, programming, and remote control. Versatility is another plus: Output can range from 5 to 520 VL-L with custom voltages available along with set frequencies of 50Hz and 60Hz and customizable frequencies from 45Hz to 500Hz.

Summary

For the most part, power sources for you equipment design and testing come from the power grid. The power grid does not always provide stable and clean AC power sources. It is, therefore, important to ensure the power sources are clean, reliable, and does not introduce errors. Follow the IEC Test Standards for AC Fault Protection. When traditional motor generators and variable transformers are used to convert frequency and voltage, ensure the outputs are clean.

Compare the traditional approach with the solid-state programmable power supply approach to obtain the optimal results. In other words, consider performance, cost, and long-term requirements to determine which approach is most suitable for the design. Compare with the traditional approach, the programmable power supplies which use solid states IGBT, can deliver more accurate and reliable AC sources.