Power quality - Analogue vs. digital: What’s the difference?
Traditionally on-site power systems depended on analogue control systems to provide
reliable service to a facility's electrical loads. Today, reliable, flexible and user-friendly digital control technology is available for every on-site power system. For traditional standby power systems, that power only emergency lighting for a small building, analogue controls may still be adequate. However with larger on-site and standby power systems, critical 24/7 power needs, more complex power distribution systems, and a mix of linear and non-linear loads digital controls provide greater performance capabilities. Jim Iverson, senior applications engineer at Cummins Power Generation explains
Analogue devices in power system control are discrete components (resistors, switches, capacitors, coils and relays) that coordinate input and output signals, and perform rudimentary logic for specific control functions. Adjustments to the system usually involve a physical adjustment such as increasing or decreasing the resistance of a variable resistor or substituting modules. Analogue control hardware communicates system status and fault conditions with indicator lights, analogue meters or alarms.
In modern generating systems, there may be more than 200 typical alarm conditions having to do with the load, the utility, other paralleled generators, the engine or the alternator. As the number of potential status or alarm conditions has grown, analogue control systems have not been able to communicate this information to operators effectively. A digital control system uses a microprocessor to control input, output and logic functions. System status can be graphically displayed on a computer, and operational adjustments are made through inputs on a keyboard or touch-screen. Changes can be made on the computer screen attached to the power system's master control, or even on a remote computer connected via a local area network or the Internet. Digital systems allow a high degree of control function integration, so one digital control can do the job of several analogue controls.
Reliability
Many power system end-users have learned to depend on their analogue control systems, feeling that they understand exactly how they work and how to fix them when they fail to work. If users don't have experience with digital generator sets, transfer switches, or paralleling controls, they may be reluctant to make the change from analogue.
This attitude is ironic, since the decision to stay with analogue controls actually makes the system less reliable and burdens a facility with controls that are, for all practical purposes, obsolete when they are installed.
Digital generator controls are demonstrably more reliable. For example, digital controls used in the PowerCommand master control systems for generator sets from Cummins Power Generation, have demonstrated a reliability of 300,000+ hours MTBF (mean time between failures). Few discrete components in an analogue control system can approach that kind of reliability. What's more, in an analogue system, all system components need to function properly for the system to operate at all. In contrast, digital systems have built-in redundancy that significantly improves reliability by allowing the system to function properly even with a component failure in one portion of the control circuit. Reliability is also enhanced because the physical electrical interconnections between logic functions have been eliminated with solid-state digital components.
On a more practical level, the number-one reason standby generators fail to start is due to dead starting batteries. Over 80% of all starting failures are from this cause. This shouldn't surprise anyone, because the same thing happens in our own cars. In order to test a battery, whether it is in your car, or on a generator set, a service technician needs to test the battery using a load bank. Basically, the load is applied, and output voltage is observed at the same time. If the voltage drops too low, too fast, the battery should be replaced. Digital power system controls have a function which detects a weak battery. With this function, battery voltage is monitored under load while the engine is cranking. If the battery voltage drops too far for too long, a weak battery alarm is sounded.
System integration
One of the primary advantages of digital controls is their seamless integration of the functional components of power systems. For example, the status of all components and values in a complex power system can be viewed and controlled from a central or remote computer screen. In addition, electro-mechanical equipment (modern gas and diesel engines, alternators, transfer switches) can also be monitored and integrated into the control strategy.
This ability of digital systems to integrate diverse functions is especially important in modern emissions-controlled diesel engines. Integrating engine control functions (fuel rate and injection timing) with fluctuations in generator load is critical for minimizing exhaust emissions from diesel engines. In fully integrated digital systems, these functions are combined in the digital master controller and not isolated in a separate engine governor. The result is better engine performance under varying loads, reduced exhaust emissions, and more stable output frequency and voltage.
The ability of a digital control system to perform logic functions is also crucial in reducing exhaust emissions while starting the generator set. In most generator sets, the engine speed control system does not "know" that the generator set is in a starting mode. Consequently, the control has a tendency to over-fuel the engine during startup, resulting in a cloud of black smoke from the exhaust. This occurs because as power is applied to the governor control, it senses that the engine is a long way from proper speed, so it applies the highest possible fuel rate in order to quickly get the engine to proper speed.
With a digitally based system, the control ‘knows' that the engine is in a starting mode, so it does not immediately try to accelerate the engine to rated speed. Instead, when the engine starts cranking, it checks for engine rotation, and then provides enough fuel to accelerate the engine gradually to rated speed. This practically eliminates black smoke upon starting. Finally, since a digital control ‘knows' what the engine temperature is, it can adjust the governor settings based on temperature, making the engine more stable on starting and more responsive as it warms up.
Reduced space requirements
The move to solid-state electronics and digital technology is not only more effective from a performance and reliability point of view, it is highly beneficial from a space-saving point of view. Depending on the application, digital power system controls can save from 25% to 40% in valuable mechanical room floor space. Digital controls are also more environmentally rugged than analogue, allowing many control systems to be located with the generator set rather than being isolated in a separate dust- and vibration-free room. The result is that digital systems occupy less floor space and require less environmental protection. These factors also help simplify installation, commissioning, and reduce maintenance and repair.
Digital systems provide superior protection
Analogue devices, such as circuit breakers, provide protection for simple power systems by sensing overloads and opening to protect wiring and, in general, the generator set. However, analogue circuit breakers do not do a good job of protecting the alternator as required by electrical codes. Excessive current in the alternator (due to even brief overloads) creates heat, which shortens insulation life and can lead to alternator failure. Molded case circuit breakers (MCCB) offer little protection against alternator overloads and heat buildup.
While today's power grid is actually more reliable than it has ever been, the cost to end-users of a power failure has steadily increased, making any power failure of any duration unacceptable. If the reliability of the standby power system is compromised because of obsolete analogue technology, then the financial risk of a power outage goes up. However, digitally controlled power systems reduce an end-users' financial risk by improving reliability.
Easy access to information
Just as digital computers have exponentially increased our access to information, digital control systems have increased our access to real-time and historical power system operating information. Once information is inserted into a microprocessor-based control system, there are numerous options for making that information available in many parts of the facility-or even remotely. Building automation systems, communication systems, security, and safety systems can all make use of the information from the power system's digital control system. The availability of information makes it easier to manage a facility efficiently and economically.
Unlike analogue systems, digital control systems provide real-time status of all major components within the system. Engine oil and coolant levels and temperatures; battery charge status; fuel levels; and the status of every transfer switch in the power distribution system-are all available on a computer screen on the digital master control in the control room, or even on a secure remote terminal connected via the Internet. With analogue systems, for example, the status of transfer switches can only be known by individual physical inspection of each switch - an antiquated approach that involves more labor and time and increases costs.
Conclusions
Digital controls for power systems offer significant advantages over traditional analogue control systems. These advantages include higher system reliability, lower system life-cycle costs, smaller size, greater operational flexibility, longer equipment life, real-time and historical operating information, easier maintenance, easy system changes through software, remote monitoring and control, and better emissions control.
Share this article with others
Post to del.icio.us
Printed from http://www.electricalreview.co.uk/features/118648/Power_quality_-_Analogue_vs_digital%3A_What%E2%80%99s_the_difference%3F.html
Comment on this article
Skip to comments
We encourage users to analyse, comment on and even challenge Electrical Review's articles, including the one above - 'Power quality - Analogue vs. digital: What’s the difference?'
User reviews and comments that include profanity or personal attacks or other inappropriate comments or material will be removed from the site.
Additionally, entries that are unsigned or contain "signatures" by someone other than the actual author will be removed. We will take steps to block users who violate any of our posting standards, terms of use or privacy policies or any other policies governing this site.