Identify Abnormal Power Quality Situations & Potential Failure

Never before has power quality and reliability been such a key issue for facility managers. Not only can the cost of downtime run into thousands, or even millions of dollars per hour, but power quality events can impact sensitive equipment such as servers, motors, process equipment and computers. This end-use equipment is often interconnected within networks, industrial processes and power infrastructure and can be negatively affected by events that arise both from the supplying power system and are generated within the facility. Power monitoring is key to maximizing uptime and ensuring all power infrastructure is functioning properly.

Proactive management of power resources: The continuous capture of all events enables users to develop trend lines and algorithms to maintain real-time illustrations of infrastructure performance and improve reliability, while automated alerting sends alarms to power managers before problems occur.

Preventive and predictive maintenance: Identifying pattern changes enables better planning of maintenance activities and avoid interruptions of critical business practices, while system data allows “just-in-time” maintenance procedures to be developed and implemented.

Early detection of problems: Power quality problems can be detected before they cause damage through benchmarking that sends “alerts” when conditions begin to deteriorate. For contract negotiation, monitoring data documents power quality and demand for utilities or energy services providers.

Troubleshooting: Power monitoring instrumentation has been designed to help identify the nature and severity of power quality problems to prevent them from recurring. Automated power quality software pinpoints the location, magnitude and duration of events to quickly remedy harmful situations

Case Study: Power Quality Problems Can be Costly; Lost Revenue, Interrupted Operations

www.dranetz.com

Despite their differences, continuous-process industries share underlying characteristics: they maintain continuous operations in facilities that represent substantial start-up costs and time, yet can be interrupted or disrupted by seemingly minor fluctuations in power quality. If the product stream is disrupted, lost productivity and lost product can create a large financial burden. For example, a voltage sag in a paper mill can waste a whole day of production and inflict a $250,000 loss, while a 5-cycle interruption at a glass manufacturing facility can cost a minimum of $200,000. It is estimated that three percent of every sales dollar in the US is spent on power quality problems. Seventy five percent of all power quality problems occur inside customer facilities, requiring power engineers and electricians to diagnose and solve these problems themselves.

Unfortunately, these percentages will only increase as loads become more sensitive to power quality events and the power utilities become more decentralized. For facility managers and engineers, understanding and managing power system infrastructure is essential to ensuring reliability of production, optimizing equipment performance, and controlling escalating energy costs. Power monitoring can potentially detect deterioration in power quality before problems arise.

The Dranetz Encore Series Power Monitoring system enables users to proactively monitor their power system, potentially identifying and correcting problems before they occur. Additionally, with the proprietary AnswerModules, this system aids in troubleshooting by quickly identifying the direction, or source, of the problems before and after they occur.

Case Study: Stalled Motors

In this case study, we look at an industrial customer with two 1250 hp motors on a 4kV circuit, whose motors would not start. To determine the problem, we installed a Dranetz PowerGuide 4400 at the motor input. The power monitoring instrument quickly identified the motor itself as the source of the problem. In fact, the motor start caused a deep sag to occur, which impacted the motor control circuitry and stopped the motor from starting. In effect, the motor was "shooting itself in the foot," creating a cycle of non-performance.

The customer was presented with several mitigation options, including adding more capacity to the circuit, installing a constant voltage transformer, or installing an uninterruptible power supply (UPS) system. The customer selected the UPS option, which was installed to protect the motor control circuitry during motor start-up and verified using the PowerGuide 4400.

Inrush currents, such as those associated with motor starting can cause interaction problems with other loads. When motors are started they typically draw 6-10 times their full load, which can cause voltage sags. These events can dim lights, cause contactors to drop out, disrupt sensitive equipment, and as in our case study, affect the successful start of a motor. The use of a power quality monitor that can capture waveforms during long duration start-ups will be quite effective in characterizing and optimizing motor starts.

The Sign of the Sine

by Richard Bingham of Dranetz

There are certain phenomena that occur in nature which show up in so many different places that it is a bit eerie. The sine wave is one of those signs that the universe wasn't just randomly put together. If one plots the speed of the pendulum as it swings back and forth, the graph would be a sine wave. At the top of the arc, the speed is zero. Then it begins to travel faster and faster until the bottom of the arc. After reaching the bottom, it begins to slow down until it hits the other top of the arc and the speed goes to zero. The process reverses as it begins to accelerate in the opposite direction. The resulting graph is show below of the speed of the pendulum versus time.

The same curve represents the change in electrical current induced in a coil of wire as it rotates past two magnetic poles, which is the basis for an alternating current electrical generator. Not wanting to restart the debate between Edison and Westinghouse over whether AC or DC is the best way to transmit electrical energy, AC is how most electrical energy is transferred between electrical suppliers and the loads. Within loads, some devices change the voltage in to DC, such as those typically called electronic loads. Integrated circuits within personal computers, VCRs, or clock radios typically run off of either 5 or 12 Vdc (though newer circuits run on lower voltages (3.3 Vdc or smaller). But it is the AC voltage, or the sine wave, around which most of power quality phenomena is measured.

A sag or swell is determined by a variation in the amplitude of the sine wave. A sag is a reduction in the RMS value of the sine wave, typically below 90% of nominal, whereas a swell is an increase, typically above 110% of nominal. A seven cycle, two stage sag is shown below.

Harmonic distortion is a series of sine waves superimposed on each other. A transient, such as a PF cap switching generated event, can be decomposed into a series of sine waves. The oscillation following the initial negative going transient usually has a frequency between 400 and1500 Hz, compared to the fundamental power frequency of 50 or 60 Hz. As Fourier's Theorem states, any periodic waveshape can be represented by the sum of a series of sine waves. Even square wave and sawtooth waves are but a series of sine waves.

Power factor is based on sine waves. The method of measuring PF in the past was based on the difference in time or phase angle between the voltage and current. This is referred to as displacement PF. Electrical motors, which make up 60% of the load in the United States, have a current sine wave that lags behind the voltage sine wave, as a motor is mostly a coil of wire,which is an inductor. Unfortunately in today's harmonic rich electrical environments, this method no longer works. Today, true power factor is defined as watts divided by voltage amperes, or how much work is done versus the energy transferred.

Though the sine wave is really magical, most designers of equipment to be powered from the AC voltage waveform have made the assumption that the shape of the waveform will be basically sinuoidal, with the amplitude between certain limits (typically +/- 10%) and frequency stable. When events, such as large motor starts or downed wires, cause the sine wave to exceed these limits, it is usually a sign that a power quality event has occurred and possibly equipment malfunction has resulted.