Power Quality Monitoring and Power Metering Tutorial

Publish Date: Apr 23, 2013 | 69 Ratings | 3.13 out of 5 |  PDF

Overview

National Instruments provides a variety of hardware and software tools for measuring and monitoring power and power quality. Using modular I/O and programmable software, you can customize a system to meet your exact needs. This tutorial provides an overview of three-phase electricity basics and then discusses three of the most common power quality events: leading/lagging power, sags/swells/interruptions, and harmonics.

Table of Contents

  1. Introduction
  2. Electricity Basics
  3. Power Quality Events
  4. NI LabVIEW Electrical Power Suite
  5. Summary
  6. Related Links

1. Introduction

Power generation and transmission today use three-phase alternating current (AC). To understand electrical power quality monitoring and electrical power metering, you must first have a basic understanding of three-phase power.

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2. Electricity Basics

Power is analogous to a water system. In a water system you have a pipe that can carry water. The larger the pipe the more water it can carry. To move the water through the pipe you need to pressurize the water and when the water has the ability to move from a high pressure area to a low pressure area (like when you open a valve) you get flow. In the electrical analogy you replace the pipe with a wire. Instead of carrying water the wire carries electrons. The larger the wire the more electrons it can carry. You pressurize the electrons by applying voltage. When the circuit is complete the electrons flow from the high voltage to the lower voltage and you get flow. The flow is known as current.

In a direct current (DC) circuit the voltage and current are constant, or with a constant load. However, in an AC circuit the voltage and the current vary in a sinusoidal manner. The instantaneous voltage and current levels vary over time based on their phase.

In a single sine wave the voltage and current levels vary according to the phase.

Phase in an AC circuit indicates the value of the voltage or current measured in degrees with 360° equal to one complete cycle. The value of the sine of the phase angle is proportional to the voltage level. Three-phase power refers to three voltages that have an offset from each other by 120° or one third of a cycle. Usually three wires carry these three voltages (and currents).

In a three phase power the sine waves are offset 120º.

Three-phase power system is the standard throughout the world because three-phase power system uses fewer and smaller conductors than multiple single-phase systems to provide the same power. Three-phase power also has the ability to drive motors with a constant torque instead of the pulsating torque of single-phase motors. Because each of the phases carries an equal voltage (and current through a balanced load) at 120° offsets, the three phases provide constant total power.

When you use electrical power you must connect a load. With AC power, the ideal configuration is to connect each of the three phases to an equal load, which is called a balanced load. Because the three phases are synchronized at 120° offsets, you can connect all three of these wires together through equal loads to a fourth wire called neutral. This connection is a wye connection because of the way it appears on a phasor diagram.

A phasor diagram showing a wye connection.

The three phases can also connect together in a delta connection. Whereas a wye connection can tolerate unbalanced loads by sending current through the neutral wire, the loads in a delta connection must be balanced because a delta connection does not have a neutral wire.

Electrical power is measured in watts (W) or kilowatts (kW) and occurs when a current flow accompanies a voltage. A watt is equal to 1 volt of potential voltage and 1 amp of current. When you apply power over a period of time you accomplish work. Work is measured by multiplying the amount of power applied by the period of time and is usually expressed in kilowatt hours (kWh). In an AC power system, you can accomplish the maximum amount of work when the voltage and current are exactly in phase and the more out of phase the voltage and current are the less useful work you can accomplish. When the voltage and current signals are 15 degrees out of phase you can accomplish 97% useful work, at 60 degrees you can accomplish 50% useful work, and at 90 degrees out of phase you can accomplish 0% useful work. The degree to which the voltage and current are in phase is expressed by power factor.

Single phase power with a 15º phase lag.

One way to understand power factor is to think about a horse pulling a barge along a canal. The horse must pull the barge from the shore; therefore, the horse is pulling the barge at an angle to the direction of travel. Because the horse is pulling at an angle, not all of the horse’s effort is used to move the barge along the canal. The effort of the horse is the total power or apparent power (kVA); the power used to move the barge is the working power or real power(kW); and the power that is trying to pull the barge to the side of the canal is the nonworking power or reactive power(kVAR). The ratio of the real power to the apparent power is known as the power factor. If the horse is led closer to the edge of the canal the angle of the rope decreases and more of the apparent power is used as the real power, increasing the power factor.

One way to understand power factors is to consider a horse pulling a barge. Only the cosine of the pulling angle multiplied by the total pull of the horse is used to move the barge.

In the case of electricity the power factor is based on the phase difference between the current and the voltage sine waves. If the phase difference is zero then all the apparent power can be used as real power and the power factor is 1. This is also called unity power factor. As the phase difference increases the power factor decreases and you need to supply more currents to give the same amount of real power.

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3. Power Quality Events

Many facilities today have sensitive computerized equipment or telecommunication equipment that use ground as the reference for all the internal operations in facilities and connect throughout the plant. Using ground as the reference makes the facilities susceptible to ground differences and to power quality problems. Whereas many people believe that most power quality problems come from power suppliers, the majority of power quality problems are introduced inside the plant.

Leading/Lagging Power

Because electrical utilities must supply additional currents to compensate for lower power factors, the electrical utilities must increase their infrastructure to generate and handle higher currents. The utilities pass this additional cost to customers as a charge based on power factor. In a plant, the power factor, or difference in the phase of the voltage and current waves, is the result of inductive and capacitive loads. An inductive load, like a motor, causes the current to lag behind the voltage. A capacitor has the opposite effect and causes the current to lead the voltage. Because average industrial sites use 80% of their power to drive motors, most industrial sites tend to have a “lagging power factor”. To help compensate for the “lagging power factor”, many sites install capacitor banks to help correct the power factor and save on utility company charges.

RMS Voltage Variations

RMS, or root mean squared, is the standard way to measure the level of a sinusoidal wave. The RMS value of a sine wave is equal to the equivalent value had the wave been a DC signal. To calculate power, you measure the voltage (and current) levels in RMS. There are three types of RMS voltage variations: a sag, a swell, and an interruption.

A sag occurs when the RMS voltage level drops to below 90% of the typical RMS level, but is greater than 10% of the nominal voltage. A swell occurs when the voltage increases to greater than 110% of the typical RMS voltage. An interruption occurs when the RMS voltage falls to below 10% of the nominal voltage.

Sags are the most common power quality disturbance and are usually the result of problems within the facility as opposed to supply problems from  electric utilities. Sags caused within the facility often come from load variations or improper wiring. One common cause of sags is starting an electrical motor. Starting a motor produces a very high inrush current (sometimes 6-10 times the normal operation current).

Interruptions in a facility typically come from fault protection from a circuit breaker or a fuse. A loose wiring connection sometimes can also cause interruptions.

Swells typically come from a rapid decrease in load such as shutting off an electric heater.

Sags and swells that last longer than three minutes are called sustained undervoltage or overvoltage conditions. A sustained undervoltage condition (also called a brownout) is the result of improper transformer tap settings or supply problems from electric utilities. Sags and interruptions can cause problems in a facility by shutting down sensitive electronics, computers, and process equipment.

Waveform Harmonics

Whereas you can think of power as a clean 60 Hz sine wave, in practice the waveform may contain harmonics. Harmonics occur at an integer multiple of the base frequency (60 Hz, 120 Hz, 180 Hz. . .). Power quality problems related to waveform harmonic usually come from equipment with a nonlinear current draw. Modern electrical equipment such as computerized equipment and telecommunication equipment often use switching power supplies to step up or step down the voltage. Using switching power supplies introduces a non-sinusoidal load that pulls current in short pulses during every cycle. Other nonlinear devices such as digital/electronic components and arching devices (for example, fluorescent lamps) can cause abnormal waveforms and serious decreases in power quality. Pulling power in a nonlinear manner introduces harmonics in an electrical system and can overheat plant distribution transformers. These harmonics can cause current transmission over a neutral power conductor in a wye power system. In an ideal balanced three-phase power system without harmonics no current transmits along the neutral conductor. When nonlinear devices induce harmonics, the current from each of the three phases no longer cancels and the current is forced through the neutral conductor. Electrical systems are especially susceptible to triplen harmonics (3rd, 6th, 9th, etc) because in a three-phase system the triplen harmonics are additive.

Single phase 120A sine wave with a third order harmonic of 35 Amps.

Three phase 120A sine waves with a third order harmonics of 35 A. Current spikes on the neutral line of 180 A peak.

This high current that transmits along the neutral conductor can overload circuits, breakers, and transformers. In some instances, plants have been forced to install power conditioning equipment or a second neutral conductor.

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4. NI LabVIEW Electrical Power Suite

The NI LabVIEW Electrical Power Suite helps you develop a custom three-phase power monitoring, metering, or quality analysis application. The following analysis functions are included with the Full version of the NI LabVIEW Electrical Power Suite and conform to the IEC 61000-4-30:2008 standard:

  • Power frequency
  • Magnitude of the supply voltage
  • Flicker
  • Supply voltage dips and swells
  • Voltage interruptions
  • Supply voltage unbalance
  • Voltage harmonics
  • Mains signaling voltage on the supply voltage
  • Rapid voltage changes (RVC)
  • Measurement of underdeviation and overdeviation parameters

The following analysis functions conform to the EN 50160:2007 standard:

  • Power measurement
  • Energy measurement
  • Aggregation (demand)

Using the NI LabVIEW Electrical Power Suite, you can combine the fixed standardized algorithms from the power industry with the custom capability of a full programming language and modular hardware. When you design systems with LabVIEW software and NI hardware, you have the flexibility to customize file formats, user interfaces/human machine interfaces (UIs/HMIs), web capability, communication protocols, and algorithms.

For more information visit the NI LabVIEW Electrical Power Suite white paper.

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5. Summary

A basic power quality system monitors the voltages and currents from each phase. The power quality system may also monitor the voltages and currents on the neutral line if unbalanced loads or harmonics are suspected. A basic power metering system monitors the RMS voltages and currents from each of the three phases and monitors the power factor. Power quality monitoring and power metering allow plants to perform predictive maintenance, energy management, cost management, and quality control.

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6. Related Links

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