AEMC Instruments Power Measurement

The measurement of power and power quality includes the basic parameters of voltage, current, watts (kW, VA, VAR) or the basic parameters of energy (kVAh, VARh, kWh). When evaluating the power utilized in a system, there are many options when choosing what to measure and whether recording of the power system is required. Based on your individual goals, purpose and problems, you may desire to measure Power, Power Quality, Power Logging, Energy Management, Event Recording, Transients, Sags, Swells, or Waveforms. Many parameters must be considered when looking at power and power quality including AC or DC voltage, AC or DC current, watts, kW, VA, VAR, PF, THD, Harmonics, or energy including kVAh, VARh and kWh.

In addition to effectively performing basic tasks like measuring voltage, current, and power (wattage), power instrumentation varies considerably and you may consider ease of use, portability, and flexibility as well as capturing waveforms, event and parameter data logging and recording parameters.

One distinction to be made is the difference between Power Measurement and Power Monitoring. Measurement generally refers to real time/instantaeous readings, while Monitoring refers to recording/data logging.

Another distinction to be made is the difference between measuring and monitoring of Energy and Power. The requirements may have overlap but there are different resolution and accuracy needs:

• Energy. Generally measured in seconds with high accuracy, particularly for billing or billing disputes.
• Power Quality. Generally measured in microseconds. High accuracy is less important, though many instruments have revenue grade accuracy.
• Many Power Quality instruments can be used for energy measurement/monitoring, but not the reverse.

Each instrument may vary on capability, but the application and solution desired will dictate which instrumentation will be perfect for the application.

Wiring Systems

There are many parameters needed to resolve most power issues and problems in commercial, industrial, and residential settings. The first consideration is always the wiring system in question. The following configurations are some of the most common wiring systems used for measurement of power and power quality with additional wiring configurations available in specific instruments, including a three phase four wire Delta and others.

Single Phase Three Wire
This is the most common residential service in North America. Line 1 to neutral and Line 2 to neutral are used to power 120 volt lighting and plug loads. Line 1 to Line 2 is used to power 240 volt single phase loads such as a water heater, electric range, or air conditioner.

Three Phase Four Wire Wye 
The most common commercial building electric service in North America is 120/208 volt wye, which is used to power 120 volt plug loads, lighting, and smaller HVAC systems. In larger facilities the voltage is 277/480 volt and used to power single phase 277 volt lighting and larger HVAC loads. In western Canada 347/600V is common.

Three Phase Three Wire Delta
Used primarily in industrial facilities to provide power for three-phase motor loads, and in utility power distribution applications. Nominal service voltages of 240, 400, 480, 600, and higher are typical.

The second consideration is the system itself and the type of Voltage and Current in the system. The following formulas are utilized for DC and AC circuits.

Power Measurement Formulas

For all DC Circuits and Resistive loads, P (power) is related to I (current) and V (voltage) and R (resistance) according to the following formula:
For all AC circuits, including the source, inductive elements and capacitive elements, the formula used for defining power is shown as: Where V_p is the peak voltage, I_p is the peak current, V_rms is the rms voltage, I_rms is the rms in amperes, and θ is the phase angle current and voltage. Cosθ is more commonly called PF, or Power Factor.
For balanced 3 phase circuits, total power can be simplified to But most real world applications are unbalanced loads, so the sum of the individual phases will be more accurate.
In AC circuits, elements such as inductors and capacitors alter the direction of energy flow. The portion of power flow that, averaged over one cycle of the waveform, is known as “real power” (also referred to as active power). That portion of power flow due to stored energy is known as "reactive power”.  In reality, most loads contain resistance, inductance, and capacitance, so both real and reactive power flows to the load. Apparent power is the vector sum of real and reactive power. Apparent power is the product of the root mean square of voltage and current and can be found displayed below in the Power Triangle.

Engineers, building managers, and consultants are concerned with apparent power, even though the current measured as reactive power does no work; it heats the system wiring, causing energy losses as well as possible heat and component failures. Energy and Power is costly. Wasting Power and Energy is not a goal for any user or owner of any system. Once problems are found, wiring systems, components, and generation equipment must be sized properly to handle all the current produced.

The Power Triangle
 
These three types of power -- true, reactive, and apparent -- relate to one another in what is referred to as the power triangle. The relationship between true power, reactive power, and apparent power is shown below as vectors.

• True Power is the horizontal vector. P (watts) = V*I*PF
• Reactive Power is the vertical vector. VAR = V*A*sin(θ)
• Apparent power is the hypotenuse of the right triangle below. The angle between the hypotenuse and the base of the triangle is the impedance phase angle. VA = V*I
• Power Factor PF = cos(θ)  =  Watts/VA

	Resistive Load Voltage and Current are in Phase Power Factor = 1 Examples: incandescent light bulb, heater
	Inductive Load Current lags Voltage. Out of Phase Power Factor < 1 Examples: motors, most power systems
	Capacitive Load Current leads Voltage. Out of Phase Power Factor > 1 Examples: power factor correction capacitors in devices, electrical panels, utility pole. Correction capacitors increase PF towards 1 for more efficiency

What is ELI the ICE man memory mneumonic?

ELI the ICE man is an old memory trick to remember AC current, voltage, and phase angle relationships in inductive and capacitive loads worth mentioning now.

Assuming a purely inductive or capacitive circuit:
ELI reminds us that voltage (E) in an inductive circuit (L) leads the current (I), by 90 degrees.
ICE reminds us that current (I) in a capacitive circuit (C) leads the voltage (E), by 90 degrees.

In a purely resistive circuit, the voltage and current arrive at the same time to the same point.

Transducer Selection

Transducers are used to measure electrical circuits that are beyond the capability of the instrument. There are two types for power measurement: Voltage and Current.

In low voltage applications, the instrument can be connected directly. Potential Transformers (PTs) are used to step down the voltage in medium and high voltage applications.

CTs are used to measure AC current using an inductive principle, which more can be learned about in our Current Clamp Adapter / Current Clamp section by clicking this link Current Clamp Adapter / Current Clamp.

• Current Transformers for power instruments are used to step down the AC current to a safe and manageable value and is typically 5A or 1A full scale.
• Current Transducers for power instruments convert the AC current measurement to a DC voltage and is typically 1.5 V or 3 V.
• Core type is an important selection:
  • With solid core CTs, the conductor must pass thru the donut, so the electrical connection must be broken to install. Typically they are less expensive and used in new installations and permanent installations where regular monitoring is needed.
  • Split core CTs open and can be clamped around the conductor in situ. Split core are available in two types:
    • Clamp on style with a rigid clamp and opening jaw.
    • Flexible style based on the Rogowski principle can be easily manipulated to fit tight spaces. They typically require external power and common lengths are 24, 36, and 48 inches. One trick to amplify low amperage signals, when using flexible CTs, is to wrap the CT in two loops but remember to double the span in the instrument.

When selecting a PT or CT, size it appropriately for the circuit and that it has the proper signal compatibility for the instrument, especially if not supplied by the instrument manufacturer. It could act as a low pass filter. Select ones with a high frequency range. Poor selection can reduce accuracy and bandwidth.

  Power Meters Glossary      Power Quality Overview
 

Power Energy Logging

Power and Energy Loggers measure and save (data log) important parameters, i.e., Voltage, Current, Frequency, Harmonics, Watts, VARS, VA and Energy (kWh and kVAh, kVAh), Power Factor & Displacement Power Factor (DPF), Crest Factor, THD, and waveforms.
 
In these modern days, rising costs of most goods and services force us to continually evaluate and modify our purchasing habits in order to obtain the greatest efficiencies. Having and maintaining this information at our finger tips allows us to react and plan. It is also understandable that as Power and Energy costs increase, we need a mechanism to be able to evaluate our needs and power and energy use. Understanding what is needed and what is being used gives us greater insight into the manner in which we may react and plan our use. The measurement of “how much and when” regarding power and energy is vital and the instruments used are Power and Energy Loggers.

Power loggers are chosen on the basis of information desired and report capability as well as parameters required, length of time for recording values, hand-held, portable, bench top, with or without displays, wiring system (single phase or three phase), and related problems in the system. Many loggers, both Energy and Power, may be programmable and chosen based on the environment capability. There are many alternate displays that may be chosen. Below are examples “only” of a digital display as well as a graphical display. Many Instruments offer both utilizing existing display panels as an integral part of the Instrument as well as software used with a laptop or PC.

Here are examples of a graphical display of a Harmonic distribution. Certainly helpful in determining system problems and establishing a mitigation plan.

• Load Studies. Understanding the system or circuit load capacity before attempting to add increased loads
• Energy Assessments. The ability to evaluate energy used before corrections and after corrections have been made. The establishment of an Energy Profile is required when looking at system data
• Harmonics Measurements. Harmonics existence can be dangerous and damaging. Understanding the existence and possible mitigation required is an advantage
• Voltage Event Capture. Seeking any information that will aid in system evaluation, i.e. surges, sags, swells, transients. etc.

One of the primary factors to consider is the time base and total length of recording for each value. It is common to consider whether the information being recorded is available in real time or in recorded intervals. Data may be collected from a time reference from every second to days and even months. This selection is based on existing problems, intermittent or average values, and costs. Utilizing a logger system offers a comprehensive picture of the Power and Energy Systems and is now available to evaluate and solve Power and Energy problems.

Applications for Power Energy Logging

• Measurement and recording of power system quality (kW, VA, VAR)
• Energy metering (kVAh, VARh, kWh)
• In plant troubleshooting of power distribution panels and individual machinery
• Monitor pad mount transformers
• Determine harmonic problems originating from source or load
• Monitor phase unbalances
• Determine transformer K-Factor
• Setup and troubleshoot variable frequency drives and UPS systems
• Verify correct operation by measuring key power quality parameters
• Harmonics measurements- Uncover harmonic issues that can damage or disrupt critical equipment
• Inrush capture. Check start-up current where spurious resets or nuisance circuit breaker tripping occurs
• Load studies. Verify electrical system capacity before adding loads
• Energy Audits
• Predictive maintenance
• Long Term Intermittent Analysis

Here are additional examples of display options. On the left, the wiring and phase choice are shown, On the right, the details of Voltage, Current and Phase angles are shown on the Phasor Diagram.
 
Power and Energy Loggers may have a multitude of displays and may be chosen using a variety of formats including Analog Data, Digital Data, Digital Graphics, Multiple Screen, Single Waveform, Multiple Waveforms, General Report generation, and Specific Report generation.  It is important to realize that Power and Energy Loggers may also be chosen for building and system monitoring, as well as residential and overall energy audits. For most data logging systems, all important power and energy data is measured, recorded, and possibly analyzed. This will allow you to understand utility costs, improve efficiency, install energy savings devices, and explore alternative energy solutions. For a complete review of the choices you have, please call one of our Power and Energy Specialists for assistance.