This post is a compact summary of data acquisition systems typically used by energy professionals for energy conservation project. References have been added throughout to guide the reader to original and greater sources of information.
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Nowadays, more and more energy applications require a combination of data acquisition systems for monitoring energy use within the end-user premises, in order to translate raw measurements into actionable, insightful information.
For example, ESCOs and facility managers require data to make the right decisions so as to develop effective strategies to reduce energy use and uncover operational and mechanical inefficiencies. Similarly, with multiple Utility-driven energy management solutions running concurrently within buildings, such as for automated equipment scheduling and demand response programs, measuring and verifying the savings achieved by each energy analytics solution is of utter importance for validating and delivering correctly monetary benefits to the end customer.
Data availability is going to play a more important role in the near future, and choosing the right data acquisition solution for specific project requirements will guarantee the success of an energy conservation project. This article guides the reader through the range of data acquisition equipment typically used within commercial and industrial environments, and provides a go-to guide for energy professionals willing to quickly review viable options.
1. Electrical meters 1.1 Electrical installations 1.2 Three-phase meters 1.3 Three-phase + neutral meters 1.4 Multi-channel three-phase meters 2. PLCs and DDC 3. Current transformers and voltage output transducers 3.1 How they work 3.2 Solid-core 3.3 Split-core 3.4 Rogowski coils 3.5 Selection criteria 4. Portable clamp-on 4.1 Clampmeter 4.2 Rogowski 4.3 Power Quality Analysers 5. Data loggers 5.1 Pulse, events and state loggers 5.2 mA current loggers 6. Data routers 7. Deployment 7.1 Daisy chain 7.2 LAN networking 7.3 Powerline communication 7.4 Wireless 7.5 Hybrid 8. Push mechanisms 8.1 FTP 8.2 HTTP 9. Conclusion
1. ELECTRICAL METERS
1.1 Electrical installations
Electricity supplied through the main distribution board of a building is typically distributed via individual circuits to other sub-boards located across the premises, which are in turn feeding other boards and end-loads such as AHUs, lighting units and appliances.
Such an electrical installation can be represented as a hierarchical tree, as illustrated in the figure below. Each electrical installation will generally be unique to a given building.
Depending on the size of the building, there may be multiple layers of distribution boards between the main distribution board and end-loads, combining heterogeneous types of boards according to the type of loads served. Multiple end-loads may also be fed from a same circuit, meaning that there is not always a separate circuit for each end-load.
A correct understanding of the ramification of an electrical installation is key for thorough and accurate electricity monitoring, as one needs to be certain of what is being measured to manage energy effectively. The analysis of circuit diagrams and labeling efforts are always a requirement prior to deploying metering equipment.
1.2 Three-phase meters
Commercial and industrial buildings are supplied with three-phase power to be able to deliver power to both single-phase end-loads e.g. lighting and appliances, and three-phase end-loads e.g. machinery and air conditioning.
Three-phase meters are generally used in non-residential settings to measure electrical energy consumption and relevant line parameters in all wiring configurations. In the figure below, one three-phase meter is used to measure the consumption of an entire distribution board using three current transformers (CTs), each clamped to a different phase of the supply line.
1.3 Three-phase + neutral meters
In any electrical installation, some current will flow through the protective ground conductor to ground [1]. This is usually called leakage current, and measuring leakage currents is recommended to test the safety of electrical equipment. Leakage current can be an indicator of the effectiveness of insulation on conductors. High levels of leakage current may cause voltages that disrupt normal operation of equipment.
It is possible to locate the source of leakage current by using three-phase meters coming with a fourth current transformer for the neutral line, or with a low current leakage current clamp, both illustrated below.
1.4 Multi-channel three-phase meters
Multi-channel three-phase meters, henceforth N-channel meters, are similar to N three-phase meters combined into one single device. N-channel meters are cost-efficient for monitoring multiple circuits fed from a same board or from multiple boards located in the same room.
In the figure below, one 12-channel three-phase meter measures the power consumption of four distribution boards using 12 x CTs, with 8 channels not being used. With 12 channels, such a meter can combine a wide range of CTs with different amperage sizes, and can be used to monitor up to 36 x 1-phase circuits, 12 x 3-phase circuits or a mix of both.
2. PLCs & DDC
PLCs and DDC controllers are often used as data loggers and data concentrators, to provide a single data collection point to SCADA systems, BMS systems and cloud-based energy management platforms.
Programmable logic controllers are industrial computer control systems that continuously monitor the state of input devices (e.g. sensors and meters), and make decisions based upon a custom program to control the state of output devices (e.g. actuators and alerts). PLC-based energy management systems have gained widespread adoption in self-sufficient and self-contained areas, but they prove to be less practical for building-wide configurations.
More recently, Direct Digital Control (DDC) has provided a more centralised network-oriented approach. All instrumentation devices are gathered by various analog and digital converters, which use the organisation or dedicated LAN network to transport signals to a central controller for decision making. Protocols such as BACNET, LON and Modbus are typically used to network DDC controllers together.
3. CURRENT TRANSFORMERS & VOLTAGE OUTPUT TRANSDUCERS
3.1 How they work
A current transformer (CT) is a device designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an AC transmission line using a standard meter [2].
The most common current transformers used for power monitoring and power controls have a 5A AC current output, but 1A AC currents are also common. A popular alternative is to add a “burden” resistor to the secondary winding to create voltage, producing an AC voltage output transducer (typically of a standard output of 333mV). For instance a 100A/333mV AC voltage output transducer will produce a 333mV output for a 100A primary current. In many occasions AC voltage output transducers are simply called current transformers because they operate using the same basic principles as a current transformer.
One benefit of AC voltage output transducers is the possibility to run longer cable lengths (up to 50 meters) as opposed to a maximum of 4-5 meters for current transformers, due to lower interferences and dissipation on voltage output levels.
Moreover, current transformers with a 1A or 5A output should not be left open-circuited or operated without a load when current flows on the primary conductor [3]. Instead, one should short-circuit the secondary terminals to avoid the risk of shock. A device called a ‘shorting block’ exists for this very purpose, and is recommended for energy projects when metering systems must be retrofitted on live circuits. When installing a 1A or 5A current transformer the secondary terminals must in a first step be shorted via the shorting block, and in a second step be connected to their load. Only then can the short-circuit (shorting block) be removed. 333mV AC voltage output transducer do not require shorting blocks because the current output flowing through their terminals is extremely low.
3.2 Solid-core
Solid-core technology for current transformers and voltage output transducers provides accuracy at low cost. Solid core current sensing devices come in various sizes and are predominant for permanent installation on new equipment and buildings.
Installing solid-core devices is generally not possible on existing machines and facilities, prohibitively expensive or even dangerous if it requires a service interruption, even for a short while (e.g. stopping a production line, a telecom or datacenter power supply, some nuclear plant equipment, etc).
3.3 Split-core
Some current transformers have a “split core” which allows them to be opened, installed, and closed, without disconnecting the circuit to which they are attached. They are convenient for retrofitting data acquisition systems onto existing equipment. But these advantages have a price, making the split core current transformers more expensive and less accurate than the solid-core transformers, although prices have significantly decreased over the years.
3.4 Rogowski coils
A Rogowski coil is used to make an open-ended and flexible sensor that easily wraps around the conductor to be measured. The Rogowski coil technology provides a very precise detection of the rate of change (derivative) of the primary current that induces a proportional voltage at the terminals of the coil. An electronic integrator circuit is required to convert that voltage signal into an output signal that is proportional to the primary current. In other words, the Rogowski coil enables the fabrication of very accurate and linear current sensors, at the price of additional electronics and calibration [5].
This kind of sensor is particularly well adapted to power measurement systems that can be subjected to high or fast-changing currents. For measuring high currents, it has the additional advantages of small size and easy installation, while traditional current transformers are big and heavy.
3.5 Selection criteria
Various parameters must be considered when selecting or designing a data acquisition system [5]:
- Accuracy: Class 1 meters require current transformers with accuracy higher than 1 percent (generally expensive), or to have the meter recalibrated for each single CT.
- Drift: A CT’s drift defines its ability to sustain a given reading over time independent from the initial system calibration. Some variations may be caused by changes in ambient humidity and temperature as well as component aging.
- Linearity: A CT’s linearity defines its ability to retain its characteristics over its full operating range. A high linearity of the analog sensing part is essential to provide accurate measurement over a wide range of primary currents, and especially at low current levels.
4. PORTABLE CLAMP-ON
4.1 Clampmeter
A clampmeter, or current clamp, is a battery-powered electrical device with two jaws that open to allow clamping around an electrical conductor, for measurement of the electric current in the conductor. Such devices are practical as they do not require electrical circuits to be disconnected.
4.2 Rogowski clamps
Rogowski clamps are beginning to replace classical current clamps for use with portable test equipment due to their light weight, high accuracy, and multiple ranges. They typically have storage capability (internal memory or SD card slot), which allow them to measure electrical energy over periods of time and export collected readings through PC-based software for further analysis. Rogowski clamps are very useful for non-permanent metering and they compare favourably to conventional clampmeters which only provide measurements snapshots.
4.3 Power Quality Analysers
A power quality analyser [6] is a piece of test equipment designed to evaluate the quality of power on its input. These devices are available in three-phase and single-phase models. They measure both current and voltage, and detect dips, swells, fast transients, harmonics, power factor, and a host of other parameters that are useful for advanced power troubleshooting. They offer logging over time and in-depth analysis using PC-based software and reporting tools.
Power quality analysers are generally used by energy professionals to assess compliance to EN50160, the European standard that defines the voltage characteristics of the electricity supplied by public distribution systems.
5. DATA LOGGERS
Data loggers can be used to monitor and record parameters in energy use and equipment operation. Typical energy use and operations monitoring systems includes devices for pressure, temperature, current voltage, pulse, event, state and motion. They are commonly used in applications such as utility usage studies, energy audits, testing and verification, building studies and in energy production processes.
5.1 Pulse, events and state loggers
Many meters use pulse outputs to transmit instantaneous energy use information from the meter to another piece of equipment. The pulse counting method involves converting energy in the form of digital pulses comprising ‘0’s and ‘1’s of varying periods in accordance with the rate of energy being consumed over a period of time [7]. In the case of an electricity meter a pulse output will be equal to a certain amount of kWh.
State data loggers are an ideal solution for monitoring and recording run-time, ON/OFF, and state changes. Typical applications include motors, pumps, furnaces, boilers, chillers and security systems.
5.2 mA current loggers
4-20 mA current data loggers capture sensor and control data that vary current as a function of measured parameters. Measurements from process current loops are popular in many applications for load, pressure, torque and other mechanical parameters.
6. DATA ROUTERS
Data routers are necessarily used when Internet access is not available to push data collected on site to remote servers. Data routers will typically provide high speed, secure GPRS/3G/4G LTE data network connectivity to remote sites with mechanisms such as dual sim, remote sms maintenance and automatic reboot to ensure that SLAs for critical applications are upheld.
7. DEPLOYMENT
The strategy to deploy meters across a site and manage data collection will typically depend on the monitoring requirements and site layout.
The traditional setup for monitoring multiple parameters throughout a site consists of deploying one central gateway unit to collect data from distributed meters and sensors, all connected through PLCs, loggers, in daisy chain, over the LAN or through wireless transmitters.
The central gateway unit will generally run data collection software, run some local pre-processing, compress and back-up data locally, and communicate data to online servers for further analysis and display.
7.1 Daisy chain
The daisy chain wiring scheme allows multiple devices to be wired together in sequence. Daisy chain also only allows communication with one device at a time. It is very practical when multiple meters are deployed in a same area, thereby reducing the amount of cabling and necessary inputs at the controller side.
Cabling and setup may however be challenging and may require the use of one or more repeaters within the chain to counteract attenuation, and there is a risk that one link failure in the chain brings down the whole network.
7.2 LAN networking
Pre-existing Ethernet communication infrastructure provides an alternative to transmit meter readings from distributed meters and other loggers to a central controller.
7.3 Power Line Communication
Power Line Communication is another media often used to connect meters and other data acquisition devices. Through PLC, the electrical installation transports communication signals between devices. Most PLC technologies limit themselves to the wiring within a single building, but some can cross between the distribution network and low-voltage building wiring.
When supported by measurement devices PLCs allow two-way communication to enable advanced monitoring and control capabilities.
7.4 Wireless
Wireless deployments enable distributed deployments without expensive cabling infrastructure. Very popular with the monitoring of environmental parameters, wireless meters transmit metered data using a robust mesh wireless protocol to the central controller, or push data directly to online servers using built-in GSM modules.
7.5 Hybrid
8. PUSH MECHANISMS
8.1 FTP
Integration of (pre-existing) metering systems via FTP, typically as a form of a .csv file uploaded once a day or every N minutes, allows key information to be made available for reporting and sharing via email, as opposed to visualisation in a graphical interface on site.
With the advent of cloud-based energy management dashboard, FTP data collection provides a seamless way for 3rd party companies to access measurements taken by pre-existing metering solutions. In such cases, a 3rd party company may provide ftp credentials to allow the metering controller (processing unit or PLC) to be configured to send data to the 3rd party server on top of the existing communication infrastructure.
8.2 HTTP
Integration of (pre-existing) metering systems via HTTP POST, typically as a form of a data stream with packets in JSON or XML format is popular for real-time data acquisition.
Data collected from the meters will typically be stored within the meter controller. The meter controller firmware can either natively push this data to an online server, or a 3rd party software can be deployed on the controller to read the data and push it online to another server. Data can be read through a variety of protocols, such as SQL queries, Modbus and OPC. In some instances, data initially pushed to one server can also be collected from that server and pushed to another server.
9. CONCLUSIONS
There is not one single data acquisition solution that fits all projects. The need to understand unique project requirements will drive the selection of the most appropriate equipment.
The type of data to collect and deployment constraints are the first elements to consider, but once a first shortlist of potential solutions is made another round of considerations must be made to ensure that the solution chosen is certified for the market and electrical system, and to balance the benefits of low-cost against measurements accuracy and other nice-to-have features.
Typical considerations are given below:
- How is the building supplied, is it a Wye or Delta electrical system?
- What type of circuit are you planning to meter, is it a single phase two wire, single phase three wire, three phase three wire, three phase four wire circuit?
- Is the data acquisition equipment certified for use and complies with the essential requirements of your territory?
- Do you have specific accuracy requirements for meter and CTs, for instance planning to use the measurements for billing?
- Do you need your device to store measurements internally for back-up in case of communication failure?
- Do you plan to use your device for EN50160 power quality analysis with current and voltage harmonics?
Wattics is a technology company developing cloud software for energy management, with a mission to assist businesses and energy professionals in turning complex energy data into clear, actionable insights. The common ingredient and enabler for these applications is the availability and access to data from heterogeneous sources, including:
- Energy data: electricity, water, gas, oil, heat, PV output etc
- Environmental data: temperature, HDD/CDD, humidity, CO2, pressure, air flow etc
- Security data: door open, liquid leaks, smoke sensors, CCTV, rack switches, glass break, vibration, occupancy, circuit breaker status, etc
as well as the possibility to remotely change setpoints, and dispatch or switch off equipment through actuators such as relays, PLCs, BMS, DCIM, 2-way thermostats, and more.
Wattics works with a variety of data acquisition system manufacturers worldwide, who are experts in specific market domains and applications. This allows the Wattics online platform to seamlessly integrate heterogeneous data streams to uncover insights and serve the market needs.
Get in touch with us to discuss your project requirements so we can advise you the best approach for your energy conservation projects!
[1] https://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/leakagebasics.htm
[2] https://www.electronics-tutorials.ws/transformer/current-transformer.html
[3] https://www.aimdynamics.com/Articles.asp?ID=254
[4] https://www.elkor.net/pdfs/AN0305-Current_Transformers.pdf
[5] https://www.altenergymag.com/emagazine/2012/06/how-to-choose-and-implement-current-sensors/1920
[6] https://www.controlbooth.com/wiki/?title=Power-Quality-Analyzer
[7] https://www.edn.com/design/sensors/4376414/Flow-metering-tutorial—Part-2–Pulse-based-counting-in-flow-meters
