An increasing number of facilities are exploring and appreciating the communications capabilities of electrical power equipment. Modern electrical system protection and metering devices measure and calculate a significant amount of digital data, and most of this data can be retrieved via communications. Due to the availability of these devices and data, systems can monitor multiple devices in a system-wide manner from a central and/or remote location. For the purpose of this article, these systems are referred to as power monitoring systems. Power monitoring systems are becoming easier to establish, including integration into existing equipment not originally equipped with communications.
This article provides a basic overview of power monitoring systems and outline one example solution. The reference point is a basic monitoring system for a low-voltage (e.g., 480V) system. However, many of the concepts and principles are applicable regardless of system voltage.
Power Monitoring System Basics
A power monitoring system usually offers one or more of the following features:
- Monitor multiple devices or entire systems from one location
- Monitor metered or calculated data (e.g., current, voltage, power, energy)
- Monitor events and alarms
- View system configuration or status
- Control system devices, configurations, and equipment
Power and Energy Monitoring
Power monitoring systems often provide power and energy information, hence the name. This information can be used to monitor and analyze use, efficiency, historical trends, peak usage, energy/cost allocation, cost control, performance, etc. For example, a facility may have a utility billing structure that incentivizes maintaining power factor above or peak demand below certain thresholds. These metrics can be monitored, and in some scenarios, the system could even provide alarms or notifications to pre-warn that values are approaching the threshold.
Overall System Monitoring
A typical power monitoring system can centrally monitor many devices or the entire system from one location. This allows system-wide monitoring of the following:
- Events and Alarms. Remotely monitoring multiple devices or an entire system from one location often translates to faster response time.
- Electrical Service Reliability and Continuity (Uptime). Operations staff can view information such as breaker positions and system voltages in a single snapshot and assess overall status. Figure 1 shows low-voltage circuit breaker data displayed in a power monitoring system human machine interface (HMI). In this example, current, voltage, power, energy, and breaker position (red = closed) information are all based on communications data obtained from the low-voltage circuit breaker trip unit. The HMI also displays breaker and load identification.
- Power Quality. Power factor and system voltages can be monitored. Advanced systems can also monitor items such as harmonics.
- Verification of Protective Device Settings. Equipment, devices, and settings can be frequently changed and interchanged. Device settings can be verified to ensure proper protection and to avoid accidental nuisance trips.
It is difficult to put a price tag on this simple benefit. Having the capability to monitor and/or operate devices without exposing personnel to equipment hazards such as shock or arc flash is significant.
Asset Management and Maintenance
Power distribution equipment communications can offer valuable information about the equipment itself and/or the load or process equipment that it feeds. This information can be used to track asset performance, loading, operating conditions, predictive maintenance, etc.
Some equipment can be controlled or operated via communications. At a basic level, this capability offers safety and operational benefits. Some more advanced systems utilize this for control schemes or protection schemes.
Troubleshoot and Diagnose
Valuable information such as protection target information, date/time, currents, voltages, and waveforms can play a key role in root-cause analysis of power system events and disturbances. Figure 2 shows current waveforms displayed on a remote HMI using communications data from a low-voltage circuit breaker trip unit. This example shows two waveform captures. The top waveform capture shows three-phase currents from a breaker in which the Phase B (red waveform) current transformer wires were discovered to be mistakenly reversed during installation. The reversed wires would result in incorrect ground fault current calculations — and a possible trip depending on settings and phase current magnitude — and incorrect power calculations. The bottom waveform capture shows the same breaker after the Phase B current transformer wiring mistake was corrected.
What Is a Communication Protocol?
A communication protocol is a system of rules that allow two or more devices to transmit and receive information with each other. Modern-day low-voltage circuit breaker trip units commonly utilize the MODBUS communications protocol, which has become an industry standard protocol for connecting industrial electronic devices.
When considering existing systems, it is important to understand that multiple communications protocols cannot successfully coexist on the same local RS-485 network. Legacy OEM trip unit proprietary protocols include Commnet and INCOM. These trip units typically cannot be placed on the same local RS-485 network as MODBUS RTU devices. However, converters are available in some applications.
How Does It Work?
A power monitoring system is made up of master equipment, devices, software and drivers, monitoring devices, and network equipment. The following explanation generally applies to most communication systems, but includes some references to RS-485 modbus serial communications, since it is common in low-voltage switchgear communications.
Typically, master equipment requests and obtains communications data from multiple devices. The data is interpreted and can be displayed on HMIs. Low-voltage circuit breaker trip units typically utilize RS-485 (serial) communications networks (twisted shielded pair, daisy-chained).
Products such as trip units and power meters are the devices, and the network is wired to a master device that interprets the data. Each device must be programmed with a unique address to allow the master to identify each device correctly.
Master equipment must be capable of the same communications protocol (language) as the devices.
Master equipment collects and/or displays the system information on some form of human machine (HMI) or graphical user interface (GUI). This equipment exists in many forms.
Typical master equipment examples include computers, servers, displays, gateways, etc. More complex and elaborate systems may utilize server equipment, gateways, and computers with customized software. However, some turnkey solutions are available, which are more straightforward to install and set up. Two turnkey solution examples are as follows:
- An HMI computer including pre-loaded software and drivers is directly connected to the devices. The same computer is used to display the system information.
- A gateway including pre-loaded software and drivers is directly connected to the devices. The gateway can be accessed by computers over Ethernet to display the system information.
Software, Firmware, Drivers
Master equipment must have software or firmware and drivers, which allow it to interpret the communications information it receives back from devices such as trip units.
Master equipment must have a driver for each device type it is connected to. This driver allows the master to know how to request and interpret specific data from each particular device type. Unfortunately, each device type organizes the communications data differently. Each device type has a published register map that documents each communication register number and the corresponding name for the data in that register. For example, a trip unit manufactured by Brand XYZ utilizes Register 7042 for current Phase A, whereas a power meter manufactured by Brand ABC utilizes Register 188. In this scenario, the master equipment needs a driver for both of these device types to allow it to properly request current Phase A.
In addition, master equipment often includes software or firmware that allows display of the communications information. This interface is often designed to allow the user or operator to view information from multiple devices, ideally in a meaningful, graphical representation. Some interfaces allow viewing multiple devices simultaneously; others only allow viewing information from one device at a time. Many present-day systems utilize Windows PCs with custom software.
Monitoring devices are typically the equipment that measures and/or calculates the data. In the case of power distribution, these devices are often trip units, power meters, or protective relays.
When equipped with communications capabilities, most or all of the metered and calculated data available in the device can be retrieved via communications. In most applications, each device must be configured or programmed with its own unique address, which allows for device identification. This is true for RS-485 MODBUS communications. Each device typically has configuration settings, for example:
- Baud Rate: Speed
- Parity and Stop Bits: Standard configuration settings to facilitate error detection that must match master equipment settings.
- Reply Delay: Setting to allow devices to process the current request before the master sends a new request
Network Wiring, Cabling, Equipment
Presently, most power monitoring systems utilize wired networks. Low-voltage trip units commonly use RS-485 networks (twisted-shielded pair cable daisy-chained from device to device). Since low-voltage power circuit breakers utilize draw-out construction, RS-485 wires must be routed through a connector that is either automatically or manually connected/disconnected when the breaker is racked in/out.
Integrating a Power Monitoring System into Existing Equipment
Constructing a power monitoring system can be involved and can require specialty staff or communications experts. As devices become more technically advanced, power monitoring systems are also becoming more advanced and complex. However, due to the increased availability of communicating devices, integrating a basic power monitoring system can be simple, and can be accomplished without significant effort or significant additional equipment.
Figure 3 depicts an example of establishing a basic 480V monitoring system using third-party retrofit trip units and an industrial HMI computer. The original switchgear and breakers were not equipped with communications capabilities. Note that this system architecture leverages modern circuit breaker trip units, which can be installed on almost any existing power circuit breaker. This allows the addition of communications to occur in conjunction with breaker maintenance or upgrades. Other than the industrial HMI, no additional dedicated communications products are needed.
Example System Components and Devices
- Trip units capable of MODBUS communications
- Industrial HMI computer with serial port capable of RS-485 connection
Example System Requirements and Installation Items
- Trip units on low-voltage circuit breakers must have communications capability if voltage, power, and energy features are desired.
- Install RS-485 network (twisted-shielded pair cable, daisy-chained from trip unit to trip unit) in switchgear and on breakers.
- Set each trip unit with a unique address.
- Configure one-line diagram view and HMI settings.
- Install HMI computer in electrical room or control room.
- In this example, software and drivers were included and pre-installed in the manufacturer’s turnkey product. In some scenarios, PC equipment, software selection, and installation may be separate.
Example System Features and Capabilities
- Field-configurable electronic one-line diagram
- Designed specifically to communicate with low-voltage trip units
- Industrial HMI computer with no moving parts
- Metering, waveforms, graphics
- Pre-installed software and drivers
- Wall-mount in electrical room or control room
Example System-View Screens, Information, and Functionality
- System overview of information from multiple devices at a time
- Breaker positions
- Metered values
- Alarms and status
Example Breaker-View Screens, Information, and Actions
- View waveforms
- View breaker trip history including waveforms
- View breaker settings
- View real-time breaker readings
- View time-current curves
- Execute remote breaker trip (requires enabled security code and local permissive setting)
- View breaker test data
- Acknowledge and reset alarms
Advanced Features and Possibilities
In addition to the features discussed in this article, power distribution communications systems can offer many advanced features. Some examples are listed below:
- Protection Schemes. High-speed communications between devices allows for many possibilities, including simple and complex protection schemes.
- Smart Predictive Maintenance. With the increase in available data, additional analysis is possible. For some equipment and systems, even a slight increase in electrical loading or energy consumption can be used to identify whether service or maintenance is necessary.
- Wireless. Wireless communications is becoming a more practical and acceptable option. Improvements in capabilities, reliability, and security will continue to make wireless communications an increasingly attractive method.
Testing and Troubleshooting
Although testing and troubleshooting is not the focus of this article, additional information and terminology is provided for reference.
- Confirm Network Wiring Integrity. This can be as simple as verifying continuity throughout an RS-485 network (twisted shielded pair).
- Confirm Device and Master Settings. Ensure configuration settings (baud rate, parity, addresses, etc.) are correct.
- Device Unit Test. Performing a simple is-it-communicating test on individual devices is often a good place to start when troubleshooting a suspected device failure.
- Network or System Tests. If the final HMI master equipment is not available, a basic software tool could be used to communicate with individual network devices one at a time over the network connection.
- Device Manufacturer Tools. Some manufacturers offer a simple test device or a simple software tool to allow testing a single unit.
- Software Tools. Several companies offer software to execute simple communications actions such as requesting MODBUS register data.
- Confirm Network Termination Circuitry Recommendations. Many manufacturers of serial communications devices require or recommend some sort of termination circuitry at the end of the line to prevent reflections. Some devices include internal circuitry that can be switched in or out with dip switches, jumpers, or software settings. Others require externally wired termination components. Others do not recommend and sometimes prohibit external termination circuitry. The bottom line is to consult device manufacturers for recommendations or requirements related to termination circuitry.
- Protocol Analyzers. These tools are available as hardware and/or software tools that capture and analyze communications data. They are typically used as an advanced troubleshooting tool.
- ANSI/NETA ECS-2015. ANSI/NETA ECS-2015, Standard for Electrical Commissioning Specifications for Electrical Power Equipment and Systems identifies general communications requirements in the Inspection and Commissioning/Testing Procedures sections.
Installing a power monitoring system offers many benefits. Establishing these systems often requires special equipment and specialty staff. However, due to the increasing availability of communicating devices and equipment, incorporating a power monitoring system might not be as challenging or expensive as you think.
Ryan McClarnon is Engineering Manager at Utility Relay Company, a manufacturer of digital trip units and conversion kits for power circuit breakers, located in Chagrin Falls, Ohio. He is an IEEE member and a Registered Professional Engineer. Ryan has been in the power distribution/systems field for over 17 years, participates in IEEE standards, and represents URC in NETA and PEARL technical activities. He received a BSEE from Cleveland State University.