Voltage sags, or dips are the most common type of power quality (PQ) event. Knowing the directivity of the sag, or where it originated, is very import when trying to locate its source and to ultimately mitigate the problem.
This application note outlines some rules of thumb to help determine the directivity of a voltage sag.
WHAT IS SAG DIRECTIVITY?
The directivity of a voltage sag is either upstream or downstream from the point in the circuit where the sag was detected. An upstream sag originated on the source side of the power supply – upstream from the monitoring point. A downstream sag originated on the load side – downstream from the monitoring point.
A good example is when measuring at the point of common coupling (PCC) with the utility, which is usually around the utility’s billing meter. This is often the first point to monitor during a PQ survey. At this point, an upstream sag originated from the utility and a downstream sag originated within the facility. This clearly determines the lines of responsibility and is crucial in deciding the next steps.
Sag directivity is determined by comparing the relationship of the voltage and the current during the sag.
UPSTREAM SAG
An upstream sag originated upstream, or on the source side of the monitoring point. During an upstream sag, the voltage and current are both reduced, or go to zero. Simply put, no voltage means no current. Examples are relays, breakers, or other protection devices opening, shorts, etc.
DOWNSTREAM SAG
The opposite situation is a downstream or load based sag. A downstream sag originated on the load side of the monitoring point. When monitoring at the PCC, something in the facility was the source of the sag.
Downstream sags are usually load based, so when comparing the voltage and current they go in opposite directions. The voltage reduction (sag) is coincident with an increase in current. A common example is when energizing a large load such as a motor.
DIRECTIVITY ANSWERMODULE®
Yes, there is an easier way! Many Dranetz instruments, including our Dranetz HDPQ family, include our AnswerModules that automate determining sag directivity and other PQ analysis functions. The Sag Directivity AnswerModule automatically determines the directivity of sags in real time and records the results with the event data. Sag directivity is viewed in the instrument or in our Dran-View 7 software as shown below.
TO CONTACT DRANETZ
Call 1-800-372-6832 (US and Canada) or 1-732-287-3680 for Technical or Sales support
Maybe you have heard of the Smart Grid on the news or from your energy provider. But not everyone knows what the grid is, let alone the Smart Grid. “The grid,” refers to the electric grid, a network of transmission lines, substations, transformers and more that deliver electricity from the power plant to your home or business. It’s what you plug into when you flip on your light switch or power up your computer. Our current electric grid was built in the 1890s and improved upon as technology advanced through each decade. Today, it consists of more than 9,200 electric generating units with more than 1 million megawatts of generating capacity connected to more than 300,000 miles of transmission lines. Although the electric grid is considered an engineering marvel, we are stretching its patchwork nature to its capacity. To move forward, we need a new kind of electric grid, one that is built from the bottom up to handle the groundswell of digital and computerized equipment and technology dependent on it—and one that can automate and manage the increasing complexity and needs of electricity in the 21st Century.
What Makes a Grid “Smart?”
In short, the digital technology that allows for two-way communication between the utility and its customers, and the sensing along the transmission lines is what makes the grid smart. Like the Internet, the Smart Grid will consist of controls, computers, automation, and new technologies and equipment working together, but in this case, these technologies will work with the electrical grid to respond digitally to our quickly changing electric demand.
What does a Smart Grid do?
The Smart Grid represents an unprecedented opportunity to move the energy industry into a new era of reliability, availability, and efficiency that will contribute to our economic and environmental health. During the transition period, it will be critical to carry out testing, technology improvements, consumer education, development of standards and regulations, and information sharing between projects to ensure that the benefits we envision from the Smart Grid become a reality. The benefits associated with the Smart Grid include:
More efficient transmission of electricity
Quicker restoration of electricity after power disturbances
Reduced operations and management costs for utilities, and ultimately lower power costs for consumers
Reduced peak demand, which will also help lower electricity rates
Increased integration of large-scale renewable energy systems
Better integration of customer-owner power generation systems, including renewable energy systems
Improved security
Today, an electricity disruption such as a blackout can have a domino effect—a series of failures that can affect banking, communications, traffic, and security. This is a particular threat in the winter, when homeowners can be left without heat. A smarter grid will add resiliency to our electric power System and make it better prepared to address emergencies such as severe storms, earthquakes, large solar flares, and terrorist attacks. Because of its two-way interactive capacity, the Smart Grid will allow for automatic rerouting when equipment fails or outages occur. This will minimize outages and minimize the effects when they do happen. When a power outage occurs, Smart Grid technologies will detect and isolate the outages, containing them before they become large-scale blackouts. The new technologies will also help ensure that electricity recovery resumes quickly and strategically after an emergency—routing electricity to emergency services first, for example. In addition, the Smart Grid will take greater advantage of customer-owned power generators to produce power when it is not available from utilities. By combining these “distributed generation” resources, a community could keep its health center, police department, traffic lights, phone System, and grocery store operating during emergencies. In addition, the Smart Grid is a way to address an aging energy infrastructure that needs to be upgraded or replaced. It’s a way to address energy efficiency, to bring increased awareness to consumers about the connection between electricity use and the environment. And it’s a way to bring increased national security to our energy System—drawing on greater amounts of home-grown electricity that is more resistant to natural disasters and attack.
Giving Consumers Control
The Smart Grid is not just about utilities and technologies; it is about giving you the information and tools you need to make choices about your energy use. If you already manage activities such as personal banking from your home computer, imagine managing your electricity in a similar way. A smarter grid will enable an unprecedented level of consumer participation. For example, you will no longer have to wait for your monthly statement to know how much electricity you use. With a smarter grid, you can have a clear and timely picture of it. “Smart meters,” and other mechanisms, will allow you to see how much electricity you use, when you use it, and its cost. Combined with real-time pricing, this will allow you to save money by using less power when electricity is most expensive. While the potential benefits of the Smart Grid are usually discussed in terms of economics, national security, and renewable energy goals, the Smart Grid has the potential to help you save money by helping you to manage your electricity use and choose the best times to purchase electricity. And you can save even more by generating your own power.
Building and Testing the Smart Grid
The Smart Grid will consist of millions of pieces and parts—controls, computers, power lines, and new technologies and equipment. It will take some time for all the technologies to be perfected, equipment installed, and systems tested before it comes fully on line. And it won’t happen all at once—the Smart Grid is evolving, piece by piece, over the next decade or so. Once mature, the Smart Grid will likely bring the same kind of transformation that the Internet has already brought to the way we live, work, play, and learn.
Reference
SmartGrid.gov n.d., The Smart Grid, U.S. Department of Energy, accessed 5 January 2021
Published by Tom Shaughnessy, Shaughnessy Consulting Services
208 Jasper Way, San Marcos, CA 92078
408-666-4009
Background
Phase rotation is something that contractors and electricians routinely check during construction and installation of motors and UPS systems. In facilities with only one service, seldom does a rotation change require utility involvement. However, complications can and do arise when a facility has more than one utility service if steps were not taken to ensure the same primary phase rotation exists for each service. This is especially critical if each utility service supplies power to delta/wye transformers and if there are plans to connect the services together at some point.
Typically, the use of fast switching automatic transfer switches inside the facility will bring the primary rotation problems to light. Figure 1 shows waveforms associated with primary rotation problems. The blue and red traces reflect phase A – to-neutral voltages for two different services – 277 volts measured from phase to neutral. There is a 60 degree phase difference between the waveforms. The 60 degree phase difference develops because the primary rotation for one utility transformer leads by 30 degrees and the other lags by 30 degrees. The result is a 60 degree phase difference between the services. It is important to note that both services have the same secondary rotation.
Figure 1: Resulting out of phase voltage waveforms
Not only is the phase difference an issue, but when one measures from Phase A of one service to Phase A of the second service, where there should be little to no voltage differential when the primary rotation angles are correct, there is now significant voltage difference – 482 volts (black trace).
At this point there are no happy answers:
Force the transfer – bad things will probably happen such as damage to automatic transfer switch, blown breakers, possibly even damage to the utility transformers. At the very least, there will be blown fuses.
Install custom phase shifting transformers to one of the primary connections to the automatic transfer switch. This approach will be costly and require significant lead times for the phase shifting transformer.
Change the utility primary transformer leads to eliminate the 60 degree phase shift. This means a serious nighttime effort and a facility shutdown.
The moral of the story is that prior to construction detailed instructions must exist advising that the primary rotation for each planned service has to match. The same applies if an additional service is added.
Please feel free to leave a question in the comments section.
Power Quality Blog 