Transient Testing of Communication-based Distribution Automation Schemes
With an ever-growing demand for a more reliable distribution grid, utilities are constantly looking for ways to optimize their distribution networks via distribution automation. Pole mounted reclosers, which offer a wide range of functionality, can be used to improve networks and reduce outages. With the introduction of built-in voltage sensing on both its source and load side, automation can easily be achieved by building in local logic to isolate a faulted section, and closing in a tie recloser to supply power to customers from a different source. Automation schemes using coordinated protection provide an easy, low-cost solution for improving reliability. The process of isolation and service restoration, however, can take up to several minutes. For critical feeder lines where long temporary outages are not acceptable, reclosers equipped with high-speed peer-to-peer communication can locate, isolate and restore power to unfaulted sections in under 400 milliseconds. With the decreasing costs of communication equipment, such schemes have proven to provide a healthy return on investment in a short time.
To put such projects into operation, utilities often rely on manufacturers to design and implement the automation scheme. Thorough testing of the whole scheme prior to installation is critical to ensure that the switching logic works as desired for various fault scenarios pertaining to the network, and that the communication equipment can support the network traffic. The testing process is integral for the verification of the system, but it can be complex, labor intensive and time-consuming. This article suggests an easy to apply method for testing any distribution automation network that will significantly reduce testing time while covering parameters that can influence the scheme’s correct operation, but is not part of conventional tests performed today.
Feeder Automation Configuration
There are different possibilities for automating feeders using reclosers. A commonly used method is to connect two feeders at an interconnection point using a normally open recloser as shown in Figure 1.
Assuming that a fault occurs as shown in Figure 2, R3 must trip first to interrupt current flow into the fault. To isolate the faulted section, R4 must open next. If the feeder supplied by Sub 2 can carry the additional load between R4 and R5, the normally open R5 would close.
Now that power is restored to customers between R4 and R5, R3 would try to reclose to determine if the fault was only of temporary nature.
Such functionality can be achieved in various ways, with the simplest method being to rely only on local logic in each recloser, which is based on a voltage function. Recloser R3 would then typically try to reclose until lockout for a permanent fault. Meanwhile a timer in R4 would start to count after it detects a permanent loss of voltage and open after a certain time. Because R5 lost voltage on either side it also will begin to count and will close after a defined time.
When purely based on voltage logic, this scheme would take a minute or more to restore power to non-faulted sections, which is not acceptable for critical feeder lines. This isolation and reconfiguration process can be significantly improved by using communication to or between the reclosers in the field. In case of a fault, the recloser could then quickly send an open or close signal downstream to the tie reclosers.
Communication Platforms and Performance
The automation scheme’s performance greatly depends on the communication platform. A wireline network using fiber-optic cables would be one of the fastest solution available, because it offers high bandwidth and low latency. Realizing schemes with new fiber-optic networks, however, is very expensive and thus not economical given the long distances that must be bridged and the number of devices that must to be connected. Wireless networks on the other hand can easily be established and expanded, and they fulfill both bandwidth and latency requirements. Utilities often have a choice to employ public or private wireless networks. The decision of whether to use a public cellular network or a private network depends on several factors. Costs certainly is one of them, but service availability and coverage are important too. Aside from the communication platform, the scheme’s performance also depends on the number of devices connected, the communication distance, and the communication technology used.
Testing Automated Schemes
Independent of the communication platform decision, a distribution automation scheme is realized with hundreds of lines of custom logic code in the recloser control or with a programmable automation controller or both. Before a scheme is deployed in the field, it should undergo thorough testing to ensure that once installed, the scheme works as expected. Because there are potentially dozens of different desired switching sequences for various faults that can occur at various locations on the feeder, each of those scenarios must be replicated when testing the scheme.
If the scheme is not based on communication and distributed logic, testing the functions could be done with less effort because secondary injection tests into a single device would be sufficient. With communication and distributed logic, on the other hand, the test must include all devices in the scheme to verify the functionality.
Typical Scheme Tests in a Lab
To verify the scheme’s functionality, all devices are set up in a lab prior to installation in the field. Using secondary injection test sets connected to each recloser control, various pre-programmed test sequences are executed to test logic, including that of the communication channels, which is an essential part of the scheme.
Typically, scheme tests require weeks to set up and complete. For every fault in every section of the scheme, load and fault values for each reclose state must be calculated, then a test sequence must be built. For the execution of each test case, the test technicians should coordinate vocally to turn each test set on and off at designated times to simulate each step of the test case. After execution, the results from each location are merged, then assessed. If problems are discovered, the automation logic must be adapted to address the issues.
In addition, it is common for the recloser switch to be connected to the controller during a scheme test in the lab, as shown in Figure 3 (page 23). The primary parting contact gives feedback to the test set and triggers test sequences. With a three-phase secondary current source, a fault is simulated and applied to the device under test. The benefit of this method is that the recloser switch’s delay is considered when testing; however, setting up the entire system in a lab environment can be cumbersome.
To synchronously test a scheme involving several reclosers, a single test set can be assigned for each recloser location. It must be synchronized by using a GPS or IRIG-B signal. In a lab environment, an alternative solution can be to use a single test set connected with additional amplifiers to expand the number of current outputs, like the setup shown in Figure 4 (page 23).
Shortcomings of a Typical Lab Test
While the previously described method has worked well for lab tests, it has some disadvantages. One is that the expected test values must be calculated and programmed into a sequence for every single device, which is time consuming. Another disadvantage is that after running the test, time must be spent comparing time stamps of each test with results on different test consoles, which can introduce additional complications.
Besides the disadvantages described, there are two factors that can influence the performance or even the basic function of the scheme, which are not taken into account when testing such a scheme using conventional sequences in a lab environment. One is the longer communication distances with possibly weak communication links in the field, and the other is waveforms of faults that are different to those static fault values typically used during lab tests.
Influence of Communication Equipment
The communication equipment used in automated high-speed distribution automation schemes is a critical component. The performance of the scheme greatly depends on the selected communication media, which defines how fast and how much information can be transmitted from one device to another. Only certain technologies provide enough bandwidth, security and reliability to wirelessly transmit information. The most used is an IEC61850 GOOSE message. Other commonly used wireless transmitting technologies are WiMAX, LTE or 4G.
The transmitting and receiving functions can be influenced by greater distances in field installations. In the worst case, a weak link or overflow on the communication channels can cause the automated scheme to fail. For this reason, a synchronized injection test into all devices in the field is recommended to test the scheme under real-life conditions.
Testing under real-life conditions requires the test values to match those of a real fault as closely as possible. Protection devices can detect and determine whether a fault is present within a few cycles. With conventional testing methods, static fault values are typically applied and then the results are measured. Under realistic conditions, a DC offset is present in cases of a fault, and depending on the algorithms of the protective device or recloser controller, one device in the scheme might detect the fault faster than another. To determine the real performance of the scheme, it is beneficial to test with transient signals.
A New Approach for Scheme Testing in the Field
It is now possible through new software to synchronously test distribution automation schemes in the field without having to connect the recloser itself. This new approach allows remote control of all test sets from one PC. In addition to addressing the issues described in this article, this approach reduces the testing time and provides comprehensive reporting and troubleshooting possibilities.
When performing this field test, protection points might be located several miles away from each other. The system, therefore, must use either an existing communication network or an external cloud service with remote connection.
A test device injects currents and voltages into each protection point in a recloser controller. The primary contact operations are also simulated by the same test device. All test devices in the scheme can communicate with each other. The associated PC runs the simulation software over the existing network. In addition, this PC must be given access to the network to communicate with all the test devices. Individual GPS antennas are used to synchronize all test devices at each protection point.
The power system under test can be modeled in a single line diagram editor, and devices such as reclosers and circuit breakers can be placed in the network, as shown in Figure 5. Infeeds are defined, for example, by entering the source-impedance-ratio (SIR), and by specifying the line data and loads in the network. Using this information, the software can calculate the resulting load and fault values for any user defined test case.
The behavior of the circuit breakers or reclosers or both should be defined within the simulation software. If the circuit breaker and/or recloser opening and closing times are known, or have been previously measured using a breaker timing test device, the software allows input of those times, which are considered when a test is performed. Accurately defined and entered breakers and/or reclosers ensure a realistic test scenario. Various secondary injection test devices have a built-in circuit breaker simulator. With these test devices, binary outputs (52a and 52b contacts) are simulated, which does not require the circuit breaker or recloser to be connected during the test.
For each test case, a fault can be defined and placed in the one single line diagram representation of the scheme. The simulation software then calculates the resulting fault values for each device at each location. When the test is executed, the simulation software sends the calculated test values into all test devices and, once all values are received, the test begins synchronously.
A closed-loop test is most ideal; meaning that after a device issues a close or open command, the software will consider the changed values when calculating the new values. Such testing solutions, however, can be prohibitively costly. This type of testing is not possible with a distributed test, but transient-based testing (Figure 6) offers the functionality of a so-called “iterative closed-loop test.” When this mode is enabled, the simulation software runs a first iteration of the test and, once a device under test in the scheme reacts, the software recalculates the test values, taking this operation into account. It is assumed that a device under test will always react in the same time if the same test values are applied.
After resetting the scheme, the test is then repeated as the second iteration, taking previous operations from the first iteration into account. This iterative process will continue to reset and run until the system under test no longer reacts. This is typically the case once the fault is isolated and power is restored to segments of the network that are not affected by the fault.
Using this test method, the scheme’s correct switching behavior can be assessed and it can be determined if the communication equipment can handle the amount of data once the equipment is installed in the field. Thanks to transient test signals, the overall performance of the automation sequence can be measured and assessed. In addition, it can be determined if corrective measures to improve the performance of the system should be implemented.
The method described here is easy to apply and allows communication-based automated distribution schemes to be tested in the field to verify both the switching logic as well as the communication equipment’s ability to carry the traffic. By adding transient test values, the test can evaluate actual scheme performance. The same test also could be used to compare different communication technologies. By using the iterative closed-loop approach, the switching operations don’t need to be defined prior to the test. The software will “learn” the behavior of the scheme for a certain fault and the engineer must only assess if the switching operations were performed correctly by the devices under test. | PGI
Editor’s note: For more information and details about scheme testing in the field, contact Robert Wang at email@example.com.
Robert Wang received a bachelor of science degree in electrical engineering from the University of Massachusetts Amherst, and a master’s degree in power systems engineering from Worcester Polytechnic Institute. In 2011, he joined OMICRON as a technical support engineer, where he has acquired a vast knowledge of the entire OMICRON family of products. Robert has more recently been positioned as the regional application specialist, focusing on recloser and sectionalizer applications. He is a member of IEEE PES. Reach him at firstname.lastname@example.org.
Stephan Geiger graduated from the Technical College of Bregenz (Austria) with a focus on electrical engineering in 2007. He joined OMICRON electronics GmbH in Austria in 2008 as an application engineer for protective relay applications, where he worked on application notes and custom applications, provided training and later took over project management responsibilities. In 2012, he became product manager with responsibility for OMICRON’s recloser control testing solutions.