Commercial Deployments of Line Current Differential Protection (LCDP) Using Broadband Power Line Carrier (B-PLC) Technology Nachum Sadan - Amperion Inc. Abstract Line current differential protection (LCDP) is a superior form of line protection that employs digital technology with stringent communications specifications. Until now only fiber optics has been deemed acceptable to this application a costly and time consuming requirement. Amperion has worked for six years with its utility partner, American Electric power (AEP) to develop an alternative to fiber. The Broadband Power Line Carrier (B-PLC) technology is a result of that effort and utilizes the existing high voltage transmission line to support LCDP. Thereby substantially reducing communications costs and commissioning time. This paper describes the first commercial deployment of LCDP operating over B-PLC installed in June 2012 - the first such installation anywhere in the world. The paper covers the technical aspects of this new technology in detail with emphasis on the challenges that have already been addressed and expected solutions for those that remain. Smart Grid applications rely on a communications platform, and B-PLC is an emerging technology that could meet these needs. With high speed and low latency, B-PLC can satisfy many Smart Grid applications at both transmission and distribution levels. The LCDP B-PLC application is particularly interesting because it is among the most technically demanding. Other applications include SCADA, substation automation, data backhaul and video monitoring for station security. 1 Amperion White Paper www.amperion.com
1. BACKGROUND Amperion is a supplier of advanced communications to electric utilities located in Lawrence MA, and the inventor of the Broadband Power Line Carrier (B-PLC) technology. Amperion is also the holder of the foundational Broadband Power Line (BPL) patents covering the method of powerline communications. American Electric Power (AEP) is the 4 th largest investor owned utility in the US, with 5.1 million customers in 11 states. AEP owns the nation s largest transmission system, a 39,000 mile network. AEP has employed pilot wire protection on short 69KV lines for decades, with about 300 installations of less than 10 miles long in service today. Pilot wire is a fairly simple to install and accurate scheme but it is now reaching the end of its useful life. The copper based wire is aging and deteriorating and phone companies are phasing out the service. Fiber is a very costly replacement alternative. Conventional PLC is another alternative, but it can t support LCDP, is expensive and is not well suited to short lines. Hence B-PLC was of great interest as a cost effective pilot wire replacement, with the added benefit of a large potential for future new applications. AEP selected Amperion as its engineering partner to develop the B-PLC solution. 2. B-PLC DEVELOPMENT The first attempts to communicate at high speeds over a high voltage transmission line took place in 2006 on a half mile 46KV line in Charleston WV. This proof-of-concept testing attempted to demonstrate permissive overreach transfer trip (POTT) protection, combined with SCADA. While not 100% successful, it worked six of seven times and provided the basis for further exploration, as well as DOE funding. The following year, a 5.1 mile three station 69KV circuit in Newark Ohio was equipped with B-PLC. The longest section of this circuit employed a repeater stage, such that the longest B-PLC segment was 2.6 miles long. A year later, this repeater stage was eliminated, thereby extending the longest unrepeated segment to 4.4 miles. In addition, a 138 KV B-PLC system was installed in Columbus OH later that year. In 2010 Amperion developed the first platform for line current differential protection. In the same year, lab tests were conducted with protective relays from GE and SEL followed by field tests that included fault simulations. In 2011 following the success of the lab and field tests, AEP decided to start commercial deployments and a number of potential sites were selected. In 2012 the first commercial installations of B-PLC were commissioned. 3. B-PLC COUPLING The design team faced a number of technical challenges. The most basic challenge was how to physically attach and inject a low power high frequency signal onto an HV line, and how to transmit a high speed digital signal in such a harsh environment. The first challenge was resolved by using a capacitive coupling technology that is conceptually based on the conventional PLC implementation with a number of improvements and adjustments. Figure 1 shows a conventional PLC system, while Figure 2 shows a broadband PLC system. 2 Amperion White Paper www.amperion.com
Figure 1 Figure 2 Note the similarities and the differences. Both employ a capacitive unit to couple the signal to the High Voltage (HV) line, but the capacitor in Figure 2 is actually a standard lightning arrester which has been selected to pass frequencies from 1 to 40MHz. In this design, the arrester not only couples the B-PLC signal, it also provides added lightning protection for the HV line. A second major difference of Figure 2 is the absence of line traps. And while the conventional PLC system typically operates at 150 KHz and is a slow analog technology, the B-PLC system operates from 2 to 35 MHz and is a digital technology that can support megabit per second data rates with millisecond latency. The second challenge was resolved using phase redundancy and noise cancellation techniques. Figures 1 and 2 are one-line diagrams that do not indicate how many phases are employed. Conventional PLC will typically use just one phase, although multiple phase operation is available. B-PLC typically employs multiple phases. A properly designed single phase B-PLC system is expected to communicate during a fault on that phase since the fault path does not drain the entire signal and the fault induced noise is usually short lived. None the less, the redundancy provided by using more than one phase provides additional confidence in communications during such faults. Also, by using a differential (multi-phase) signaling method, any common mode noise on the two communicating phases is cancelled and emissions from B-PLC signals are reduced. Future versions are under development that will employ all three phases and multiple frequency bands for even greater reliability. See Figures 3 & 4 below. Figure 3 Figure 4 3 Amperion White Paper www.amperion.com
4. B-PLC OPERATION The next set of challenges involved extending the link distance using frequency separation and increasing signal to noise ratio (SNR) while co-existing with other operating frequencies. The solution involved the implementation of a flexible filtering scheme with multiple band pass filters dividing the operating frequency range into separate channels. This enabled the use of different frequency bands in each repeated segment to prevent adjacent segment interference without a need for frequency blockers such as line traps. In order to co-exist with other frequency users such as short wave radio station and avoid interference, a notching scheme was implemented. The notch s central frequency, the width of the notch in KHz, and the depth in db, can all be set as software programmable parameters. Figure 5 Figure 6 Figure 5 shows the entire frequency spectrum and Figure 6 demonstrates the effective use of notching. B-PLC operates in the frequency range from 2 to 34MHz. Since a power line also acts as an antenna, the B-PLC signal radiates and can be a source of interference to other users in this band. This was particularly a concern of amateur radio operators who feared their ability to communicate may be impacted. This issue has been resolved by notching out those frequencies used by ham radio enthusiasts. The notching is highlighted in yellow and clearly illustrated in Figure 6. 5. B-PLC FOR LINE CURRENT DIFFERENTIAL PROTECTION The addition of optical communications (fiber) on modern electric transmission lines has enabled high speed communications between connecting stations. One particularly attractive application for this communications link is LCDP. Important new transmission lines are often equipped with optical ground wires (OPGW) that support this relaying application. But older existing lines do not have such communications channels and adding them is expensive and time consuming. Yet their existing pilot wire protection is wearing out. These factors led AEP to partner with Amperion in developing LCDP over B-PLC. AEP knew that successful development would lead to superior protection of existing lines, while avoiding the costs of fiber communications channels. LCDP is a specialized application with some demanding requirements that B-PLC had to meet. In particular, maximum allowable loop latency of 66 milliseconds and link asymmetry of less than 1.5 milliseconds represented a challenge. Adding to these tight limits on jitter, and it became clear that a 4 Amperion White Paper www.amperion.com
specially designed B-PLC package was required. Figure 7 shows the configuration that was implemented to meet these requirements. Using GE L90 LCDP relays and specially designed TDM converters with jitter buffering, the B-PLC system shown below was tested in the AEP lab with 100% success. Figure 8 shows the implementation diagram of the jitter buffer inside the TDM converter. The TDM to Ethernet converter with its integrated jitter buffer allowed a plug and play connection with the relay interface. This feature emulates a 64KB or 128KB synchronous data stream over optical fiber. The relay is configured to operate with a fiber optic communications channel and does not know the difference. Figure 7 Figure 8 Figure 9 Figure 10 Figure 9 shows the B-PLC field test setup and Figure 10 shows one of the fault simulation test results. Multiple tests were done for faults both within the zone of protection and outside of it. All tests were 100% successful. The success of the field tests was the basis of AEP s decision to proceed with commercial deployments. Figures 11 and 12 show commercial applications AEP and Amperion deployed in 2012. 5 Amperion White Paper www.amperion.com
Figure 11 Figure 12 Figure 11 shows two substations in Portsmouth Ohio that are connected with B-PLC. Figure 12 shows two links in Ashland Kentucky. The short link has LCDP enabled with B-PLC. The long link uses a repeater stage and is running B-PLC however LCDP has not been enabled yet. Figure 13 Figure 14 Figure 13 shows a typical panel installation with an Amperion Phoenix B-PLC gateway, a GE L90 relay and a SEL 311L relay inside a station control building. Figure 14 shows three B-PLC couplers installed on a pedestal in the switchyard and connected to three 69kV phases. It also includes the differential combiner box in the center that connects the couplers to the B-PLC modems inside the Phoenix gateway using low loss coax cables. 6 Amperion White Paper www.amperion.com
Figure 15 Figure 16 Figure 15 shows the performance of the B-PLC link in Mbps. This graph was taken from Amperion s NMS, a Network Management System that continuously monitors the health of the B-PLC links and gathers and reports statistics. The 24/7 monitoring function which also includes an alert feature insures early detection and correction of any channel communications issues. Figure 16 shows a typical LCDP implementation with B-PLC in a point to point configuration between two stations. The B-PLC solution currently supports any line voltage at 138KV and below. The data from the relay in station A goes through the Phoenix, on to the line attached couplers in station A, over the high voltage transmission lines to the other side at station B where the attached couplers deliver the signal to the Phoenix in station B and to its connected relay. 7 Amperion White Paper www.amperion.com
SUMMARY: LINE CURRENT DIFFERENTIAL PROTECTION (LCDP) WITH B-PLC LCDP standing for Line Current Differential Protection is the most accurate line protection scheme utilities use today to identify a fault on the line and reduce the effect of a power outage. The line is usually protected by very fast relays that are responsible to execute the following sequence of actions in a matter of milliseconds: 1. Identify the location of the fault; this is done using inter-relay communications using fiber or Amperion s B-PLC. 2. Isolate the fault to avoid propagation to other areas of the grid; this is done by opening breakers. 3. Restore power to the affected area from a secondary backup line; this is called service restoration. Amperion s role is to provide the communications medium that connects the relays between the substations. There are a number of line protection methods. Some require a metallic connection for communications and some don t. Those that do not use communications are based on estimates and therefore are not considered to be very accurate. The estimation method uses protection zones and estimates line impedances to identify whether the fault is within zone or out of zone. A fault to ground changes the impedance and the relay detects the change and determines where the fault had occurred. Since impedance changes can also happen due to variance of load this is not the most accurate method. It is widely used because of its low cost but is not so simple to implement since protection engineers have to calculate a number of impedance values to feed into the relays and requires adjustment when the line configuration changes. On short lines such as in the sub-transmission category the use of LCDP is more effective due to the coordination challenges. Hence B-PLC is a good fit for sub-transmission line protection. In contrast to impedance based protection methods, pilot based protection schemes that require a communication channel are very accurate because they rely on a simple and very effective method of comparing currents at the local and remote stations on both ends of the protected line. Under normal conditions the sum of currents in every closed circuit should be zero according to Kirchhoff law, which means that the two currents should be equal. If a fault occurs than the current flows into the fault and subtracting the two measured current values will be greater than zero. Measuring the current values and their ratios can also point to the actual location of the fault without having to guess. Measuring is always a more preferred method than estimating. This makes LCDP a very simple and accurate protection scheme. The key is having a reliable and fast communications channel and this is what Amperion provides with B-PLC. 8 Amperion White Paper www.amperion.com