MONITORING For different reasons, one or two phases supplying the power transformer can get to open phase condition while transformer is in operation ABSTRACT Detection of open This article deals with open phase events in power transformers, mainly related to nuclear power plants where phase condition in several cases have led to a global ini- tiative for a prompt solution for this vulnerability. Different solutions are power transformers possible, and one of them, which has already been tested at a Spanish nu- clear power plant, is presented here. Transformer monitoring: Open phase condition Field test results are presented to- 1. Introduction transformer can get to open phase condi- gether with simulations. Power transformers are key components tion while transformer is in operation. in nuclear power plants and have a clear KEYWORDS impact on the operational safety. Some This is an Open phase condition event transformers supply critical components (OPC). Standard protection systems may power transformers; transformer in the plant where safety is a first prior- not detect this situation because the trans- monitoring; open phase operation; ity. Most transformers are supplied by a former magnetic core restores the voltages nuclear power plants three-phase system. For different reasons, in the affected phase, hence not tripping one or two phases supplying the power the protection devices. This situation cannot 82 TRANSFORMERS MAGAZINE | Volume 6, Issue 2 | 2019 Jorge MANZANO, Jordi MENA, Andrés TABERNERO, Marco FERREIRA, Sérgio CRUZ Standard protection systems mayay notn detectdete open phase condition because thee transformer’strans ormer’s magnetic core restores the voltagesoltages in the affected phase problem should be studied in greater overvoltage protection and 46, negative depth. This resulted in the WANO (World sequence overcurrent protection) [7]. Association of Nuclear Operators) SOER (Significant Operating Experience Re- In the case of Trillo Nuclear Power Plant port) 2015-01 "Safety Challenges from in Spain, the applicable regulation is RSK Open Phase Events" [5], in which the pos- 467 (Reactor Security Commission of sible non-detection of the event by the ex- Germany) [8], in which the necessary ac- isting electrical protection systems is iden- tions are established, that must be under- tified and how that influences important taken by the plants. It permits protecting safety equipment. security equipment without including new equipment in the reactor protection The NRC (Nuclear Regulatory Commis- system, because it presupposes that there sion) has established the criteria by which is no qualified equipment. It also suggests the protection design and its ability to that the OPC can be detected by measur- tackle the OPC phenomenon will be eval- ing the voltage or reverse sequence cur- uated, with the Standard Review Plan, rent. BTP-8.9 (Branch Technical Position). The NEI (Nuclear Energy Institute) has de- For its part, the IAEA (International veloped an initiative to give a proactive Atomic Energy Agency) has issued the response from the nuclear industry to the safety report "Impact of Open Phase problem. The VGB (International techni- Conditions on Electrical Power Systems cal association for generation and storage of Nuclear Power Plants" [9]. It indicates of power and heat) has coordinated stud- again the recommendation to detect the ies of motor behavior in an OPC [6] and problem by measuring the voltage or re- has made recommendations to install new verse sequence current. It also reflects that protections. They recommended detec- the option of detecting a single OPC with tion of the OPC phenomenon by measur- toroidal current transformers of higher ing the voltage or reverse sequence cur- class and precision for new protections rent protections (47, negative sequence (51G neutral overcurrent protection) [7] lead to serious damage of the trans former, but if loads like motors are connected to the secondary side, those can end up being supplied by an unbalanced system leading to an over temperature trip or possible damage, depending on their own protection mechanisms. A relevant open phase condition was first recorded in Byron Station (Illinois, USA) 2012 and several other incidents have been reported since then [1]. This has re- sulted in an international need to define the problem and design upgraded solu- tions to avoid it, either at an operational, protection, or electrical level. Subsequently, similar events occurred in several nuclear power plants and, togeth- er with a review of the operational expe- rience, it was concluded that open phase Figure 1. Example of EMS TCM monitoring system. Courtesy of Enging. www.transformers-magazine.com 8 3 MONITORING that generate alarms only, should be pro- ing systems for power transformers are By using this technology, a Portuguese grammable and adjustable depending on subject to a continuous evolution in tech- company named Enging – Make Solu- the load of the transformer (algorithm nology. What is demanded in monitoring tions developed a system that can con- detection). is noise discrimination, high reliability (if tinuously monitor several variables from possible with a longer life time than that of voltage and current measurements (Fig- Technology is continuously supporting the transformer), lesser amount of data to ure 1), compare them with a trans former this evolution and there are different solu- work with (instrument needs to be smart) model and also check magnetization tions being implemented in each nuclear and reliability in diagnosis. Monitoring current. This way, the monitoring system power plant. The idea is to have a 100% Technologies from most common to new- can detect and discern if there is a prob- coverage of all possible situations from no er solutions are: lem either in the windings, magnetic core load to full load of a power transformer. • Dissolved gas analysis (DGA, continu- or tap-changer. The system is called EMS ous monitoring) TCM (Transformer Condition Monitor). • Bushing monitors (either testing C1 2. Power Transformer leakage or on-line tan delta) Monitoring • Tap changer evaluation 3. Open phase solution Traditional maintenance of power trans- • Partial discharge measurement Some open phase conditions can be de- formers was related to field testing pro- • Current and Voltage monitoring tected with protection relays, but usually gram during planned plant outages. Then either the relay doesn’t cover the full ran ge it was performed as a set of usual tests on This last technology lies between protec- of operation or the protection relay is the power transformer: DC Insulation, tion and monitoring. Systems record data special ly dedicated for the application that Tan delta, FRA (Frequency Response from actual current and voltage trans- makes the cost higher. Analysis), FDS (Frequency Domain Spec- formers to diagnose problems in evolution troscopy), etc. This work [2] has been up- [3, 4] in the transformer. The advantage In general, international organizations graded with improved test sets and soft- is that sensors are already there: current that have analysed the problem, recom- ware support. and voltage transformers. Technology is mend monitoring the phenomenon based non-invasive, and assumes connecting on measuring the voltage and reverse se- Off-line testing has been complemented in parallel to potential transformers and quence current (protections 47 and 46), with transformer monitoring, especially picking a current probe around secondary as well as the currents that are drained to in critical transformers. Actual monitor- wires of current transformers. neutral of the transformer connected to ground (protection 51G). The detection of an OPC simple that affects Some open phase conditions canan be detectedtecte a transformer in no load, or with a low load, if the neutral on the high voltage side with protection relays, but thehe problem is of the transformer is connected to ground, how to cover the full range off operation oro presents great difficulties, since we have voltage levels and reverse sequence cur- the relay’s cost rent that are very small, regenerating the Figure 2. Schematic representation of the system under analysis, for open phase detection (OPD). 84 TRANSFORMERS MAGAZINE | Volume 6, Issue 2 | 2019 voltages and then, neither conventional International organizations that have protections 46, 47 nor 51G can be used. analysed the problem, recommendreco mend With an adaptation/extension of the EMS monitoring the phenomenonenon based TCM system, the EMS OP-TCM system was developed for open phase event de- on measuring the voltage and reverseeverse tection. sequence- and neutral currentt Enging’s OP-TCM is a fully integrated solution designed to solve a problem that has appeared in the nuclear power indus- remotely, the open phase detection sta- (severity factors) is selected with a pro- try: how to detect open-circuits faults for tus through an alarm connection for the prietary algorithm. If the fault indicator off-site auxiliary power transformers that control room, visual LED indication or a is above that threshold, a counter is incre- are serving nuclear generating stations. dedicated software. mented. When the counter reaches 3, this The developed system is capable of detect- means an open phase fault was detected. ing such scenario in both unloaded and The open phase detection algorithm, in The role of this counter is to increase the loaded conditions. The solution lies in the each iteration, checks if any other