Experimental study on ultra-high voltage transmission line protection device based on RTDS

RTDS (Real Time Digital System) can more realistically reflect the fault characteristics of the actual power system, and compared with the analog dynamic model test system, it has the advantages of easy control of fault conditions and reproducible faults, which makes it play an important role in the research and development of high-voltage line protection devices. While introducing the model and requirements of the dynamic model test of high-voltage line protection devices, this paper focuses on analyzing several difficulties in the current research of high-voltage line protection devices, and gives our processing strategies and methods in the development of a new generation of high-voltage line protection products DF3621. A large number of RTDS tests have shown that DF3621 performs well in special conditions and conversion faults, and the overall performance of the device is reliable and can meet the protection requirements of high-voltage lines. This device still needs to be further tested in on-site trial operation.
Keywords : RTDS; high-voltage line protection; oscillation lockout

Test Study of Extra-High-Voltage Line Protection Based on RTDS

Mao Peng ¹ ， Yang Li-fan ¹ ， Du Xiao-gong ¹ ， Li Xiao-bing ¹ ， Suonan Jia-le ²

(1 Dongfang Electronic Group, Yantai, 264001; 2 Xi'an Jiaotong University, Xi'an, 710049)

Abstract : RTDS (Real Time Digital System) can really simulate fault states of the practical power system, and has more excellences than analogy dynamic simulation system, such as being easy to control and fault state could reappear etc. Because of its characteristics, RTDS plays an important role in the R&D process of extra high voltage(EHV) line protection relay. This paper introduces the test model and requirements of EHV line protection relay, and analyzes several study emphases of line protection device in detail. Scheme and method for these problems in developed new high voltage line protection DF3621 are presented. Lots of RTDS test results show that DF3621 has high reliability and satisfactory performances to special system states and faults such as faults during swing and translating faults etc.. The protection still requires more practical running tests .
Keywords : RTDS, High Voltage Line Protection, Swing Block


0 IntroductionDigital simulation of power systems is not limited by the scale and structural complexity of prototype systems, can ensure the safety of the systems being studied and tested, and has good economy and convenience. It has attracted the attention of researchers and plays an important role in power system planning and design, device research and development, operator training, etc. ^[1] As we all know, power system protection devices, especially those for high-voltage/ultra-high-voltage power grids, require Sufficient reliability, adaptability to various working conditions of the power system, and sufficient sensitivity under any fault type to quickly and accurately remove the fault to ensure grid stability and equipment safety. In the field, the device is inactive for a long time except for a short time of action. Therefore, there is very little actual fault experience for reference, and even less opportunities for actual power grid testing. As a result, high-voltage power grid protection devices have always been a In the development and research of this high-tech and high-threshold industry, practical experience in power systems and dynamic model tests play an important role. Compared with the traditional dynamic model test, the fully digital RTDS dynamic model system has many advantages, such as convenient model construction, multiple system models, and repeatable reproduction of similar fault conditions, which provides an extremely favorable means for the development and research of protection devices.and elaborates on several special processes in the RTDS test. The paper analyzes the test results of the current ultra-high voltage microcomputer line protection, points out several difficulties in the current research, and gives our treatment plan and test results for reference by peers. Research on special problems needs to be carried out, and the devices developed and manufactured are also Further on-site trial operation is required. 1 RTDS dynamic simulation test system 1.1 RTDS test model ^[2] RTDS is a digital dynamic simulation system developed by the Maniloba DC Research Center (HVDC) in Canada and is specifically used for real-time research on power systems. The power system component models and simulation algorithms are based on the EMTP and EMTDC that have been recognized by the industry and widely used. The simulation results are consistent with the actual conditions of the actual system on site. The system has been promoted and used in many countries and regions around the world. At present, many units in China have introduced RTDS systems of varying sizes. The basic component of RTDS is a group (RACK). Multiple RACKs are connected through a bus. The number of RTDS determines the scale of the simulation system. By inputting the real-time simulation of electrical quantity and switch quantity into the protection device under test, and then introducing the output signal of the device into the switch quantity board of the simulation system, the closed-loop real-time simulation test of the protection device can be realized . The key to RTDS relay protection product testing is the establishment of a test model system. Referring to the test model system of the Electric Power Research Institute and combining the development positioning of this device: applied to single-circuit lines with voltage levels of 220KV and above, the following line models were constructed: To be closer to the actual situation on site and make the test results convincing, all line models adopt distributed parameter models with a voltage level of 500kv. (1) Single power supply no-load long line model (I) Infinite power supply with 400km single-circuit overhead line, infinite The power supply short-circuit capacity is 3000MVA. It is mainly used to test the transient overrun performance of distance protection. (2) Dual-power dual-circuit long-line model (II) The simulation system model is shown in Figure 1. The protection devices P1 and P2 under test _are installed _on the N side and L side of the NL1 line respectively. The line voltage signal required for protection The line current signal required for protection is provided by a 500kV/0.1kV capacitive voltage transformer, and the line current signal required for protection is provided by a 1250A/1A current transformer. The N system is a regional equivalent system with a short-circuit capacity of 3000MVA. The maximum load capacity is 1000MW, of which the motor load accounts for 65% and the resistance load accounts for 35%. 150Mvar shunt reactor. Under normal circumstances, the power flow P=1000MW, Q=480Mvar. Main parameters of transmission line: z ₁ = z ₂ = 0.0193 + j0.2793 Ω/km, z ₀ = 0.1788 + j0.8412 Ω/km, c ₁ = 0.013 μF/km, c ₀











=0.0092μF/km.

(3) Short-line ring network model (III)
The short-line ring network system model is shown in Figure 2. The main line parameters are the same as those in model II. The tested protection devices are installed on the L side and N side of the NL line respectively. M plant, N plant and L system are interconnected through a 500kV short-distance transmission line. M plant is equipped with a generator M1 with an equivalent capacity of 1050MW, and N plant is equipped with a generator M2 with an equivalent capacity of 1050MW. N plant is also connected to a load transformer FB, the capacity of the load transformer is 1200MVA, and the maximum load capacity is 1000MW, of which the motor load accounts for about 65% and the resistive load accounts for about 35%. The L system is a regional equivalent system with two operating modes, large and small, and its corresponding short-circuit capacity is 3000MVA and 20000MVA.

Special attention should be paid to the construction of certain component models determined by the characteristics of the ultra-high voltage power grid line itself, and the design should be as consistent as possible with the actual situation. Its authenticity will directly affect the performance indicators of the protection device. Among them, the use of capacitive voltage transformer (CVT) leads to the secondary signal obtained after the fault has obvious transient characteristics, which is an important test for distance protection in transient overrun test and outlet fault test; in addition, due to the large attenuation time constant of the ultra-high voltage power grid, the non-periodic component decays slowly under certain fault conditions, resulting in saturation of the current transformer (CT). This is also a very important assessment content for the protection device. It can be seen that the characteristics of the high-voltage power grid put forward higher requirements for the protection device. For different channel connection methods, we use the "programmable logic function" of the SEL protection device to simulate and obtain good test results.
1.2 Test content and results
In order to make the developed protection device have a higher starting point and meet the various requirements of high-voltage/ultra-high voltage line protection with excellent performance, the content of SD286-88 "Technical Conditions for Dynamic Model Test of Line Relay Protection Products" is taken as the basic requirement. In addition, considering that this standard was formulated more than 10 years ago, some of the content is no longer suitable for the needs of the current form. Therefore, combined with the actual needs of the current system, we draw on the enterprise standards of other domestic protection manufacturers to formulate our test content and performance requirements.
The line protection device DF3621 we developed is positioned in the high-voltage/ultra-high voltage power system. The basic configuration: mainly based on longitudinal distance protection, three-stage distance protection, stage zero-sequence protection and inverse time zero-sequence protection as backup protection, and a complete set of protection devices with complete auxiliary functions. The device adopts an advanced and reliable software platform, and the hardware platform adopts a 32-bit CPU+DSP mode, which provides a reliable foundation for the overall performance of the protection. The protection principle is complete and advanced. On the basis of absorbing the advantages of similar protection products in China, some mature research results such as self-adaptation and pattern recognition are added, so that this device can meet the basic requirements of current protection devices and also have satisfactory results for faults under special working conditions.
This RTDS test belongs to the means of the development stage, so the test items fully include all the test items of the Electric Power Research Institute and some relevant standards of the network bureau, and we have added some test contents: complex faults, ultra-high resistance (greater than 300) grounding faults, grounding faults through transition resistance during oscillation, and severe CT saturation conditions. The basic test results of DF3621 are shown in Table 1.

Other items such as PT disconnection, CT disconnection and saturation, manual unloaded line and fault line were also fully tested. After thousands of tests, it was shown that all indicators of DF3621 high-voltage line protection device can meet the requirements, the performance is stable and reliable, and the fault response under special working conditions has also obtained relatively satisfactory results. 2 Several difficulties of high-voltage line protection device ^[3-6] China's microcomputer protection products have been put into practical use for more than ten years. On the basis of completing all the functions of the previous integrated protection, the actual performance improvement in terms of protection function is not large. It can also be said that the advantages of microcomputers have not been fully utilized in terms of protection principle. Especially with the rapid development of soft and hard technologies, some mature advanced algorithms and intelligent analysis methods can be fully introduced into the protection. Of course, the development of protection devices is based on reliability. We are also based on this principle and have conducted experimental attempts to solve the problems that are not ideal in the current protection devices. We also hope to attract the attention of peers to such problems. 2.1 State recognition and adaptation Strictly speaking, state recognition and adaptation involve many aspects. Due to the limited space and focus of this article, several main aspects are proposed for discussion. At present, for domestic protection devices, after the start element is actuated, the subsequent fault handling process is carried out based on the start time. For example, after the sudden change is actuated, the fast stage, steady state stage, oscillation lockout stage, post-tripping stage and non-full-phase stage are successively executed. Other contents such as PTDX and acceleration are included in it. It should be said that this scheme is unreasonable. The protection processing scheme we have formulated is for the primary and secondary states outside the protection device. Except for simple single faults, the program processing and system state are consistent. Most of them will lead to inconsistency and affect the overall performance of protection. A few examples are listed below: (1) In order to ensure that the start element can be actuated under various possible faults, the setting sensitivity is very high, so the start element action is often not the actual time of the fault occurrence; in order to take into account the rapid removal of the near-end fault and the transient overshoot at the end, the distance protection generally adopts the method of releasing the protection range based on the start time as the reference stage. This strategy will inevitably lead to the result that the transient overshoot test action time will not be too fast. In addition, for faults caused by small disturbances that occur some time after the start, transient overshoot will inevitably occur. (2) At present, microcomputer distance protection generally adopts short-time opening, and enters the oscillation lockout processing module after 150ms (based on the start time), and opens the protection by adding conditions. It should be said that this scheme focuses on reliability considerations and has positive practical significance for the stability of China's power grid. However, the oscillation lockout program is only run 150ms after the sudden start element is activated. It should be said that this element is too sensitive, which leads to the failure of the protection delay after the small disturbance causes the start under the condition of system stability. (3) During the non-full-phase operation of the system, due to the existence of zero-sequence current and other characteristics, the protection device automatically puts into the corresponding protection while exiting some protection elements that will malfunction. Therefore, once the non-full-phase state is missed, it will inevitably affect the overall performance of the protection device. At present, some protection devices only judge whether the system is in a non-full-phase state by whether the three-phase circuit breaker at the protection end is closed. This is inaccurate and will cause the protection device on the priority reclosing side after a single trip to mistakenly believe that the system has resumed three-phase operation. The fundamental reason for the emergence of the above problems is that the protection device fails to make full use of its powerful memory and analysis functions to accurately identify the system state. The corresponding processing module is invested through the state identification results. This process itself reflects the adaptive idea. The newly developed line protection device DF3621 changes the previous practice of adjusting the protection range based on the start-up time, and uses the waveform of the current and voltage used by the component (the concept of waveform coefficient is proposed) as the basis for adaptive selection of digital filters, adjustment of protection range, adjustment of direction component action range and other links. Theoretically, it can be seen that: at different fault moments, the non-periodic components and higher harmonic components in the fault transient component are different. After the use of adaptive adjustment filters, the possibility of amplifying harmonics in certain cases by uniformly using differential Fourier algorithm can be avoided. After CT saturation, the short data window is used adaptively to ensure the authenticity of the calculated amount. This is directly positioned as a processing method that reflects the credibility of the component input quantity, which fundamentally solves the problem that signal distortion caused by any reason will not affect the protection performance, and also ensures the possibility of rapid fault removal within the transient exceeding index range under any fault condition. The emergence of system oscillation state has precursors and requires a process, which provides us with the possibility of oscillation state identification. We can make full use of the strong memory function of microcomputer protection and adopt the state prediction algorithm to ensure that the oscillation lockout processing module is put into use in time before the system oscillates. The practice of considering the system as oscillating 150ms after the sudden change is started is cancelled. The action performance of the protection device is ensured when the system fails again when there is no oscillation after the small disturbance is started. This improves the performance index of the protection device as a whole. For the identification of non-full-phase state, while monitoring the tripped phase quantity and switch quantity, the sensing behavior of each protection element and the change behavior of the system zero-sequence quantity should be comprehensively utilized to identify the non-full-phase state of the primary system. 2.2 Phase selection processing during oscillation ^[7] The processing scheme of the oscillation lockout module, starting from the aspects of ensuring the stability of the power grid, has the basic requirements that the protection does not malfunction in the case of oscillation, and secondly, the protection can operate correctly when a fault occurs during oscillation. At present, the protection devices can basically meet the first performance index, while the second index is generally achieved by delaying the protection action and reducing the performance index, that is, whether the fault can correctly select the phase, correctly calculate the impedance, and withstand the transition resistance in the oscillation is not too strict. As a protection technology staff, we hope that the protection performance index will not be affected under this working condition, which is also the focus of further research. At present, the phase selection element in the oscillation is only realized by the sequence component and the calculated impedance after the fault opening condition is met. It is bound to lead to certain fault conditions. It can only be opened when the potential angle between the two ends is within a certain range, so that the protection delays the action; or even cannot be opened, resulting in protection refusal to operate or no selective exit. In the newly developed line protection device DF3621, a new phase selection element based on the model identification method is specially developed for the system oscillation state. Its basic principle is to comprehensively utilize the relationship between the various sequence quantities at the protection installation and the relationship between the various sequence quantities at the fault point obtained by calculation, and finally give the correct fault type. Since the opening conditions required by this phase selection element are relatively sensitive, it ensures that the phase can be selected under various working conditions, and the blind area that does not work in the oscillation cycle is greatly reduced, which improves the protection selection accuracy and action time. A large number of RTDS dynamic model tests have proved this point. 2.3 Conversion faults For the developmental faults of different fault types at the same point, the phase selection element and the impedance element are used for parallel real-time calculation. After the fault type changes, the element that first meets the entry section is fixed first, which can meet the test indicators; for the secondary fault, that is, the interval between the two fault conversions is relatively long, and the first fault has been successfully removed. At this time, according to the state detection scheduling, the recurring fault will be removed by the non-full-phase fault processing module, and all performance indicators can be met by general protection devices. The difficulty lies in the forward and reverse complex fault situation, that is, the forward outlet and reverse outlet fault points exist at the same time. If they are different phases, for single-channel high-frequency protection, the remote protection device can only choose three trips without selection, while the near-end protection device hopes to be able to select the correct trip. For this special fault type, if no special treatment is made, the phase selection can only be guaranteed to be correct when the fault power generated by the additional power supply of the fault in the area felt at the protection installation is much greater than the power generated by the additional power supply of the fault outside the area, thus failing to ensure the consistency of protection performance under various power grid structures. To this end, we add a complex fault detection element with sufficiently high sensitivity, and use this element to activate a special phase selection element under this working condition. It comprehensively utilizes the phase selection results of the current sequence component and the voltage sequence component, as well as the historical and current perception of the protection measurement element, and finally gives the correct phase selection result after unified analysis.
















2.4 High-resistance grounding fault
The ability of grounding protection to withstand transition resistance is an important test item for distance protection elements. Distance protection elements are measuring elements, which are different from measuring elements. Within the protection range, even if the measurement error is large, the protection element can operate correctly. It can be seen that the ability to withstand transition resistance has different performances for different points in the protection range. All distance elements based on metallic faults will have weaker ability to withstand transition resistance at the end of the protection range. In this regard, we use a distance measurement algorithm based on zero-sequence current correction as a measuring element for high-resistance single-phase grounding faults at the end of the protection range, thereby ensuring that no matter where the fault is within the protection range, it has a high ability to withstand transition resistance and improves the overall performance of the device. 3 Conclusion After a large number of RTDS tests, it is shown that the performance indicators of the newly developed DF3621 line protection device are relatively satisfactory. Of course, it still needs the test of long-term field trial operation. The RTDS digital dynamic model test system plays a very important role in the development of protection devices. It has changed the original device development model based on experience-based device development, simulated dynamic model test, and long-term on-site actual operation inspection. Making full use of RTDS test as an important means of protection device development will shorten the development cycle and improve the overall performance of the protection device to varying degrees. In addition, we feel in the process of research and development: on the basis of fully understanding the current protection device product processing solutions, inheriting the advantages, for some processing methods that have development era limitations and cannot meet the current actual on-site needs well, new research results can be adopted under the premise of sufficient testing to improve the comprehensive performance indicators of the protection device; with the development of technology, the further improvement of user needs, and the breakthrough in the research of the protection principle itself, the protection device products should also be timely upgraded or developed new products.