I. Introduction Due to the rapid development of optoelectronic technology and computers, new optical voltage and current transformers are increasingly showing attractive prospects and strong vitality, and gradually replacing electromagnetic transformers is a development direction of relay protection. The construction of digital substations should fully embody the design concepts of intelligent primary equipment and networked secondary equipment. The focus of the primary equipment intelligence is the application of photoelectric transformers and intelligent circuit breakers. The networking of secondary equipment should implement and implement the IEC 61850 standard. The digital substation is designed according to the three-layer structure of process layer, bay layer and station control layer. The application of photoelectric transformers and the general object-oriented substation event (GOOSE) communication technology will completely solve the problem of current transformer saturation and AC-DC crosstalk of secondary cables problem. The construction of digital substations should be based on the premise of safe, reliable and economic operation of the power grid, and effectively solve the problem of repeated investment in data acquisition equipment and interoperability of secondary intelligent equipment. The digital substation raises some new problems for the optical fiber differential protection. This article will explain in detail how the optical fiber differential protection solves the above problems in conjunction with the 220kV digital substation system plan. 2. Application environment of optical fiber longitudinal difference protection The line protection, main transformer protection and busbar protection in the digital substation realize the sharing of the data source and the intelligent operation mechanism through the process-level interval LAN. A pair of optical fiber differential protection between digital substations realizes data sharing and synchronization of sampling clock through leased optical fiber channels. The data source of the digital substation comes from the merging unit (Mu). Mu's sampling adopts the unified clock source SYN of the whole station. The line differential protection of each station also uses the same clock source. Mu data is provided to the relay protection equipment at intervals by the process layer switch, including line protection, bus differential protection, etc. The data of line differential protection comes from two different substations. When there is no unified clock signal in the system, the sampling of the two stations is not synchronized. Differential protection needs to solve the problem of synchronization of sampled data on both sides. 3. The requirements of digital substation for data collection The MU of each line interval of the digital substation provides Ia, Ib, Ic, ua, Ub, Uc, 3Uo, 31o, and one-phase bus voltage data required for line protection. Line protection, main transformer protection and bus protection pass through the process level interval switch Communicate with the MU according to the IEC 61850-9-1 / 2 standard to obtain the above data, and realize the sharing of data sources. Since bus protection has high requirements for synchronization of data collection between intervals, the design of digital substations requires that the synchronization signal for the data collection of the entire station comes from the same clock source. At present, a pair of optical fiber differential protection between two substations adopts the method that the acquisition clock of a certain substation optical fiber differential protection is the main clock, and the other substation optical fiber differential protection adjusts its own acquisition clock to synchronize with the main clock. Increasing MU violates the principle of data source sharing, and realizes the data collection synchronization between substations through the global positioning system (GPS), which violates the rules of safe and reliable operation of the power grid. This leads to the main problem to be discussed in this article: How to synchronize data acquisition between substations? Fourth, the key issues of optical fiber longitudinal differential protection (1) Synchronization of line differential protection In the digital substation, the exchanges collected by bus differential protection and transformer protection are all in one substation. When a unified clock source is used at the entire station, the Mu samples are synchronized and there is no synchronization problem. For line differential protection, the collected power is the current and voltage of 2 different substations. Under the premise that the entire power system does not use the same clock source, line differential protection needs to achieve data acquisition synchronization between substations. Different from the traditional line differential protection sampling synchronization, the acquisition of the AC volume of the digital substation is completed by Mu, and the sampling cannot be adjusted in real time, which requires a new scheme for synchronization. 1. Clock signal synchronization based on ping-pong principle Digital differential substation line differential protection adopts sampling synchronization adjustment scheme based on clock signal synchronization based on ping-pong principle Taking the differential protection of the M station as an example, when the device receives the synchronization signal SYNM of the own station, the "synchronization signal frame" is added to the data frame transmitted to the N station; at the same time, the "synchronization signal frame from the N station" is received. ", Record the time difference Tma at this moment relative to the local synchronization clock, and send back a frame of" synchronization confirmation frame "on the N side. Tma is included in the "synchronization confirmation frame". The sending and receiving process of the N-side device is the same. When the M station differential protection receives the "synchronization confirmation frame" transmitted from the N station, it records the time difference Tnb at this moment relative to the local synchronization clock. After a round trip, the M station protection device knows the time difference △ t = Tnb / 2-Tma of the synchronization clock between the local protection device and the opposite protection device. In the same way, the time difference of the synchronous clock detected by the N station between the local protection device and the opposite protection device is Δt = Tmb / 2-Tna. △ t has positive and negative points. When △ t is positive, it means that the local synchronization clock is ahead of the contralateral synchronization clock: when △ t is negative, it means that the local synchronization clock is behind the contralateral synchronization clock. Digital substation fiber split-phase longitudinal differential protection requires the same acquisition frequency of Mu on both sides. Assuming that the frequency of Mu sending data frames is fs, after calculating Δt, the interval Nd between the differential protection devices on both sides can be calculated, Nd = round (fS △ t) In the formula: the round function is to round the real number and round the remainder. When △ t is positive, move the sampled value of the local side backward by Nd point and synchronize with the opposite side; when Δt is negative, move the sampled value of the local side forward by Nd point and synchronize with the opposite side With this synchronization method, the theoretical maximum phase difference δ = ω / (2fS) after synchronization between the two sides, where ω is the system angular frequency.
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