Yin Yonghua

Studying on Measures of Restraining DC Current through Transformer Neutrals

Monopole ground return mode may cause DC current through the transformer neutrals which are connected to the ground and make the transformers DC biases. With the increase of HVDC transmission power, some transformers through which there will be more DC current going may be saturated, which brings some problems such as acutely shaking, increased noises and overheating when power system is in the normal operation. Not only the safety of transformer but also the normal operation of power system may be impacted in this case. There are three main measures to restrain DC current through transformer neutrals. They are: (1) putting a resistor in series between transformer neutral and ground, (2) putting a capacitor in series on AC transmission line and (3) putting a capacitor in series between transformer neutral and ground. Combining with the real instance of the transformers at Lingao and Dayawan nuclear power stations, these three measures to restrain the DC current caused by monopole ground return mode of HVDC transmission system are studied by building a module of power system with ground grid according to the DC current values measured at the transformer neutrals of Lingao and Dayawan nuclear power stations. The results of study show that the optimum measure to restrain DC current through transformer neutrals at Lingao and Dayawan nuclear power stations is to put a capacitor in series between transformer neutral and ground. This measure is studied further. Some main technology parameters are put forward according to the results of study

Coordinated out-of-step protection system based on WAMS

This paper describes a novel coordinated out-of-step protection system. The purpose of the protection system is to prevent total system collapse caused by step-out in large-scaled interconnected power grid. Different from traditional out-of-step protection which is based on local power grid information, this system is based on wide area measurement system, the protection system can observe power systems more comprehensively, the protection principle can detect out of step in main transmission interfaces in the power grid, so it can improve the coordination and selection of liquidating out of step fault in large power grid. The paper provides the principle and characteristics of the protection

Development of Transformer Neutrals DC Current Blocking Device in High Power System

The DC current through transformer neutrals was harmful for transformer operation and influenced system stability. Among several methods of limiting or eliminating neutrals DC current, series capacitor method, composed by capacitor and fault bypass system, was an ideal way. Several criterions must be followed, they were: no or less influence to relevant relays and automations; tolerance worst fault current and keep safety of transformers and neutrals; in normal system mode must ensure neutrals in capacitor ground state; once neutrals voltage overpass threshold, bypass system should be put into operation in limited time; economical and feasible for installation, testing & maintenance, etc. Capacitor had been adopted as main component, which had been inserted between transformer neutrals and earth. Bypass system had paralleled with capacitor and composed of a full- wave diode bridge rectifier, thyristor trigger circuit, pre-test circuit before capacitor-ground, fast breaker and other auxiliary components. When transformer neutrals in direct-ground state, and DC current through transformer neutrals over setting up value, a pre -trigger process of thyristor started to check whether bypass system run correctly, if right, device transferred neutral grounding state from direct-ground to capacitor-ground; when in capacitor-ground state, if system fault event occurred and terminal voltage surpassed threshold, trigger circuit had been fired to make neutrals to direct ground state as quickly as 20 us and simultaneously fast breaker operated and ensured metal ground below 25 ms. Built in control unit and recorder unit improved device automation and reliability .By remote terminal several devices could be monitored. For engineering required, the prototype model device had been produced. Factory test including big current surge endurance and automation characteristic showed this device could satisfy power system field application.

Studies on resonance phenomena caused by energization of transformer

This paper presents an analysis of high frequency resonance caused by switching on the unload transformers, including its equivalent circuit, basic equations, resonance conditions, origin of harmonic energy and its analytical methods. Two typical resonance phenomena from HVDC and HVAC transmission systems are studied by means of EMTP, and the simulation results provided a good agreement with the field measurement. The potential problems caused by energization of transformer and its countermeasures for different system conditions are discussed.

Power connection technology for full-digital real-time simulation and analog simulator

The study of UHVAC/UHVDC hybrid power grid demands various means in every stage, such as in planning, designing, building and operating. Full-digital real-time simulator has the merits such as very large simulation scale and very easy modified model. And analog HVDC simulator has the merits such as reliable and precision simulation results. To bring the merits of these two simulators into play, a power connection technology for combining them is developed, which is developed by distributed parameters transmission line model. Using this power connection technology, a hybrid simulation model for a simple AC/DC hybrid power grid is set up. Results of load flow, start-up and fault test show that this power connection technology is precision, stable and practical, thus it suits to realtime simulation for parallel operation of AC/DC hybrid power grid.

Analysis of main technical problems for XS and JS ±800kV UHVDC projects system test

The main technical problems of Xiangjiaba to Shanghai (abbr. XS) and Jinping to Sunan (abbr. JS) 800kV UHVDC transmission projects system test were analyzed in this paper. Based on the system stability calculation results, 46 kinds of main circuit connection modes and referring to 500kV HVDC system commissioning test experiences, the system test program and test plan were prepared, which was divided into four parts that were single converter system test instructions, monopole system test instructions, bipolar system test and de-icing mode system test instructions. Duo to rectifier side system was weak in the initial operation stage, AC bus voltage control and reactive power control had been solved. The main technical problems in XS and JS UHVDC project system test were analyzed in detail, which included VBE and control interface function verification, island mode test and de-icing connection mode test. These technical problems had been solved during system commissioning test, which guaranteed to complete XS and JS UHVDC project system commissioning test and put into operation in time.