Analysis of the steady-state and transient characteristics of electromagnetic instrument transformers reveals the following features: Few primary turns, many secondary turns; low magnetic flux density during normal operation, high magnetic flux density during faults, exhibiting saturation; high internal resistance; small secondary load, secondary cannot be open-circuited; weak electrical insulation, bulky size; may lead to ferroresonant overvoltage; analog output.
Electronic instrument transformers feature: simple and reliable insulation; small size and weight; wide dynamic range of the current transformer (CT), no magnetic saturation; no resonance phenomenon in the physical transformer (PT); the secondary of the CT can be open-circuited; digital output; strong resistance to electromagnetic interference. Current transformers and voltage transformers can be combined into one, called a current-voltage combined transformer (ECVT).
With the standardization needs and technological development of industrial control information exchange, a new information exchange standard based on the concept of "one world, one technology, one standard"-IEC 61850-has been proposed internationally. In China, the various shortcomings of existing information exchange technologies in substation automation have severely hampered the improvement of new production management technologies. Therefore, adopting IEC 61850 to standardize information exchange has become a consensus in the domestic power automation industry. Simultaneously, the State Grid Corporation of China has proposed a strategic policy of "building a digital power grid and creating an information-based enterprise," leading to the emergence of intelligent substations.
Since intelligent substations use IEC 61850 as the standard for information exchange standardization, whether or not electronic instrument transformers are used exclusively is not the distinguishing factor between conventional and intelligent substations. A comparison of electromagnetic and electronic instrument transformers shows that the higher the voltage level, the more obvious the advantages of electronic instrument transformers. However, for low voltage levels, using electronic instrument transformers is not very meaningful; conventional instrument transformers should be used for the following reasons:
Electronic instrument transformers are used to solve the problem of instrument transformer saturation, while low-voltage conventional instrument transformers generally do not have saturation problems.
Electronic instrument transformers were used to solve the problem of long-distance transmission of secondary signals from instrument transformers. However, since conventional low-voltage instrument transformers and protection devices are already installed in the switchgear, the long-distance transmission problem has been solved.
The main factor limiting the reduction of switchgear size is the volume occupied by the operating mechanism, not the volume of the instrument transformers. Therefore, the advantages of small size and light weight of electronic instrument transformers installed in switchgear are not realized.
(Low-voltage electronic instrument transformers output small analog voltage signals, which are not easily shared directly. They need to be converted into digital signals by a merging unit before sharing, which undoubtedly increases the cost of the merging unit. In contrast, the signals output by conventional instrument transformers are easily shared by various protection and control devices.
Therefore, future substations will neither see a dominance of electronic instrument transformers nor the demise of traditional electromagnetic instrument transformers. The author believes that the two will complement each other and ultimately coexist.
With the establishment of intelligent substations, coordination with conventional substations (where conventional and electronic instrument transformers will coexist) is inevitable, and the issue of sampling synchronization between the two should be given attention. The sampling synchronization issue among the various instrument transformers within an intelligent substation should also be emphasized. This can be addressed in the following ways: The sampling time between conventional and intelligent substations can be adjusted to compensate for the difference in sampling route delays on both sides, ensuring that the currents on both sides for calculating the differential current are at the same time. Within an intelligent substation or between an intelligent substation and a conventional substation, the sampling synchronization can be based on the Global Positioning System (GPS).) /The BeiDou satellite navigation system uses time pulses for synchronous sampling.
It is corrected by software algorithms of protection and control devices.