I. Analysis of Current Protection Methods in Distribution Systems
Current protection is a basic type of relay protection in any distribution system. Currently, the common current protection methods mainly include the following three types:
(1) Instantaneous overcurrent protection. For current protection that operates instantaneously in response to an increase in the magnitude of short-circuit current, it is called instantaneous overcurrent protection.Transformer Comprehensive Test Bench is mainly used for testing the no-load and load characteristics of power transformers and distribution transformers, and can also perform frequency-doubled induction and power-frequency withstand voltage tests on the tested transformer. To ensure its selectivity, it generally can only protect a part of the line. Instantaneous overcurrent protection has the advantages of being simple, reliable, and fast in operation, so it has been widely applied; its disadvantage is that it cannot protect the entire length of the line, and its protection range is directly affected by changes in the operating mode.
(2) Time-delayed instantaneous overcurrent protection. Since selective instantaneous overcurrent protection cannot protect the entire length of the line, it can be considered to add a section of protection with time delay, which is time-delayed instantaneous overcurrent protection. The requirements for this protection are: first, it must be able to protect the entire length of the line under any circumstances and have sufficient sensitivity; second, under the premise of meeting the above requirements, it should strive to have a small operating time delay; when a short circuit occurs in the downstream line, it ensures that the downstream protection gives priority to clearing the fault, meeting the selectivity requirements.
(3) Definite-time overcurrent protection. As a remote backup protection when the main protection of the downstream line refuses to operate and when the circuit breaker refuses to operate, and as a near backup protection when the main protection of this line refuses to operate, and also as protection against overload, overcurrent protection is generally used. Overcurrent protection usually refers to protection whose starting current is set according to avoiding the maximum load current, and it starts when the current amplitude exceeds the maximum load current value.
II. The Impact of Power Electronic Equipment on Relay Protection in Distribution Systems
The impact of power electronic equipment on relay protection operating in the distribution system varies depending on the type of protection and the installation location. The situations that may cause relay protection to malfunction or refuse to operate mainly include the following aspects:
(1) Being electrically close to large harmonic sources; (2) The installation location of the protection device has conditions for severe harmonic amplification or is close to harmonic resonance conditions, for example, the installation location has shunt capacitors, etc.; (3) The operating setting value of the protection device is very small, especially for relay protection devices connected to differential current circuits, zero-sequence circuits, or negative-sequence circuits, whose setting values are usually only a few percent of the phase current or phase voltage; (4) The components or operating principles selected in the relay protection device are highly sensitive to harmonics, especially overcurrent-type relay protection that reflects changes in instantaneous values and effective values of voltage and current; (5) When there are unbalanced loads or fundamental negative-sequence currents from inrush currents in the system, and they coexist with harmonic currents, the relay protection cannot operate correctly and reliably. Based on the above analysis, the greatest impact of power electronic equipment on the relay protection system of the distribution system lies in the harmonics derived from power electronic equipment. Currently, there are many measures commonly adopted at home and abroad to reduce the impact of harmonics on relay protection, which can be roughly summarized into two categories: one is auxiliary measures, whose starting point is to use other devices in the distribution system to achieve the goal of correct and reliable operation of relay protection, usually including:
① Using frequency testers installed in the distribution system to test harmonic frequencies, thereby assisting relay protection devices in measuring and comparing voltage and current values; ② Adding filtering devices to make the input signals in the protection undistorted; ③ Using devices that respond to "increments", that is, connecting a differentiating circuit before the actuating element of the relay protection device so that it only responds to sudden changes and not to steady-state quantities; ④ Installing harmonic blocking links among various protections in the distribution system. The second category starts from the relay protection itself, by reasonably setting its operating setting values to achieve correct and reliable operation. Currently, more and more advanced technologies and improved algorithms are applied to fault detection and parameter setting of relay protection. For example, relying on the application of microcontroller technology in stepped current protection, it is proposed that microcontrollers can replace a large number of relays, avoiding malfunction and refusal to operate of protection devices caused by complex wiring and unstable components, thereby improving the safety and reliability of power supply.
