With the continuous growth of the power industry, generator capacities are increasing, and the demand for advanced protection systems is becoming more critical. The foundation of relay protection lies in a thorough understanding of fault patterns. To enhance the performance of main protection systems for large generators, it's essential to deeply analyze internal faults. This deep insight allows for the development and improvement of protection schemes tailored to specific fault characteristics, enriching the theoretical framework of generator protection.
**Generator Differential Protection**
A key feature of generator differential protection is its ability to remain stable and reliable during external faults, avoiding any unnecessary tripping. The DG2 differential protection relay meets these requirements effectively and has gained positive feedback from users. The basic DG2 relay can also be enhanced with additional printed circuit boards to expand its capabilities. Using a new technique for processing current signals, the relay can detect whether a current transformer is saturated due to internal or external faults, helping decide if the generator should trip or continue operating. This enhanced version (DG2-Sat) is particularly suitable for protecting critical or high-risk generators within power systems.
In cases of short circuits between the stator windings and their lead-out wires, longitudinal differential protection must be implemented. Depending on the wiring configuration, it can be categorized into full or incomplete longitudinal differential protection. Ratio brake fully differential protection serves as the primary safeguard against internal phase-to-phase short circuits. Its use as a primary protection method for such faults has a long history. After the introduction of digital technology, research into digital longitudinal protection began. A scheme based on instantaneous sampling values was proposed, followed by methods using correlation functions to calculate current phasors at the generator end and neutral point side, enabling differential protection. Proportional differential or differential current squared serves as a braking criterion, improving selectivity and sensitivity for single-sided power supply components. To further improve sensitivity, the fault component principle is widely applied to enhance traditional differential protection schemes.
While traditional differential protection is effective for phase-to-phase faults between the generator and its leads, its scope is limited. Recently, an incomplete differential protection scheme has been developed abroad and is being used in large hydro turbines. This approach connects the neutral point side of traditional differential protection to the phase-connected branch circuit of each phase, expanding the protection to include phase-to-phase, inter-turn short circuits, and open welds in branches. However, this expansion comes at the cost of reduced sensitivity in certain areas compared to full or transverse differential protection. Additionally, acquiring the neutral current for incomplete differential protection is challenging and involves complications with TA installation.
**Measures to Improve the Sensitivity Factor of Differential Protection:**
1. Minimize unbalanced current by selecting TAs with similar characteristics, such as the same model, and reducing the load on the secondary circuit.
2. Enhance the operational characteristics of differential protection. Currently, line-rate braking differential protection is widely used in large units, improving both sensitivity and braking characteristics.
The protection system for phase-to-phase short circuits in the generator’s stator winding and lead-out wires includes the following key features:
1. It incorporates harmonic and proportional braking to prevent malfunctions during external faults and overexcitation.
2. It can issue an alarm when the current transformer is disconnected.
3. It can operate during two-point ground faults on the same phase, even if they occur slightly outside the protection zone.
4. The operating current setting range is 0.1 to 1.0 times the rated current.
5. The operation time (when set to twice the current) is no more than 30ms.
6. After the differential protection trips, a shutdown relay is activated to disconnect the generator.
**The exit logic diagram of the stator differential protection is shown in Figure 7-6.**
The 998 generator differential protection device uses a 32-bit imported DSP chip and a "main and rear integration" design, making it suitable for various generator types and capacities.
**Key Features of the 998 Generator Differential Protection Device:**
1. It adopts a modular design, featuring a high-performance imported DSP chip, ratio differential protection, turn-to-turn protection (transverse difference), quick-break differential protection, non-electrical protection, and CT disconnection alarms.
2. It provides high-precision reverse power protection, with soft switching between measuring and protection level current transformers. It uses different channels for small and large currents, along with multi-level angle compensation to ensure accurate reactive power measurement.
3. The device features a full metal casing and dual A/D redundant hardware design, enhancing its resistance to interference and improving measurement accuracy.
4. It includes a standard RS-485 interface and can be equipped with other communication interfaces as needed, ensuring compatibility with GZP-SCADA monitoring systems for remote control.
5. As a specialized main protection device, it can work alongside other products like the 988 generator protection and control unit, 989 grounding protection device, and 988 backup protection device to form a complete generator protection system.
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