With the continuous development of the power industry, the capacity of generator sets is becoming increasingly larger, placing higher demands on the performance of generator protection. The theoretical foundation of relay protection is based on a clear understanding of fault patterns. Further improving the performance of main protection for large generators requires a deep and extensive understanding of the patterns of internal generator faults. Only with a profound understanding of internal generator faults can we propose and improve generator main protection schemes based on fault characteristics, thus enriching and perfecting the theory of generator main protection.
Generator differential protection
A key characteristic of generator differential protection is that it must remain absolutely stable and reliable against faults or other abnormal symptoms outside the protected area, and should not malfunction. The DG2 generator differential protection relay fully meets these requirements and is highly popular with users.
The basic DG2 relay can be further expanded with the addition of an additional printed circuit board. By employing a new technology using operational current signals, the relay can determine whether the current transformer's magnetic circuit saturation is caused by an internal or external fault in the protected area, thus determining whether to trip the generator or maintain stable operation. This extended-function relay (DG2-Sat) is particularly suitable for protecting critical generators or generators located in power systems with high failure rates.
For phase-to-phase short circuits in the stator windings and leads of the generator, longitudinal differential protection should be installed.
Depending on the wiring method and location, differential protection can be further divided into fully longitudinal differential protection and incomplete longitudinal differential protection. Ratio-restrained fully differential protection is the main protection for phase-to-phase short-circuit faults within the generator. Longitudinal differential protection, as the main protection for phase-to-phase short circuits, has the longest history of application, and digital longitudinal differential protection was the first area of research after the advent of digital technology. Following the proposal of a differential protection scheme based on instantaneous sampling values, a scheme using the correlation function method to calculate the phasors of the generator terminal and neutral point currents to achieve differential protection was proposed. Furthermore, using proportional differential protection or a product-restrained criterion based on the square of the differential current action quantity achieved good selectivity and sensitivity for components with a single-sided power supply. To eliminate the influence of load components and further improve sensitivity, the fault component principle is widely used to improve traditional longitudinal differential protection schemes.
Traditional longitudinal differential protection is only effective for phase-to-phase short circuits of the generator and its leads, resulting in a narrow function. In recent years, incomplete longitudinal differential protection schemes have been proposed abroad and have been initially applied in large hydro turbines. This protection combines the principles of traditional transverse and longitudinal differential protection, reconnecting the current transformers installed on the neutral point side of the traditional longitudinal differential protection in phase-by-phase configurations to a portion of the parallel branch circuit of each phase. This expands the protection function to include phase-to-phase, turn-to-turn short circuits, and branch weld breaks. However, this functional expansion comes at the cost of compromising the original single-item performance; for phase-to-phase or turn-to-turn short circuits in certain locations, the sensitivity of the incomplete differential protection scheme will be lower than that of longitudinal or transverse differential protection. Furthermore, obtaining the neutral point current in incomplete differential protection is also relatively difficult, and it also faces the same challenges related to current transformer (CT) installation.
Measures to improve the sensitivity coefficient of differential protection mainly include two aspects:
(1) Minimize unbalanced current, including selecting current transformers (CTs) with the same characteristics as possible, such as selecting differential protection CTs of the same model, and minimizing the load on the secondary circuit.
(2) Improve the operating characteristics of longitudinal differential protection. At present, longitudinal differential protection with piecewise linear ratio braking characteristics is widely used in the longitudinal differential protection of large units, which effectively improves both the sensitivity coefficient and braking characteristics.
Protecting the generator stator windings and their leads from phase-to-phase short-circuit faults has the following main functions:
(1) It has harmonic braking and proportional braking characteristics to prevent maloperation due to faults outside the zone, and prevents the generator from maloperating when over-excited.
(2) When the current transformer is disconnected, an alarm signal can be issued.
(3) When two grounding faults occur on the same phase (one inside the zone and one outside the zone), the output can be activated.
(4) The setting range of the operating current is 0.1 to 1.0 times the rated current.
(5) The action time (when the current is twice the setting current) shall not exceed 30ms.
(6) After the differential protection operates, the generator protection shutdown 1 operation output relay is used.
The output logic diagram of the stator differential protection is shown in Figure 7-6.
The 998 generator differential protection device mainly adopts a 32-bit imported DSP chip and the "main and rear integrated" design principle, and is a measurement and control device specially designed for the main protection of generators of various capacity levels and types.
Features of the 998 generator differential protection device:
1. This product adopts a modular design concept, high-performance imported DSP chip, and integrates protection functions such as differential protection, winding inter-turn protection (transverse differential), differential instantaneous overcurrent protection, non-electrical quantity protection, and CT disconnection alarm.
2. The product provides high-precision reverse power protection. The internal measuring-level current transformer and the protection-level current transformer are soft-switched. Under low current, the measuring-level current transformer sampling channel is used, while under high current, the protection-level current transformer sampling channel is used. Through multi-stage angle compensation, high-precision power measurement is guaranteed over a wide range of reactive power.
3. The product adopts an all-metal casing and a dual A/D redundant hardware design, which improves both the device's resistance to dryness and the accuracy of its measurements.
4. The product provides a standard RS-485 interface, and can also be equipped with other communication interfaces according to user requirements. It can be well networked and communicated with the GZP-SCADA microcomputer background monitoring system to realize remote control.
5. This product is a professional device for the main protection of generators. It can be used in conjunction with our company's 988 generator protection and control device, 989 generator grounding protection device, and 988 generator backup protection device to form a complete generator protection system.
