Ground Differential Protection Research Articles

Presenting an Adaptive Restraint Method to Improve Performance of Ground Differential Protection of Power Transformer

The ground differential (GD) relay is used to detect internal ground faults on the power transformer windings and terminals. In fact, this relay is employed as a complement to the phase differential (PD) relay. However, its stability is affected by magnetic saturation in current transformers (CTs) cores so that the spurious differential current caused by CT saturation may lead to the relay malfunction. In this paper, by modifying the restraining current definition in a conventional method, an adaptive restrain method is proposed. To evaluate the proposed method performance compared with conventional method, a real 230/63 kV power transformer under a large number of internal fault, external fault and in rush current cases are simulated in PSCAD/EMTDC environment. It is worth noting that Jiles-Atherton (JA) model is utilized to simulate CTs. MATLAB software is used to implement and analyze these methods using obtained simulation. Obtained results show that the proposed method is quite superior to the conventional method, despite its simplicity.

Advanced Generator Ground Fault Protections in Pulp and Paper Mill Applications

Ground faults in field/rotor and generator stator circuits are serious events that can lead to damage, costly repair, extended outages, and loss of revenue. This paper explores advances in field/rotor circuit ground fault and stator ground fault protection. These advanced protection strategies employ ac injection, ground differential protection, and the use of hybrid grounding to reduce both internal generator ground fault levels and facility ground levels in utility-paralleled operating modes.

Improving Pulp and Paper Plant MV Transformer Protection

In pulp and paper plants, power transformers play a critical role in process continuity. These transformers are subject to internal short circuits, external short circuits, and abnormal operating conditions. The following protection challenges to power transformers will be explored, and methods to improve the protection are provided. First, remanence in a current transformer (CT) may cause misoperation of phase differential protection due to compromised CT performance. Heavy through-faults, sympathetic inrush, and recovery inrush all cause high current. This, combined with high remanent flux, can create a security issue. IEEE CT performance calculations will be used to support the use of dual slope differential characteristics to promote secure differential protection operation when challenged with unequal CT performance. Second, on transformer energizing, the second harmonic current has been traditionally used as a means to prevent phase differential misoperation. Certain transformers may not exhibit high enough the second harmonic, causing a dependability issue if the restraint is set too low. The use of the second and fourth harmonics for inrush detection will be shown to enhance the reliability during energizing inrush situations. Third, overexcitation can occur from abnormal operation of the utility system or the plant's excitation control. Causes of overexcitation will be outlined and the use of volts per hertz protection is explored. With overexcitation occurring from system voltage rise, the phase differential protection has been traditionally blocked using the fifth harmonic restraint. This may cause an undesired nonoperation of the phase differential protection if an internal fault occurs while the transformer is overexcited, causing a delay in tripping and severe damage to the transformer. A technique using adaptive phase differential pickup value will be illustrated to overcome this challenge. Finally, on resistance-grounded power transformers, phase differential protection sensitivity for ground faults near the neutral is decreased. The use of ground differential protection will be explored and the increased sensitivity gained will be demonstrated.

Protection and Commissioning of Digital Transformer Relays: Improvements in Medium-Voltage Industrial Transformer Protection

The application of multifunctional digital relays to protect medium voltage power transformers has become a common industrial practice. Industrial transformers, unlike utility transformers, frequently use neutral grounding resistors to limit ground current during faults to the 200-400 A range on medium voltage systems. This article discusses why these types of transformers require sensitive ground differential protection. The basics of transformer protection are also reviewed, including phasing standards, through-fault withstand capability, differential/fusing/overcurrent protection, slope, current transformer (CT) requirements, and harmonic restraint. How to best communicate these requirements when programming and commissioning new digital relays is discussed. The rationale for providing transformer overexcitation protection on all major transformers within industrial facilities is also addressed.

Protection and Commissioning of Multifunction Digital Transformer Relays at Medium Voltage Industrial Facilities

The application of multifunction digital relays to protect medium voltage power transformers has become a common industrial practice. Industrial transformers, unlike utility transformers, frequently use neutral grounding resistors to limit ground current during faults to the 200-400-A level on medium voltage systems. This paper will discuss why these types of transformers require sensitive ground differential protection. The paper will also discuss the basics of transformer protection including phasing standards, through-fault withstand capability, differential/fusing/overcurrent protection, slope, current transformer (CT) requirements, and harmonic restraint, and communicating these properly to new digital relays. The rationale for providing transformer overexcitation protection on all major transformers within mill facilities is also addressed. Advancements in digital technology have allowed relay manufacturers to include more and more relay functions within a single hardware platform as well as address increasingly more transformer winding configurations. This has resulted in digital transformer relays requiring an Einstein to set and an Edison to commission. Since there are few Einsteins or Edisons among us, the next generation of transformer relays needs to concentrate on this complexity issue in addition to technical improvements. This paper addresses these issues that the author believes are the major shortcomings of existing digital transformer protective relays.

Applying CTs with digital ground relays

Industrial power distribution system substation transformers and generator step-up transformers in power stations often use resistance-grounded wye secondary windings for medium-voltage power distribution. The purpose of this is to limit damage due to ground-fault currents, while providing sufficient fault current for the operation of ground-fault relaying. The relaying used to protect against ground faults in the system may not provide sufficient protection of the transformer winding against internal faults because the backup ground overcurrent relay in the transformer neutral-to-ground connection must be set to coordinate with downstream relays. In order to protect the winding itself, special relays are utilized. Ground differential protection can be provided by digital overcurrent relays in conjunction with auxiliary ratio matching transformers. Ground differential protection can also be provided in multifunction digital relays. Transformer protection relays may include this feature with one of the schemes used with component relays. If a feeder-protection relay is used on the secondary, in some cases, this may have a ground-directional feature that can be utilized for ground-differential protection.

Application of transformer ground differential protection relays

Application of transformer ground differential protection relays

Ground-differential protection revisited

The use of ground differential for the protection of impedance-grounded transformers and generators has been increasing. Because of its inherent selectivity and speed of operation, ground differential provides excellent protection against ground faults within the grounded winding of the equipment. However, several factors should be considered when applying ground-differential protection to a power system. These factors include the type and ratings of the equipment being protected, whether it is a new installation or a retrofit project, and the type of relay being used. Both static or electromechanical relays may be applied for ground-differential-protection schemes on impedance-grounded transformers and generators. Each application should be evaluated regarding current transformer burden and saturation to ensure proper operation. The protection engineer should carefully consider the application factors to ensure that the desired level of protection is provided.

The Practice of Ground Differential Relaying

The basic concepts are presented of ground differential protection for transformers and generators. Ground differential relaying provides fast and sensitive ground fault protection for wye-connected and ground electrical power equipment such as rotating generators or power transformers. Because it is a differential system, there is no need to incorporate undesirable time delay or to sacrifice sensitivity for the sake of system coordination. The authors first explain ground differential relaying for power transformers and generators, and then explain the ground differential scheme using a commercially available product type relay. Finally, they illustrate a method of current transformer selection that will help better implementation of the ground differential scheme. >

Innovations in power distribution in cement plants

A few of the many new product applications and protection methods first used in the decade between 1971 and 1981 for cement plant power distribution systems are surveyed. References noted in each brief discussion contain application details as well as equipment descriptions, ratings, and application advantages of the innovations discussed. Transformers: Transformer designs, transformer through fault protection. Motors: Thermal tracking motor protection wound rotor motor protection, dual drive protection, high efficiency 460 V motor design, large horsepower SCR-DC fan drives, medium voltage load commutated inverter drives. System: Medium voltage vacuum circuit breakers, pilot wire tie line protection, transformer ground differential protection, zinc oxide station class surge arresters.