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1.ENR-BNR neutral grounding resistor cabinet

1.Overview

Baoding Yinuoer Electrical Equipment Co.,Ltd.is a high-tech enterprise with long-term experience in the production of neutral grounding resistors for power systems.The company's ENR-BNR series of neutral grounding resistor cabinets for 6kV-35kV power systems are specialized complete sets of devices used for neutral grounding through resistors in power grids.The products are characterized by high precision,good linearity,reliable operation,convenient installation,and attractive appearance.

Currently,35kV resistors are primarily used in domestic systems such as wind power,thermal power,energy storage,fishery-photovoltaic integration,and offshore platforms.Due to the angle connection method on the 35kV side of 110kV or 220kV transformers at many project sites,a 3-phase Z-type grounding transformer is required to export the neutral point,which is installed on site in conjunction with the resistor.

II.Model Description

III.Introduction to the supporting three-phase Z-type grounding transformer

3.1 Overview of principle

Generally,the 35kV side of a 220/10kV or 110/10kV transformer adopts a delta connection form.Therefore,in operation,the 35kV system has no neutral point,and thus the neutral point operation mode cannot be discussed.

For a distribution system with a delta connection,an earthing transformer must be connected to form the system's neutral point.There are two types of earthing transformers:Z-type earthing transformers(ZN,ZN,yn)and star/delta connection transformers(YN,d).Currently,Z-type earthing transformers are more commonly used,and their neutral points can be connected to neutral point resistors.

The Z-type grounding transformer is structurally identical to a common three-phase core power transformer,except that the windings on each phase core are divided into two parts with equal turns,one above and one below,connected in a zigzag pattern.Due to different wiring methods,it is further divided into two types:ZN,yn1 and ZN,yn11.

The direction of zero-sequence current in the two half-windings on the same column of a Z-type grounding transformer is opposite,resulting in a small zero-sequence reactance and no choke effect on the zero-sequence current.When the neutral point of the Z-type grounding transformer is connected to an arc suppression coil or grounding resistor,it allows the compensation current of the arc suppression coil or the resistance current to flow freely.Therefore,Z-type transformers are generally used as grounding transformers.

The Z-type grounding transformer can also be equipped with a low-voltage winding and connected in a star neutral grounding(yn)configuration,serving as an auxiliary transformer.Z-type grounding transformers are available in both oil-immersed and dry insulation types,among which resin casting and DuPont paper insulation are examples of dry insulation methods.

Schematic diagram of grounding transformer:

For 35kV and 66kV distribution networks,transformer windings typically adopt a Y connection with a neutral point,eliminating the need for a grounding transformer.In contrast,for 6kV and 10kV distribution networks,transformer windings usually employ aΔconnection without a neutral point,necessitating the use of a grounding transformer to derive a neutral point.The primary function of a grounding transformer is to provide a neutral point connection for arc suppression coils or grounding resistors when the system is configured in aΔor Y connection without an accessible neutral point.

Grounding transformers utilize a Z-type connection(also known as a zigzag connection),where each phase winding is split across two magnetic cores.The zero-sequence magnetic fluxes generated by the two-phase windings cancel each other out,resulting in a very low zero-sequence impedance(typically less than 10Ω)for Z-type grounding transformers.This design ensures minimal no-load losses and allows over 90%of the transformer's capacity to be utilized.In comparison,conventional transformers exhibit significantly higher zero-sequence impedance,limiting arc suppression coil capacity to no more than 20%of the transformer's rated capacity and neutral point resistor capacity to no more than three times the transformer's rating.Thus,Z-type connected transformers are an optimal choice for grounding applications.

When system imbalance voltage is substantial,Z-type transformers with balanced three-phase windings can adequately meet measurement requirements.For systems with minor imbalance voltage(e.g.,fully cable-based networks),the neutral point of a Z-type transformer should be adjusted to generate an imbalance voltage of 30V to 70V to satisfy measurement needs.

In addition to accommodating arc suppression coils and neutral point resistors,grounding transformers can also support secondary loads,serving as station service transformers.When carrying secondary loads,the primary capacity of a grounding transformer should equal the sum of the arc suppression coil capacity and the secondary load capacity.

Summary of the fundamental roles of three-phase Z-type grounding transformers:

①Formation of artificial neutral point

②Allow for a low zero-sequence impedance.

③Can reduce the balance in system voltage.

III.Ordering Instructions:

Model;

Rated capacity;

Rated voltage;

Other special requirements.

III.Appendix:

Introduction to the selection principles for user grounding transformer capacity and resistance parameters.

1)The standard configuration for this type of product is as follows:

2)For the on-site conditions,it is first necessary to determine the system's single-phase grounding capacitive current to provide a basis for selecting the technical parameters of the resistor and transformer.The methods for obtaining the data include:

A.Obtain technical data such as the length of on-site cable and overhead line installations,as well as the model,specifications,and cross-sectional area of the cables or overhead lines from relevant design plans,and use the corresponding formulas to calculate the specific capacitive current data;

B.Alternatively,the actual cable manufacturer can provide the specific capacitive current value per kilometer of the cable to estimate the capacitive current value.

C.Use a capacitance current tester for actual measurement.