NEMA US 80018-2022 设计规范考虑因素与建议

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NEMA US 80018-2022

Transformers in DC Applications:

Design Spec Considerations and Recommendations

Disclaimer

The standards or guidelines presented in a NEMA standards publication are considered technically sound

at the time they are approved for publication. They are not a substitute for a product seller’s or user’s own

judgment with respect to the particular product referenced in the standard or guideline, and NEMA does

not undertake to guarantee the performance of any individual manufacturer’s products by virtue of this

standard or guide. Thus, NEMA expressly disclaims any responsibility for damages arising from the use,

application, or reliance by others on the information contained in these standards or guidelines.

NEMA US 80018-2022

Page 2

Intro and Scope

Use cases for AC transformers within DC applications are becoming more common with the growth of

renewable generation. For example, project developers might use a medium-voltage DC bus to directly

interconnect solar PV and a battery energy storage system (BESS). Step-up and step-down transformers

are required on both ends of the bus (≈1 kV – 24 kV). Grid planners might also connect a load center

such as small towns/cities with MV DC connections to facilitate greater power flow control and facilitate

renewable penetration. The links are generally in the 24-36 kV range.

In both of the above examples, configurations that involve AC transformers require the use of power

conversion through an inverter or rectifier and may also entail interconnection directly with BESS. For

these use cases, there are special design considerations for the AC transformer to ensure optimal

performance. The purpose of this document is to provide guidance for developers and planners—anyone

charged with specifying the transformer design—to better understand these considerations.

Table

The table below provides a brief overview of the design considerations for AC transformers in various

applications that will be discussed throughout this document.

Design Considerations

Applications

Solar Inverter

BESS

Drive/Rectifier

Voltage Transients

Electrostatic Shield

Harmonic Currents

Harmonic Voltages

Flux Density

Grounding

Partial Discharge

Voltage Transients and Electrostatic Shielding

One of the unique aspects of medium-voltage AC transformers connected to inverters in battery energy

storage system (BESS) and/or solar photovoltaic applications is that the inverters connected to the low-

voltage side of the transformer can cause fast transient voltages. This happens when the IGBTs of the

inverter are firing to create a sinusoidal waveform (i.e., alternating current or voltage). These transients

can have a dV/dt of 1600 V/µs or higher (2400 V/µs, etc.), with peak voltages of more than seven times

the rated line-ground voltage of the winding. For these applications, we recommend specifying “inverter

duty” insulation, suitable for operation while connected to a pulsed inverter and capable of handling

differential-voltage-to-differential-time (dV/dt) surges. This is not a standard insulation level for a typical

distribution application.

For a solar inverter application, we recommend adding an electrostatic shield between the high-voltage

and low-voltage windings in order to isolate the high-voltage winding from the effect of the fast-rising

voltage on the low-voltage winding. The electrostatic shield acts as an additional dV/dt filter and filters the

voltage gradient of the pulsed inverter output. The electrostatic shield should have single-point grounding

only to avoid circulating current within the shield. See Figure 1.

NEMA US 80018-2022

Page 3

Figure 1

Schematic of the Transformer and Inverter Winding Appropriate for DC Applications

To validate that the transformer design is able to withstand these transient voltage levels, we recommend

that a lightning impulse test be performed on the transformer design as a type test. We also recommend

that an induced potential test be performed as a routine test on each transformer unit, with an induced

voltage of two times the rated voltage of the transformer.

Harmonic Currents

For a BESS application or a rectifier/drive application, transformer loading is non-sinusoidal. The

customer should specify a harmonic factor (i.e., “K-factor”) for the application as per ANSI/IEEE C57.110.

It is common to specify the transformer with a factor of K-4, which accounts for some overheating caused

by harmonics generated from the inverter.

Harmonic Voltages

For a solar inverter application, the transformer may be subjected to non-linear harmonic voltages, which

cause additional heating in the transformer core and need to be addressed in the design. We recommend

specifying a harmonic voltage factor that corresponds to the harmonic voltage generated by the inverter.

Flux Density

For solar inverter and BESS applications, flux density of the transformer core will vary in operation

depending on the level of active and reactive loads on the transformer. In order to ensure that the

transformer operating flux density does not exceed the saturation flux density of the core steel used in the

transformer, product designers should source appropriate materials such as grain-oriented electric steel

(GOES) or amorphous electrical steel for the transformer core. Saturation of the core would add

additional harmonics to the system along with other negative consequences such as transformer heating

and high noise levels.

To ensure minimum harmonic generation, the saturation flux density of the transformer should be higher

than the maximum flux density reached during normal operation. This maximum flux density is obtained

at the highest secondary voltage, at full capacitive power generation, and at the highest reference

voltage, with minimum continuous frequency.

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标签： #设计规范

摘要：

NEMA US 80018-2022《设计规范考虑因素与建议》是面向相关工程设计、合规与项目管理人员的重要参考文件。本文对该标准规范进行系统梳理，概括其适用范围、编制背景、核心设计原则、关键考虑因素与实施建议，重点解析设计流程、风险识别、安全与性能要求、选型安装、验证审查及文档管理等内容。通过条理化的解读，帮助工程师、设计人员、合规负责人和项目管理方快速掌握 NEMA US 80018-2022 的要点，提升标准应用效率，降低设计与合规风险，并为项目规划、设计审查、采购决策和工程实施提供实用参考

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