Why Are NGRs Typically Rated for a 10-second Fault Duration？

Writer： admin Time：2025-12-23 14:57:01 Browse：53℃

The fault current duration for Neutral Grounding Resistor (NGR) is typically set to 10 seconds. This is not an arbitrary value but the result of a trade-off between protection sensitivity, economic considerations, and thermal capacity design in power system engineering. Below is a detailed explanation and application analysis for various scenarios:

1. Why is it typically 10 seconds? 

A. Protection coordination requirements
The 10-second time window is sufficient to cover the protection action sequence of most low-resistance grounding (LRG) systems.

1. Action delay: Even in a complex power grid with multiple levels of selectivity, the main protection usually operates within a few hundred milliseconds, while the most remote backup protection (such as zero-sequence current protection or time-limited overcurrent protection) typically trips within 1-3 seconds.

2. Tolerance margin: The 10-second limit allows for multiple reclosing attempts (if applicable) or provides a sufficient buffer for backup protection action, ensuring the resistor does not burn out before the fault is fully cleared from the system.

B. Balance between thermal capacity and cost
The temperature rise of the resistor is proportional to the energy absorbed (I²Rt). If the design allows for longer durations (such as continuous operation), the resistor will have to be very large and require complex heat dissipation structures, making it extremely costly. Ten seconds is a technical threshold: it allows the resistor to utilize its own thermal mass to absorb heat without needing external cooling. Within this time, the resistor can typically heat up to 760℃ (according to IEEE 32 standards).

C. International and industry standards
The IEEE 32 standard specifies the short-time rating durations for grounding devices as 10 seconds, 1 minute, 10 minutes, etc. Ten seconds is recognized as the "default standard" for protection trip systems.

Case 1: QFSN-300-2 Generator

A 300MW generator in a power plant with a stator rated voltage of 20kV uses a neutral grounding resistor at the secondary side of a single-phase distribution transformer, with an equivalent resistance of 1060Ω on the primary side, and a fault current rating of about 11A, with a fault duration of 10 seconds. When a single-phase fault occurs inside the generator, the zero-sequence voltage protection trips within 0.3 seconds. The 10-second limit not only satisfies the coordination of protection levels but also provides safety margin during the residual energy release in the generator demagnetization process.

Case 2: SZ11-50000/110 Main Transformer

A 50MVA main transformer at a city substation has a 35kV side with a JZ-35/400-10s low-resistance grounding device. The resistance is 50.5Ω with a rated current of 400A and a current flow time of 10 seconds. During a single-phase fault caused by external force at a 10kV outgoing line, the fault current reaches 380A. The substation feeder protection cuts off the faulted line within 2.0 seconds. The 10-second rated time ensures that the resistor’s thermal capacity remains within safe limits during multi-level feeder protection actions and reclosing attempts.

Applications for Times Longer or Shorter than 10 Seconds

Longer than 10 Seconds:
When the 10-second rating is insufficient to meet specific operational requirements, power systems may adopt longer durations for neutral grounding resistors. Common specifications include 1 minute (60s), 10 minutes, or even continuous operation.

1. 1-minute (60s) rating: Commonly used in systems with multiple branches and frequent faults. In systems with many outgoing lines, such as those in thunderstorm-prone areas, multiple single-phase ground faults may occur in a short time. The 10-second thermal capacity might cause the resistor to overheat due to the accumulation of heat from multiple faults.

2. 10-minute rating: Applied in industrial sites with special process requirements. In such scenarios, the system does not allow immediate tripping upon fault detection, so a longer time window is necessary for manual intervention and orderly load disconnection, providing operators enough time to handle the situation.

3. Continuous operation: Specifically designed for high-resistance grounding (HRG) systems. These systems typically have fault currents controlled between 5A and 10A. Since the system operates under a "alarm-only" condition during a single-phase fault and continues running for hours to locate the fault, the resistor must be designed for continuous operation to ensure safe long-term operation.

Shorter than 10 Seconds:
While 10 seconds is the general standard, in some cases where extreme efficiency or specific equipment protection is required, shorter time ratings (such as 2 seconds or 5 seconds) may be used. In modern digital substations, where protection actions are extremely fast (milliseconds), the rated time may be reduced to 5 seconds or even shorter to minimize the size and cost of the resistor. In generator neutral grounding systems, in order to minimize damage to the stator core from a single-phase fault, the protection system may require ultra-fast tripping, allowing the resistor’s withstand time to be reduced. However, to ensure safety, the industry still tends to retain some margin in these cases.

This translation provides a clear technical explanation of the Neutral Grounding Resistor fault current time settings, applicable cases, and various time ratings used in different scenarios.