⚡ electrical engineering

Fast detection method for AC short-circuit faults in shipboard power systems based on decoupled double synchronous rotating frame

This paper proposes a fast detection method for AC short-circuit faults in shipboard power systems using a decoupled double synchronous rotating frame (DDSRF) to precisely separate voltage sequences and construct a fault state variable, achieving detection times under 2 ms with over 50% improvement in speed compared to traditional RMS methods.

Original authors: Shengbin Zhuang, Xianfeng Mao, Zerong Zhou, Hongxing Chen

Published 2026-07-09

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Shengbin Zhuang, Xianfeng Mao, Zerong Zhou, Hongxing Chen

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Problem: A Ship's Power System is a "Fragile House"

Imagine a ship's electrical system as a high-tech, isolated house floating in the middle of the ocean. Unlike a city power grid (which is huge and heavy), this ship's power system is small, light, and has very little "inertia" (momentum).

In this house, the electricity comes from batteries, solar panels, and generators, all connected by a complex web of wires. The most critical part of this house is the inverter, which acts like a translator, converting DC power (from batteries) into AC power (for the ship's motors and lights).

The Danger: If a short circuit happens (a "spark" or a "short" in the wiring), the voltage drops instantly, and the current (electrical flow) spikes dangerously high. In a small ship system, this can happen so fast that it damages equipment or shuts down the entire ship before traditional safety switches can react.

The Old Way (The "Slow Watchman"):
Traditional methods for detecting these faults are like a watchman who checks the voltage and current every few seconds or waits for a full cycle of the wave to pass before making a decision. By the time this watchman says, "Hey, there's a problem!", the damage is already done. They are too slow and get confused easily by normal fluctuations (like a motor starting up or clouds passing over solar panels).

The New Solution: The "Decoupled Double Synchronous Rotating Frame" (DDSRF)

The authors propose a new, super-fast detection method. To understand it, let's use an analogy of two spinning merry-go-rounds.

The Two Merry-Go-Rounds:
Imagine the electricity in the wires as a dance. Sometimes the dance is perfect (symmetrical), and sometimes it's messy (asymmetrical).
- The new method uses two special coordinate systems (merry-go-rounds) spinning in opposite directions.
- One merry-go-round spins to track the "good" part of the electricity (Positive Sequence).
- The other spins to track the "bad" or messy part (Negative Sequence).
- Because they spin in opposite directions, they can separate the good from the bad instantly, without getting confused. This is the "Decoupled" part.
The "Fault State Variable" (FSV): The Ship's "Stress Meter"
Once the system separates the good and bad electricity, it creates a single number called the Fault State Variable (FSV). Think of this as a Stress Meter for the ship's power system.
- Normal Operation: The Stress Meter sits at 0. The voltage and current are balanced.
- Disturbance (e.g., Clouds or Motor Start): The meter might wiggle slightly up or down, but it quickly returns to 0. It knows this isn't a disaster.
- Short Circuit: The moment a short circuit happens, the voltage crashes (like a floor dropping out) and the current explodes (like a flood). The Stress Meter (FSV) shoots up instantly and stays high.

How It Works in Practice

The researchers built a small-scale model of a ship's power system (a 1kW inverter) to test this.

The Test: They simulated various types of electrical shorts (like a wire touching another wire, or a wire touching the ground).
The Result:
- Old Method (RMS): Took anywhere from 8 to 22 milliseconds to realize something was wrong. That's like a human blinking their eyes 10 times.
- New Method (FSV): Detected the fault in less than 2 milliseconds, and in some cases, as fast as 0.05 milliseconds. That is roughly 50 times faster than the old method.

Why This Matters for Ships

The paper claims this speed is a game-changer for ships because:

Speed is Safety: In a small ship system, a fault can destroy equipment in the blink of an eye. This new method acts like a reflex, detecting the problem before the "flood" of electricity reaches its peak.
No False Alarms: The system is smart enough to ignore normal changes, like a motor starting up or the sun going behind a cloud. It only screams "Help!" when there is a genuine short circuit.
Versatility: It works for all types of short circuits, whether it's a three-phase mess or a single wire touching the ground.

The Bottom Line

The authors have created a "super-speed radar" for ship electrical systems. By using two spinning reference frames to separate good and bad electricity, they built a "Stress Meter" that can spot a dangerous short circuit in the blink of an eye (microseconds). This allows the ship's safety systems to cut the power and isolate the problem before any real damage occurs, keeping the ship running safely even in rough electrical weather.

Technical Summary: Fast Detection Method for AC Short-Circuit Faults in Shipboard Power Systems Based on Decoupled Double Synchronous Rotating Frame

Problem Statement
Shipboard power systems are evolving into isolated microgrids with limited capacity and low inertia, increasingly integrating renewable energy sources like photovoltaics and energy storage. In this environment, the three-phase two-level grid-connected inverter is a critical interface. However, traditional short-circuit fault detection methods, which rely on steady-state power-frequency electrical quantities (such as RMS values), suffer from slow operating speeds and weak anti-interference capabilities. These limitations make it difficult to meet the rapidity requirements necessary to prevent equipment damage, cascading failures, and total power loss in modern shipboard systems, particularly given the complex transient conditions introduced by propulsion and daily loads.

Methodology
The paper proposes a fast detection method for AC short-circuit faults based on the Decoupled Double Synchronous Reference Frame (DDSRF). The methodology proceeds through the following steps:

System Modeling and Fault Analysis: An equivalent circuit model of a three-phase grid-connected inverter is established. The authors analyze the fault response characteristics for both symmetrical and asymmetrical faults. Unlike traditional systems where asymmetrical faults primarily affect the faulted phase, the inverter's d- and q-axis decoupling control strategy causes three-phase current amplitudes to increase synchronously regardless of fault symmetry.
Voltage Vector Construction: The DDSRF is utilized to construct two synchronous rotating coordinate systems with opposite rotation directions. This allows for the precise separation of positive-sequence and negative-sequence voltage components, even under unbalanced grid conditions.
Fault State Variable (FSV) Construction: A Fault State Variable (FSV) is defined to integrate positive and negative sequence features. The FSV is a normalized scalar time-varying vector calculated as the difference between the normalized current ratio and the normalized voltage ratio:
$FSV = k_f \frac{i_m}{I_{pv}} - \frac{u_m}{U_N}$
Where $i_m$ and $u_m$ are the instantaneous current and voltage amplitudes, $I_{pv}$ and $U_N$ are rated values, and $k_f$ is a compensation coefficient.
Detection Criterion: Under normal operation or non-fault disturbances (e.g., PV fluctuations), the FSV remains near zero. During an AC short-circuit fault, the voltage drops and current surges, causing the FSV to rise significantly above zero. A threshold ( $FSV_{ac} = 0.15$ ) is set to distinguish faults from disturbances.

Key Contributions

Novel Detection Metric: The introduction of the Fault State Variable (FSV), which fuses positive and negative sequence characteristics into a single scalar value, enabling rapid detection without phase-by-phase analysis.
Decoupled Double Synchronous Reference Frame Application: The application of DDSRF specifically for fault detection in shipboard inverters, leveraging its ability to decouple sequence components to handle both symmetrical and asymmetrical faults effectively.
Robustness to Disturbances: The method is designed to be insensitive to common non-fault disturbances such as photovoltaic output fluctuations and motor starting, which often trigger false positives in traditional RMS-based methods.

Experimental Results
The method was validated using a 1 kW three-phase two-level grid-connected inverter experimental prototype and simulation models.

Detection Speed: The proposed method achieved detection times of less than 2 ms for various fault types. Specifically, detection delays were recorded as 0.34 ms for three-phase short circuits, 1.61 ms for single-phase ground faults, and as low as 0.05 ms for two-phase ground and two-phase short-circuit faults.
Comparison with Traditional Methods: Compared to the traditional Root-Mean-Square (RMS) method, the proposed FSV method demonstrated a speed improvement of more than 50% (and up to an order of magnitude in some cases). For instance, the RMS method took up to 22.4 ms to detect a single-phase short circuit, whereas the FSV method took 1.61 ms.
Stability and Robustness: The method maintained stable performance across different initial fault phase angles. Furthermore, under non-fault disturbances (simulated light intensity changes and motor starting), the FSV fluctuation remained well below the detection threshold (maximum fluctuation of 0.07), confirming no false judgments occurred.

Significance
The paper claims that this method provides an effective technical means for fast fault protection in shipboard power systems. By achieving microsecond-level detection (with minimum delays of 0.05 ms), the method offers an extremely early identification basis for solid-state circuit breakers and fast current limiters. This capability allows for fault isolation before the short-circuit current reaches its peak, thereby preventing large-area voltage sags that could disconnect critical loads such as propulsion motors and high-power radars. Ultimately, the approach aims to significantly improve the continuous operation capability and self-healing levels of shipboard power systems, particularly in complex AC/DC hybrid or DC microgrid configurations.

The Problem: A Ship's Power System is a "Fragile House"

The New Solution: The "Decoupled Double Synchronous Rotating Frame" (DDSRF)

How It Works in Practice

Why This Matters for Ships

The Bottom Line

More like this