Ungrounded System: Applications, Advantages and Ground Fault Detection

A. What is an Ungrounded System?

An ungrounded system is an electrical power system in which no intentional connection is made between the system conductors (usually the neutral) and earth (ground). This configuration is mostly found in industrial and critical applications, such as in process plants, shipboards, and aerospace systems.

In an ungrounded system:

The neutral point of transformers or generators is not connected to earth ground.
Phase-to-ground voltages exist due to capacitance between system conductors and ground, but no current flows under normal conditions.

B. Advantages of Ungrounded Systems

Advantage	Description
✅ Continued Operation during Single Line-to-Ground Fault	The system can continue running with one ground fault (no tripping), providing higher availability.
✅ Reduced Fault Current	Fault current during a single line-to-ground fault is very small (capacitive), reducing equipment stress.
✅ Better System Continuity	Used in critical systems (e.g., petrochemical, refineries) to avoid sudden shutdowns.
✅ Reduced Arcing Risk	Lower ground fault current reduces risk of arc flash during a single-phase fault.

C. Disadvantages of Ungrounded Systems

Disadvantage	Description
⚠️ Difficult Fault Detection	First ground fault doesn't cause high current, making detection tricky.
⚠️ Transient Overvoltages	Arcing ground faults can lead to dangerous transient overvoltages on healthy phases.
⚠️ Insulation Stress	Prolonged operation with a ground fault can damage insulation on healthy phases.
⚠️ Maintenance Requirement	Requires regular insulation monitoring and ground fault detection systems.

D. Methods of Ground Fault Detection in Ungrounded Systems

Since no large current flows during a ground fault, special detection methods are needed:

Method	Description
Ground Fault Detectors (GFDs)	Monitor voltage and current imbalance to detect ground faults.
Voltage Balance Detection	Compares phase-to-ground voltages; imbalance indicates fault.
Insulation Monitoring Devices (IMDs)	Continuously monitor system insulation resistance to ground.
Pulse Injection / Fault Location System	Inject low-frequency pulses and track return path to locate the fault.
Zero-Sequence Voltage Detection	Monitors zero-sequence voltage at transformer/generator neutral.

E. Standards Relevant to Ungrounded Systems & Ground Fault Detection

Standard	Description
IEEE Std 242 (Buff Book)	Details on protection and coordination, including grounding practices.
IEEE Std 142 (Green Book)	Grounding of industrial and commercial power systems.
NFPA 70 (NEC)	U.S. National Electrical Code – Articles 250 (grounding), 215 & 230.
IEC 61557-8/-9	Specifies performance requirements for insulation monitoring and fault location.
IEEE Std 1100 (Emerald Book)	Power quality and grounding for sensitive electronic equipment.
UL 1053	Ground fault sensing and relaying equipment.

F. Typical Applications of Ungrounded Systems

Process industries (chemical, petrochemical)
Aerospace and marine power systems
Medium-voltage AC and DC systems
Battery banks and UPS systems
Control and instrumentation panels

G. Applications of DC Ungrounded Systems

Application Area	Description
Marine & Shipboard Power Systems	Often use ungrounded DC systems (e.g., 110 V or 220 V DC) to prevent shutdown from a single ground fault and ensure continuous power to navigation, control, and safety systems.
⚡ Battery Energy Storage Systems (BESS)	Many large-scale or critical backup battery banks use ungrounded DC for safety and fault isolation.
Industrial Control & Instrumentation	24 V or 48 V DC systems in control panels are commonly ungrounded to avoid unintended shutdowns due to minor insulation faults.
UPS & Telecom Power Systems	48 V DC telecom power and DC UPS systems are often ungrounded or floating for reliability and ease of monitoring.
Hospitals & Critical Loads	In life-safety systems and operating rooms, ungrounded DC control circuits may be used to maintain service continuity.
Railway Signaling & Rolling Stock	Ungrounded DC is used in control, auxiliary, and signaling systems to ensure uninterrupted operation and detect early ground faults.

G.1. Why Use Ungrounded DC Systems?

Single line-to-ground fault does not trip the system.
Allows continued operation while locating and fixing the fault.
Minimizes the risk of short circuit damage.
Reduces fire and arc flash hazards due to low ground fault current.

G.2. Ground Fault Detection Methods in DC Ungrounded Systems

Because DC faults don't behave like AC, traditional overcurrent relays don’t work. Instead, you need:

Method	Description
Insulation Monitoring Devices (IMD)	Continuously measure the insulation resistance between DC buses and ground. Triggers alarms below a threshold (e.g., < 50 kΩ).
Pulsed Signal Injection	Sends low-frequency signals on DC lines to detect and locate ground faults based on the return path or voltage drop.
Symmetry Voltage Detection	Checks the voltage of both positive and negative DC poles to ground. Asymmetry indicates a ground fault.
Digital Fault Location System	Advanced systems can pinpoint fault location using transient behavior or reflectometry.
Differential Current Monitoring	Compare current entering and leaving the DC system; any difference may indicate a fault to ground.

G.3. Relevant Standards for DC Ground Fault Detection

Standard	Focus
IEC 61557-8	Insulation monitoring devices for ungrounded AC and DC systems.
IEC 61557-9	Equipment for locating insulation faults in ungrounded systems.
UL 1053	Ground-fault sensing and relaying equipment.
IEEE 142	Covers DC system grounding and detection techniques.
NFPA 70 (NEC)	Applies to grounding practices and protective devices for DC systems.
EN 50178 / IEC 62020	Standards for protective devices in power electronic systems, including fault detection.

G.4. 6.9 kV DC Ungrounded BESS – Why It's Done

For a 6.9 kV DC BESS, ungrounded operation is quite common, especially when high availability, personnel safety, and insulation monitoring are critical. Here's a focused breakdown for your case:

Safety: No intentional connection to ground limits ground fault current, reducing arc flash risks.
Continuity: System remains operational during a single ground fault—crucial for critical grid support.
Isolation: Prevents circulating currents or fault propagation through ground paths.

G️.5. Ground Fault Detection for 6.9 kV Ungrounded BESS

At this voltage level, ground faults can stress insulation and escalate into phase-to-phase faults. Thus, early fault detection is vital.

G.5.1. Recommended Detection Methods:

Method	Description
Insulation Monitoring Devices (IMDs)	Measure insulation resistance to ground. Should be rated for MVDC (6.9 kV). Triggers alarms if resistance drops below a setpoint (e.g., 100 kΩ).
DC Voltage Symmetry Monitoring	In symmetrical ±3.45 kV systems, monitor for imbalance between positive/negative poles and ground.
Transient or Pulse Fault Location	Works with IMDs to inject test signals and locate faults accurately. Useful for fast troubleshooting.
HV Differential Insulation Monitors	Some vendors offer fiber-optic isolated systems specifically for MVDC applications to reduce leakage current noise.

G.5.2. Applicable Standards

Standard	Use
IEC 61557-8	IMDs for AC and DC ungrounded systems. Devices must be suitable for MVDC.
IEC 61557-9	Fault location systems in ungrounded systems.
IEEE 1653.1	Design of DC traction power systems (relevant for high-voltage BESS designs).
IEEE 1547 / UL 9540	Interconnection and safety for BESS—emphasizes protection and fault response.
NFPA 70 (NEC) – Article 706	Covers energy storage systems, including insulation and fault protection.

G.5.3. Recommended Equipment for 6.9 kV DC BESS

Vendor	Device	Notes
Bender	ISOMETER® iso685-D-P + EDS440	Rated for MVDC systems; supports insulation monitoring + fault location.
Littelfuse	IRDH575 or IRDH265	High-voltage IMDs for BESS; includes remote communication.
Megger	IMD-250 or similar	Suitable for continuous monitoring in MVDC ungrounded systems.
Schneider	Vigilohm IM400	Adaptable to medium voltage with external coupling devices.

G.5.4. Integration Considerations

Signal Isolation: Use fiber optic communication or isolated analog signals to avoid capacitive coupling issues.
Alarm Response: Tie IMD alarms into SCADA/EMS for predictive maintenance or automatic switchover.
Arc Flash Studies: Even with ungrounded systems, consider arc flash hazard analysis for personnel protection.