In medium-voltage power distribution systems, grounding transformers (earthing transformers) play a critical role in system protection and reliability. Understanding their principles and applications is essential for engineers involved in 6 kV, 10 kV, and 35 kV networks.
1. What Is a Grounding Transformer?
A grounding transformer is a special-purpose transformer used to create an artificial neutral point in systems where the neutral is not naturally available.
Classification
- By insulation medium
- Oil-immersed grounding transformer
- Dry-type grounding transformer
- By phase
- Three-phase grounding transformer
- Single-phase grounding transformer
Main Function
Grounding transformers provide a neutral point for ungrounded or isolated-neutral systems, allowing the connection of:
- Arc suppression coils (Petersen coils)
- Neutral grounding resistors (NGR)
This reduces capacitive earth-fault current during single-phase-to-ground faults and significantly improves power supply reliability.
2. Why Grounding Transformers Are Needed in MV Systems
In many 6 kV / 10 kV / 35 kV distribution networks, the system operates with an ungrounded neutral.
- The low-voltage side of the main transformer is often delta-connected, with no accessible neutral point.
- When a single-phase ground fault occurs:
- Line-to-line voltages remain symmetrical
- The system can continue operating for 1–2 hours
- Capacitive earth-fault current is usually small (≤ 10 A)
This allows some temporary faults to self-clear, improving supply continuity.
3. Problems Caused by Increased Capacitive Earth-Fault Current
With urban grid expansion and the rapid increase in cable feeders, system-to-ground capacitance rises sharply. When capacitive earth-fault current exceeds 10 A, serious risks appear:
Major Risks
- Arc grounding overvoltage
- Intermittent arc extinction and reignition
- Overvoltage up to 4 × U (U = phase voltage peak)
- Long duration, severe insulation stress
- Phase-to-phase short circuits
- Continuous arcing ionizes air
- Insulation breakdown between phases
- Ferroresonance overvoltage
- Damage to voltage transformers (VTs)
- Surge arrester failure or explosion
These phenomena can cause line tripping, equipment damage, and fault escalation, threatening safe grid operation.
4. Role of Grounding Transformers in Fault Current Compensation
To reduce capacitive earth-fault current, arc suppression coils or resistors must be connected to the system neutral.
Since ungrounded systems lack a natural neutral, a grounding transformer is installed to create an artificial neutral point, enabling:
- Neutral grounding through arc suppression coils
- Reduction of earth-fault current
- Improved system stability and safety
5. Three-Phase Grounding Transformer (Zig-Zag Type)
Zig-Zag (Z-Type) Connection
Three-phase grounding transformers commonly use a Zig-Zag (Z-type) winding connection.
Key structural feature:
- Each phase winding is split into two sections
- Wound in opposite directions on the same core limb
Advantages of Zig-Zag Grounding Transformers
- Zero-sequence flux flows through the core limbs, not leakage paths
- Very low zero-sequence impedance (typically around 10 Ω)
- Enables effective zero-sequence current flow
In contrast, ordinary transformers have much higher zero-sequence impedance.
Capacity Advantage
- Ordinary transformers with arc suppression coils:
- Limited to ≤ 20% of transformer capacity
- Zig-zag grounding transformers:
- Can carry 90%–100% of arc suppression coil capacity
Additional Benefits
- Can supply secondary auxiliary loads
- May replace station service transformers
- Helps reduce overall project investment
6. Single-Phase Grounding Transformer
Single-phase grounding transformers are mainly used in:
- Generator neutral grounding systems
- Neutral grounding resistor cabinets for large transformers
Purpose
- Provide a grounding point for systems that already have a neutral
- Reduce the size and cost of neutral grounding resistor cabinets
7. Operating Characteristics of Grounding Transformers
A well-designed grounding transformer has the following characteristics:
- Low zero-sequence impedance
- Ensures sufficient zero-sequence current during ground faults
- High magnetizing impedance
- Reduces no-load current
- Low no-load loss
- Minimizes daily operating energy consumption
These features ensure reliable operation while maintaining high efficiency.
8. Typical Applications
Grounding transformers are widely used in:
- Medium-voltage distribution substations
- Cable-dominated urban power networks
- Industrial power systems
- Wind farms and renewable energy substations
They are often installed together with:
- Arc suppression coils
- Neutral grounding resistors
- Protection and monitoring systems
Conclusion
Grounding transformers are essential components in modern medium-voltage power systems. By providing an artificial neutral point, they effectively:
✔ Reduce earth-fault current
✔ Suppress arc grounding overvoltage
✔ Prevent ferroresonance
✔ Improve system safety and reliability
As distribution networks continue to expand, grounding transformers will play an increasingly important role in safe and stable grid operation.