Solid earthing (or grounding) Working Principle

Solid earthing (or grounding) works by directly connecting the neutral point of a generator or transformer to the earth using a conductor with negligible resistance and reactance. This creates a low-impedance path that keeps the neutral at earth potential, prevents transient overvoltages, and allows for rapid, high-magnitude fault current detection, which ensures swift protection device operation.

Key Working Principles and Characteristics

• Direct Connection: The neutral point is connected directly to the ground without any intentional insertion of impedance (resistors or reactors).
• Low-Resistance Path: The path offers minimal impedance, allowing a large, high-magnitude earth fault current (typically 25% to 100% of the three-phase fault current) to flow, which helps in quick isolation of the faulty circuit.
• Voltage Stabilization: The neutral point remains tied to the earth potential, which prevents the voltage of healthy phases from rising significantly during a single-line-to-ground fault.
• Fault Current Limitation: While the fault current is high, the system is designed to keep this current within safe limits for equipment.

Applications

• Commonly used in low-voltage distribution systems.
• Used in high-voltage systems, particularly above 220 kV.
• Ideal for systems where rapid clearing of ground faults is essential for safety.

The operating principle of solid (or "effective") earthing consists of directly connecting the neutral point of a three-phase system (such as a transformer or generator) to earth via a conductor with negligible impedance.

How It Works

• Potential Maintenance: The direct connection "locks" the neutral potential to ground (0 Volts) under all operating conditions.
• Fault Behavior: If a ground fault occurs on one phase, the voltage of the faulted phase drops to zero, while the voltages of the other two "healthy" phases relative to ground remain virtually unchanged (they do not exceed the normal phase voltage).
• Low Impedance Return Path: The system creates an easy path for the fault current to return to the source. This generates a high fault current, which is necessary to quickly trip protective devices such as circuit breakers or fuses.

Advantages and Disadvantages

Common Applications

• Low-voltage (LV) systems: Widely used in residential and commercial environments (e.g., 400/230V systems) to ensure personal safety.
• High-voltage (HV) systems: Universally used for voltages above 33-123 kV where insulation costs become a critical factor.

Conclusion

The solid grounding of neutral point has the following advantages:

1. The neutral is effectively held at earth potential.
2. When earth fault occurs on any phase, the resultant capacitive current IC is in phase oppo­sition to the fault current IF. The two currents completely cancel each other. Therefore, no arcing ground or over-voltage conditions can occur. Consider a line to ground fault in line B as shown in Fig. above. The capacitive currents flowing in the healthy phases R and Y are IR and Iy respectively. The resultant capacitive current IC is the phasor sum of IR and Iy. In addition to these capacitive currents, the power source also supplies the fault current IF. This fault current will go from fault point to earth, then to neutral point N and back to the fault point through the faulty phase. The path of IC is capacitive and that of IF is inductive. The two currents are in phase opposition and completely cancel each other. Therefore, no arcing ground phenomenon or over-voltage conditions can occur.
3. When there is an earth fault on any phase of the system, the phase to earth voltage of the faulty phase becomes zero. However, the phase to earth voltages of the remaining two healthy phases remain at normal phase voltage because the potential of the neutral is fixed at earth potential. This permits to insulate the equipment for phase voltage. Therefore, there is a saving in the cost of equipment.
4. It becomes easier to protect the system from earth faults which frequently occur on the When there is an earth fault on any phase of the system, a large fault current flows between the fault point and the grounded neutral. This permits the easy operation of earth-fault relay.

Applications

Solid grounding is usually employed where the circuit impedance is sufficiently high so as to keep the earth fault current within safe limits. This system of grounding is used for voltages upto 33 kV with total power capacity not exceeding 5000 kVA.

Calculation

Low-reactance Grounding

ased on network constant ratios, the criteria to qualify as reactance grounded is: 

Xₒ/X1 > 3

as seen at fault, but less than the value necessary for resonant grounding. 

Inserting a low-reactance such that 

Xₒ/X1 ≤ 3

at fault, is not reactance grounding.
When employing a solidly grounded grounding transformer, its reactance may be such that

Xₒ/X1 > 3

and the system is deemed reactance grounded. 

Recommendations

It is not proper to endanger a generator winding with fault currents higher than the three-phase current at the terminals. In generators, the ground-fault currents are higher than the three-phase fault currents because the internal impedance seen by the ground-fault is less than the impedance to three-phase faults. The high current produces excessive heating and mechanical forces.

A suitable current limiting impedance, like a low-reactance reactor, should be installed in the neutral to avoid generator damage.

In transmission and distribution systems, without directly connected rotating machines, the neutrals of the transformers are usually effectively grounded and reactors are not typical.

The phase-to-ground fault current should range from 25% to 100% of the three-phase fault current. Less than 25% may cause damaging transient overvoltages. Choose the value of reactance needed to limit the ground-fault current to the preferred amount.

The ratio is 

• Xₒ/X1 = 10 when 25%

• Xₒ/X1 = 1 when 100%

• Xₒ/X1 = 3 when 60%

Limiting the ground-fault current to 60% of the three-phase fault current is the borderline between effective grounding and reactance grounding.
When installing a 100% reactor in a generator neutral, the system is not reactance grounded but effective grounded, by definition, and the maximum fault-current contribution of this generator to a line-to-ground fault anywhere in the system outside of the generator will be its three-phase fault current.

Low-resistance Grounding

In the USA, low-resistance grounding is the most popular method utilized to limit ground-fault current. The value of resistance is much lower than the high-resistance method and ranges from 5% to 20% of the three-phase fault current. Some applications limit the ground current to around 50A to 600A.
A typical resistor of 400A will allow enough current flow to operate the protective relays for fast fault clearing. These resistors are also time-rated. A standard figure is 10s because, like in effective grounding, the branch will shut down after the first ground-fault.

Medium voltage transformer with star point connection to earth

Medium voltage (MV) transformers with a star (Y) connected winding, particularly on the secondary side, often have the neutral point grounded to ensure system stability, safety, and proper fault protection. This configuration allows for phase-to-neutral loading, reduces transient overvoltages, and limits fault currents to safe levels, often utilizing a Neutral Grounding Resistor (NGR).

Key Aspects of Star Point Grounding in MV Transformers

• Grounding Methods: The neutral point is typically connected to the ground directly or through an impedance, such as a resistor (NGR) or a Petersen coil, to limit ground fault currents.
• Purpose: The primary reason for grounding is to provide a low-impedance path for fault current, which allows protective relays to quickly detect and isolate ground faults.
• Voltage Limitation: It prevents neutral shifting and keeps the phase-to-earth voltage at a stable level (phase voltage = line voltage (√3), reducing the insulation requirement for equipment. 
• Common Applications: While MV primary sides are often delta-connected, the secondary (e.g., 11kV or 20kV) is often star-connected to allow for direct grounding, specifically in industrial, mining, or distribution systems.
• Safety & Protection: Grounding helps mitigate transient overvoltages, protects against lightning, and reduces personnel risks by ensuring fault currents are handled effectively.

In some cases where the main transformer is delta-connected, a separate grounding transformer (e.g., a Zigzag transformer) is used to create a star point for grounding purposes.

In medium voltage (MV) systems (typically 1–35 kV), a transformer with a star point (neutral) connection to earth serves as a critical safety and stability mechanism. While the low-voltage (LV) side is almost always solidly grounded, MV grounding methods vary based on the desired fault current control.

Common Grounding Configurations

• Solid Earthing: The star point is connected directly to a ground rod. This is common in some high-voltage grids but can lead to high fault currents in MV systems.
•  Resistor Earthing (NGR/NER): A Neutral Grounding Resistor is placed between the star point and earth to limit earth-fault currents to a specific value (e.g., 100 A or 1,000 A). This reduces damage to equipment and prevents transient overvoltages.
• Resonant Grounding (Petersen Coil): The star point is connected via an inductance (coil) that resonates with the system's capacitance, effectively "extinguishing" the fault current during a single line-to-ground fault.

Key Technical Benefits

• Safety: Provides a reliable path for fault currents, protecting personnel from electric shock.
• System Stability: Establishes a voltage reference point, which helps maintain balanced phase-to-ground voltages.
• Fault Detection: Enables protection relays to detect and clear ground faults quickly.
• Harmonic Suppression: The grounded star point allows for the flow of triplen harmonics (3rd, 9th, etc.), reducing voltage distortion in the system.

Use of Earthing Transformers

If the main MV winding is Delta-connected (which has no natural star point), an Earthing Transformer (Zig-zag or Star-Delta) is used to create an artificial neutral for grounding purposes.
Would you like to know the specific Neutral Grounding Resistor (NGR) sizing requirements for a particular MV system voltage?

Active/Automatic Load Sharing

Active/Automatic Load Sharing systems automatically redistribute electrical loads across multiple transformers or power sources based on real-time demand, typically using PLCs, microcontrollers, or relays to manage parallel operations. This technique protects equipment from overheating and failure by connecting secondary ("slave") units when the main transformer exceeds capacity, improving efficiency, reducing voltage drops, and increasing reliability in industrial or distribution networks.

Key components of automatic load sharing systems often include:

• Control Unit: A microcontroller or PLC (Programmable Logic Controller) continuously monitors load conditions and voltage levels.
• Sensors: Used to measure real-time current, voltage, and temperature to determine when to share the load.
• Switching/Relays: Automatically connect or disconnect additional transformers in parallel based on signals from the controller.

Key Benefits

• Preventing Overloads: Protects transformers from burning out by ensuring they operate within their rated capacity.
• Improved Efficiency: Optimizes the performance of transformers by avoiding overloading.
• Voltage Regulation: Maintains stable voltage levels even during peak loads.
• Reliability: Provides uninterrupted, or "bumpless," power transfer in parallel systems.

Applications

• Substations: Managing load distribution among multiple transformers.
• Industrial Areas: Handling high-demand machinery.
• Generator Sets: Balancing power generation across multiple generators.
• Shopping Malls & Rural Areas: Managing fluctuating, high-density loads.

Isochronous Control Sharing

Isochronous control sharing enables multiple generators in an islanded, parallel system to maintain constant, stable frequency (e.g., 50/60 Hz) regardless of load changes, preventing system shutdowns. By using communicating electronic governors, units share load proportionally, avoiding the instability of uncontrolled, independent isochronous operation.

Key Aspects of Isochronous Load Sharing

• Function: Unlike droop control, which allows frequency to drop as load increases, isochronous control maintains a strict speed and frequency setpoint.
• Load Sharing Mechanism: To avoid "fighting" (where machines compete to take the entire load), units are connected via a communication line or a master controller (e.g., RTAC) that balances the active power (kW) across all units.
• Applications: Primarily used in islanded, isolated power plants, or industrial microgrids where high-quality, stable power is required.
• Benefits:

• Eliminates Frequency Drift: Maintains constant frequency under fluctuating load.
• Improved Reliability: Reduces the need for, or eliminates, generator load-shedding schemes.
• Proportional Load Distribution: Ensures all generators share the load according to their capacity.

While one machine can operate in simple isochronous mode, parallel operation requires coordinated, electronic load sharing to maintain stability.

Isochronous load sharing is a control method used in power generation to allow multiple generators to operate in parallel while maintaining a constant system frequency (typically 50 or 60 Hz), regardless of the total load.

Key Characteristics

• Constant Frequency: Unlike droop control, where frequency decreases as load increases, isochronous control forces the system to return to its nominal speed immediately after a load change.
• Proportional Sharing: All participating generators communicate via a "load sharing line" (analog or digital, such as CAN-bus) to ensure each unit carries a proportional share of the total load based on its capacity.
• Islanded Operation: This mode is primarily used in islanded systems (e.g., ships, remote microgrids, or isolated industrial sites) where no connection to a massive utility grid exists to dictate frequency.

How It Works

• Communication: Each generator's electronic governor is linked to a common communication backbone. In modern systems, this is often handled by a Real Time Automation Controller (RTAC) or integrated engine control modules like Wärtsilä’s UNIC.
• Coordination: Without communication, multiple isochronous units would "fight" each other, leading to instability as they each try to independently correct the frequency. The sharing line provides a common reference signal.
• Active Trimming: The control system dynamically calculates the required output for each unit. If the load increases, all units ramp up simultaneously to maintain the target frequency.

Comparison with Droop Control

System Risks

Single Point of Failure: Because it relies on constant communication, a break in the load-sharing line can cause units to drift or become unstable. Many systems are designed to automatically revert to droop mode if a communication fault is detected.

Reactive Power (kVAR) Sharing

Reactive power (kVAR) sharing in parallel generators is controlled by adjusting the alternator field excitation (Automatic Voltage Regulator - AVR) to balance reactive load and maintain system voltage. Increasing excitation (over-excitation) makes a generator take more load, while decreasing it (under-excitation) makes it take less, without significantly affecting real power (kW)

Key Principles of kVAR Sharing

• Method: The alternator excitation system dictates the proportion of the total system kVAR load each generator carries.
• Regulation: Proper sharing prevents one generator from becoming overloaded (over-excited) while another is under-utilized, and avoids excessive circulating currents.
• Voltage vs. kVAR: While AVRs maintain system voltage, the relative excitation level between paralleled machines dictates kVAR distribution.
• Droop Control: A common technique where the voltage setpoint decreases slightly as reactive load increases, aiding in stable sharing.
• Proportionality: Units should share the total kVAR based on their kVA ratings.

How to Adjust kVAR Sharing

• Increase Load: Increase the field excitation (increase voltage setpoint) on a generator.
• Decrease Load: Decrease the field excitation (decrease voltage setpoint) on a generator.
• Note: If generators are not sharing properly, ensure voltage regulators are in droop mode and that all droop settings are identical.

• Field Excitation Control: Adjusting the excitation level changes a generator's kVAR output without significantly altering the system voltage.

• Over-excitation: Increases the kVAR delivered by that unit and decreases its power factor.
• Under-excitation: Decreases the kVAR delivered and increases its power factor.

• Reactive Droop Compensation: A method where the alternator's output voltage is allowed to "droop" (decrease) slightly as the reactive load increases. This prevents units from fighting each other for control, ensuring they share the total reactive load in proportion to their individual ratings.
• Cross-Current Compensation: A method used to balance reactive load without intentional voltage droop. It uses interconnected current transformers between the AVRs of paralleled units to sense and eliminate "circulating" reactive currents between machines.

Why Effective Sharing is Critical

• Preventing Overloads: Poor kVAR sharing can cause one generator to carry excessive current even if the kW load is balanced, leading to tripped breakers or alternator overheating.
• System Stability: Unmatched excitation levels lead to undesirable circulating currents (cross currents) between alternators, which reduces efficiency and can damage equipment.
• Voltage Regulation: Proper sharing ensures that the overall system voltage remains stable under varying inductive loads (like motors and transformers).

Modern Control Systems