Okay, this is a classic scenario that arises when integrating solar and BESS with an existing power system, particularly one with delta-connected transformers. Let's break down the interaction and how to approach a solution.

Understanding the Problem: The Delta-Wye Grounding Mismatch

The core issue is the difference in grounding philosophies between the original delta-delta system and the new wye-grounded PT setup: Delta-Delta (70kV-12kV Transformer)

Delta-connected windings inherently block zero-sequence currents. This means there's no intentional path for ground fault currents through the transformer itself. In a traditional delta system, ground faults are often allowed to persist (high impedance grounded) until maintenance can be scheduled (if there's no sensitive equipment affected). Relaying is usually basic, focused on phase-to-phase faults.

Wye-Delta (Plant Distribution)

The delta-connected secondary of the distribution transformer provides a similar benefit to the delta-delta transformer by providing a path for circulating currents on the 480V and blocking zero sequence currents.

Wye-Grounded PTs (Potential Transformers)

The wye-grounded PT bank provides a solidly grounded neutral reference. This is essential for sensitive ground fault protection schemes (like SEL protection) because it provides a low impedance path for zero-sequence currents to flow. With a solidly grounded system, ground faults are expected to be cleared quickly.

The Conflict

The problem occurs when the wye-grounded PTs see a circulating ground current, even when there isn't a real ground fault. Here's how this can happen:1.

Harmonics

Inverter-based resources (solar PV and BESS) can introduce harmonic currents into the system. Even if the inverters are designed to minimize harmonics, some level is always present. Certain harmonics (especially triplen harmonics – 3rd, 9th, 15th, etc.) are zero-sequence in nature. These harmonics circulate in the delta windings and can be seen by the wye-grounded PTs as a ground fault.2.

Charging Capacitance

The MV cables within the plant have inherent capacitance to ground. This capacitance creates a path for charging currents to flow from the energized phases to ground. While small under normal conditions, these charging currents can be significant enough to be detected by sensitive ground fault relays, especially when combined with the grounding provided by the wye-grounded PTs.3.

Unbalances

Even small imbalances in the voltage or impedance of the system can cause currents to flow in the neutral. These imbalances can be amplified by the presence of the wye-grounded PTs.4.

Interaction with BESS

The BESS system can contribute to the issue in several ways:

Inverter Grounding Configuration

The BESS inverter itself may have a grounding scheme that conflicts with the existing system grounding. It might inadvertently be sourcing or sinking ground current.

Control System Issues

The BESS control system might be reacting to voltage fluctuations or imbalances in a way that causes it to inject ground current.

Transition Issues

When energizing/de-energizing, the BESS system can create voltage sags or swells, which temporarily upset the system balance and trigger ground fault protection.5.

Resonance

The system capacitance and inductance (especially the transformer magnetizing inductance) can create a resonant circuit at certain frequencies. This resonance can amplify harmonic currents, making the ground fault detection more sensitive.

Why the Inconsistency?

The intermittent nature of the problem suggests the following: Varying Solar Output

The amount of harmonic current injected by the solar inverters will vary with solar irradiance. Higher output may lead to more harmonics.

BESS Operation

The BESS may only be active during certain times of day, or under certain loading conditions.

System Configuration Changes

Switching of loads or changes in the MV network configuration can alter the system impedance and capacitance, affecting the ground fault currents.

Temperature

Cable capacitance can change with temperature.

A Systematic Approach to Fixing the System

Here's a step-by-step approach to diagnose and resolve this issue:1. Thorough System Study

Conduct a comprehensive power system study, including:

Load Flow Analysis

Model the entire system, including the utility connection, transformer impedances, cable lengths and types, motor loads, solar inverters, BESS, and the wye-grounded PTs.

Harmonic Analysis

Model the harmonic sources (solar inverters, BESS) and the system impedance at harmonic frequencies. This will help identify potential resonance points.

Grounding Analysis

Evaluate the effectiveness of the existing grounding system and the impact of the wye-grounded PTs. Determine the expected ground fault current levels under various fault conditions.

Transient Analysis

Simulate switching events (energizing/de-energizing the system) to identify potential voltage sags, swells, and inrush currents that could trigger the ground fault protection.

Short Circuit Analysis

2.

Data Logging and Monitoring



Ground Current

Install high-resolution current transformers (CTs) on the neutral of the wye-grounded PTs to monitor the ground current. Record the current over time, especially during system energization/de-energization.

Voltage and Current Harmonics

Use a power quality analyzer to measure the voltage and current harmonics at various points in the system, including: The MV bus where the PTs are connected The output of the solar inverters The output of the BESS

BESS Status

Log the operating status of the BESS, including charging/discharging rates, voltage, current, and any fault conditions.

MV Voltage

Log voltage phase to ground on the medium voltage system, including neutral voltage3.

Evaluate Grounding Options

Consider these alternatives to the current wye-grounded PT setup. This is the most critical step.



High Resistance Grounding (HRG)

Replace the solid grounding of the PTs with a high-resistance grounding resistor. This will limit the ground fault current to a safe level (typically 5-10 amps) while still allowing for sensitive ground fault protection. HRG provides several advantages: Reduces stress on equipment during ground faults. Prevents arc flash hazards. Allows for continued operation during a single ground fault. Provides sufficient current for ground fault relaying.

Ungrounded System (Delta)

While generally not recommended for modern systems, it may be possible to operate without intentional grounding. However, this can lead to overvoltages during ground faults. This is generally only considered as a last resort. You would need to analyze the risk of overvoltages and ensure adequate surge protection is in place.

Wye-Delta Transformer with Grounded Wye Source

If feasible, replacing the delta-delta utility transformer with a wye-delta transformer (with the wye side grounded) would fundamentally change the grounding paradigm of the entire system. This is a major undertaking and may not be practical.4.

Harmonic Mitigation



Inverter Filtering

Ensure that the solar inverters and BESS inverters are equipped with appropriate harmonic filters (passive or active).

Harmonic Traps

Install harmonic filters (traps) at strategic locations in the system to absorb specific harmonic frequencies that are causing problems.

Reactor Sizing

Adjust sizing of your system reactors such as step up transformers, if this is a resonant frequency it will worsen the issue.5.

BESS Control and Grounding Configuration



Review BESS Grounding

Carefully examine the BESS inverter's grounding configuration. Ensure it is compatible with the overall system grounding.

Control System Optimization

Fine-tune the BESS control system to minimize voltage fluctuations and imbalances during switching events. Work with the BESS vendor to adjust parameters related to grid support functions (e.g., voltage regulation, frequency regulation).6.

Relay Coordination and Settings



Re-evaluate Relay Settings

Recalculate the ground fault relay settings (SEL protection) based on the chosen grounding method and the expected ground fault current levels.

Coordination Study

Conduct a thorough relay coordination study to ensure that the ground fault protection is properly coordinated with other protective devices in the system. Avoid nuisance tripping.

Implement Blocking/Time Delay Logic

You might need to implement blocking or time delay logic in the ground fault relays to prevent tripping during transient conditions (e.g., energization/de-energization). However, be careful not to compromise safety.7.

Cable Shield Grounding

Verify that the MV cable shields are properly grounded at both ends (or grounded at one end and open at the other, depending on the cable type and application). Improperly grounded cable shields can contribute to circulating ground currents.8.

Procedure Development



Energization/De-energization Procedures

Develop detailed procedures for energizing and de-energizing the system, including specific steps for the solar inverters, BESS, and other critical equipment.

Regular Maintenance

Implement a regular maintenance program for all electrical equipment, including the solar inverters, BESS, transformers, and relays.

Specific SEL Relay Considerations



SEL Logic

SEL relays are highly configurable. You can implement custom logic to differentiate between real ground faults and nuisance tripping caused by harmonics or transients.

Harmonic Blocking

SEL relays often have harmonic blocking functions that can be used to prevent tripping on harmonic currents.

Adaptive Settings

Some SEL relays support adaptive settings that can automatically adjust the relay settings based on system conditions (e.g., solar output, BESS status).

Waveform Capture

Use the SEL relay's waveform capture feature to record the voltage and current waveforms during tripping events. This data can be invaluable for diagnosing the cause of the tripping.

Example: Implementing High Resistance Grounding (HRG)If you choose to implement HRG:1.

Select HRG Resistor

Select a resistor value that will limit the ground fault current to a desired level (e.g., 5 amps). The resistor is typically sized based on the system voltage and the desired fault current.2.

Connect HRG Resistor

Connect the resistor between the neutral point of the wye-grounded PTs and ground.3.

Adjust Relay Settings

Recalculate the ground fault relay settings to be sensitive to the limited ground fault current.4.

Monitor Neutral Voltage

Install a voltage relay on the neutral to monitor the neutral voltage. This relay can be used to detect ground faults.

Safety Considerations



Arc Flash

Any changes to the grounding system will affect the arc flash hazard levels. Conduct an arc flash study to determine the new hazard levels and ensure that personnel are properly protected.

Training

Provide thorough training to all personnel who will be working on the electrical system. The training should cover the new grounding system, the relay settings, and the energization/de-energization procedures.

In Summary:The problem you're facing is complex and requires a systematic approach. By conducting a thorough system study, monitoring the system behavior, evaluating grounding options, mitigating harmonics, and optimizing the BESS control system, you can resolve the issue and ensure the safe and reliable operation of your power system. It's crucial to engage qualified electrical engineers with experience in power system studies, grounding design, and relay protection to guide you through this process. Do not make changes to your grounding system without expert consultation. Flag for review