Maximum Load for Two 100kVA Transformers Configured in an Open Delta

When two transformers, each rated at 100kVA and with a voltage rating of 13200/240 volts, are connected in an open delta (V) configuration, the maximum load that can be supported is limited due to the unique characteristics of this connection.

Understanding the Open Delta Configuration

An open delta or V connection is a specific configuration used for three-phase systems where only two of the three phases are connected, leaving the third phase unconnected. This setup can be advantageous in situations where a three-phase supply is not available or necessary.

Calculating the Maximum Load in Open Delta Configuration

To understand the load capabilities of two 100kVA transformers in an open delta connection, it is important to first calculate the total capacity of the two transformers. The total capacity is simply the combined capacity of the two individual transformers.

Given that each transformer is rated at 100kVA, the total capacity is:

Total capacity 100kVA 100kVA 200kVA

However, it's not as simple as adding their individual ratings directly, as the load that can be supported in an open delta configuration is limited to about 57.7% of the total capacity.

The factor 0.577 comes from the geometric properties of the V connection, where the voltage between any two phases is equal to the square root of 3 (approximately 1.732) times the line-to-neutral voltage.

Therefore, the maximum load that can be supported in the open delta configuration is calculated as:

Maximum load 200kVA * 0.577 ≈ 115.4kVA

Comparison with Parallel Connection

It's important to note that if the same transformers were connected in parallel, the maximum load would be the sum of their individual capacities, which is:

Parallel connection maximum load 100kVA 100kVA 200kVA

This is the full capacity of the transformers and is valid under the assumption of unity power factor, meaning the load is purely resistive.

Three-Phase Transformer Banks with Open-Delta Configurations

For three-phase systems where two single-phase transformers are configured open-delta to open-delta, the total capacity of the transformer bank is limited. A typical three-phase transformer bank with this configuration has a capacity of approximately 86.6% of the combined transformer apparent power ratings.

In the case of two 100kVA transformers, the combined apparent power rating is 200kVA. Assuming balanced loading between the three phases, the maximum load for this bank is:

Maximum load ≈ 200kVA * 0.866 ≈ 173kVA

Load Considerations and Safety

Utilities often load distribution transformers above their nameplate rating for short periods to accommodate peak demand. However, continually operating at these higher loads can lead to increased temperature rise, reducing the life expectancy of the transformer bank.

To ensure long-term reliability, it's crucial to monitor and manage the load to prevent continuous overloading. Consulting with a professional electrical engineer or the transformer manufacturer can provide specific guidance on load limits and operational safety.