In modern substations, accurate power system design requires a clear understanding of instantaneous (transient) loads and how they impact equipment sizing, particularly for battery chargers in DC systems. This article breaks down the concept of transient loads, their implications, and how to properly calculate charger capacity under such conditions.

1. What Is an Instantaneous (Transient) Load?

A transient load refers to a short-duration, high-power demand event that typically lasts from a few milliseconds to several seconds. These loads are characterized by:

(1)High amplitude current or power

(2)Brief duration

(3)Unpredictable timing

Common Examples:

Inrush currents during motor startup or transformer energization (up to 5–10× rated current)

Switching surges from welding machines or high-frequency induction equipment

Fault events, such as lightning strikes or short circuits, causing brief but intense load spikes

2. Why Are Transient Loads Important?

(a) Impact on Electrical Equipment

Electrical components like circuit breakers, transformers, and cables must be rated to handle:

Peak withstand currents

Thermal stress during surges:
Failing to do so can result in insulation breakdown, overheating, or equipment failure.

(b) Challenges for Protection Devices

Protective relays may misinterpret transient loads as fault conditions. It's critical to:

Set appropriate protection thresholds

Discriminate between normal transients and actual faults

3. How to Calculate Charger Capacity (Substation DC Systems)

Scenario:

A charger supplies power to a DC system that supports batteries, control protection, and emergency loads. Proper sizing must account for:

Battery charging demand

Steady-state DC loads

Potential transient (instantaneous) loads

4. Step-by-Step Calculation

(a) Determine Charging Current

For lithium or lead-acid batteries, constant-current charging is typically 0.1C–0.2C, where C is the rated capacity (Ah).

Example:
Battery capacity = 200Ah
Charging current = 0.1C = 20A

(b) Calculate Charging Power

Charging voltage = 2.35V/cell × 110 cells = 258.5V
Charging power = 20A × 258.5V = 5.17kW

(c) Add Other Loads

Steady-state load: e.g., control/protection devices ≈ 1kW

Transient load: e.g., circuit breaker closing ≈ 3kW (short duration)

(d) Account for Efficiency and Safety Margin

Charger efficiency (η): typically 85%

Margin factor: usually 20–30%

5. Final Formula

P_charger=P_charging+P_{regular load}+P_{transient load  /} η ×(1+margin)

6. Example Calculations

Case A: Charging and transient load do not overlap

Charging power = 5.2kW

Regular load = 1kW

Transient load = excluded

P=(5.2+1)×1.2/0.85≈8.75 kW⇒select 10kW charger

Case B: Charging and transient load occur simultaneously (worst-case scenario)

Transient load = 3kW included

P=(5.2+1+3)×1.2/0.85≈13.06 kW⇒select 15kW charger

Conclusion

Designing for transient load conditions is essential in ensuring substation reliability. Underestimating peak demands can lead to equipment failure and system downtime. When calculating charger capacity, always consider:

Battery charging requirements

Steady-state and transient load overlap

Efficiency and safety margins

With these considerations, you can ensure that your DC power systems operate safely, efficiently, and reliably—even during momentary surges.