DC Power Supply System Panel Capacity Selection Guide for Substations
Core Selection Logic
The DC power supply system panel provides an uninterruptible DC control power supply for protection, control, communication, and circuit-breaker closing circuits within the substation. Battery capacity selection must simultaneously satisfy two operating condition verifications, with the maximum value adopted as the final selection to ensure system safety and stability under contingency conditions:
| Condition Type | Core Control Objective |
| Emergency Continuous Power Supply | After AC power loss, the battery bank reliably supports all DC loads of the substation for the specified duration |
| Surge Load Resistance | During circuit-breaker closing, the DC bus voltage shall not drop below allowable limits to prevent mal-operation and failure to trip of protection and control equipment |
- Spring-operated mechanisms (mainstream type): Closing surge current is small (20–50 A). Capacity is generally governed by the emergency discharge duration.
- Electromagnetic (solenoid) operating mechanisms (legacy type): Closing surge current is extremely high (≥ 100 A). Surge voltage verification becomes the governing condition and must be given priority validation.
DC Load Classification (Strictly in accordance with DL/T 5044-2014)
DC loads are categorized into three types based on operational characteristics. Statistical rules shall comply strictly with standard requirements to guarantee complete statistics without duplication or omission:
| Load Type | Operating Characteristic | Typical Equipment | Statistical Rule |
| Continuous Load Izc | Operates continuously under normal conditions, and remains energized during emergency (mandatory sustained operation) Protection relays, control & monitoring units, DC indicator lamps, online monitoring devices 100% full inclusion | Protection relays, control & monitoring units, DC indicator lamps, online monitoring devices | 100% full inclusion |
| Emergency Load Isg | Energized only upon AC power loss as required for safe shutdown and communication; normally de-energized | Emergency lighting, communication UPS, DC emergency auxiliary loads | Full inclusion subject to the specified emergency discharge duration |
| Surge Load Ihz | Short-duration random surge load during closing (duration ≤ 5 s, superimposed at the end of the sustained discharge period) | High-voltage circuit-breaker closing coils | Adopt the maximum closing surge current of a single device within the substation |
Capacity Calculation Methods (Dual-Algorithm Compliance System)
Method 1: Simplified Calculation Method (for 11 kV / General 33 kV Substations)
Applicability: Spring-operated mechanisms, non-hub substations without high-current electromagnetic closing devices. This method is only applicable for scheme design, preliminary equipment selection and tender quotation. It shall NOT be adopted for official grid review and approval.
Core Parameters (fixed values specified by the standard):
- Reliability coefficient: Kk=1.4Kk =1.4 (covers battery aging, ambient temperature variation, load fluctuation and safety margin)
- Capacity conversion coefficient Kc: Obtained from Appendix C of DL/T 5044-2014 according to emergency discharge duration and battery end-of-discharge voltage
Sustained Capacity Calculation Formula:
C=Kk⋅(Izc+Isg)⋅T/Kc
Surge Verification Requirement: The initial 1-minute surge operating condition at the start of emergency discharge must be independently verified to avoid excessive instantaneous voltage dip.
Method 2: Stepped Load Calculation Method (for Grid Review / Substations of 110 kV and above)
This is the official standard algorithm specified in Appendix C of DL/T 5044-2014. It is the only code-compliant algorithm acknowledged for detailed engineering design, grid review & approval, project acceptance and registered electrical engineer qualification examinations.
Core Calculation Steps:
| Step | Description |
| Load Profile Segmentation | Divide the entire emergency discharge process into three load stages: initial surge stage (1 min), sustained discharge stage and random surge stage (5 s, superimposed at the end of sustained discharge) |
| Parameter Determination | Obtain the corresponding capacity conversion coefficient Kc from tables for each stage based on discharge duration and end-of-discharge voltage |
| Capacity Calculation | Uniformly adopt the standard reliability coefficient Kk=1.4; calculate the required battery capacity for each stage independently |
| Final Selection | Adopt the maximum calculated value among all stages, take the random surge load effect into consideration, and round up to the nearest standard battery capacity rating |
| Voltage Verification | In accordance with Appendix D, verify that DC bus voltage at the end of emergency discharge shall not be less than 85% of rated voltage (≥ 187 V for 220 V DC system) |
Charging Module Configuration Specifications
The DC charging device consists of high-frequency switch-mode rectifier modules operating in parallel. All substations shall adopt an N+1 hot-standby redundancy configuration to guarantee uninterrupted power supply when a single module fails.
| Parameter | Calculation Method |
| Battery boost charging current | ≈0.1⋅C (where C is the rated capacity of the finally selected battery bank, in Ah) |
| Total required charging current | Boost charging current + substation-wide continuous load Izc |
| Quantity of modules | Total required current ÷ rated current per module; round up to integer, then add one module for redundancy |
Engineering Calculation Example (35 kV Industrial Park Substation)
Project Basic Conditions: 35 kV substation equipped with 13 sets of high-voltage switchgears. All switchgears adopt spring-operated mechanisms without electromagnetic closing devices. The specified emergency discharge duration is 2 hours.
Step 1: Load Summary
| Load Type | Operating Current | Description |
| Continuous Load Izc | 12A | Protection, control and monitoring devices of the whole substation |
| Emergency Load Isg | 10A | Communication UPS and station emergency lighting |
| Surge Load Ihz | 30A | Maximum closing surge current of single circuit-breaker |
Step 2: Simplified Method Capacity Calculation
Check the “Capacity Conversion Coefficient Table for Valve-Regulated Lead-Acid (VRLA) Batteries” in Appendix C of DL/T 5044-2014. For a 2-hour discharge duration (end-of-discharge voltage: 1.80 V/cell), the corresponding capacity conversion coefficient is
Kc≈0.344. The standard reliability coefficient is Kk=1.4.
C=1.4⋅(12+10)⋅2/0.344=179.1 Ah
The calculated capacity is 179.1 Ah, which shall be rounded up to the nearest standard specification. The final selection is a 200 Ah battery bank.
Submission Note: The result obtained by the simplified method is only used for preliminary selection and quotation. For official grid review and as-built document filing, verification shall be conducted with the stepped load method, and a formal standard calculation report shall be issued.
Step 3: Charging Module Configuration Calculation
| Calculation Item | Process | Result |
| Base boost charging current | 0.1×200 | 20 A |
| Total required substation current | 20 A + 12 A | 32 A |
| Basic module quantity (10 A per module) | 32 ÷ 10 ≈ 4 units | 4 units |
| N+1 redundancy configuration | 4 + 1 | 5 units |
Final Selection: 200 Ah DC power supply system panel equipped with 5 × 10 A charging modules, cabinet-mounted installation.
Standard Selection Reference Table by Application Scenario
| Application Scenario | Recommended Battery Capacity | Charging Module Configuration | Mounting Type |
| 10 kV Distribution Room (≤ 8 panels) | 40Ah | 2 × 5A (N+1 redundancy) | Wall-mounted for capacity ≤ 40 Ah / Cabinet-mounted |
| General 35 kV Substation (13–20 panels) | 65–100 Ah | 3–4 × 10 A (N+1 redundancy) | Cabinet-mounted |
| 110 kV Critical Substation | 100–150 Ah per bank (dual independent banks) | 3–4 × 10 A per bank (independent redundancy) | Dual cabinet-mounted |
Core Code Compliance Summary
| Key Point | Standard Specification |
| Design Basis | Fully implemented in accordance with DL/T 5044-2014 Appendix C |
| Standard Reliability Coefficient | Uniform value Kk=1.4, a fixed value mandated by the standard |
| Applicability of Simplified Method | 10 kV and general 35 kV substations; only for preliminary selection and quotation |
| Applicability of Stepped Load Method | All grid review projects, substations of 110 kV and above and critical hub stations; the sole recognized compliant algorithm |
| Voltage Verification Rule | DC bus voltage at the end of emergency discharge shall not be lower than 85% of rated voltage |
| Configuration Requirement for Critical Substations | Substations of 110 kV and above shall be equipped with dual independent battery banks and dual independent charging systems |
| Charging Module Configuration | Uniform N+1 hot-standby redundancy configuration for all substations to ensure operational reliability |
