1 DC System Composition and Overview

Substations are crucial links for power transmission, exchange and distribution in the power system, as well as the main monitoring points for grid operation. The DC power supply system serves as the "power heart" of power plants and substations, providing DC power for signal equipment, protection devices, automatic devices, emergency lighting, backup power supplies, and the opening/closing operations of circuit breakers. It is applicable to thermal power plants, hydropower plants, various types of substations and other places where DC equipment is used.

Main Components:

1. AC Distribution Unit: Controls the automatic switching of two AC power supplies in the DC panel cabinet to prevent over-discharge of storage batteries.
2. Charging Device: Mainly adopts the high-frequency switching power supply type charging device.
3. Storage Batteries: Valve-regulated sealed lead-acid batteries are commonly used. The charging modes include floating charge mode, equalizing charge mode and emergency discharge mode.

   

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2 DC System Maintenance Items and Requirements

Charging Panel Test: Including inspection of charging module current-sharing function, voltage stabilization function, current stabilization function, ripple coefficient test, etc. Appropriate maintenance strategies shall be implemented according to the results of condition assessment.

Battery Discharge Test: Specifications require that newly installed valve-regulated batteries shall undergo a verification charge-discharge test. After that, a verification charge-discharge test shall be conducted once every 2 years. For valve-regulated batteries in operation for more than 4 years, a verification charge-discharge test shall be performed annually.

Battery Internal Resistance Test: The internal resistance of a battery increases as its capacity decreases, i.e., with the continuous aging of the battery and the continuous reduction of its capacity, the internal resistance of the battery will increase. By comparing the internal resistance values of the entire battery bank or tracking the internal resistance changes of individual battery cells, the underperforming batteries in the bank can be identified and the aging degree of the batteries can be assessed. The measured internal resistance value of a battery shall be compared with the average internal resistance value of the entire bank and the internal resistance value measured in the previous test, with an allowable deviation range of ±10%. When the internal resistance of 20% of the batteries in the bank is 10% higher than the factory value, the entire bank needs to be replaced.

△ Application Site

3 DC System Test Equipment

FDT-220/110 DC System Comprehensive TesterIt supports charging characteristic tests of DC chargers (voltage stabilization, current stabilization, ripple coefficient, etc.) and discharge tests of storage batteries. It mainly consists of a host, a voltage regulator, a wireless module and a load box, which can be freely combined according to on-site test requirements. The host can conduct discharge tests on batteries with a capacity of 500Ah; when equipped with a load box, it can perform discharge tests on large-capacity battery banks with a discharge current of up to 180A, meeting the charge-discharge test requirements of large thermal and hydropower plants with heavy loads and large discharge current capacities. It is suitable for municipal power supply companies under State Grid and China Southern Power Grid, large thermal power plants, hydropower plants, power transmission and transformation companies, electric power construction companies and other large engineering enterprises.

MDJ-800 DC System Differential Coordination Characteristic TesterIt meets the requirements of DL/T 5044-2014 Technical Code for Design of DC Power Supply Systems in Electric Power Engineering. It can test the correct probability of protection operation differential coordination of DC circuit breakers at all levels of the substation DC circuit, and verify the differential coordination selection of DC circuit breakers at all levels according to the test result report. From the battery bank to the electrical equipment in the substation, power distribution is generally carried out through three levels, and DC circuit breakers are mostly used as protection electrical appliances at each level. During the operation of the DC system, if the protection operation characteristics of the upper and lower level DC circuit breakers are mismatched, a short-circuit fault of the lower-level electrical equipment will cause overstep tripping of the upper-level DC circuit breaker, which will further lead to power outage accidents of other feeder lines.

FR-3 Battery Internal Resistance TesterIt supports internal resistance and capacity test analysis of single-cell/entire battery banks. It can quickly test the internal resistance and voltage of batteries, perform tests on single battery cells or continuous tests on each cell of the entire bank, and permanently store parameters of 999 battery groups. The upper computer data management software provides functions such as data management, printing, analysis and automatic report generation.

BD-500A DC Circuit Breaker Time-current Characteristic TesterIt supports time-current characteristic tests, and the constant current can be expanded to 1000A. Power test specifications require that when a current of 1.13I is applied to the circuit breaker for the specified time, the circuit breaker should not trip; then the current should be stably increased to 1.45I within 5 seconds, and the circuit breaker should trip within the specified time.

HD-2612 Battery Cell Activation InstrumentIt can activate aging single battery cells in the battery bank to achieve 80% capacity of new battery cells, and supports charging, discharging and activation of 2V, 6V and 12V single battery cells.