Remote Capacity Verification of Storage Batteries | Hard Interlock Logic Design of Electric Switches

Release Date: May 19, 2021Wuhan Haomai Electric Power Automation Co., Ltd.

For storage battery maintenance, capacity verification discharge is one of the most effective methods to measure the power supply capacity of batteries at present. A periodic check discharge is usually required once a year. However, manual capacity verification is cumbersome and consumes a lot of human and material resources. The MDC-2000 Battery Comprehensive Monitoring and Remote Automatic Maintenance System can remotely start capacity verification operations on the background server, and the station terminal automatically controls the discharge process until the entire battery charging and discharging process is completed.

△ MDC-2000 Battery Comprehensive Monitoring and Remote Automatic Maintenance System

During the normal operation of the DC power supply system, the output of the charger bears all DC bus loads and also performs floating charging for the battery bank. When the battery bank undergoes regular discharge operations, it is necessary to disconnect the battery bank from the charger and the DC bus, and invert the DC power stored in the battery bank into AC power to feed back to the AC power grid. The Measures for Preventing Major Accidents in 18 Power Grid Items of State Grid Corporation of China stipulates in the regulation on preventing power loss of station DC systems: "When the station DC power supply system is in operation, the battery bank is prohibited from being disconnected from the DC bus". Therefore, the safety protection measures for the electric switch process of disconnecting the battery bank from the bus are crucial, and the switch action sequence must be correct to ensure that a standby battery bank is connected to the DC system during capacity verification operations and the normal operation of the DC system is guaranteed.

Electric Switch Block Diagram

△ MDC2000 System Block Diagram

DK1: Bus tie electric switch for two bus sections

DK11: Electric switch from Section I charger to Section I DC bus

DK12: Electric switch from Section I battery bank to Section I DC bus

DK13: Electric switch from Section I charger to Section I battery bank

DK14: Electric switch from Section I battery bank to inverter discharge device

DK15: Section I discharge protection circuit breaker

DK21: Electric switch from Section II charger to Section II DC bus

DK22: Electric switch from Section II battery bank to Section II DC bus

DK23: Electric switch from Section II charger to Section II battery bank

DK24: Electric switch from Section II battery bank to inverter discharge device

DK25: Section II discharge protection circuit breaker

JK1: AC contactor

JK2: AC circuit breaker

△ Electric Switch Diagram

An electric switch consists of a DC molded case isolating switch, an electric operating mechanism and auxiliary contacts.

The DC molded case isolating switch includes input terminals 1, 3, 5 and output terminals 2, 4, 6, with multiple built-in auxiliary contacts to indicate the switch opening and closing status.

The auxiliary contact has three wiring points: 11 is the common terminal, 12 is the normally closed contact, and 14 is the normally open contact.

The electric operating mechanism is fixed on the DC molded case isolating switch, and pushes the handle of the isolating switch to open and close through motor rotation to achieve electric control effect.

△ Wiring Diagram of Electric Operating Mechanism

Three terminals PE, P1, P2 supply power to the electric operating mechanism: PE is grounded, P1 is connected to the positive pole of the DC power supply, and P2 is connected to the negative pole of the DC power supply. There are three control terminals S1, S2, S4: S1 is the common terminal; the closing operation is performed when S1 is connected to S2, and the opening operation is performed when S1 is connected to S4.

To control the action sequence between electric switches, the switch hard contact interlock action logic can be realized only by connecting auxiliary contacts to the opening and closing control circuit.

Electric Switch Interlock Logic

Taking the check discharge of Section I battery bank as an example, the operation process and interlock logic of electric switches are explained.

△ Hard Interlock Control Circuit

Normal Operation Status of Each Switch

DK1 bus tie switch is open, and the two DC bus sections operate independently. DK11, DK12, DK21 and DK22 are all in the closed state; the charger sets of Section I and Section II respectively bear the DC loads, and the battery banks are on floating charge for standby.

Switch Operation Process for Discharge Operation (Under Safe and Fault-Free Operation of the Two DC Power Supply Systems)

1. Close the bus tie switch DK1 to realize parallel operation of the two bus sections.
2. Open the Section I battery bank input switch DK12 to disconnect the output of the Section I battery bank.
3. Open the Section I charger input switch DK11 to disconnect the output of the Section I charger and start battery discharge.
4. After the discharge is completed, close DK13 and charge the Section I battery bank with the Section I charger.
5. After the charging is completed, open DK13; after the battery bank is left to stand, close DK11 and DK12.
6. Finally, open DK1 to restore the independent operation state of the two bus sections, and the check discharge operation is completed.

The action sequence of electric switches must be strictly executed in accordance with the process, and the action interlock logic is realized by connecting the switch hardware auxiliary contacts in series into the opening and closing control circuit.

Interlock Logic Table

Switch	Action	Interlock Condition	Description
DK1	KC1 Closed	Four normally open auxiliary contacts of DK11, DK12, DK21, DK22 are connected in series	Before discharge, all charger and battery output switches are normally closed and operating, and the voltage difference between the two DC bus sections is less than 5V
DK12	KC6 Opened	Normally open contact of DK1 and normally open contact of DK22 are connected in series	The bus tie switch is closed and Section II battery bank is connected to ensure backup power for the bus
DK11	KC4 Opened	Normally open contact of DK1 and normally open contact of DK21 are connected in series	Disconnect the Section I charger after the Section I battery bank is disconnected
DK13	KC7 Closed	Normally closed contact of DK11 is connected in series	The charger charges the battery bank after discharge is completed
DK13	KC8 Opened	None	The battery bank is left to stand after charging is completed
DK11	KC3 Closed	Normally open contact of DK13 is connected in series	Connect the charger output
DK12	KC5 Closed	Normally closed contact of DK13 is connected in series	Connect the battery bank output
DK1	KC2 Opened	Two normally open auxiliary contacts of battery switches DK12 and DK22 are connected in series	Restore the charger and battery output switches after discharge, and disconnect the bus tie switch

The hardware interlock control method with built-in auxiliary contacts of electric switches is safe and effective, and the matching software logic control ensures the reliability of remote charging and discharging of battery banks. The bus is guaranteed not to lose power during the entire process, and each electric switch opens and closes in an orderly manner under the framework of pre-set program logic. The built-in auxiliary contacts of electric switches serve as a part of the logic control judgment, which ensures the safety of remote charging and discharging of battery banks.