DL/T 1498.5-2019Technical specification for on-line monitoring device of transformation equipment. Part 5: On-line monitoring device of transformer core earthing current (English PDF)

4.1 General technical requirements

The safety performance, reliability, basic functional requirements, insulation performance requirements, electromagnetic compatibility performance requirements, environmental performance requirements, mechanical performance requirements, enclosure protection performance, structure and appearance, and other general technical requirements of the on-line monitoring device of transformer core earthing current shall comply with the relevant requirements of DL/T 1498.1-2016.The connection safety performance and functional requirements of the on-line monitoring device of transformer core earthing current shall comply with 4.2 and 4.3 of this part.

4.2 Connection safety requirements

4.2.1 The connection of on-line monitoring devices shall not lead to safety hazards to the monitored equipment or adjacent equipment.

4.2.2 For monitoring devices with current limiting function, their earthing current loop

shall have protection against open circuit and meet the protection requirements for personnel and equipment.

4.3 Functional requirements

4.3.1 The basic functional requirements for the device are as follows.

a) It has the function of continuous or periodic automatic monitoring of the transformer core/clamp earthing current.

b) The monitoring device shall have the ability to work stably for a long time, meet the requirements of on-site verification, and have the functions of not losing data when power is off, self-diagnosis and self-recovery.

c) It has functions, such as over-limit alarm, device abnormality alarm, and communication abnormality alarm.

4.3.2 The monitoring device with current limiting function (the structural diagram of

the current-limit unit is shown in Figure 1) shall also meet the following functions.

a) The current setting value for controlling the current-limiting resistor on and off is 100 mA. If there are special requirements, the current-limiting resistor setting shall be changed in consultation with the equipment manufacturer.

When switching the current-limiting resistor on and off, the earthing current can be limited to the range required by the regulations, and an automatic alarm shall be triggered. Switching the current-limiting resistor on and off shall not cause a dead loop.

b) The current-limit unit shall have an overvoltage protection function. When the voltage exceeds the set value, the protection unit shall operate reliably.

c) It has automatic/manual mode switching and branch switching indication functions.

4.3.3 For monitoring objects with high harmonic content of transformer core/clamp

earthing current (such as converter transformers), the device shall have the function of monitoring the fundamental effective value.

NOTE 1.The accuracy class of the high-precision current measuring device for laboratories is not lower than Class 1.

NOTE 2.When the measured value of the on-line monitoring device is less than 20 mA, the measurement error shall be calculated according to formula (1). When the measured value of the on-line monitoring device is greater than or equal to 20 mA, the measurement error shall be calculated according to formula (2).

Table 1 -- Technical specifications of on-line monitoring device of transformer core earthing current

5.1 Test environment

Except for environmental impact tests, other tests shall be conducted in the following test environments.

a) ambient temperature. 15 °C~35 °C;

b) relative humidity. 25%~75%;

c) atmospheric pressure. 80 kPa~106 kPa.

NOTE. If, for some reason, the device cannot be tested under the above conditions, the actual climatic conditions shall be recorded in the inspection report.

5.2 Functional and performance tests

5.2.1 Test for general technical conditions

The general technical condition test items include the basic function testing, insulation performance testing, electromagnetic compatibility performance testing, environmental adaptability performance testing, mechanical performance testing, enclosure protection performance testing, and structural and appearance inspection. The test methods for these items and the performance requirements that the on-line monitoring device shall meet after the tests are specified in DL/T 1498.1-2016.

5.2.2 Measurement error test

Within the effective measurement range of the monitoring device, ten different current values for measurement are selected, including the on-line monitoring device's minimum measurable current value (allowable current deviation of 0%~30%), the maximum measurable current value (allowable current deviation of -10%~0%), two current values in the range of 5 mA~20 mA, two current values in the range of 20 mA~100 mA, and four current values in the range of 100 mA~10 A. The measurement error shall conform to the specifications in Table 1.

5.2.3 Data transmission function check

The testing device is connected to the on-line monitoring device to be tested. The data stored in the monitoring device shall be able to be saved locally, exported, and uploaded.

5.2.4 Impulse voltage test for the current-limit unit

Under normal test atmospheric conditions, the current-limit unit shall be able to withstand a short-time impulse voltage test of a standard lightning wave of 1.2/50 µs.

The open-circuit test voltage of the power supply unit is 5 kV. After the test, the current- limit unit shall show no insulation damage or component damage.

5.2.5 Test for current-limiting resistor switching and alarm function

In automatic operation, when the core earthing current exceeds the switching-on set value of the current-limiting resistor, the current-limiting resistor can automatically engage, limiting the earthing current within the required range and issuing an alarm signal. After the current-limiting resistor is engaged, when the earthing current falls below the current-limiting resistor's switching-off current value, the device will automatically disengage the current-limiting resistor of a certain resistance value to ensure that the earthing current is limited within the set value range.

5.2.6 Test for the device's current-carrying capacity

Without the current-limiting resistor in operation, the main circuit of the monitoring device shall be able to withstand a current of 3 times the maximum measured current value for 30 minutes, and there shall be no abnormalities in the main circuit of the monitoring device.

5.2.7 Test for the current-limit unit's current-carrying capacity

Change the current limiting device to manual mode, adjust the loop current to 1.2 times the maximum measuring current, and then change the current limiting device to automatic mode. At this point, the device shall automatically engage the current limiting resistor, and the corresponding indication shall be correct. The device shall have no insulation damage or component damage, and show no abnormalities. The test shall be performed three times, with each current-bearing time being 10 minutes and a 30- second interval between tests.

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