Secondary side overvoltage protector HLC of current transformer

1、 Overview

In the power system, current transformers (CTs) are used for measuring and controlling primary current. During normal operation, the secondary side of the transformer is in an approximately short-circuit state, and the output voltage is very low. If the secondary winding is open or abnormal current flows through the primary winding during operation (such as lightning current, resonant overcurrent, capacitor charging current, inductor starting current, etc.), it will generate overvoltage of thousands or even tens of thousands of volts on the secondary side. This not only poses a threat to the insulation of the secondary system, but also causes the transformer to overheat and burn out, and even endangers the safety of the operating personnel.

Based on the actual needs of the power system, our company has developed protectors with various functions, including action contact output, automatic light display alarm, automatic locking differential protection, manual reset function after action, and multiple combinations, on the basis of domestic protectors. This product adopts new automatic control technology and high reliability components, with a long service life and can reliably operate more than tens of thousands of times. Fast action speed, strong overload capacity, low static leakage current, can meet the needs of various CT protections.

2、 Product protection principle

The basic component of the protector is a special Zn0 varistor developed by our company, which is connected in parallel to both ends of the CT secondary protected winding. During normal operation, the voltage at both ends of the varistor is less than 20V. At this time, the varistor is in a high resistance state of approximately open circuit, and the current passing through it is called leakage current, which is less than 8 μ A (U1mA ≤ 800V) or 5 μ A (U1mA>800V). The impact on the protection action value and meter accuracy of the circuit can be ignored.

When the secondary circuit is open or the primary winding experiences abnormal overcurrent, the voltage generated in the secondary winding is much higher than the normal operating voltage (the value depends on the parameters and operating conditions of the CT itself), and the parallel connected varistor instantly enters a conducting state. Due to the inherent characteristics of Zn0 varistors, overvoltage is effectively limited to a selected value and enters a stable short-circuit state, thereby completely avoiding overvoltage hazards. The protector can reliably short-circuit the secondary winding and display light within 20ms of overvoltage, and can provide an open circuit (or overvoltage) signal and a contact for blocking differential protection. After troubleshooting, it can be reset and reused, with a lifespan of up to tens of thousands of actions and stable and reliable operation.

3、 Use　way

The HLC series current transformer secondary overvoltage protector is mainly used for abnormal overvoltage protection on the secondary side of various CTs. The protector is fixedly connected to both ends of the CT secondary winding, and during normal operation, the leakage current is extremely small, showing a high resistance state; When the CT secondary circuit is open or there is abnormal overcurrent in the primary winding, the overvoltage generated by the secondary winding causes the protector to quickly act and short-circuit the CT secondary side. The device automatically displays the fault location and outputs a passive contact signal. After troubleshooting, press the manual reset button to restore the circuit to its original state and resume normal operation.

4、 Wiring principle

In general, current transformers are connected to phases A, B, and C, with a few connected to two phases and a few connected to one phase. The majority of the secondary winding connections of current transformers are star connections, with a few being delta connections. This protector is a secondary winding star connection, with secondary windings A, B, and C connected to the terminals of protector A, B, and C. The neutral point (N) of the three-phase secondary windings A, B, and C is connected to the N terminal of the protector. If only A and B windings are used, the C phase can be left unconnected without affecting the normal operation of the protector.

The DC220V power supply is directly connected to the DC220V terminal of the protector, with a pair of normally open and a pair of normally closed contacts (sharing the middle movable terminal) as passive output terminals for users to use according to their actual needs.

5Characteristics

The input impedance is greater than 100M Ω, and it does not affect any measurement and protection performance after connection;

The secondary side protection voltage can be set by the user to meet different usage scenarios;

After protection, there is a display of the location of the fault point, which facilitates troubleshooting;

After protection, there is a contact signal output for easy system connection and use;

After protection, there is a manual button reset, and the device can be reused;

Adopting a design that saves space and tracks for easy installation and use.

VIProduct Technical Standards and Parameters

Product Technical Standards

The technical parameters of the secondary overvoltage protector for the HLC series current transformers were designed with reference to the following standards and strictly comply with or exceed the relevant requirements of these standards.

GB/T1408-1989 Test Method for Power Frequency Electrical Strength of Solid Insulation Materials

GB/T2423.46-1997 Environmental Testing for Electrical and Electronic Products

DL/T620-1997 Overvoltage Protection and Insulation Coordination for AC Electrical Installations

GB7251.1-1997 Low Voltage Switchgear and Control Equipment

Q/HL08-2006 HLC Series Current Transformer Secondary Overvoltage Protector

Product Technical Parameters

● Nominal voltage U1mA

The voltage at both ends of the protector when passing a DC current of 1mA. The range of U1mA is 500-2000V, with 1 level for every 100V, and the error does not exceed ± 10%.

● Residual pressure ratio Kr

The ratio of the peak terminal voltage U5AP (residual voltage) to U1mA when the protector passes through an effective AC current of 5A.

Kr＝U5AP／U1mA＜1.70

Leakage current I20V

The current (AC RMS) passing through the two ends of the protector when subjected to an AC RMS voltage of 20V.

When U1mA ≤ 1000V, I20V ≤ 8 μ A

When U1mA>1000V, I20V ≤ 5 μ A

Maximum impulse current I1s

The maximum current (AC RMS) allowed to pass through the protector within 1 second. I1s=120A

● Conducting voltage limit time Ts

The protector operates with a current of 5A (AC effective value) and automatically limits the voltage, maintaining the residual voltage U5AP value for a certain period of time.

The range of Ts is 15-400ms, and after Ts time, the residual voltage drops or short circuits to zero.

Between the insulation level voltage limiting element and the signal contact, between the contact fracture, between the conductive part and the shell and ground, withstand AC 50Hz with an effective value of 2kV for 1 minute.

● Output contact capacity

Resistive 3A/380V AC or 5A/220AC, with a lifespan of 20000 cycles.

7、 Model Description

Selection Catalog

8、 Terminals and

9Installation, on-siteTesting and maintenance

■Installation and maintenance

●Installation

The installation point of the protector should be as close as possible to the protected CT and convenient for inspection.

When the lead is long, try to use armored cables to prevent wire breakage.

The required external power supply is DC. 220V, with a current of ≮ 0.6A;

●Maintenance

During normal operation, the button (TEST) can be used to test the integrity of the indicator light. At this time, the test terminals T1 and T2 terminal groups cannot be short circuited.

After troubleshooting, simply press the reset button (RESET) to reset the active protector.

No maintenance is required for any model of HLC.

It can be inspected once a month and tested every four years. If U1mA deviates by 20% from the nominal value, it should be replaced.

■Attention

The replacement of the protector should be carried out during a power outage. If the power cannot be cut off, the open circuit CT secondary winding must be reliably short circuited before proceeding.

Before the open circuit fault is resolved, it cannot be reset, otherwise it will cause another open circuit or burn out the protector

10、 Normal usage conditions

Environmental temperature: -40 ℃ to+40 ℃.

Environmental humidity: The average maximum relative humidity in the wettest month is less than 90%.

● Altitude: not exceeding 2000 meters.

● Atmospheric conditions: There are no obscene, corrosive, or explosive media in the atmosphere that seriously affect the insulation of the device casing.

Environmental conditions: The location where the product is used must not have severe vibrations and impacts.