BENDER WR Transformer Selection

BENDER WR Transformer Selection

The reliability of residual current monitoring and insulation fault location in industrial power distribution, data centers, ships, mines, rail transit, and IT ungrounded systems largely depends on the selection and installation of measuring current transformers. The BENDER WR series rectangular measuring current transformers, including WR70x175S (P), WR115x305S (P), WR150x350S (P), and WR200x500S (P), are designed as high-sensitivity measuring elements for RCM, RCMS, and EDS series evaluation devices. They convert the primary side AC current into an assessable secondary signal for residual current monitoring, insulation fault location, and system status analysis. For engineers who are replacing discontinued modules, optimizing existing monitoring systems, or building new insulation fault location solutions, understanding the structure, parameters, installation specifications, and common fault troubleshooting methods of the WR series is key to ensuring long-term stable operation of the system.

Product positioning and core applications

The WR series belongs to the rectangular open type measuring current transformer, whose main function is to proportionally convert the alternating current generated by the current carrying conductor passing through its inner hole into a measurement signal that can be recognized by the evaluator. It does not directly display residual current, but works in conjunction with RCM, RCMS series residual current monitors or evaluators to form a complete residual current monitoring system. In IT ungrounded systems, the WR series can also be used in conjunction with insulation fault location devices such as EDS470, EDS460/490, EDS440, etc. to measure the positioning current generated by PGH positioning current injectors or ISOMETER IRDH. In other words, the WR series is both a sensor for residual current monitoring and a front-end acquisition unit for insulation fault location systems.

This series of products covers a variety of inner hole sizes, ranging from 70 × 175 mm to 200 × 500 mm, and can adapt to cable, busbar, and conductor combinations of different cross-sections. For ordinary load circuits, WR... S type can be selected; For busbar systems or situations with high load currents, WR... SP type is more suitable. The SP type is equipped with integrated shielding, which can effectively suppress the misoperation of RCM, EDS and other devices caused by high load current or surge current. This feature is particularly important in large-scale industrial power distribution, bus ducts, and transformer outgoing lines.

Model, inner hole size and selection logic

The main models of the WR series are distinguished by inner hole size: WR70x175S (P) inner hole 70 × 175 mm, WR115x305S (P) inner hole 115 × 305 mm, WR150x350S (P) inner hole 150 × 350 mm, WR200x500S (P) inner hole 200 × 500 mm. The "S" in the model represents the standard type, and the "SP" represents the integrated shielding type. The internal dimensions determine the number and cross-section of conductors that can pass through. When selecting, the first step is to confirm the number of conductors, cable outer diameter, or busbar size that need to be passed through simultaneously in the monitored circuit. If the inner hole is too small, the conductor cannot pass through or the installation stress is too high; If the inner hole is too large, although it is easy to install, it may affect the measurement sensitivity. Therefore, the size of the inner hole must match the actual conductor layout.

Next, it is necessary to confirm the load current level. The standard WR... S type is suitable for general load circuits, while the WR... SP type is specifically designed for busbar systems and situations where the load current is ≥ 500 A. SP type integrated shielding can reduce the risk of false alarms caused by external magnetic field interference and high load currents. For frequency converters UPS、 The WR series, in conjunction with RCM and RCMS evaluators, can cover the monitoring needs of AC, DC, and pulsed DC residual currents for devices such as rectifiers and battery chargers that may generate DC components or high-frequency residual currents. If the system is an IT ungrounded system and insulation fault branches need to be located, a WR transformer compatible with the EDS system should be selected, and the evaluator model should be confirmed to support the corresponding positioning current measurement.

Application of SP shielding type and busbar system

In large-scale power distribution systems, busbars typically carry currents of hundreds or even thousands of amperes. High load current will form a strong magnetic field around the transformer. If the shielding is insufficient, interference signals may be induced on the secondary side, leading to false alarms from the residual current monitor or misjudgments from the EDS positioning system. The WR... SP series measuring current transformers have built-in shielding layers, which can significantly reduce such interference. Its design goal is to avoid the erroneous tripping of RCM or EDS devices connected by high load currents or surge currents in the monitored system. Therefore, WR... SP series should be given priority in applications such as bus ducts, main distribution cabinets, transformer outgoing lines, and high-power variable frequency drive cabinets.

It should be noted that although the SP type enhances anti-interference ability, basic specifications still need to be followed during installation. The shielding layer and PE conductor must not pass through the inner hole of the transformer, otherwise it will damage the principle of residual current measurement. All current carrying conductors, including phase and N-wires, must pass through in the correct direction simultaneously. For three-phase systems, L1, L2, L3, and N should all pass through; For single-phase systems, L and N should pass through simultaneously. If only a portion of the conductor is passed through, the residual current vector sum will no longer be accurate, resulting in measurement distortion. For busbar systems, busbars are usually wider, and during installation, it is important to ensure that the busbars are aligned with the center of the transformer holes to avoid measurement errors caused by eccentricity.

Collaboration with RCM, RCMS, EDS systems

The WR series transformers can be compatible with various BENDER evaluation devices. RCMS460/490 and RCMS420 series residual current monitoring systems are suitable for centralized monitoring of multiple circuits; The RCM series residual current monitor is suitable for single circuit or small systems; EDS470, EDS460/490, and EDS440 insulation fault locators are used for IT system branch positioning. The connection method usually uses a two-wire cable, connecting the S1 (k) and S2 (l) terminals on the secondary side of the transformer to the corresponding input terminals of the evaluator. When wiring, pay attention to polarity, and the P1 (K) and P2 (L) directions should be consistent with the requirements of the evaluator. Wrong direction can cause the phase of the positioning current to be reversed, affecting the judgment of the faulty branch.

In the insulation fault location system, the positioning current injected by IRDH or PGH will return along the fault branch. WR transformers are installed on each branch to measure the positioning current. If there is an insulation fault in a certain branch, the transformer of that branch will detect a strong pulse signal, and the EDS evaluator will judge the fault channel based on this. The sensitivity, accuracy, and anti-interference ability of WR transformers directly affect the success rate of positioning. Therefore, in the positioning system, the transformer should be as close as possible to the monitored branch to avoid multiple branch cables crossing the same transformer, otherwise it is impossible to distinguish the faulty branch.

Interpretation of Key Technical Parameters

The technical parameters of WR series measuring current transformers determine their applicable boundaries. Its insulation coordination complies with IEC 61869-2, with a maximum system voltage Um of AC 720 V and a rated impulse withstand voltage Uimp of 3 kV. The rated transformation ratio of the measurement circuit is 600/1, the rated load is 180 Ω, the rated primary current is ≤ 10 A (100 A), the lower limit of the rated primary current is ≥ 10 mA, the rated power is 50 mVA, the rated frequency is 50... 400 Hz, and the internal resistance is 5... 8 Ω. The secondary overvoltage protection adopts P6KE6V8CP suppression diode with an accuracy level of 5. Rated continuous thermal current 100 A, rated short-time thermal current 14 kA/1 s, rated dynamic current 35 kA/30 ms. These parameters indicate that the WR series can measure both small residual currents and withstand high short-term fault current surges.

In terms of environmental adaptability, the equipment can withstand 15 g/11 ms impact, 40 g/6 ms collision, 1 g/10... 150 Hz working vibration, and 2 g/10... 150 Hz transportation vibration. Working temperature range -10...+50 ° C, storage temperature -40...+70 ° C, climate level 3K22. The connection adopts screw terminals, rigid/flexible conductors of 0.2... 4 mm ² or 0.2... 2.5 mm ², flexible wires with or without plastic sleeves of 0.25... 2.5 mm ², AWG 24... 12. When connected to the evaluator, single wire ≥ 0.75 mm ² can be used for 0... 1 m; twisted pair single wire ≥ 0.75 mm ² can be used for 0... 10 m; shielded cable ≥ 0.6 mm ² can be used for 0... 40 m. It is recommended to J-Y (ST) Y min. 2 × 0.6, and the shielding layer should be single ended with PE. These wiring requirements are very important to ensure signal integrity.

Installation specifications and key points for threading

The first principle when installing the WR series is: do not pass shielded cables through the measuring current transformer. The second principle is that PE conductors and low resistance conductor circuits shall not pass through transformers. The third principle is that all current carrying conductors must pass through in the correct direction. Specifically, for a three-phase system, L1, L2, L3, and N should pass through simultaneously; For single-phase systems, L and N should pass through simultaneously. P1 (K) side usually corresponds to the power supply side, and P2 (L) side corresponds to the load side, depending on the requirements of the evaluator. If the direction is reversed, the phase of the residual current and positioning current will reverse, affecting measurement and positioning.

The conductor should be aligned with the center of the inner hole of the transformer as much as possible to avoid eccentricity. Eccentricity can lead to uneven magnetic field distribution and reduce measurement accuracy. A conductor can only be bent within the minimum bending radius specified by the manufacturer, excessive bending may damage insulation or alter magnetic field distribution. For multiple conductors, it should be ensured that they are tightly arranged to avoid the formation of additional current loops. If the monitored circuit contains a PE line, the PE line must bypass the transformer and cannot be threaded through. For the busbar system, the busbar should pass through the inner hole smoothly to avoid friction with the inner wall of the transformer. The installation location should avoid strong vibration, high temperature, humidity, and corrosive gases. Transformers can be installed in any position, but it should be ensured that the screws are firmly fixed to prevent loosening during operation.

Selection verification when replacing discontinued modules

If you are replacing a discontinued measuring current transformer or residual current monitoring module, you cannot just look at the external dimensions. The following items should be carefully checked: whether the original system evaluator model is RCM, RCMS, or EDS; Whether the transformer ratio and accuracy level match; Can the inner hole size pass through existing cables or busbars; Is the load current ≥ 500 A and does it require SP shielding type; Is the secondary side connection method two-wire or other; Whether the cable length exceeds the allowable value; Does the shielding layer require single ended grounding; Is the installation space sufficient; Is the ambient temperature within the allowable range; Whether the short-term thermal current and dynamic current meet the system's short-circuit capacity.

If the original system is an IT ungrounded system for insulation fault location, it is necessary to confirm whether the new transformer is suitable for EDS positioning current measurement. If the original system is a TN/TT system for residual current monitoring, it is necessary to confirm the compatibility between the new transformer and the RCM/RCMS evaluator. If the original system uses busbars, it is recommended to prioritize the WR... SP series. If the original system cable is long, shielded cables should be used and single ended with PE, with a length controlled within 40 meters. If the original system requires high accuracy, it should be confirmed whether accuracy level 5 is met. If there are DC components such as frequency converters and UPS in the system, it should be confirmed whether the combination of evaluators and transformers supports AC/DC sensitive monitoring.

Common troubleshooting

1. Residual current monitoring false alarm. Check if the PE line passes through the transformer, if the N line is mixed with other circuits, if the conductor does not pass through in the correct direction, if the shielding layer is grounded at both ends, and if there is interference from high current busbars or frequency converters nearby. If the load current is high, consider replacing the WR... SP shielded type.

2. EDS positioning is inaccurate. Check whether each branch transformer only passes through the conductor of this branch, whether there are multiple branches mixed, whether the P1/P2 directions are consistent, whether the positioning current is sufficient, and whether the address and channel settings of the evaluator are correct. If the signal is weak, check whether the inner hole of the transformer is too large, whether the conductor is eccentric, and whether the connecting cable is too long.

3. The measured value is low or zero. Check whether the conductors pass through at the same time, whether the N wire is missing, whether the PE wire is mistakenly threaded, whether the secondary side wiring is loose, and whether the polarity is reversed. If it is zero, it may be a fault in the input channel of the evaluator or an open circuit on the secondary side of the transformer.

4. The measured values fluctuate greatly. Check for high-power equipment start stop, frequency converter carrier interference, busbar vibration, and poor grounding nearby. Use shielded cables and single ended grounding, and if necessary, add filtering or adjust the response delay of the evaluator.

5. The transformer heats up or makes abnormal noises. Check if the load current exceeds the rated continuous thermal current of 100 A, if the conductor is rubbing against the inner hole, if the screws are loose, and if the installation environment temperature is too high. If the short-term fault current exceeds 14 kA/1 s or dynamic 35 kA/30 ms, it may damage the transformer.

6. The shielding effect of SP is not significant. Check whether the shielding layer is reliably grounded, whether the shielding cable passes through the transformer, and whether the SP type installation direction is correct. The shielding layer should be connected to PE at one end and cannot form a ground loop.

7. The evaluator displays over range. Check whether the residual current is truly too high, whether the response value is set too low, whether the conductor is threaded incorrectly, and whether there is an external strong magnetic field. If it is a busbar system, confirm whether SP type is selected.

8. Unable to thread after installation. Verify the size of the inner hole with the outer diameter of the cable and the size of the busbar. If there is insufficient space, a larger inner hole model should be replaced instead of forcibly threading. Forcefully threading may damage the insulation of the transformer.

Debugging and maintenance suggestions

When debugging, first confirm that the evaluator model matches the transformer model. Disconnect the monitored circuit, install the transformer and thread the wires. Confirm that the PE line does not pass through, and all current carrying conductors pass through in the direction. Connect the secondary cable, use the recommended shielded cable and single ended PE. After powering on, check the zero points of each channel on the evaluator. If there is zero drift, check the shielding and grounding. Conduct TEST testing to verify alarm and positioning functions. For the EDS system, simulate the faulty branch and confirm that the positioning channel is correct. After operation, regularly check the fixing of transformers, terminal fastening, cable shielding, and surface cleaning. At least one functional test shall be conducted annually to ensure reliable residual current monitoring and insulation fault location.