Introduction

The ZZW-35 33kV current transformer (CT) is a cast-resin insulated instrument transformer designed for accurate current measurement and protection in medium-voltage electrical networks operating at the standardized IEC voltage level of 33kV. Developed in strict compliance with the international standard IEC 61869-2, this device ensures reliable performance under demanding operational conditions typical of industrial, utility, and infrastructure applications.

Current transformers serve as critical components in power systems by stepping down high primary currents to standardized secondary values (typically 1 A or 5 A), enabling safe interfacing with metering, protection relays, and monitoring equipment. The ZZW-35 series leverages advanced epoxy resin casting technology to provide superior dielectric strength, mechanical robustness, and environmental resistance—essential attributes for installations exposed to pollution, humidity, or thermal cycling.

This article examines the practical application scenarios of the ZZW-35 33kV CT, supported by real-world case studies, technical advantages aligned with IEC 61869-2 requirements, and frequently asked engineering questions relevant to system designers and asset managers.

Application Overview

The ZZW-35 33kV current transformer is engineered for deployment in a variety of medium-voltage (MV) power system configurations where precise current transformation is required for both metering and protective functions. Its design adheres to the dimensional, insulation, and accuracy class specifications outlined in IEC 61869-2: “Instrument transformers – Part 2: Additional requirements for current transformers.”

Typical application environments include:

• Utility substations: Used on 33kV feeders for revenue metering and feeder protection schemes.
• Industrial plants: Integrated into switchgear panels for motor protection, load monitoring, and energy management systems.
• Railway traction networks: Deployed in 33kV overhead catenary supply systems to monitor traction loads and detect earth faults.
• Renewable energy interconnections: Installed at the point of common coupling (PCC) for solar or wind farms connected to 33kV distribution grids.
• Municipal infrastructure: Applied in water treatment facilities, district heating plants, and public lighting networks operating at 33kV.

The cast-resin construction eliminates the need for oil or gas insulation, reducing fire risk and maintenance requirements—particularly advantageous in indoor switchgear rooms or environmentally sensitive locations. Additionally, the transformer’s compact footprint facilitates retrofitting into existing switchgear bays originally designed for older CT models.

Compliance with IEC 61869-2 ensures that key performance parameters—including rated primary current, accuracy class (e.g., 0.2S for metering, 5P20 for protection), burden rating, and short-time thermal current withstand—are rigorously validated through type testing and routine verification procedures.

Case Studies

Case Study 1: Urban Substation Modernization in Southeast Asia

A municipal utility in Thailand undertook a substation modernization program to replace aging oil-filled instrument transformers with maintenance-free alternatives across its 33kV distribution network. The ZZW-35 33kV cast-resin CT was selected due to its compliance with IEC 61869-2, compact dimensions, and Class E insulation system suitable for tropical climates.

The installation involved replacing legacy 35kV-class CTs (non-standard per IEC) with standardized 33kV units to align with regional grid harmonization efforts. Over 120 units were deployed across six urban substations. Post-commissioning tests confirmed accuracy class 0.5 for metering and 5P10 for protection, meeting both regulatory and operational requirements. The elimination of oil handling reduced site preparation time by 40% and eliminated long-term leakage risks.

Case Study 2: Industrial Motor Protection in a Petrochemical Facility

A petrochemical complex in Saudi Arabia required robust current sensing for high-inertia induction motors rated at 6.6 MW, fed from a 33kV internal distribution bus. Due to stringent safety codes prohibiting flammable insulation materials in hazardous areas, oil-immersed CTs were excluded from consideration.

The ZZW-35 CT was specified with dual cores: one core calibrated to accuracy class 0.2S for precision energy billing, and a second core rated 5P20 for differential and overload protection. The cast-resin housing provided IP54 ingress protection and withstood ambient temperatures up to 55°C without derating. After three years of continuous operation, no degradation in ratio error or phase displacement was observed during periodic calibration checks, validating the long-term stability promised by IEC 61869-2’s aging test protocols.

Case Study 3: Renewable Energy Integration in Europe

A 20 MW solar photovoltaic (PV) farm in Spain was interconnected to the national grid via a 33kV collector substation. Grid code compliance mandated the use of IEC-standardized instrument transformers with documented type test reports per IEC 61869-2.

The ZZW-35 units were installed on each of the four 33kV outgoing feeders to provide current signals for anti-islanding protection, power quality monitoring, and SCADA telemetry. The CTs’ low remanence characteristics ensured rapid saturation recovery after fault clearance—a critical requirement for fast reconnection following transient grid disturbances. Independent verification by the transmission system operator (TSO) confirmed compliance with EN 50178 and IEC 61869-2, facilitating swift grid connection approval.

Technical Benefits

The engineering advantages of the ZZW-35 33kV cast-resin current transformer stem directly from its adherence to IEC 61869-2 and modern manufacturing practices. Key technical benefits include:

1. Standardized Voltage Rating (33kV)

The unit is explicitly rated for 33kV, the nominal system voltage defined in IEC 60038 for medium-voltage networks. This avoids confusion with non-standard 35kV ratings sometimes used regionally but not recognized under IEC frameworks. The insulation coordination—including lightning impulse withstand (170 kV peak) and power frequency withstand (70 kV rms for 1 min)—is validated per IEC 61869-2 Clause 6.

2. Cast-Resin Insulation System

Epoxy resin casting provides homogeneous dielectric properties, high tracking resistance (CTI ≥ 600 V), and excellent mechanical integrity. Unlike oil-filled designs, there is no risk of leakage, fire hazard, or environmental contamination. The material also exhibits low thermal expansion, minimizing stress cracks during thermal cycling.

3. Dual-Core Flexibility

Available with single or dual secondary windings, the ZZW-35 supports simultaneous metering and protection functions. Typical configurations include:

• Metering core: Accuracy class 0.2S or 0.5S (IEC 61869-2 Table 101)
• Protection core: Accuracy class 5P or 10P with defined limiting factor (e.g., 5P20)

This eliminates the need for multiple CTs per circuit, reducing cost and space requirements.

4. High Short-Time Withstand Capability

Rated to withstand thermal short-circuit currents of up to 25 kA for 3 seconds (or 40 kA for 1 second), the ZZW-35 meets the dynamic and thermal stress requirements of modern MV networks with increasing fault levels. This is verified per IEC 61869-2 Clause 8.4.

5. Environmental and Operational Resilience

Designed for operation in ambient temperatures from -40°C to +55°C, the CT maintains performance under extreme conditions. UV-stabilized resin allows outdoor use without housing degradation. Pollution degree 3 compatibility ensures reliability in industrial atmospheres.

6. Compliance Documentation

Each unit is supplied with a test report referencing IEC 61869-2, including data on ratio error, phase displacement, knee-point voltage (for protection cores), and partial discharge levels (< 10 pC at 1.2 × Um/√3).

Frequently Asked Questions (Application Scenarios)

Q1: Can the ZZW-35 33kV CT be used in a 35kV system?

No. The ZZW-35 is explicitly rated for 33kV systems as defined in IEC 60038 and tested per IEC 61869-2. While some regions historically refer to “35kV” networks, the standardized nominal voltage under IEC is 33kV. Using a 33kV-rated CT on a true 35kV system (i.e., with highest voltage for equipment Um = 36kV or higher) would violate insulation coordination requirements and void compliance with IEC 61869-2.

Q2: What accuracy classes are available for metering and protection applications?

For metering, the ZZW-35 supports accuracy classes 0.2S, 0.5S, and 1.0 per IEC 61869-2 Table 101, suitable for revenue and technical metering. For protection, classes 5P and 10P are available with standard limiting factors (e.g., 5P10, 5P20). The specific class and burden must be specified at order to ensure proper core design.

Q3: Is the ZZW-35 suitable for outdoor installation?

Yes. The cast-resin housing is UV-resistant and rated for outdoor service in pollution degree 3 environments per IEC 60815. However, terminal boxes should be oriented downward or shielded to prevent direct water ingress, and creepage distance complies with IEC 61869-2 minimum requirements for 33kV (≥ 630 mm).

Q4: Can the CT be retrofitted into existing switchgear designed for older CT models?

In many cases, yes. The ZZW-35 follows common dimensional envelopes used in IEC-based switchgear (e.g., 35–40 mm window diameter, standard mounting hole patterns). However, a mechanical compatibility check—including primary conductor clearance, secondary terminal accessibility, and weight—is recommended prior to retrofit.

Q5: Does the ZZW-35 require periodic maintenance?

No routine maintenance is required. Cast-resin CTs are sealed for life and do not contain consumable insulating media. Periodic verification of ratio error and insulation resistance may be performed as part of predictive maintenance programs, but no servicing (e.g., oil top-up, gas pressure checks) is needed.

Conclusion

The ZZW-35 33kV cast-resin current transformer represents a technically robust solution for modern medium-voltage power systems operating at the IEC-standardized voltage of 33kV. Its design, fully compliant with IEC 61869-2, delivers accuracy, reliability, and safety across diverse application scenarios—from urban substations and industrial complexes to renewable energy integration points.

By eliminating flammable insulation, supporting dual-core functionality, and offering resilience to harsh environmental conditions, the ZZW-35 addresses key challenges faced by system engineers in today’s evolving grid landscape. Real-world deployments confirm its performance under thermal, electrical, and mechanical stress, while adherence to international standards ensures interoperability and regulatory acceptance.

For new installations or modernization projects involving 33kV networks, the ZZW-35 provides a future-proof, low-maintenance instrument transformer that aligns with global best practices in power system instrumentation.