Medium-Voltage Substations: What Is a Medium-Voltage Current Transformer?
The Indispensable Dielectric Guardian for Medium-Voltage Power Distribution Switchgear
Inside every medium-voltage electrical substation, industrial power distribution room, compact ring main unit (RMU), and renewable wind/solar grid interconnection bay operating at 10kV, 12kV, 24kV, and 33kV/36kV, measuring current accurately while isolating high voltages is a critical safety imperative.
Medium-voltage switchgear busbars carry thousands of amperes under severe continuous operating voltages. If dangerous 10kV–35kV potentials were allowed to flash over into secondary control wiring, delicate digital protection relays and energy meters would vaporize instantly, endangering human substation operators.
Furthermore, when heavy short-circuit faults erupt downstream—such as underground cable punctures or transformer phase-to-phase flashovers—fault currents explode to 25,000 or 31,500 amperes within milliseconds.
The Cast Epoxy Resin Medium-Voltage Current Transformer (Model LZZBJ9 / LZBW Series) provides the universal engineering benchmark: a robust dry-type pillar instrument transformer vacuum-cast from high-grade cycloaliphatic epoxy resin, delivering multi-core Class 0.2S precision metering and Class 5P10 protection under full IEC 61869-2 certification.
Engineering Anatomy: Inside a Model LZZBJ9 Medium-Voltage Cast Resin Current Transformer
- Monolithic Vacuum-Cast Epoxy Resin Body: Manufactured using the Automatic Pressure Gelation (APG) process with electrical-grade cycloaliphatic epoxy resin and micro-fine silica filler. Provides impervious hermetic sealing, zero oil leakage hazard, high mechanical cantilever rigidity, and permanent track-resistant insulation.
- High-Conductivity Primary Copper Terminals (P1, P2): Thick silver-plated electrolytic copper terminal pads engineered with standard dual-bolt hole patterns to accept direct bolting of medium-voltage copper busbars with minimal joint resistance.
- Independent Dual Magnetic Cores: Houses two completely isolated secondary windings wound on separate magnetic cores: a high-permeability nanocrystalline core for Class 0.2S revenue billing, and a high-saturation CRGO silicon steel core for Class 5P10 / 10P10 protection relaying.
- Enclosed Secondary Terminal Box with Lead-Sealing Cover: Flame-retardant clear terminal compartment at the base houses robust brass screw terminals (1S1-1S2, 2S1-2S2) and dedicated earth terminal, complete with wire sealing holes to prevent commercial energy theft.
- Short-Circuit Thermal Rating ($I_{th} = 31.5$ kA / 3s): Engineered internal copper conductor geometry and mechanical resin bonding withstand violent electromagnetic repulsion forces ($I_{dyn} \ge 80$ kA peak) and thermal short-circuit energy without internal cracking.
Comparison: Vacuum Cast Resin MV CT (LZZBJ9) vs. Legacy Oil-Immersed MV CT vs. Low-Voltage CT with Cable Sleeve
| Engineering Metric | Low-Voltage CT over Insulated Cable | Legacy Oil-Immersed Medium-Voltage CT | YOMIN Cast Epoxy Resin MV CT (LZZBJ9 Series) |
|---|---|---|---|
| Insulation Voltage & Dielectric Safety | Unsafe: Relies entirely on cable outer sheath; zero BIL lightning protection. | High: Insulating transformer mineral oil; but prone to moisture ingress. | Superior Solid Dielectric: Vacuum-cast resin withstands 95kV BIL & 42kV power frequency. |
| Fire & Explosion Hazard in Switchgear | High risk of flashover if cable insulation degrades over time. | Severe Hazard: Combustible oil can rupture violently during internal arc faults. | Zero Fire Hazard: Solid self-extinguishing epoxy resin eliminates oil explosion risks. |
| Maintenance & Oil Sampling Requirements | Frequent cable inspection required. | High: Requires annual oil dielectric breakdown testing and gasket replacement. | 100% Maintenance-Free: Hermetically sealed solid-state dry casting lasts 30+ years. |
| Measurement Accuracy & Dual Core Capability | Single core only; lower accuracy due to large cable air gap. | Dual core available; but bulky oil expansion bellows increase footprint. | Certified Dual Cores: Class 0.2S Revenue Metering + Class 5P10 / 10P10 Protective Relaying. |
| Switchgear Space & Mounting Orientation | Requires extra cable compartment height. | Must remain upright; cannot be tilted or mounted horizontally. | Compact Pillar Design: Mounts upright or horizontal inside standard 11kV/33kV switchgear cubicles. |
Technical Specifications
| Technical Specification Parameter | Engineering Value & Performance Rating |
|---|---|
| Rated System Operating Voltage Class ($U_m$) | Medium-voltage distribution ratings: 10kV / 12kV, 24kV, and 36kV / 40.5kV AC (50/60 Hz, 3-Phase AIS/GIS) |
| Rated Primary Current Range ($I_{pn}$) | Standard primary ratings: 5A, 10A, 15A, 20A, 30A, 50A, 100A, 200A, 400A, 600A, 1000A, 1250A, up to 3150A |
| Rated Secondary Output Currents ($I_{sn}$) | Standard switchgear secondary dual-output ratings: 5 A or 1 A (Core 1: 1S1-1S2; Core 2: 2S1-2S2) |
| Dual Core Configuration & Accuracy Classes | Dual-core architecture: Core 1: Class 0.2S / 0.5 (Revenue Metering); Core 2: Class 5P10 / 10P10 (Protection Relaying) |
| Short-Time Thermal Current Withstand ($I_{th}$) | Heavy short-circuit thermal withstand: up to 31.5 kA for 3.0 seconds (Dynamic peak current $I_{dyn} \ge 2.5 imes I_{th}$) |
| Insulation Material & Casting Technology | Automatic Pressure Gelation (APG) vacuum-cast cycloaliphatic epoxy resin (Class E / Class B insulation rating) |
| Partial Discharge (PD) Level Certification | Ultra-low partial discharge: $< 10$ pC at $1.2 U_m / \sqrt{3}$ (guarantees 30+ year insulation service life) |
| Power-Frequency & Lightning Impulse (BIL) | Dry power frequency withstand: up to 95 kV (1 min); Lightning impulse withstand (BIL): up to 200 kV peak (1.2/50 μs) |
Frequently Asked Questions
What is a Medium-Voltage Current Transformer (MV CT) and why is cast epoxy resin insulation essential?
A Medium-Voltage Current Transformer—frequently designated as an MV instrument transformer, cast resin CT, or 10kV–35kV measuring/protection transformer—is a specialized instrument transformer designed to operate in medium-voltage power distribution networks (10kV to 35kV) to safely scale down heavy primary phase currents to standardized secondary values (5A or 1A) for digital protection relays, tariff energy meters, and SCADA monitoring equipment. In medium-voltage switchgear and distribution substations: <ul><li><strong>The Dielectric Isolation Barrier:</strong> Low-voltage 0.66kV CTs rely on simple plastic shells and cannot withstand kilovolts of operating potential or transient lightning impulses (reaching 75kV at 12kV, or 170kV at 36kV). YOMIN LZZBJ9 series MV CTs utilize vacuum-cast cycloaliphatic epoxy resin processed via Automatic Pressure Gelation (APG). The monolithic solid resin casting forms a void-free, moisture-proof hermetic encapsulation that withstands 42kV power frequency voltage and 95kV lightning impulses, completely isolating sensitive low-voltage digital relays from dangerous medium-voltage potential.</li><li><strong>Zero Maintenance & Non-Flammable Safety:</strong> Unlike legacy oil-filled transformers that leak oil, require maintenance, and pose severe fire/explosion hazards inside indoor switchgear rooms, dry-type cast resin CTs are self-extinguishing, maintenance-free, and environmentally benign.</li></ul>
Why do Medium-Voltage CTs feature Dual Cores: Class 0.2S Metering vs. Class 5P10 Protection?
A single magnetic core cannot simultaneously satisfy the conflicting physics of tariff metering and short-circuit fault protection: <ul><li><strong>Core 1 — High-Precision Revenue Metering (Class 0.2S, Instrument Security Factor FS5):</strong> Wound from high-permeability nanocrystalline or nickel-iron alloy with an extremely narrow hysteresis loop. It maintains ±0.2% measurement accuracy even at tiny load currents (from 1% to 120% of rated current). Crucially, its core is designed to <strong>saturate early (FS ≤ 5)</strong> during short-circuit faults, capping secondary current and protecting delicate digital power meters from thermal burnout.</li><li><strong>Core 2 — Protective Relaying (Class 5P10 / 10P10, Accuracy Limit Factor ALF10/ALF20):</strong> Wound from high-saturation cold-rolled grain-oriented silicon steel (CRGO). It is engineered <strong>NOT to saturate</strong> even when primary currents spike to 10 or 20 times nominal current ($10 imes I_n$) during severe short-circuit faults. This guarantees that protection relays receive linear secondary current signals to execute instantaneous trip commands in under 20 milliseconds.</li></ul>
What is Partial Discharge (PD) and why must it be certified below 10 pC under IEC 61869-2?
Partial discharge refers to localized microscopic electrical sparks occurring inside tiny air bubbles, voids, or delamination cracks trapped within the solid epoxy resin casting during manufacturing. Under continuous 10kV to 35kV electrical stress, ongoing partial discharges emit ozone, ultraviolet radiation, and nitric acid, gradually eroding the solid epoxy from the inside out until a catastrophic flashover shatters the CT and burns out the switchgear panel. YOMIN LZZBJ9 series CTs are manufactured using advanced vacuum de-gassing and high-pressure APG injection molds, guaranteeing factory routine partial discharge levels certified <strong>below 10 pico-coulombs (< 10 pC at 1.2 Um)</strong>, ensuring a proven operational lifespan exceeding 30 years without dielectric degradation.
Medium-Voltage Substations: What Is a Medium-Voltage Current Transformer?
Designing medium-voltage air-insulated switchgear (AIS), compact ring main units (RMU), renewable substation metering bays, or distribution feeder panels requiring certified IEC 61869-2 cast epoxy resin current transformers (10kV to 35kV, dual 0.2S / 5P10 cores, Ith up to 31.5kA)? YOMIN manufactures precision LZZBJ9 series MV CTs.
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