220 kV is the backbone voltage of transmission networks in Egypt, across the Middle East, and in many African and Southeast Asian grid systems. A 220 kV power transformer is a fundamentally different class of equipment from a distribution transformer — the insulation system, cooling architecture, short-circuit withstand requirements, and testing regime all shift to a higher level of engineering complexity.
The 220 kV Insulation System: Switching Impulse Becomes Mandatory
For a 220 kV system, the highest voltage for equipment (Um) is 245 kV in IEC terminology. At this level, switching impulse tests become mandatory — they are not optional or buyer-specified additions. IEC 60076-3:2013 explicitly defines switching impulse levels for all Um > 72.5 kV and requires them for the 245 kV class.
Typical insulation levels for a 220 kV class transformer:
Parameter Typical Value (245 kV Class)
Highest voltage for equipment (Um) 245 kV
Rated lightning impulse withstand (LI) 850–950 kVp
Rated switching impulse withstand (SI) 650 kVp
Rated power frequency withstand 395–460 kVrms
These values are defined in IEC 60076-3 and aligned with IEC 60071-1:2010 insulation coordination tables.
The third edition of IEC 60076-3 introduced several important revisions for high-voltage transformers, including new requirements for impulse waveshape (k factor) and a revised induced voltage test procedure with PD measurement to ensure adequate phase-to-phase test voltages. These changes directly affect the factory testing protocol for 220 kV units.
Cooling Method Selection: ONAN, ONAF, OFAF, or OFWF?
For 220 kV transformers in the 100–300 MVA range, cooling method selection is not a minor detail — it determines the transformer's footprint, noise level, overload capability, and maintenance requirements.
Cooling Class Method Typical Rating Range Best For
ONAN Natural oil, natural air Up to 60 MVA Low-noise sites, low load density
ONAF Natural oil, forced air (fans) 10–60 MVA Sub-transmission, moderate loads
OFAF Forced oil, forced air 75 MVA+ Power station step-up (GSU), industrial loads
OFWF Forced oil, forced water 120 MVA+, 220 kV+ Hydroelectric GSU, HVDC converter
For a 220 kV transformer rated above 120 MVA, OFWF is commonly specified, particularly for hydroelectric step-up applications and large transmission substations. For 220 kV transformers below 40 MVA, OFAF or even ONAF may be sufficient depending on load profile.
The practical implication for buyers is that cooling method affects overload capability. A 220 kV transformer specified with ONAF can typically handle 1.5× nameplate for short periods when fans are running; an OFWF unit can sustain 2× or more under continuous forced circulation. For grid operators in Egypt and Africa managing summer peak loads, this overload headroom can mean the difference between maintaining supply and load shedding.
Short-Circuit Withstand: The Critical Test That Separates Designs
IEC 60076-5 governs the ability to withstand short-circuit. For 220 kV class transformers, the network short-circuit apparent power can be 15,000 MVA or higher, producing enormous mechanical forces on windings during a fault.
The short-circuit withstand test is a type test — not performed on every unit — but it is the most demanding mechanical test a power transformer can undergo. A transformer that passes the test must show:
No displacement or deformation of windings beyond acceptable limits
Post-test reactance increase not exceeding 1% for transformers above 100 MVA (per IEC 60076-5)
No damage to clamping structures, leads, or tap changer mechanisms
For 220 kV transformers procured for Egyptian or African transmission projects, short-circuit withstand test certificates from a nationally accredited laboratory (or equivalent) are typically required as part of the bid submission.
Factory Type Testing for 220 kV Transformers
The type test regime for 220 kV transformers is more extensive than for distribution-class units. The following tests are typically required:
Test Type Standard Reference
Lightning impulse (LI, LIC) Type/Routine IEC 60076-3
Switching impulse (SI) Type IEC 60076-3
Temperature rise Type IEC 60076-2
Short-circuit withstand Type (special) IEC 60076-5
Partial discharge (ACLD) Routine IEC 60076-3
FRA (SFRA) Routine/Special IEC 60076-18
Partial discharge (PD) testing is particularly critical for 220 kV units. PD measurement per IEC 60270 detects localized electrical discharges that can progressively damage insulation and lead to eventual failure. For 220 kV transformers, PD measurement is performed both during the induced overvoltage test at the factory and, per many utility specifications, again during site commissioning.
Frequency response analysis (FRA) has become a standard tool for detecting winding deformation. A baseline FRA sweep is performed at the factory after all other tests, and repeat measurements are taken at site after transport. For a 220 kV three-phase transformer with HV/LV/tertiary windings, a full test set may require 12 sweeps. Comparison of factory and site FRA signatures is the most reliable method for confirming that the transformer has not been damaged in transit.
220 kV Transformers in Egypt's Transmission Grid
Egypt's transmission network operates at 220/500 kV for primary transmission, with 66/132 kV as sub-transmission. EETC's recent procurement includes three 175 MVA transformers rated 11/66/220 kV for the Sadat Main and Tahrir Badr substations, part of the Alexandria and West Delta Electricity Zone reinforcement. Elsewedy Electric alone has supplied more than 40 units of 220 kV class transformers to EETC substations, reflecting the scale of ongoing grid investment.
Egypt's grid is also interconnected with Sudan, Libya, Jordan, and Saudi Arabia, meaning that 220 kV transformers deployed in the Egyptian network must be compatible with cross-border power flow requirements.
How Hualai Serves 220 kV Projects
Jiangsu Hualai Electric Technology Co., Ltd. manufactures 220 kV class power transformers from 63 MVA to 300 MVA at 220/66 kV, 220/33 kV, and 220/11 kV configurations. Our capabilities include:
Full insulation coordination per IEC 60076-3 with switching impulse testing
Cooling options: ONAF, OFAF, or OFWF depending on rating and site conditions
OLTC with ±10% tap range, available with separate or common conservator
Short-circuit withstand verification with third-party accredited laboratory certification
Factory FRA baseline with site comparison support
Partial discharge measurement during induced overvoltage test
Every 220 kV unit undergoes a complete type and routine test program at our facility, with witnessed FAT available for customer representatives. Full documentation packages — including type test reports, routine test reports, FRA sweeps, and PD measurement records — are provided for utility bid submissions and site acceptance.
📩 For 220 kV transformer specifications or to discuss your transmission project requirements, contact hualaipower@gmail.com or WhatsApp: +86 18914882008.
