22kV Transformer Procurement Checklist: A Field Guide for Indonesian and Southeast Asian Power Projects

Introduction

The 22 kV voltage level occupies a unique niche in global power distribution. It is the dominant medium-voltage standard in Indonesia (PLN), is widely used in Thailand (PEA/MEA), appears in Vietnam (EVN), and appears in isolated pockets of Myanmar, the Philippines, and parts of West Africa. Yet it is almost entirely absent from IEC 60076's standard voltage tables, which jump from 17.5 kV to 24 kV to 36 kV.

This means procurement engineers working on a palm oil mill in Sumatra, a nickel smelter in Sulawesi, or a data center in Jakarta are specifying equipment against a de facto regional standard that international manufacturers may not stock. The risk of a specification mismatch — wrong BIL, wrong tapping range, wrong accessories — is higher for 22 kV than for any other common voltage level.

This article is a practical procurement checklist written from experience supplying dozens of 22 kV transformers into PLN-governed networks, mining concessions, and agro-industrial sites across Southeast Asia.

Why 22 kV Exists

Indonesia inherited its 20 kV nominal system from the Dutch colonial era (the Netherlands used 10 kV and 20 kV before adopting 10 kV exclusively). Over time, the operating voltage drifted upward to 22 kV to reduce losses on long rural feeders, and PLN formally standardised on 22 kV as the nominal distribution voltage with a maximum system voltage Um of 24 kV.

Other 22 kV countries followed similar paths — Thailand's PEA operates extensive 22 kV rural networks, and Vietnam uses 22 kV for urban distribution in parallel with 35 kV for longer transmission feeders.

The practical consequence for procurement: a "22 kV transformer" is technically a 24 kV-class transformer (Um = 24 kV) operated at a nominal 22 kV. Any IEC 60076-3 insulation level for Um = 24 kV applies. The confusion arises because many Chinese and Indian manufacturers list their standard products as "11 kV / 22 kV / 33 kV" but the actual insulation levels differ.

Core Specification Table

Copy and paste this table into your RFQ (Request for Quotation). Values shown are the minimum specification I recommend for any 22 kV project in Southeast Asia.

Parameter	Minimum Specification	Notes
Rated power	_____ kVA	ONAN rating
HV rated voltage	22,000 V ± _____	Nominal 22 kV
LV rated voltage	_____ V (L-L)	400/230 V (IEC) or 415/240 V (legacy)
Highest voltage for equipment (Um)	24 kV	Per IEC 60076-3 for 22 kV class
Rated frequency	50 Hz	Indonesia, Thailand, Vietnam
Number of phases	3	Single-phase only for pole-mount ≤ 100 kVA
Vector group	Dyn11	Standard for distribution
BIL (LI) – HV	125 kV peak	IEC 60076-3, Um=24 kV
BIL (LI) – LV	6 kV peak	If separately tested
Power-frequency withstand – HV	50 kV rms	1 minute, 50 Hz
Impedance voltage	_____ % at 75°C	4.0–6.0% depending on kVA
Cooling	ONAN	ONAF for dual-rated ≥ 2500 kVA
Tapping range	±2×2.5%	Off-circuit, 5 positions

Why BIL 125 kV, Not 95 kV or 150 kV

This is a recurring point of confusion. IEC 60076-3, Table 2, lists the following insulation levels for Um = 24 kV:

Um (kV)	Rated short-duration power-frequency withstand (kV rms)	Rated lightning impulse withstand (kV peak)
24	50	125
24	50	95
24	50	150

Three BIL options exist: 95, 125, and 150 kV.

• BIL 95 kV: This is a reduced insulation level permitted by IEC for systems with effective overvoltage protection (e.g., close-coupled surge arresters). I have never seen it specified for 22 kV in Southeast Asia, and I would not recommend it for tropical environments where lightning activity is intense. Sumatra and Kalimantan, for instance, experience isokeraunic levels exceeding 150 thunderstorm days per year, and a reduced BIL is a false economy.

• BIL 125 kV: This is the standard insulation level for 22 kV class equipment and is what PLN, PEA, and EVN normally specify. It provides a 20% safety margin above BIL 95 kV for an incremental cost of roughly 3–5% on the unit price.

• BIL 150 kV: This exceeds what a 22 kV (Um=24 kV) transformer requires and is typically associated with 33 kV class (Um=36 kV). Specifying BIL 150 kV on a 22 kV unit is technically permissible but forces the manufacturer to adopt 36 kV-class clearances throughout, increasing dimensions and cost by 10–15% with no operational benefit.

> ☑ Recommendation: Specify BIL 125 kV unless you have a very specific project reason to deviate.

Tapping Range: Why ±2×2.5% Is Not Always Enough

The ±2×2.5% (five-position off-circuit) tap changer is the factory default for most imported transformers. But in Indonesian and Thai rural networks, the 22 kV feeder voltage can swing from as low as 19 kV (heavy daytime load, long feeder) to as high as 24 kV (light nighttime load, nearby generation).

A five-position tap changer with ±5% range (±2×2.5%) gives you:

• Position 1: +5% (23.1 kV)
• Position 2: +2.5% (22.55 kV)
• Position 3: 0% (22 kV nominal)
• Position 4: –2.5% (21.45 kV)
• Position 5: –5% (20.9 kV)

If your feeder drops to 19 kV, even the –5% tap leaves you 10% low, and the secondary voltage will be correspondingly depressed.

For rural and long-feeder projects, specify:

> ±5% in 2.5% steps (positions: +5%, +2.5%, 0%, –2.5%, –5%)

This requires a five-position tap changer with a wider range. Some manufacturers offer ±10% in 2.5% steps (9 positions) for extreme voltage conditions, but that is uncommon on distribution-class units and typically triggers an OLTC discussion.

Online Oil Filter – When to Specify

Online (continuous) oil filtration systems are an optional accessory that circulates transformer oil through a filter element while the transformer is energized, removing moisture, dissolved gases, and particulates.

For a 22 kV transformer in the following environments, I strongly recommend specifying an online oil filter:

• Palm oil mill substation: High ambient temperature (35–40°C), high humidity (80%+), and intermittent heavy loading from mill motors create aggressive ageing conditions. Oil oxidation accelerates exponentially above 80°C top-oil temperature.

• Mine power supply (coal, nickel, gold): Dust-laden air contaminates breather silica gel within weeks. Frequent load cycling from crushers, conveyors, and hoists creates thermal stress. Online filtration extends oil life from 10–15 years to 20+.

• Coastal installation (< 1 km from sea): Salt-laden air enters through the breather and accelerates corrosion of tank internals. An online filter with a sealed conservator system (rubber bag or membrane) is the standard defence.

• Remote, unattended substation: Where oil sampling for DGA (Dissolved Gas Analysis) is logistically difficult, an online filter with integrated DGA monitoring provides a continuous health picture without site visits.

The incremental cost of a skid-mounted online oil filter (typically 3–5 L/min flow rate, with silica gel / activated alumina cartridges) is 5–8% of the transformer cost for units below 5 MVA. This is cheap insurance against premature failure in hostile environments.

> ☑ Specification blank: Online oil filtration system: REQUIRED / NOT REQUIRED > > If required: flow rate _____ L/min, filtration grade _____ micron absolute, DGA monitor integrated: YES / NO

Scene-Specific Configuration

Palm Oil Mill (Kelapa Sawit)

Parameter	Recommendation	Reason
LV voltage	400/230 V	Standard for mill auxiliary supply
Additional tertiary winding	Consider if plant has 6.6 kV motor loads	22/6.6 kV + 0.4 kV, three-winding unit
Corrosion class	C4 (ISO 12944)	High humidity, organic acids in atmosphere
Breather	Silica gel, 1.0 kg charge	Standard, increase charge for tropical humidity
Tank paint	RAL 7035 (light grey)	Reflects solar radiation, reduces tank temperature

Mining Power Supply

Parameter	Recommendation	Reason
Impedance	6.5–7.0%	Limit fault current for downstream motor starting
LV neutral earthing	NER (Neutral Earthing Resistor)	Limit earth-fault current to 100–400 A
Temperature monitoring	Winding temperature indicator (WTI) + remote RTD	Essential for remote monitoring
Auxiliary power	220 V DC from station battery	Tripping circuit reliability

Data Center / Commercial Complex

Parameter	Recommendation	Reason
Insulating liquid	Natural ester (IEC 62770) or dry-type	Fire safety, indoor installation
Noise level	≤ 55 dB(A) at 1 m	Occupied building proximity
Harmonic loading	K-factor ≥ 4	UPS and VSD loads
Enclosure	IP23 minimum	Indoor substation

PLN Compliance – What Matters

For any 22 kV transformer being installed within PLN's network in Indonesia:

• SNI certification: The Indonesian National Standard (SNI) certification is mandatory for distribution transformers. Your supplier must hold an active SNI certificate with the specific product scope covering 22 kV distribution transformers.

• PLN technical specification SPLN: PLN publishes detailed technical specifications (SPLN documents). While not always publicly accessible, your supplier should be familiar with SPLN D3.002-1:2007 (distribution transformers) or its current revision.

• PLN type test acceptance: If the unit is the first of its design to be installed in a PLN network, a full type test witnessed by PLN representatives is typically required. Budget 6–8 weeks for type testing plus report review.

• Local content (TKDN): For government-funded projects, Indonesian local content requirements (Tingkat Komponen Dalam Negeri) may apply. Foreign manufacturers without a local assembly facility may be precluded from bidding on these tenders.

• After-sales service: PLN requires a local service partner or representative office. Confirm that your supplier has an in-country presence capable of commissioning, warranty service, and emergency oil processing.

FAQ

Q: I see transformers listed as 20 kV in PLN documents but 22 kV in tender specifications. Which is correct?

A: Both are correct in different contexts. PLN's distribution network operates at a nominal 20 kV, but in practice the system voltage at the substation busbar is maintained at approximately 21–22 kV to compensate for voltage drop along feeders. PLN's own technical specifications for transformers reference a rated voltage of 20 kV with taps up to +10% (22 kV), and many PLN-purchased units are nameplated at 20/0.4 kV but designed to operate continuously at 22 kV. For a new procurement, specify "22,000 V rated, with tappings as per schedule." A 20 kV nameplate voltage only guarantees continuous operation up to 20 kV — you need the manufacturer to explicitly confirm 22 kV continuous rating.

Q: My project is a floating production platform for oil & gas. Can I specify a standard 22 kV transformer?

A: No. Offshore and marine transformers must comply with additional standards beyond IEC 60076, primarily IEC 60092 (Electrical installations in ships) and classification society rules (ABS, DNV, BV, Lloyd's Register). Key differences from a land-based 22 kV unit include: reinforced mechanical bracing for pitch/roll motion, enclosed terminal chambers (IP56 minimum), corrosion class C5-M high (ISO 12944), synthetic ester or silicone liquid for fire safety, and shock/vibration withstand per the specific vessel design specification. A standard land-based transformer will not pass marine classification survey. Inform your supplier that offshore/marine certification is required at the RFQ stage — it is not retrofittable.

Q: We are connecting a 22 kV transformer to a generator set, not a grid. Does the specification change?

A: Yes. Generator-supplied transformers experience different stress profiles from grid-connected units. Key adjustments: (1) Impedance should be at the lower end of the range (4.0–5.0%) to minimise voltage regulation from a source that already has limited short-circuit capacity. (2) The transformer must be rated for the genset's fault contribution, which is typically 3× rated current for 10 seconds — confirm the short-circuit withstand per IEC 60076-5 against the genset's declared fault level, not an infinite grid. (3) If the genset is island-mode (no grid synchronisation), the transformer sees no lightning surge exposure, so BIL 95 kV may be acceptable — but only if the 22 kV bus has no overhead line exposure. (4) The genset's voltage regulator and the transformer's tap changer must be coordinated: specify the tap position that delivers correct secondary voltage at the genset's nominal output, not the grid voltage.

Q: What's the practical difference between specifying ONAN and ONAN/ONAF for a 22 kV transformer?

A: ONAN is a single rating — e.g., 1000 kVA ONAN means the transformer delivers 1000 kVA continuously with natural oil and air cooling only. ONAN/ONAF is a dual rating — e.g., 1000/1250 kVA ONAN/ONAF means the transformer delivers 1000 kVA naturally (ONAN) and 1250 kVA with fans running (ONAF). The dual-rated unit uses the same core, coils, and tank as the ONAN unit, but includes fan sets mounted on the radiators. When the load exceeds the ONAN rating, the fans start (automatically via WTI or manually) and provide additional cooling capacity, raising the rating by typically 25–33%. For a 22 kV distribution transformer, specify ONAN only unless: (a) your peak load exceeds the ONAN rating for more than 2 hours daily, or (b) you expect future load growth and want the transformer to accommodate it without replacement. The fan option adds 3–5% to cost but can defer a transformer upgrade by 5–10 years.

Q: The local PLN inspector is insisting on a separate HV neutral bushing even though our transformer is Dyn11. What do I do?

A: A Dyn11 vector group has a delta-connected HV winding with no neutral connection. There is physically no HV neutral point to bring out. The inspector may be confused with a YNd11 configuration, which has a star-connected HV with an accessible neutral. Ask the inspector to confirm which PLN specification clause requires the HV neutral bushing, and share the transformer's vector group designation. If PLN genuinely requires an HV neutral for earthing or protection purposes, you must change the specification to YNyn0 or YNd11. This is a vector group change, not an accessory change — it affects the entire winding design, insulation coordination, and manufacturing lead time, typically adding 4–6 weeks. Confirm this requirement during the design review, not at the factory acceptance test.

References and Standards

Standard	Title
IEC 60076-1:2011	Power transformers – Part 1: General
IEC 60076-3:2018	Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in air
IEC 60076-5:2006	Power transformers – Part 5: Ability to withstand short circuit
IEC 60076-22-7:2020	Power transformer and reactor fittings – Accessories and fittings
IEC 60296:2020	Mineral insulating oils for electrical equipment
IEC 62770:2013	Unused natural esters for transformers
IEC 60092 (series)	Electrical installations in ships
ISO 12944	Corrosion protection of steel structures by protective paint systems
SPLN D3.002-1:2007	PLN Technical Specification – Distribution Transformers (Indonesia)
SNI	Standard Nasional Indonesia – distribution transformer certification

About the Author

Du Fu is a Production Engineer at ZY POWER, with direct experience in designing and supplying 22 kV transformers for PLN-governed networks in Indonesia, PEA/MEA networks in Thailand, and industrial captive power plants across Southeast Asia. He has participated in Factory Acceptance Tests witnessed by PLN inspectors since 2019.