{"id":10523,"date":"2026-08-29T23:43:04","date_gmt":"2026-08-29T15:43:04","guid":{"rendered":"https:\/\/q8bbzjnpm5.wpdns.site\/?p=10523"},"modified":"2026-08-29T23:43:04","modified_gmt":"2026-08-29T15:43:04","slug":"transformer-ratio-the-regulating-valve-for-stable-operation-of-power-systems","status":"publish","type":"post","link":"https:\/\/subian-electric.com\/de\/news\/transformer-ratio-the-regulating-valve-for-stable-operation-of-power-systems\/","title":{"rendered":"Transformatorverh\u00e4ltnis: Das \u201cRegelventil\u201d f\u00fcr einen stabilen Betrieb von Stromversorgungssystemen"},"content":{"rendered":"

A substation operator looked at his electronic meter today when a thunderstorm arrived and changed the 110 kV line of power from 108 kV to 116 kV. In spite of that, the 20 kV bus on the secondary side showed values between 19.9 and 20.1 kV at the same time. The instrument that works there is called an on-load tap changer: the transformer ratio is corrected step by step.<\/p>\n

The article interprets what transformer ratio is, how it is calculated, what the relationship between transformer ratio<\/strong> and voltage stability is, how tap changers make transformer ratio stable at that very moment, and what consequences one can get if there is a transformer ratio error. Examples of calculations are provided in the paper together with references to IEC 60076 and IEEE C57 standards, typical tap range, the cost of tap changers, and others.<\/p>\n

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What Is Transformer Ratio? (Definition & Formula)<\/h2>\n

Transformer ratio or turns ratio defines the ratio of primary winding turns to the number of secondary winding turns. It is equal to the ratio of primary voltage to secondary voltage and the inverse ratio of primary to secondary currents in the theoretical case.
\nN1\/N2 = V1\/V2 = I2\/I1.
\nReal transformers are not ideal. In real transformers, leakage reactance and resistance of the winding give rise to voltage drops making the actual voltage lower than the voltage on no load. The difference in the voltage in percent is called voltage regulation<\/strong> which is around 2%-6% depending on the transformers\u2019 characteristics.
\nThe following definition is used interchangeably in the contracts:<\/p>\n\n\n\n\n\n\n
Transformer Ratio Terminology<\/caption>\n
Term<\/th>\nBedeutung<\/th>\nBeispiel<\/th>\n<\/tr>\n
Turns ratio<\/td>\nN1 \/ N2 (pure winding geometry)<\/td>\n550 \/ 100 = 5.5<\/td>\n<\/tr>\n
\u00dcbersetzungsverh\u00e4ltnis<\/td>\nV1 \/ V2 at no load<\/td>\n110 kV \/ 20 kV = 5.5<\/td>\n<\/tr>\n
Regulation<\/td>\nVoltage drop from no-load to full load<\/td>\n2.5%\u20136%<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

How to Calculate Transformer Ratio: Worked Examples<\/h2>\n

Determining the ratio is easy if one knows voltage or number of turns.<\/p>\n

For example:
\n110 kV\/20 kV voltage transformer has 110:20 with 2200 turns in the primary it means that there are 400 turns in the secondary.
\nIn the case of a higher voltage transformer 18 kV – 400 kV ratio becomes (400\/18 = 22.2).
\nDistribution transformer usually has a voltage ratio of 11 kV\/0.4 kV which means the ratio is 11\/0.4 = 27.5:1.
\nFor Owing to the construction of the auto-transformer which shares a part of the winding system it has lower number of turns for the same voltage ratio of 2:1.<\/p>\n

In the case of transformers that operate in a three-phase system the line-to-line voltage should be used while the vector group (Dyn11, YNd11) is important in terms of determining the relationship of the phase shift and the magnitude ratio of the system.<\/p>\n

Why Ratio Is the Regulating Valve of the Grid<\/h2>\n

Voltage fluctuations on a transmission line occur due to several factors, namely modifications in load, generator failures, reactive power losses, and distance from the source. Without adjustment to the ratio, a feeder may experience voltage fluctuations of 5% to 10% throughout a single day. The transformer ratio acts as the practical means of compensating for the advantages of voltage variability, as there is a direct relationship between turns and the output voltage, with any changes leading to corresponding fluctuations.
\nSystem operators must keep voltage within the legal limits set by regulations, which require holding voltage at \u00b1 5% in the distribution sector and \u00b1 10% in the transmission sector. The most widely used method in this process is the OLTC, which adjusts the transformer ratio. In fact, OLTC is so widely in use that having a transformer without OLTC needs a justification in writing.<\/p>\n

Tap Changers: The Mechanism That Adjusts Ratio<\/h2>\n

Two families of tap changers exist, and choosing between them is mostly a duty-cycle question:<\/p>\n\n\n\n\n\n\n\n\n\n
Off-Circuit vs. On-Load Abgriffwechselger\u00e4te<\/caption>\n
Aspekt<\/th>\nOff-Circuit (DETC)<\/th>\nOn-Load (OLTC)<\/th>\n<\/tr>\n
Betrieb<\/td>\nDe-energized only<\/td>\nUnder load, automated<\/td>\n<\/tr>\n
Typical tap range<\/td>\n\u00b12.5% to \u00b15%<\/td>\n\u00b110% to \u00b116%<\/td>\n<\/tr>\n
Steps<\/td>\n3\u20135<\/td>\n13\u201317 (1.25%\u20132.5% each)<\/td>\n<\/tr>\n
Regulation capability<\/td>\nSeasonal only<\/td>\nContinuous, automatic (\u00b12% holding)<\/td>\n<\/tr>\n
Price premium<\/td>\nBasislinie<\/td>\n+$15,000\u2013$60,000 depending on rating<\/td>\n<\/tr>\n
Common on<\/td>\nVerteilungstransformatoren<\/td>\nPower transformers \u22655 MVA<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

While OLTCs are operating with electrical current, they use resistors or reactors and fast vacuum interrupters in order to minimize electrical arcing on the main contacts in the process of switching from fixed taps. A contemporary motor-controlled OLTC takes approximately 2 to 10 seconds to finish a cycle. It can be noted that the controller in this case uses voltage feedback from the secondary side of the transformer.<\/p>\n

Ratio Matching for Parallel Operation<\/h2>\n

When using two transformers to supply the same bus simultaneously, the ratios need to be adjusted to be closely matched. A difference in ratios of \u0394% generates a so-called circulating current (participating in the circuit even in idle condition).<\/p>\n

One advice is to make sure we keep the circulating current below 10% of the capacity of the smaller transformer.
\nThe effect of a 1% difference in ratio is usually a circulating current of 2%-5% of the capacity of two transformers with capacity of 20 MVA \/ 110 kV, which depends on impedance (normally 10%).
\nThe transformers should be operated in accordance with some basic conditions for this, namely: same group of vector, ratio must match, and impedance should not vary more than 10%, if at all.<\/p>\n

To account for this, operators need to change the tap position for each transformer to make those equal, and many of modern AVRs have a master-follower scheme for this purpose.<\/p>\n

Effects of Wrong Ratio: Circulating Current & Protection Issues<\/h2>\n

Relaying misoperation, due to relay rating, can lead to malfunctioning of protections with erroneous relay settings resulting in either protector operation without proper fault or failure to detect real internal defects because of 2%-5% errors.
\nVoltage changes can force the voltage along the creek into outside limits.
\nImbalance in load situation implies that a unit with a higher ratio consumes a considerable share of both active and reactive power leading to acceleration of its aging.
\nThat is why ratio check became one of the steps of commissioning according to IEC 60076-1 requirements stating that the voltage ratio on the main tapping must differ by no more than \u00b10.5% from design value and every tap must be verified against the nameplate data.<\/p>\n

Ratio Testing Methods & Standards<\/h2>\n

The standard field test is the turns-ratio test performed with a transformer turns ratio (TTR) meter:<\/p>\n\n\n\n\n\n\n
Ratio Testing Methods<\/caption>\n
Methode<\/th>\nPrinciple<\/th>\nGenauigkeit<\/th>\nTypische Verwendung<\/th>\n<\/tr>\n
TTR meter (TTR test)<\/td>\nCompares injected and induced voltage on each tap<\/td>\n\u00b10.1%<\/td>\nCommissioning, routine maintenance<\/td>\n<\/tr>\n
Voltage ratio check at no load<\/td>\nApply reduced voltage, measure both sides<\/td>\n\u00b10.3%<\/td>\nOn-site verification<\/td>\n<\/tr>\n
Capacitance ratio \/ low-voltage test<\/td>\nEquivalent ratio via capacitive coupling<\/td>\n\u00b10.5%<\/td>\nFactory type tests<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Both factory tests and site tests originate from IEC 60076-1 and IEEE C57.12.90. The TTR test is considered a primary diagnostic test as the movement in the ratio between different tests done on the same unit oftentimes signifies a shorted turn in the winding, making yearly ratio tests an essential element of monitoring.<\/p>\n

Typical Ratios and Tap Ranges by Application<\/h2>\n\n\n\n\n\n\n\n\n
Typical Ratios & Tap Ranges Across Applications<\/caption>\n
Anwendung<\/th>\nTypical Ratio<\/th>\nTap Range<\/th>\nTap Changer<\/th>\n<\/tr>\n
GSU at a power plant<\/td>\n18\/400 kV (22.2:1)<\/td>\n\u00b17.5% in 5 steps<\/td>\nOff-circuit, sometimes OLTC<\/td>\n<\/tr>\n
Transmission substation<\/td>\n220\/110 kV (2:1)<\/td>\n\u00b116% in 17 steps<\/td>\nOLTC<\/td>\n<\/tr>\n
Sub-\u00dcbertragung<\/td>\n110\/20 kV (5.5:1)<\/td>\n\u00b112% in 13 steps<\/td>\nOLTC<\/td>\n<\/tr>\n
Industrial power<\/td>\n33\/11 kV (3:1)<\/td>\n\u00b110% in 9 steps<\/td>\nOLTC or off-circuit<\/td>\n<\/tr>\n
Verteilung<\/td>\n11\/0.4 kV (27.5:1)<\/td>\n\u00b12.5% off-circuit<\/td>\nAbgeschaltet<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Distribution transformers require only periodic adjustments, so off-load taps in steps of \u00b12.5% and \u00b15% are adequate to account for seasonal load variation. By contrast, transmission and sub-transmission transformers areconfigured with OLTC because large groups of customers are directly affected by the action of the transmission voltage on them.<\/p>\n

Tap Changer & Transformer Price Impact<\/h2>\n

Tap-changer choice is a real line item in any transformer budget. Planning ranges:<\/p>\n\n\n\n\n\n\n
Price Impact of Tap Changer Selection<\/caption>\n
Transformator<\/th>\nWithout OLTC<\/th>\nWith OLTC<\/th>\nPremium<\/th>\n<\/tr>\n
5 MVA \/ 33 kV<\/td>\n$55,000\u2013$90,000<\/td>\n$75.000\u2013$130.000<\/td>\n+$15,000\u2013$40,000<\/td>\n<\/tr>\n
20 MVA \/ 110 kV<\/td>\n$150,000\u2013$260,000<\/td>\n$175,000\u2013$320,000<\/td>\n+$20,000\u2013$60,000<\/td>\n<\/tr>\n
50 MVA \/ 110 kV<\/td>\n$360,000\u2013$700,000<\/td>\n$400,000\u2013$800,000<\/td>\n+$25,000\u2013$90,000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The charges differ according to the type of specifications, the brand, and the area. The high expense involved in grid-tied units pays off in short time: the On-load tap changer (OLTC) can recover the cost quickly because it keeps the voltage regulated and diminishes the energy losses in feeders.<\/p>\n

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Brands & How Ratio Design Differs<\/h2>\n

Ratio mathematics remains the same irrespective of the originating company \u2014 physics is not up for negotiation; however, manufacturers distinguish themselves based on the type of tap-changer design, control, quality management, and pricing. Reliability benchmarks are set by the MR (Maschinenfabrik Reinhausen) and ABB tap changers as Asian companies increasingly make use of licensed or self-developed OLTC designs at cheaper rates.<\/p>\n\n\n\n\n\n\n\n\n
Transformer Brands & Tap-Change Capability<\/caption>\n
Marke<\/th>\nLand<\/th>\nTap-Changer Strengths<\/th>\nIndicative Price (20 MVA\/110 kV, with OLTC)<\/th>\n<\/tr>\n
Hitachi Energie<\/td>\nSchweiz\/Japan<\/td>\nProprietary OLTC, AVR integration<\/td>\n$450,000\u2013$700,000<\/td>\n<\/tr>\n
Siemens Energie<\/td>\nDeutschland<\/td>\nMR OLTC options, digital control<\/td>\n$430,000\u2013$680,000<\/td>\n<\/tr>\n
Schneider Electric<\/td>\nOptimierungsma\u00dfnahmen in der Praxis<\/td>\nDistribution focus, MV units<\/td>\n$280.000\u2013$450.000<\/td>\n<\/tr>\n
TBEA<\/td>\nChina<\/td>\nIn-house OLTC, scale<\/td>\n$180.000\u2013$320.000<\/td>\n<\/tr>\n
Jiangsu Subian Electric Power<\/td>\nChina<\/td>\nCustom ratios & taps, MR or in-house OLTC<\/td>\n$160.000\u2013$300.000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The cost is determined by specification and area. In projects that require customized ratio and tap specifications \u2013 unique vector groups, special tap ranges, or particular brands of OLTC \u2013 Jiangsu Subian Electric Power is a reliable partner: it manufactures distribution and power transformers with capacity from 50 kva to 220 kV, provides quotes for individual tap configurations, and installs the required MR or similar on-load tap changers. Each unit is delivered with ratio test records on all taps under the IEC 60076-1 standard, and factory prices are usually 30%-50% lower than prices for European companies.<\/p>\n

H\u00e4ufig gestellte Fragen<\/h2>\n

How is transformer ratio calculated?<\/h3>\n

The transformer ratio is determined by the number of turns in the primary coil divided by the number of turns in the secondary coil (N1\/N2), which is equal to the no-load voltage ratio (V1\/V2). For example, the transformer applied at 110 kV\/20 kV will have a 5.5:1 ratio, while for the 11 kV\/0.4 kV transformer the ratio is 27.5:1. Because of the inverse relationship, it is evident that in a step-up transformer (ratio smaller than one; such as 0.045 for the transformer with 18\/400 kV voltage), the output voltage will be high while the output current is low.<\/p>\n

What is the tolerance on transformer ratio?<\/h3>\n

Per IEC 60076-1, the ratio at the principal tap must be within \u00b10.5% of the nameplate value, and the same tolerance applies at every tap position unless otherwise specified. Field TTR meters measure to \u00b10.1%, so a deviation beyond 0.5% on a commissioning test warrants investigation for shorted turns or a wrong connection.<\/p>\n

Why do transformers need tap changers for voltage regulation?<\/h3>\n

Due to fluctuations in line voltage based on load, generation, and distance from the source, without the ratio adjustment, the distribution feeder can fluctuate 5%\u201310% throughout the day. OLTC varies the ratio by 1.25%\u20132.5% in steps within a range of \u00b110% to \u00b116% to provide a secondary voltage of about \u00b12% automatically. This ensures the motors, electronics, and the customer equipment are safe from overvoltage and undervoltage.<\/p>\n

What happens if two transformers in parallel have different ratios?<\/h3>\n

An electric current circulates between the two units, even when there is no load coming from outside. In other words, a circulating current flows in a transformer even when it is on standby mode. When we take the case of 20 MVA\/110 kV units, even a 1%-ratio mismatch will cause a circulating current of around 2%-5% of full-load rated current. Moreover, if the mismatch increases, the circulating current will also increase accordingly. The action of operators, who equalize the ratio through the tap selection process, will prevent circulating currents from rising more than 10% of the rating of the small transformer.<\/p>\n

How much does an on-load tap changer add to transformer cost?<\/h3>\n

OLTC generally makes it possible to add between $15,000 and $60,000 to the cost of power transformers rated between 5 MVA and 50 MVA, with higher rated transformers costing more. The investment usually pays off due to better voltage control and lower losses of power as well as absence of equipment failures.<\/p>\n

Referenzen<\/h2>\n