MVA stands for megavolt-ampere. It is a unit of apparent power used for transformers, generators, substations, and other large AC power systems.
The conversion is exact:
1 MVA = 1,000 kVA = 1,000,000 VAMVA is not the same as MW. MVA describes the total RMS voltage-current product, while MW describes the real power converted into useful work or heat. Power factor connects the two.
What Does MVA Mean?
The letters represent:
- ม.: mega, the SI prefix for 10^6;
- วี: volt;
- ก: ampere.
Volt-amperes measure apparent power in an AC circuit. The same quantity may be written as VA, kVA, or MVA depending on scale.
| การจัดอันดับ | Equivalent value |
|---|---|
| 1 kVA | 1,000 VA |
| 1 MVA | 1,000 kVA |
| 1 MVA | 1,000,000 VA |
| 10 MVA | 10,000 kVA |
| 100 MVA | 100,000 kVA |
The uppercase M is important. In SI notation, M means mega, while lowercase m means milli, or 10^-3.
MVA, MW, and Mvar: The Power Triangle
AC power is commonly described through three related quantities:
| ปริมาณ | เครื่องหมาย | หน่วย | ความหมาย |
|---|---|---|---|
| กำลังไฟฟ้าปรากฏ | S | VA, kVA, MVA | Combined RMS voltage-current requirement |
| Real power | พี | W, kW, MW | Power converted into useful output, motion, heat, or light |
| Reactive power | Q | var, kvar, Mvar | Power exchanged with magnetic and electric fields |
For a sinusoidal system:
S^2 = P^2 + Q^2
Power factor = P / S
MW = MVA × power factorFor example, a load drawing 10 MVA at a power factor of 0.80 consumes:
MW = 10 MVA × 0.80 = 8 MWThe remaining relationship is reactive rather than additional real energy consumption. Using the power triangle:
Mvar = sqrt(10^2 - 8^2) = 6 MvarIf the power factor changes while MW remains constant, the required MVA changes. This is why poor power factor increases current and consumes transformer, cable, and switchgear capacity without delivering an equal increase in real power.
For the corrective-design process, see การปรับปรุงค่าตัวประกอบกำลัง: การกำหนดขนาดคาปาซิเตอร์แบงก์, ฮาร์มอนิก, และการป้องกัน.
MVA Formula for Single-Phase and Three-Phase Systems
Single-phase apparent power
MVA = V × I / 1,000,000where V is RMS voltage in volts and I is RMS current in amperes.
Three-phase apparent power
For a balanced three-phase system using line-to-line voltage and line current:
MVA = sqrt(3) × V_L-L × I_L / 1,000,000Rearranging to calculate current:
I_L = MVA × 1,000,000 / (sqrt(3) × V_L-L)These equations calculate apparent power from RMS values. They do not include power factor unless the required result is real power.
Worked Example: Current of a 10 MVA Transformer
Consider a three-phase 10 MVA transformer rated 33/11 kV.
On the 33 kV side:
I = 10,000,000 / (sqrt(3) × 33,000)
I ≈ 175 AOn the 11 kV side:
I = 10,000,000 / (sqrt(3) × 11,000)
I ≈ 525 A
The apparent-power rating is approximately the same on both sides, but the lower-voltage winding carries more current. Actual loading must also respect the transformer’s cooling class, temperature limits, tap position, harmonics, installation conditions, and manufacturer instructions.
Why Are Transformers Rated in MVA Instead of MW?
Transformer heating is driven mainly by voltage-dependent and current-dependent losses. The transformer must carry RMS voltage and current regardless of the load’s power factor. Because power factor belongs primarily to the connected load, transformer capacity is expressed in VA, kVA, or MVA rather than W or MW.
This does not mean power factor is irrelevant. At the same MW output, a lower power factor requires higher MVA and higher current:
| Real load | ตัวประกอบกำลัง | Required apparent power |
|---|---|---|
| 8 MW | 1.00 | 8 MVA |
| 8 MW | 0.90 | 8.89 MVA |
| 8 MW | 0.80 | 10 MVA |
| 8 MW | 0.70 | 11.43 MVA |
For a deeper explanation of smaller transformer ratings, see การทำความเข้าใจค่า kVA สำหรับหม้อแปลงไฟฟ้า.
How to Read an MVA Nameplate Rating
An MVA value should never be read alone. Check it together with:
- rated primary and secondary voltages;
- number of phases and frequency;
- rated current for each winding;
- vector group and connection;
- percentage impedance;
- tap range and tap position;
- cooling method;
- temperature-rise and insulation information;
- applicable transformer standard.
The MVA rating does not by itself define the available short-circuit current. Transformer impedance and upstream-system strength are also required. It also does not state how much energy the transformer delivers over time; energy is measured in units such as MWh, not MVA.
Common MVA Mistakes
Treating MVA as MW
MVA equals MW only when power factor is 1.00. For other power factors, use MW = MVA × power factor.
Confusing power with energy
MVA is an instantaneous apparent-power rating. MWh is energy accumulated over time. They are not interchangeable.
Omitting the square-root-of-three factor
The balanced three-phase formula using line voltage and line current includes sqrt(3). The single-phase formula does not.
Using nominal voltage without checking the winding
A transformer has different rated currents on its high- and low-voltage sides. Use the voltage of the winding being evaluated.
คำถามที่พบบ่อย
How many kVA are in 1 MVA?
One MVA equals 1,000 kVA.
How many MW are in 1 MVA?
It depends on power factor. At unity power factor, 1 MVA equals 1 MW. At 0.8 power factor, 1 MVA corresponds to 0.8 MW.
Is a higher MVA rating always better?
No. The rating must match the system load, voltage, fault level, cooling conditions, expansion plan, efficiency objectives, and protection design. Oversizing can add cost and alter system performance without solving poor power factor or harmonic problems.
Final Definition
MVA means megavolt-ampere, a unit of apparent power equal to one million volt-amperes. Use MVA with voltage to calculate AC current, and use power factor to convert between MVA and MW. When reading a transformer or generator rating, verify the complete nameplate rather than treating the MVA number as real power, energy, or fault capacity.
