12 essential parameters required for rating large generators

Describing the generator’s soul

When specifying generators, the first thing that comes to mind is a rating. The machine’s rating is a set of parameters that, simply speaking in engineering terms, describe the generator. These values indicate the generator’s available power output as well as its capability in terms of electrical, thermal, and mechanical constraints.

With enough practice, a qualified person can frequently deduce further information regarding the generator’s size and basic design aspects.

The most common way to describe a generator is to assign it a rating. The generator’s rating is supplied at the machine’s maximum continuous power output capability point. Each of the below parameters is a design quantity that describes the generator’s capabilities or limitations. In some situations, they also specify operational restrictions that, if exceeded, will result in excessive stress on one or more of the generator’s components (mechanical, thermal, or electrical).

These parameters and some others not mentioned here (stator/rotor winding insulation and temperature class, overspeed capability, etc.) are considered in the design of all major generators, and they are all represented in the design standards for generators.

Currently, gas-turbine generators with ratings of up to 400-500 MVA are being developed. Currently, steam-turbine generators with ratings of up to 1700 MVA are being produced, but there are designs with ratings of up to 2000 MVA.

Figure 1 – Trend in MVA rating of large turbogenerators

The below-listed parameters have precise ranges that are outlined in design specifications and discussed in publications about proper operating practices for large generators.

The following are the terms commonly used to indicate the rating:

1. Apparent power (in MVA, or Mega volt-amperes)
2. Real power (in MW, or Megawatts)
3. Power factor pf (dimensionless quantity) and Reactive power (in MVARs, or Mega volt-amps reactance)
4. Stator terminal voltage Vt (alternating voltage)
5. Stator current Ia (alternating current, in amperes)
6. Field voltage Vf (direct voltage)
7. Field current If (direct current, in amperes)
8. Speed rpm (revolutions per minute)
9. Hydrogen pressure psi (pounds per square inch)
10. Hydrogen temperature (°C)
11. Short-circuit ratio
12. Volts per Hertz and Overfluxing

1. Apparent Power

The rating of a turbine generator is referred to as apparent power. It is generally always indicated in mega-volt-ampere (MVA) units in big generators, though it can alternatively be stated in kVA. Although real power (usually always expressed in megawatts [MW], though it can also be expressed in kilowatts [kW]) is commonly used to describe equipment, it is the apparent power that better represents the rating.

This is because the physical size of a machine is mostly determined by the product of voltage and current (MVA). The MVA in a three-phase power system is calculated using the following formula:

• MVA = √3 (Generator’s line current in kA) × (Line voltage in kV), or
• MVA = 3 (Generator’s line current in kA) × (Phase voltage in kV)

Also, MVA = MW / Power factor

Figure 2 depicts an example of a nameplate that might be placed on a large generator.

Figure 2 – Steam turbine power generator nameplate

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2. Real Power

The product of the generator’s rated apparent power (in MVA) and rated power factor is the rated power (in MW). The rated power of the turbogenerator as a whole is determined by the turbine. To take advantage of additional output that may become available from the turbine, boiler, or reactor, the rated power of the generator is frequently set and constructed to be slightly higher than that of the turbine.

This parameter is measured and monitored in order to maintain track of the machine’s load point and to allow the operator to adjust the generator’s functioning.

Transient MW events from the system or internally in the machine will also show up as transients in the stator current and/or terminal voltage.

Suggested course – Electrical Machines Course: DC, Synchronous and Induction Machines and Transformers

Electrical Machines Course: DC, Synchronous and Induction Machines and Transformers

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3. Power Factor

It was shown in many previously published articles, that the power factor is a measure of the angle between the current and the voltage in a particular branch or a circuit. In mathematical terms, the power factor is the cosine of that angle. Within the context of a generator connected to a system, the power factor describes the existing angle between the voltage at the terminals of the generator (Vt), and the current flowing through those terminals (I1).

The angle between the current and the voltage is characterized as positive in the workings of generators when the current lags behind the voltage, and negative when the current leads behind the voltage. As a result, the power factor is used to characterize whether the generator is “lagging” or “leading” in terms of power factor.

“Overexcited” or “inductive” for lagging power factor operation, and “underexcited” or “capacitive” for leading power factor operation are other terms for indicating whether the unit is producing or consuming VARs.

Figure 3 – Reactive power control for a synchronous generator

A power factor of one is referred to as a unity power factor. Generator operators frequently refer to their units as “boosting” or “backing” VARs. Boosting means overexcited or inductive in this context, while bucking means underexcited or capacitive. For the uninitiated, these several labels for the same manner of operation can be perplexing.

For most turbogenerators, the rated power factor is in the range of 0.85 to 0.90 lagging (overexcited).

The power factor (which reflects the flow of reactive power) has a significant impact on the power system since it can alter the voltage. The ability of the system to transport the appropriate levels of power, and hence its stability, is affected by the change in voltage.

A simple example is provided to demonstrate this vital idea. A generator is shown in Figure 4 supplying a single radial circuit with a load at the end.

Figure 4 – Schematic representation of a generator feeding a load through a line

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4. Terminal Voltage

The line-to-line terminal voltage at which a three-phase generator is designed to operate continuously is known as the rated or nominal voltage. Large generators’ rated voltage is typically between 13,800 and 27,000 volts.

Generators built to IEEE standards and comparable standards can operate continuously at 5% above or below the specified voltage at the rated MVA. When a power system’s unique requirements necessitate a greater terminal voltage range, the manufacturer must factor this into the generator’s design, resulting in a larger and more expensive unit.

The circumstances in which this type of variation is required are determined by the generator’s location and the power system’s requirements for interaction.

The terminal voltage of the generator must be matched in magnitude, phase, and frequency to that of the system voltage before closing the main generator breakers. This is to ensure smooth closure of the breakers and connection to the system and to deter faulty synchronization.

Figure 5 – Generator equivalent circuit

Where:

• E = Induced EMF
• Xs = Synchronous reactance
• Zs = Synchronous impedance
• Ra = Armature resistance
• Vt = Terminal voltage
• Ia = Armature (stator) current

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5. Stator Current

Stator’s current capability in large generators depends largely on the type of machine in question. In the simplest machines (i.e., the indirectly air-cooled generator) the capability of the stator winding defines the rated stator current. The capability of an indirectly hydrogen-cooled generator winding is significantly sensitive to the hydrogen pressure within the machine.

Reduced capabilities are commonly stated for below-rated pressures, down to 15 psig (103 kPa), and at slightly above atmospheric pressure. Modern generators may be found operating with hydrogen pressures up to 75 psig (518 kPa).

A direct hydrogen-cooled stator winding is directly dependent on hydrogen pressure. Capabilities are commonly stated in increments of 15 psi (103 kPa) below rated hydrogen pressure.

In Figure 6 the dependency of the generator’s rating on the pressure of the cooling hydrogen can be seen.

Figure 6 – Typical capability curves for a synchronous generator

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6. Field Voltage

In proportion to the rotor winding resistance, increasing the field voltage increases the field current. The voltage in the field is monitored, but it is rarely used for alerts or trips. It’s utilized to figure out the rotor winding resistance, as well as the rotor winding average and hot spot temperature.

Problems with the automatic voltage regulator (AVR) can cause the field voltage to become excessively high, causing the excitation to exceed design limitations.

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7. Field Current

The field current at the rated apparent power, the rated power factor, and the rated terminal voltage are all used to determine the rotor winding’s capabilities. All of the capability issues outlined for hydrogen-cooled stator windings, both indirectly and directly, apply to the rotor winding as well.

It’s worth noting that increasing the field current will:

1. Augment the MVARs exported to the power system
2. Increase armature (stator) current if the unit is already in the boost or overexcited region
3. Tend to increase the differential of potential at the generator’s terminals

Suggested reading – Four abnormal operating conditions of a generator that could shut down the power plant

Four abnormal operating conditions of a generator that could shut down the power plant

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8. Speed

Unlike an induction machine, the synchronous generator can only generate power at one speed, called the synchronous speed of that unit. That unique speed is related to the system’s frequency and the number of poles of the machine, by the following equation:

Synchronous speed (rpm) = 120 × System frequency (Hz) / Number of poles

Practically all large turbogenerators are of the two- or four-pole design. Therefore, almost without exception, the following apply:

60 Hz system:

• 3600 rpm for two-pole generators
• 1800 rpm for four-pole generators

50 Hz system:

• 3000 rpm for two-pole generators
• 1500 rpm for four-pole generators

Suggested course – The Essentials of Rotating Electrical Machines: AC & DC Electric Motors and Generators

The Essentials of Rotating Electrical Machines: AC & DC Electric Motors and Generators

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9. Hydrogen Pressure

The required pressure of hydrogen in the generator when it is loaded to its nominal rating is known as the rated hydrogen pressure. It’s usually the highest hydrogen pressure at which the generator is capable of operating. Up to 75 psig** is the range of rated hydrogen pressures for generators currently being manufactured (518 kPa).

**The unit psig is pounds per square inch “gauge,” relative to standard atmosphere.

Therefore one of the sources of a drop in hydrogen pressure in the generator may be into the stator cooling water system if a leak exists.

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10. Hydrogen Temperature

The temperature of the hydrogen cooling gas is kept at a precise level, just like the pressure, to ensure proper cooling of the internal generator components. As the hydrogen gas runs over the various sections of the generator’s internals, it absorbs heat and transfers it to raw water circulating through hydrogen coolers in the generator.

As a result, the gas entering the coolers is considerably hotter than the gas leaving them. The temperatures of hydrogen gas are commonly referred to as hot and cold.

The hydrogen gas temperatures are usually maintained by an arrangement of four coolers, as being the most common. A balance between these is then maintained by adjusting the flow of raw water through the coolers, and locking the inlet valves in those positions. The balance is generally kept as close as possible and under 2°C on cold outlet gas temperature from the coolers.

Temperature control of the hot and cold hydrogen gas is accomplished by installing thermocouples (TCs) or resistance temperature detectors (RTDs) in the gas path. These can then be monitored and set with alarm points to notify operators when limits are exceeded.

Figure 7 – Technologies applied to 900 MVA-class indirect hydrogen cooled generator

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11. Short-Circuit Ratio (SCR)

The short-circuit ratio (SCR) is the ratio of the field current necessary to produce rated terminal voltage on an open circuit to the field current required to produce rated stator current on a sustained three-phase short circuit with the machine running at rated speed. The change in excitation varies inversely as the SCR during operation to maintain constant voltage for a given change in load. For the same load change, a generator with a lower SCR requires a bigger change in excitation than a machine with a higher SCR.

The short-circuit ratio of a generator in a power system determines its inherent stability. It is a measure of the field winding’s relative influence on the level of useable magnetic flux in the generator compared to the stator winding.

On the other hand, machines with a higher SCR aren’t always the most stable in a given setting. The response times of the voltage regulator and excitation systems, the match between the turbine and generator time constants, control functions, and the combined inertia of the turbine and generator are also important.

In recent years, the short-circuit ratio for turbine generators has been in the range of 0.4 to 0.6.

Suggested course – Power Engineering Course: Generators, Transformers and Transmission Lines

Power Engineering Course: Generators, Transformers and Transmission Lines

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12 Volts per Hertz and Overfluxing Events

The term “volts per hertz” has been borrowed from the operation of transformers. In transformers, the fundamental voltage equation is given by:

V = 4.44 × f × B_max × Area of core × Number of turns

where B_max is the vector magnitude of the flux density in the core of the transformer.

By rearranging the variables, the following expression is obtained:

• V/f [V/Hz] = 4.44 × Bmax × Area of core × Number of turns, or
• B_max[tesla] = constant × (V/f)

Or, in another notation: B_max ∝ V/Hz

The maximum flux density in the core of a transformer is proportional to the terminal voltage divided by the frequency of the supply voltage, according to the last equation. V/Hz is the abbreviation for this ratio.

A similar set of equations can be derived for the armature of an AC machine. Winding features such as pitch and distribution variables are included in the constant in this case. The end effect is the same: in AC current machine, the maximum core flux density in the armature is proportional to the terminal voltage divided by the supply frequency (or V/Hz).

Increasing the volts per turn in the machine (or transformer) raises the flux density above the knee of the saturation curve (see Figure 8).

Figure 8 – Typical saturation curve for transformers and generators

Consequently, large magnetization currents are produced, as well as large increases in the core loss due to the bigger hysteresis loop created (see Figure 9). Both of these result in substantial increases in core and copper losses, and excessive temperature rises in both core and windings.

In fact, if a unit becomes excessively overfluxed (i.e., the maximum V/Hz has been exceeded) for just a few seconds, complete failure of the core may result in a short time, or after some time of operation.

Figure 9 – Hysteresis losses under normal and abnormal conditions

The IEEE Std 67-2006 standard states that generators are normally designed to operate at rated outputs of up to 105% of rated voltage. ANSI/IEEE C57 standards for transformers state the same percentage for rated loads and up to 110% of rated voltage at no load.

In practice, the operator should make sure (by consulting vendor manuals and pertinent standards) that the machine remains below limits that may affect the integrity of both the generator and the unit transformer.

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Sources:

1. Operation And Maintenance Of Large Turbo-generators by Geoff Klempner And Isidor Kerszenbaum
2. Development of Large Capacity Turbine Generators for Thermal Power Plants by YASUNORI SATAKE, KAZUHIKO TAKAHASHI, TAKAMI WAKI, MITSURU ONODA, and TAKAYASU TANAKA
3. IEEE Application Guide for IEEE Std 1547™, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems