Basic Operation Modes of the Synchronous Machine-An Extract from Electric Generator Study

This article is aimed to refresh basic knowledge of mode of operations of large generator. The content has been extracted from study of Electric Generator.

In this article following, the most elementary principles of, operation of synchronous machines will be presented:

·      No-Load Operation

·      Motor Operation

·      Generator Operation

As it is known that all large turbogenerators are three-phase machines. Thus the best place to start  describing the operation of a three-phase synchronous machine is a description of its magnetic field.

Briefly a current flowing through a conductor produces a magnetic field associated with that current and was by coiling the conductor, a larger field is obtained without increasing the current’s magnitude.If the three phases of the winding are distributed at 120 electrical degrees apart, three balanced voltages are generated, creating a three-phase system.

By a simple mathematical analysis it can be shown that if three balanced currents (equal magnitudes and 120 electrical degrees apart) flow in a balanced three-phase winding, a magnetic field of constant magnitude is produced in the airgap of the machine. This magnetic field revolves around the machine at a frequency equal to the frequency of the currents flowing through the winding.

The importance of a three-phase system creating a constant field cannot be stressed enough. The constant magnitude flux allows hundred of megawatts of power to be transformed inside an electric machine from electrical to mechanical power, and vice versa, without major mechanical limitations. It is important to remember that a constant magnitude flux produces a constant-magnitude torque. Now try to imagine the same type of power being transformed under a pulsating flux (and therefore pulsating torque), which is tremendously difficult to achieve.

It is convenient to introduce the fundamental principles describing the operation of a synchronous machine in terms of an ideal cylindrical-rotor machine connected to an infinite bus. The infinite bus represents a busbar of constant voltage, which can deliver or absorb active and reactive power without any limitations. The ideal machine has zero resistance and leakage reactance, infinite permeability, and no saturation, as well as zero reluctance torque.

The production of torque in the synchronous machine results from the natural tendency of two magnetic fields to align themselves. The magnetic field produced by the stationary armature is denoted as φs. The magnetic field produced by the rotating field is φf. The resultant magnetic field is

φr = φs + φf

The flux φr is established in the airgap (or gasgap) of the machine. (Bold symbols indicate vector quantities.)

When the torque applied to the shaft equals zero, the magnetic fields of the rotor and the stator become perfectly aligned. The instant torque is introduced to the shaft, either in a generating mode or in a motoring mode, a small angle is created between the stator and rotor fields. This angle (λ) is called the torque angle of the machine.

No-Load Operation

When the ideal machine is connected to an infinite bus, a three-phase balanced voltage (V1) is applied to the stator winding (within the context of this work, three-phase systems and machines are assumed). As described above, it can be shown that a three-phase balanced voltage applied to a three-phase winding

evenly distributed around the core of an armature will produce a rotating (revolving) magneto-motive force (mmf) of constant magnitude (Fs). This mmf, acting upon the reluctance encountered along its path, results in the magnetic flux (φs) previously introduced. The speed at which this field revolves around

the center of the machine is related to the supply frequency and the number of poles, by the following expression:

ns = 120 (f/p)

where

f = electrical frequency in Hz

p = number of poles of the machine

ns = speed of the revolving field in revolutions per minute (rpm)

If no current is supplied to the dc field winding, no torque is generated, and the resultant flux (φr), which in this case equals the stator flux (φs), magnetizes the core to the extent the applied voltage (V1) is exactly opposed by a counterelectromotive force (cemf) (E1).

The underexcited condition

If the rotor’s excitation is slightly increased, and no torque is applied to the shaft, the rotor provides some of the excitation required to produce (E1), causing an equivalent reduction of (φs). This situation represents the underexcited condition shown in below figure.

When operating under this condition, the machine is said to behave as a lagging condenser, meanings it absorbs reactive power from the network.

The overexcited condition

If the field excitation is increased over the value required to produce (E1), the stator currents generate a flux that counteracts the field-generated flux. Under this condition, the machine is said to be overexcited, shown in below figure.

The machine is behaving as a leading condenser; that is, it is delivering reactive power to the network.

The load/torque angle (δ)

Under no-load condition both the torque angle (λ) and the load angle (δ) are zero. The load angle is defined as the angle between the rotor’s mmf (Ff) or flux (φf) and the resultant mmf (Fr) or flux (φr). The load angle (δ) is the most commonly used because it establishes the torque limits the machine can attain in a simple manner. One must be aware that in many texts the name torque angle is used to indicate the load angle. The name torque angle is also sometimes given to indicate the angle between the terminal voltage (V1) and the excitation voltage (E1). This happens because the leakage reactance is generally very much smaller than the magnetizing reactance, and therefore the load angle (δ) and the angle between (V1) and (E1) are very similar. Here name power angle is used for the angle between (V1) and (E1). In above two figures, the power angle is always shown as zero because the leakage impedance has been neglected in the ideal machine.

It is important to introduce the distinction between electrical and mechanical angles. In studying the performance of the synchronous machine,all the electromagnetic calculations are carried out based on electric quantities;that is, all angles are electrical angles. To convert the electrical angles used in the calculations to the physical mechanical angles, we observe the following relationship:

Mechanical angle = (2/p) Electrical angle

Motor Operation

The turbogenerator units seldom operate as a motor. (One such example is when the main generator is used for a short period of time as a motor fed from a variable speed converter. The purpose of this operation is for starting its own prime-mover combustion turbine). If a breaking torque is applied to the shaft, the rotor starts falling behind the revolving-armature-induced magnetomotive force (mmf) (Fs). In order to maintain the required magnetizing mmf (Fr) the armature current changes. If the machine is in the underexcited mode, the condition motor in above Figure represents the new phasor diagram.

On the other hand, if the machine is overexcited, the new phasor diagram is represented by motor in above Figure. The active power consumed from the network under these conditions is given by

Active power = V1 × I1 × cos ?1 (per phase)

If the breaking torque is increased, a limit is reached in which the rotor cannot keep up with the revolving field. The machine then stalls. This is known as “falling out of step,” “pulling out of step,” or “slipping poles.” The maximum torque limit is reached when the angle δ equals π/2 electrical.

The convention is todefine δ as negative for motor operation and positive for generator operation. The torque is also a function of the magnitude of φr and φf. When overexcited, the value of φf is larger than in the underexcited condition.

Therefore synchronous motors are capable of greater mechanical output when overexcited. Likewise, underexcited operation is more prone to result in an “out-of-step” situation.

Generator Operation

Let’s assume that the machine is running at no load and a positive torque is applied to the shaft; that is, the rotor flux angle is advanced ahead of the stator flux angle. As in the case of motor operation, the stator currents will change to create the new conditions of equilibrium shown in above figures, under generator.

If the machine is initially underexcited, underexicted condition in above figure obtains. On the other hand, if the machine is overexcited, overexcited condition in above Figure results.

It is important to note that when “seen” from the terminals, with the machine operating in the underexcited mode, the power factor angle (?1) is leading (i.e., I1 leads V1). This means the machine is absorbing reactive power from the system.

The opposite occurs when the machine is in the overexcited mode. As for the motor operation, an overexcited condition in the generating mode also allows for greater power deliveries.

As generators are normally called to provide VARs together with watts, they are mostly operated in the overexcited condition.