SHAFT CURRENT, BEARING INSULATION, AND PHASE SEQUENCE TEST OF SYNCHRONOUS GENERATORS

Shaft Current and Bearing Insulation

Irregularities in the SG magnetic circuit lead to a small axial flux that links the shaft. A parasitic current occurs in the shaft, bearings, and machine frame, unless the bearings are insulated from stator core or from rotor shaft.

The presence of pulse-width modulator (PWM) static converters in the stator (or rotor) of SG augments this phenomenon. The pertinent testing is performed with the machine at no load and rated voltage. The voltage between shaft ends is measured with a high impedance voltmeter.

The same current flows through the bearing radially to the stator frame. The presence of voltage across bearing oil film (in uninsulated bearings) is also an indication of the shaft voltage.

If insulated bearings are used, their effectiveness is checked by shorting the insulation and observing an increased shaft voltage. Shaft voltage above a few volts, with insulated bearings, is considered unacceptable due to bearing in-time damage.

Generally, grounded brushes in shaft ends are necessary to prevent it.

Phase Sequence

Phase sequencing is required for securing given rotation direction or for correct phasing of a generator prepared for power bus connection. As known, phase sequencing can be reversed by interchanging any two armature (stator) terminals.

There are a few procedures used to check phase sequence:

• With a phase-sequence indicator (or induction machine)

• With a neon-lamp phase-sequence indicator (Figure 8.1a and Figure 8.1b)

• With the lamp method (Figure 8.1b)

When the SG no-load voltage sequence is 1–2–3 (clockwise), the neon lamp 1 will glow, while for the 1–3–2 sequence, the neon lamp 2 will glow. The test switch is open during these checks.

The apparatus works correctly if, when the test switch is closed, both lamps glow with the same intensity (Figure 8.1a).

FIGURE 8.1 Phase-sequence indicators: (a) independent (1–2–3 or 1–3–2) and (b) relative to power grid.

With four voltage transformers and four lamps (Figure 8.1b), the relative sequence of SG phases to power grid is checked. For direct voltage sequence, all four lamps brighten and dim simultaneously. For the opposite sequence, the two groups of lamps brighten and dim one after the other.

GENERATING STATION POWER REQUIREMENTS BASIC INFORMATION

Power requirements for unit auxiliaries at startup (or shutdown) are dependent on the size and type of unit and the degree of reliability. The design should include considerations for continuous load, short time and transient overload, and a margin for future growth.

Continuous Capability

The continuous capability should supply maximum power requirements of all electrical auxiliaries in any combination in which they may be used during plant operation from the station service system.

Where more than one unit is supplied from a common station service system, consideration should be given to the possibility that a unit in normal operation may trip from full load and transfer its shutdown auxiliaries to the common station service system during the startup of another unit's auxiliaries.

Instead of designing a system to supply auxiliaries requirements for two units, it may be acceptable to impose engineering or administrative load limitations. The system should also be able to react in the event that station load is suddenly lost.

Short-Time Overload Capability

Certain auxiliaries served by station service systems may be required, for limited periods, to carry loads greater than those existing at plant full load.

Examples are circulating water pumps filling the system before full syphon conditions are established, induced-draft fans handling cold air, motor-driven startup boiler feed pumps that will be replaced by turbine-driven pumps at a more advanced stage of startup, and electric boilers not used during full-load operation.

Affected equipment or components of the service system should be capable of carrying these overloads for appropriate periods without producing voltage profiles or temperature rises outside prudent limits.

While reviewing equipment load requirements that are greater during starting than at full-load operating conditions, equipment whose load requirements are lower during starting should also be noted.

A typical example would be the forced-draft fans of a unit, which operate substantially below nameplate rating during startup. Incorporating both types of load in a common portion of the system may reduce the overall supply requirements.

Transient Overload Capability

Station service systems should have the capability of supplying electrical auxiliaries during the starting or restarting of the largest load. When the largest load is a motor driving high-inertia loads, such as an induced-draft fan, the starting transient duration may be up to 60 s.

Voltage profiles during such transients are not required to satisfy criteria established for continuous operation or short-time overload, but they must be adequate to assure that no running motor will stall and no motor-starter contactor will drop out. Acceptable equipment operating voltage ranges should be confirmed with the manufacturer.

If load tap-changing (LTC) transformers are used in the power system, it should be assumed that these tap changers do not operate fast enough to apply significant correction during such transients. The system should be capable of supplying unit auxiliaries required during an automatic transfer of the buses from the normal to the startup/shutdown source.

Voltage profiles immediately following such transfers should meet the same criteria as those established for the motor-starting transient. Within approximately 60 s, the profiles should meet criteria established for continuous operation. If transformers with LTC are used, it should be assumed that significant correction is not applied until after the 60 s period.

Margin for Load Growth

Station service system design should allow for auxiliaries that may be added after the plant is placed in service. An acceptable margin for future loads should be included in transformer and feeder cable capacity and space should be provided for switchgear and motor control center additions.

Short-circuit withstand capability of switch-gear buses should be sufficient to avoid transformer voltage regulation problems and allow for future growth. This last margin generally will be created by increasing transformer impedance at the expense of voltage regulation. For that reason, it is often impractical to make large allowances for future load growth.

SEPARATE EXCITATION SYSTEM OF GENERATORS BASIC AND TUTORIALS

Separate exciter. The most common way of supplying dc current to the rotor winding without brushes and sliprings is shown in Fig. 5.17.

Fig. 5.17 Separate brushless exciter

The output of the AVR drives a dc current If through the pole windings of the exciter, which are mounted in a stator frame. The poles produce a stationary field which induces a voltage in the exciter rotor winding as it turns.

The ac voltage produced by the rotor winding of the exciter is converted to dc by a bridge rectifier which is also mounted on the rotor shaft.

The dc output of the rectifier is connected to the main rotor windings by conductors laid in a slot along the rotor shaft. The inductance of the main generator rotor coils is usually sufficient to smooth out the ripple in the bridge rectifier output.

The power supply to the AVR is either provided by a separate excitation winding in the main generator stator, or by a small permanent-magnet generator mounted on the shaft of the main generator, often referred to as a ‘pilot exciter’.

The advantage of the pilot exciter is that the generator has a source of power available once the shaft is turning; the voltage supplied to the AVR is completely independent of generator load and there is no reliance on residual flux in the magnetic circuit of the main generator to start the self-excitation process.

The pilot exciter also enables the generator to supply current to a connected network even when a short circuit occurs, enabling the high current to be detected by protection relays which will then disconnect the faulty circuit.

If the AVR is supplied from an excitation winding in the main generator stator, the supply voltage is very small when the stator windings experience a short circuit, and the AVR is unable to drive an adequate rotor excitation current.

One manufacturer uses two excitation windings to provide a voltage from the AVR under short-circuit conditions, so that sufficient current is supplied into the fault to trip the protection system.

During a short circuit the air gap flux density in these machines shows a pronounced harmonic component. This component induces voltage in coils of one of the excitation windings, which are short pitched and therefore deliver a voltage to the AVR even under short-circuit conditions.

The second excitation winding is fundamental-pitched and provides the major drive for the AVR under normal operating conditions. It is claimed that the performance of this system is comparable to a machine using a permanent-magnet exciter.

Another method used to provide voltage to the AVR under short-circuit conditions is a series transformer driven by the generator output current.

AUTOMATIC VOLTAGE REGULATOR (AVR) BASIC INFROMATION AND TUTORIALS

What is an Automatic Voltage Regulator (AVR)?

While some small generators have an inherent ability to produce a reasonably constant voltage as the load varies, it is clear from the previous explanations that some form of automatic voltage control is required in the usual form of generator. The Automatic Voltage Regulator (AVR) already referred to in the preceding sections is based on a closed-loop control principle.

The basis of this closed loop control is shown in Fig. 5.14. The output voltage is converted, usually through a transformer or resistor network, to a low voltage dc signal, and this feedback signal is subtracted from a fixed reference voltage to produce an error signal.

Fig. 5.14 Closed-loop voltage control

The error signal is processed by a compensator before being amplified to drive the rotor excitation current. The change in rotor excitation current produces a variation in output voltage, closing the control loop.

If the gain of the control loop is large enough then only a small error is required to produce the necessary change in excitation current, but a high gain can lead to instability in the circuit, with oscillations in the output voltage. The purpose of the compensating circuit is to enable small errors to be handled in a stable way.

The most common form of compensator is a PID circuit in which the error is amplified proportionately (P), integrated (I) and differentiated (D) in three parallel circuits before being added together. Many AVRs have adjustment potentiometers which allow the gains of each channel to be varied in order to achieve the best performance.

The integral term enables compensator output to be achieved at zero error, and this produces the minimum error in output voltage. Many AVRs are now offered with digital circuitry.

The principle of the feedback loop remains the same, but the feedback signal is converted to digital form using an analogue-to-digital converter. The calculations are performed digitally in a microprocessor and the output is on or off, using pulse width modulation (PWM) to vary the average level of dc supplied to the rotor excitation winding.

Alternatively, the phase angle of a thyristor bridge can be used to vary the output level; this is known as phase-angle control. The continuous improvement in power electronic controls and processor power is bringing further advances in voltage and speed control, with more flexible protection of the generator and its connected circuits.

This consists of a permanent-magnet generator driven by a variable-speed engine and feeding a power electronic frequency-changer circuit, which delivers output at constant frequency.

A microprocessor is used to control the switching of the output devices and to regulate the engine speed depending upon the load applied to the generator. At low power demand the engine speed is reduced to minimize noise, increase efficiency and extend life. The result is a saving in the volume and weight of the generator.

GENERATOR PARALLEL OPERATIONS OPERATING LIMITS BASIC INFORMATION

When a generator operates in parallel with a system, the power supplied by the generator is controlled by the torque control on the prime mover, for instance the rack of a diesel engine fuel injection pump.

The power factor is controlled by the field or excitation current of the generator. By increasing the field current the phase angle of the stator current progressively lags that of the terminal voltage, and conversely a reduction in field current results in the stator current phase angle leading that of the terminal voltage.

The power supplied by the generator will remain constant during this process if the prime mover torque is kept constant. The operator or automatic controller must be aware of the limits of the generator otherwise excessive current may occur in the field or stator windings.

To this end, an operating chart is often provided by the manufacturer (sometimes referred to as the capability chart). An example is given in Fig. 5.13.

The vertical axis represents output power and the horizontal axis represents output kVAr. The output voltage is shown as OV and the output current is represented by OB, shown here lagging the voltage by φ.

The field current is represented by AB. The output power is given by OD (or VI cos φ), and the output VAr is represented by OG (VI sin φ). The limits for generator operation are represented by the shaded area, where:

● BH represents the field current temperature limit

● KB represents the power limit of the prime mover

● KJ represents the practical limit of stability. Reducing field current reduces the peak torque capability of the generator, and the rotor load angle moves progressively towards the peak output angle.

Any fluctuations in voltage or load can then lead to pole slipping. Automatic relay protection of generators will produce an alarm if the generator operation comes outside this area.

LOADED GENERATOR OPERATION BASIC INFORMATION AND TUTORIALS

In a three phase circuit currents flow in each phase. If an inductive or capacitive load is connected, then the current waveforms will respectively ‘lag’ or ‘lead’ the voltage waveforms by 90°.

GENERATOR NO LOAD OPERATION BASIC INFORMATION AND TUTORIALS

The basic operation of all these generator types can be explained using two simple rules, the first for magnetic circuits and the second for the voltage induced in a conductor when subjected to a varying magnetic field.

The means of producing a magnetic field using a current in an electric circuit have shown that the flux Φ in a magnetic circuit which has a reluctance Rm is the result of a magneto-motive force (mmf ) Fm, which itself is the result of a current I flowing in a coil of N turns.

Φ = Fm/Rm and Fm = IN

The main magnetic and electrical parts of a salient-pole generator are shown in Fig. 5.4. In Fig. 5.4(a), dc current is supplied to the rotor coils through brushes and sliprings.

The product of the rotor or field current I and the coil turns N results in mmf Fm as in eqn 5.2, and this acts on the reluctance of the magnetic circuit to produce a magnetic flux, the path of which is shown by the broken lines in Fig. 5.4(b).

As the rotor turns, the flux pattern created by the mmf Fm turns with it; this is illustrated by the second plot of magnetic flux in Fig. 5.4(b). When a magnetic flux Φ passes through a magnetic circuit with a cross section A, the resulting flux density B is given by B = Φ/A Figure 5.4(a) also shows a stator with a single coil with an axial length l.

As the rotor turns, its magnetic flux crosses this stator coil with a velocity v, an electromotive force (emf ) V will be generated, where V = Bvl (5.4)

The direction of the voltage is given by Fleming’s right-hand rule, as shown in Fig. 2.6. Figure 5.4(b) shows that as the magnetic field rotates, the flux density at the stator coil changes. When the pole face is next to the coil, the air gap flux density B is at its highest, and B falls to zero when the pole is 90° away from the coil.

The induced emf or voltage V therefore varies with time (Fig. 5.5) in the same pattern as the flux density varies around the rotor periphery. The waveform is repeated for each revolution of the rotor; if the rotor speed is 3000 rpm (or 50 rev/s) then the voltage will pass through 50 cycles/second (or 50 Hz).

This is the way in which the frequency of the electricity supply from the generator is established. The case shown in Fig. 5.4 is a 2-pole rotor, but if a 4-pole rotor were run at 1500 rpm, although the speed is lower, the number of voltage alternations within a revolution is doubled, and a frequency of 50 Hz would also result.

The general rule relating the synchronous speed ns (rpm), number of poles p and the generated frequency f (Hz) is given by f = nsp/120 The simple voltage output shown in Fig. 5.5 could be delivered to the point of use (the ‘load’) with a pair of wires as a single-phase supply.

If more coils are added to the stator as shown in Fig. 5.4(a) and if these are equally spaced, then a three-phase output as shown in Fig. 5.6 can be generated. The three phases are conventionally labelled ‘U’, ‘V’ and ‘W’. The positive voltage peaks occur equally spaced, one-third of a cycle apart from each other.

The three coils either supply three separate loads, as shown in Fig. 5.7(a) for three electric heating elements, or more usually they are arranged in either ‘star’ or ‘delta’ arrangement in a conventional three-phase circuit (Fig. 5.7(b)).

In a practical generator the stator windings are embedded in slots, the induced voltage remaining the same as if the winding is in the gap as shown in Fig. 5.4(b). Also, in a practical machine there will be more than the six slots shown in Fig. 5.6(a). This is arranged by splitting the simple coils shown into several subcoils which occupy separate slots, each phase still being connected together to form a continuous winding. Figures 5.1 and 5.2 show the resulting complexity in a complete stator winding.

SOURCES OF THE ELECTRIC ENERGY—GENERATION BASIC AND TUTORIALS

There are a number of ways to produce electricity, the most common commercial way being the use of a synchronous generator driven by a rotating turbine. The combination is called a turbine-generator.

The most common types of turbine generators are those where a fossil fuel is burned in a boiler to produce heat to convert water to steam which drives a turbine.The turbine is attached (coupled) to the rotating shaft (armature or rotor) of a synchronous generator where the rotational energy is transformed to electrical energy.

In addition to the use of fossil fuels to produce the heat required to change the water to steam, there are turbine generators which rely on the fission of nuclear fuel to produce the heat. Other types of synchronous generators are those where the turbines are driven by moving water (hydro turbines) and gas turbines which are turned by the exhaust of a fuel burned in a chamber containing compressed air.

For each type system, there are many variations incorporated in the power plant in order to improve the efficiency of the process.Hybrid systems are also in use; an example is a combined cycle system where the exhaust heat from a gas turbine is used to help provide heat for a steam driven turbine.

Typically, more than one these generating facilities were built at the same site to take advantage of common infrastructure facilities, that is, fuel-delivery systems, water sources and convenient points to connect to the delivery system.

A small, but not insignificant, segment of the electric generation in the country includes technologies that are considered more environmentally benign than traditional sources; that is, geothermal, wind, solar, biomass. In many of these technologies dc power is produced and use is made of inverters to change the dc to the alternating current (ac) needed for transmission and use.

Generators are selected, sized and built to supply different parts of the daily customer load cycle. One type generator might be designed to operate continuously at a fixed level for the entire day. This is a base loaded generator.

Another generator might be designed to run for a short period at times of peak customer demand.This is a peaking generator.Others might be designed for intermittent type service.

One important aspect of the selection of a particular generator is the trade off between its installed cost and its operating costs. Base loaded generators have much higher installed costs per unit of capacity than peaking generators but much better efficiency and lower operating costs.

Included in this decision is the availability and projected cost of fuel. Prior utility practice has been to have enough generation available to meet the forecast customer seasonal peak demand plus an adequate reserve margin.

Reserve margins were determined by conducting probability studies considering, among other things, the reliability of the existing generation and potential future loads. Systems that were mainly hydro generation based had lower reserve margins (~12%) than systems that had nuclear, coal, or oil fired generation (~16–24%). The availability of aid from neighboring systems during shortages also had a large impact on the required reserve.