TRANSIENT REQUIREMENTS OF EXCITATION SYSTEM BASIC INFORMATION

Ceiling Current

Low ceiling voltage exciters, normally less than 150% of rated value, can usually be allowed to attain their ultimate ceiling current. Where high ceiling voltages are employed for improved transient performance, the ceiling current, if unrestricted, may reach high values and require excessive exciter capacity.

An inclusion of a field current limiter should be considered to limit the ceiling current to a specified value. Ceiling voltage would then still be available to force the rapid change in current.

The ceiling current of the excitation system should have a transient time capability equal to or greater than the short time overload capability of the synchronous machine to which it is connected. ANSI C50.13-1989 [3] and ANSI C50.14-1977 [4] give the field winding short-time thermal overload requirements.

Note that these overloads are based on the voltage (rather than current) applied to the field windings. Presently there is no corresponding requirement in ANSI C50.12-1982 [2] for salient pole machines.

Ceiling Voltage

The ceiling voltage of an excitation system is normally not specified directly but is a function of the excitation system's nominal response requirement. This is one area where it is easy to specify conflicting requirements, and the specification writer is cautioned to be sure that some other reference to ceiling voltage does not conflict with the response requirement.

The response should be specified by the user and the selection of the ceiling voltage left up to the manufacturer. For systems that obtain their energy from an ac source, the per unit voltage and (if applicable) current values of this source at which the nominal response requirement shall be met should be specified.

Present standards base the rating of an exciter on its continuous output parameters and its time response to transient change. It is understood that the equipment must function in the transient mode and achieve ceiling output conditions without any detrimental effects.

The ratio between ceiling and normal operation voltages will increase as higher nominal response systems are specified. For certain special cases, a negative ceiling voltage may be required to control machine overvoltage conditions. Due to firing angle margin requirements of thyristor exciters, the negative ceiling is normally specified to be not more than

Fault and Pole-Slipping Duties

The excitation system must withstand, without damage, any faults or abnormal operation of the synchronous machine. Faults on the synchronous machine ac terminals will induce large positive currents into the field (adding to the normal field current).

In addition, the induced current will have an ac component at the power frequency. This is important when rectifier exciters supplied at the power frequency are involved since the peak current occurs at the same point each cycle and tends to overload one phase of the rectifier.

The magnitude and time duration of this induced current is a function of machine and system reactances. Refer to IEEE C37.18-1979 (ANSI) [5] for a table of suggested values of induced currents for various types of machine construction.

In addition to the positive induced field current under faults, there can be negatively induced currents (subtracting from the normal field current). These negative currents can be induced into the field circuit during pole-slipping events.

When the negative induced current is so large that the total current becomes negative, and if the negative current is not allowed to flow, then the resulting voltage may become excessive. Excitation systems that employ solid-state rectifiers normally conduct current only in the positive direction.

Some machines are inherently self-protecting due to additional current paths in the rotor. These may be damper windings or a solid steel structure. In machines where there is a possibility of large voltages, protective equipment may be supplied to protect both the exciter and machine field circuit.

While the magnitude of the induced negative field current is a function of the machine design, the time the current flows is a function of the number of pole-slipping cycles, the system operating procedures, and the protective relay settings involved.

The maximum time that any potentially damaging negative field current will flow should be specified in order to ensure there is sufficient energy capacity in any protective equipment.

EXCITATION SYSTEM VOLTAGE AND CURRENT SPECIFICATION TUTORIALS

Rated Current

The continuous current rating should be specified to be equal to or larger than the maximum required by the synchronous machine field under any allowed continuous operating conditions. Note that some machines have a continuous overload rating.

In addition, ANSI C50.12-1982 [2], ANSI C50.13-1989 [3], and ANSI C50.14-1977 [4] allow all machines to operate at rated MVA and within ± 5% of rated terminal voltage. Some machines may require an even wider operating range.

The need for off-frequency operation must also be considered in establishing the rating. Some machines, such as combustion-turbine-drive units, have a variable rating depending on ambient air temperature.

The excitation system for these machines may require a variable rating based on ambient air temperature. In the past, some excitation systems have had a small continuous negative current rating. This was utilized when the machine was operated as a synchronous condenser and the negative field current allowed a slightly greater transmission line charging capability.

The negative current was relatively easily supplied from commutator-type exciters, with little additional complexity. Modern exciters employing solid-state rectifiers do not normally have inherent capability for negative currents.

It could be obtained with great complexity, which cannot justify the associated minimal performance improvement (except for synchronous condensers). Some modern exciters will generate a transient negative voltage to force the decay of field current toward zero.

This should not be confused with the concept of a continuous negative current rating, which is mentioned above. For exciters that are specified with redundant current paths or cooling elements, the continuous rating should apply with the redundant parts out of service. The exciter efficiency and losses should be measured at the rated current and voltage point with all redundant parts in service, since this is the normal operating mode.

Rated Voltage

The continuous voltage rating of a system should be such that the voltage is sufficient to supply the necessary continuous current to the synchronous machine field, with the field at its maximum temperature under rated load conditions.

In addition, the continuous voltage capability should allow operation of the synchronous machine at rated MVA and within ±5% of rated terminal voltage unless otherwise specified.

In determining the required voltage for the continuous as well as the transient ratings mentioned later, all voltage drops, including interconnecting bus or cable run voltage drop, up to the field winding terminals should be considered. Any brush drop voltage should be considered part of the synchronous machine field circuit.

CAPACITOR EXCITATION SYSTEM OF GENERATORS BASIC AND TUTORIALS

Although some generators are still produced with brushes and sliprings, most now have a brushless excitation system. One of the main techniques for synchronous generators is capacitor excitation and this is described in the following sections.

The use of this technique is usually restricted to single phase generators with a rated output less than 10 kW. A separate excitation winding in the stator has a capacitor connected directly across its output as shown in Fig. 5.19.

 Fig. 5.19 Capacitor excitation

The rotor is usually of salient-pole construction as described previously, but in this case the rotor winding is shorted through a diode. On starting, the residual flux in the rotor body induces a small voltage in the stator excitation winding and a current flows through the capacitor.

This current produces two waves of magnetic flux around the air gap of the generator. One wave travels in the same direction as the rotor, to create the armature reaction.

The second wave travels in a direction opposite to the rotor, and induces a voltage in the rotor windings at twice the output frequency. The current circulated in the rotor windings by this induced voltage is rectified by the diode to produce a dc current.

This dc current increases the magnetic flux in the machine, which in turn drives more current through the stator excitation winding, which in turn produces more rotor current. This self-excitation process continues until the flux reaches a point at which the magnetic circuit is saturated, and a stable voltage results.

The process also produces an inherent AVR action, since any load current in the output stator winding induces more rotor current to offset the armature reaction effect.

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.

EXCITATION SYSTEM CEILING VOLTAGE OF SYNCHRONOUS GENERATORS

Fast control of excitation current is needed to preserve SG transient stability and control its voltage. Higher ceiling excitation voltage, corroborated with low electrical time constants in the excitation system, provides for fast excitation current control.

Today’s ceiling voltages are in the range of 1.6 to 3.0 P.U. There is a limit here dictated by the effect of magnetic saturation, which makes ceiling voltages above 1.6 to 2.0 P.U. hardly practical.

This is more so as higher ceiling voltage means sizing the insulation system of the exciter or the rating of the static exciter voltage for maximum ceiling voltage at notably larger exciter costs.

The debate over which is best — the alternating current (AC) brushless exciter or static exciter (which is specified also with a negative ceiling voltage of –1.2 to 1.5 P.U.) is still not over.

A response time of 50 msec in “producing” the maximum ceiling voltage is today fulfilled by the AC brushless exciters, but faster response times are feasible with static exciters.

However, during system faults, the AC brushless exciter is not notably disturbed, as it draws its input from the kinetic energy of the turbine-generator unit.

In contrast, the static exciter is fed from the exciter transformer which is connected, in general, at SG terminals, and seldom to a fully independent power source.

Consequently, during faults, when the generator terminal voltage decreases, to secure fast, undisturbed excitation current response, a higher voltage ceiling ratio is required.

Also, existing static exciters transmit all power through the brush slipring mechanical system, with all the limitations and maintenance incumbent problems.

EXCITATION SYSTEM OF HYDRO POWER GENERATOR BASIC INFORMATION

The excitation system fulfills two main functions:

1. It produces DC voltage (and power) to force current to flow in the field windings of the generator. There is a direct relationship between the generator terminal voltage and the quantity of current flowing in the field windings.

2. It provides a means for regulating the terminal voltage of the generator to match a desired set point and to provide damping for power system oscillations.

Prior to the 1960s, generators were generally provided with rotating exciters that fed the generator field through a slip ring arrangement, a rotating pilot exciter feeding the main exciter field, and a regulator controlling the pilot exciter output.

Since the 1960s, the most common arrangement is thyristor bridge rectifiers fed from a transformer connected to the generator terminals, referred to as a “potential source controlled rectifier high initial response exciter” or “bus-fed static exciter” (IEEE, 421.1-1986; 421.2-1990; 421.4-1990; 421.5-1992).

Another system used for smaller high-speed units is a brushless exciter with a rotating AC generator and rotating rectifiers.

Modern static exciters have the advantage of providing extremely fast response times and high field ceiling voltages for forcing rapid changes in the generator terminal voltage during system faults. This is necessary to overcome the inherent large time constant in the response between terminal voltage and field voltage (referred to as T do , typically in the range of 5 to 10 sec).

Rapid terminal voltage forcing is necessary to maintain transient stability of the power system during and immediately after system faults.

Power system stabilizers are also applied to static exciters to cause the generator terminal voltage to vary in phase with the speed deviations of the machine, for damping power system dynamic oscillations.

Various auxiliary devices are applied to the static exciter to allow remote setting of the generator voltage and to limit the field current within rotor thermal and under excited limits. Field flashing equipment is provided to build up generator terminal voltage during starting to the point at which the thyristors can begin gating. Power  for field flashing is provided either from the station battery or alternating current station service.