02 excitation basics general a1(notes pages)

UNITROL® 6000 Service and Commissioning Training

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Fundamentals of Excitation Systems

Chapter 2

ABB Switzerland AG

Learning CenterPower ElectronicsTurgi, Switzerland


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2. Fundamentals of Excitation Systems

Content:

What is an Excitation System?

Synchronous Machine Operation Modes and Characteristics

Basic components of the excitation system

Closed loop control and features


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2.1 What is an Excitation System

North

South

Current

Rotor The rotor of a synchronous machine is an electromagnet.

The effect of the rotating flux on the stator windings

produces an induced voltage.

The principle of voltage generationThe production of voltage in the synchronous machine is based on the induction low. This means if the flux changes in the stator winding of the synchronous machine there will be a voltage induced.

The flux is produced by the current supplied from the excitation system to the rotor winding. The change of flux in the stator winding is caused by the movement of the rotor. This induces the voltage in the stator winding as illustrated in the figure below:

L1

L2

L3

U

Stator

Rotor

Rotor winding

Stator winding


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Excitation System

Current Control

Voltage Regulation Voltage

Rotor Current Production

Power Supply

In any excitation system, several components can be identified. Depending on the age and type of the system, the equipment may vary greatly, however the basic components can still be classified.

Rotor Current ProductionThe rotor of the machine must be supplied with a current. For example this could be by:A large power electronic converter (direct), or a small current supply feeding an excitation machine, which in turn produces the large rotor current. (indirect system).

Power SupplyThe excitation system needs a power supply in order to produce a current. There are many different configurations.Shunt Supply – The supply is taken from the machine terminals.Line Supply – The supply is taken from an auxiliary supply.Permanent Magnet Generator – A small permanent magnet generator is mounted on the same shaft as the main machine.

Current ControlNo matter how the current is produced, there must be some method of controlling how much current is produced. In the case of a state of the art control system the rotor current is controlled by semi conductive rectifiers.

Voltage RegulationVoltage regulation is done in the control system by the Automatic Voltage Regulator (AVR). The voltage regulator various the rotor current automatically in order to maintain the terminal voltage of the synchronous machine even in case of load change.


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STEP UP TRANSFORMER

GENERATORBREAKER

1GOVERNOR

1

AC & DCAUXILIARY SYSTEMS

LV SWITCHGEAR

AUX.TRANSF.

EXCITATIONSYSTEM

PT’s&CT’s

STARPOINT

CUBICLE

CONTROL SYSTEMS

CONTROL ROOM

SYNCHRONIZING

HV SYSTEM HV- BREAKER

PROTECTION

EXCITATIONTRANSFORMER

SYNCHRONOUSGENERATORTURBINE


The Excitation System in the Power Plant

The picture above shows the connections to the excitation system in a power plant. The excitation system is usually located close to the synchronous machine. The main power supply for the production of the rotor current is taken from the generator terminals and fed via the excitation transformer to the excitation system.The output of the excitation system supplies the direct current via slip rings to the rotor winding. The terminal voltage and machine current is measured by means of Potential transformers PT’s and current transformer CT’s. These signals are used to control the generator voltage and reactive power.The excitation system is operated by the operators in the control room via the control interface as illustrated .


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Primary Mechanical Electrical ConsumerEnergy Energy Energy

Turbine Generator

Excitation System

GeneratorVoltage

FieldCurrent

Chain of energy conversion Chain of energy conversion

2.2 The Synchronous Machine

The excitation system in the chain of energy conversion

The primary energy in the form of water, fossil fuels, wind etc. is transformed by the turbine into mechanical energy. This energy is then transformed with the aid of the generator into electrical energy, which is then fed to the consumers.The generators used today are mostly so-called synchronous machines which, in addition to converting mechanical energy into electrical energy, also allow the network voltage to be generated and regulated. The influencing of the generator voltage and the resulting reactive power flow to the network is achieved through the magnetisation, or excitation as it is also called, of the synchronous machine. For this purpose, a direct current is fed into the so-called exciter winding in order to generate a magnetic field. For this reason, this current is also referred to as the field current. The exciter winding is embedded in the rotating part of the synchronous machine, the rotor. Thus, in order to increase the generator voltage, the magnetisation or the excitation current must be increased. In order to regulate the generator voltage, a voltage regulator is therefore used which forms part of the excitation system.


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Disturbance

If Ug

Controlled Object

SynchronousMachine

Network

ExcitationSystem

The closed loop regulating circuit

The closed loop regulating circuit of the synchronous machine can be represented for the electrical variables as shown in the figure. The output voltage UG of the synchronous machine is picked up by the voltage regulator of the excitation system and compared with the setpoint. The output of the excitation system in the form of the excitation current If is the input to the synchronous machine, which closes the regulating circuit.For a synchronous machine coupled to an electrical network, the network simply acts as a disturbance value. Disturbances in the network such as the shutting down of large consumers or short circuits influence the generator voltage in an undesirable way. It is the function of the excitation system to balance out these undesirable changes immediately and to operate the machine stably on the network.


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The solid pole synchronous machineThe solid pole synchronous machine

High speed application for speed range > 1500 rpm

Rotor Stator

The synchronous machine

The synchronous machine essentially consists of two parts: the rotating part, the rotor, and the static part, the stator.In order to cover the wide range of rotational speeds of possible turbines, two different types of synchronous machine are available.

The solid pole machine (Turbogenerators)In thermal turbines, rotational speeds >1500 rpm are usually required. In this case, so-called solid pole machines, as shown in the diagram, are used. The full pole machine is also referred to as a turbogenerator.

The salient pole machineIn river-driven power stations, Kaplan turbines are usually used which have low rotational speeds of < 1500 rpm . In these cases, so-called salient pole machines are used, as shown in the following diagram. In contrast to the full pole machine, in these machines the diameter of the rotor is very large and the length short.

The full pole and salient pole machines basically function in the same way. They only differ, in some cases, in their behaviour under load.


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The salient pole synchronous machineThe salient pole synchronous machine

Slow speed application for speed range < 1500 rpm

Rotor

Stator

The figure shows a typical salient pole machine with an output of 120 MVA

A distinctive feature is the very large diameter of the rotor, which can exceed 20 m in very large machines.


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2.2 The Synchronous MachineSynchronous machine triphase representation

URIR

US

IS

UT

IT

UfIf

IDR

IDT

IDS

Rotor

Stator

120° 120°

120°

3-phase representation of the synchronous machine

The diagram shows the synchronous machine with the three phases. Each phase is displaced physically by 120° and, viewed in terms of electrical values, essentially consists of two reactances, the main reactance and the secondary reactance formed by the damper winding. Both reactances are associated with ohmic resistances, which are not of importance in considering the excitation system. A further reactance is found in the rotor winding with the associated winding resistance.


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2.2 The Synchronous Machined-q axes representation

UdId

Uf If

IdD

Uq

Iq

IQ2

IQ1

Ψd

ΨfΨq

ΨdD

ΨQ1 ΨQ2

D axis

Q axis

δω

ra

ra

rdD

rf

rQ1 rQ2

Rotor

Stator

D-Q axis representation of the synchronous machine

The D-Q axis representation is used to explain the behaviour of the synchronous machine. The 3-phase system can be transformed into a “single-phase” representation by means of a mathematical operation. The mathematical operation will not be discussed here.In order to explain the behaviour of the synchronous machine, the two resulting axes, the quadrature axis (Q-axis) and the direct axis (D-axis), are given different impedances and reactances, together with the associated resistances, which are given the corresponding index q or d. These impedance values can be found in the detailed data sheets provided by the manufacturer of the synchronous machine.

The meaning of the individual reactances will not be examined here. Rather, we will carry out a substitution of the different reactances in order to explain the behaviour of the synchronous machine.


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The simplified equivalent circuit for the synchronous machine

E

Xfσ Xaσ

XmRotor Stator

P

Xd,q

EP

Synchronous Reactance

Uf

Rotor Stator

If

ω

UG

q-axis

d-axis

Fig. a Fig. b Fig. c

The equivalent circuit diagram for the synchronous machine

In order to explain the behaviour of the synchronous machine in stationary operation, we simplify the complex structure of the synchronous machine. Taking into consideration the q-axis and d-axis, one can represent the synchronous machine as bipolar, see Fig.a. The rotor with the field winding is fed from the excitation system. The excitation current generates a magnetic field which induces a voltage in the stator winding through the rotation of the rotor, according to the induction principle. This voltage can be measured at the output terminals of the generator when the machine is in no-load operation.This physical interpretation of the way the synchronous machine functions can be represented as the equivalent circuit diagram Fig. b) with the main reactance Xm and the control reactances Xfσ and Xaσ as shown in the diagram. The voltage source Ep stands for the voltage induced in the stator windings which is determined by the excitation current and the rotational speed of the machine. Ep is also referred to as EMF (electromotive force) or air gap voltage . The structure of this equivalent circuit diagram is also used for transformers. In fact, the synchronous machine acts like a transformer with an air gap. The reactances shown in Fig. b) can be further condensed and transferred to the very simple equivalent circuit diagram Fig. c). This equivalent circuit diagram is sufficient to describe the stationary behaviour of the synchronous machine. Essentially, it simply consists of the “internal“ voltage source and an “internal resistance“ which essentially appears in the form of a reactance, the so-called synchronous reactance Xd or Xq. The synchronous reactance has a great influence on the electrical behaviour of the machine. The value in the direct axis Xd and in the quadrature axis Xq are almost equally in solid pole machines. In salient pole machines, is Xd > Xq.


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2.2 The Synchronous MachineWhat values can you find on the name plate of your synchronous machine?

Physical values of your machine AbsoluteValue

Per unitvalue

Unit

Link to data sheet of real SM


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Generator no load characteristic

Ug

IfIfo

UGn

Saturation

No load field current

Generatornominal voltage

Xd

EpUG

Speed n = constant

If ,n

The operating behaviour of the synchronous machine

Generator no load characteristicsStarting out from the simple equivalent circuit diagram, the generator terminal voltage in no-load operation is essentially determined by the excitation current If and the rotational speed n. In considering excitation, one can assume that the machine rotates at nominal speed. This means the induced voltage Ep is only determined by the excitation current. The relationship between excitation current and generator voltage can be seen from the graphic. If one starts to slowly increase the excitation current, the generator voltage increases in proportion with the excitation current. An important point here is the excitation current required in order to reach the generator nominal voltage. This current is called the no-load field current Ifoand is one of the important characteristic values of the synchronous machine.


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2.2 The Synchronous MachineGenerator short circuit characteristic

I g

IfIfo

I G

No load field current

Generatorcurrent at Ifo

Xd

Ep UG = 0

Speed n = constant

If ,n

Ep = UGn

UGn

For If = Ifo ⇒ Xd = IGn/IG IG at (If = Ifo)Example:Measurement at If = Ifo: IGn/IG = 2.43

⇒ Xd = 2.43 pu

Generator short circuit characteristicsFor the short circuit test the machine terminals must be short circuited. Be aware that the machine current can go up the its nominal value. While the machine is running at rated speed the field current will be slowly increased. At the same time the machine current must be read in order to gain the short circuit characteristic of the synchronous machine. The ratio between IG(If

=Ifo)/IGn determines the synchronous reactance Xd of the machine, where Ifo is the no load field current and IG the measured machine current at no load field current.

Where: Xd Sychronous reactance direct axisIG Machine current at no load field currentIGn Machine rated current

Gn

Gd I

IX =


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Generator on load

ϕ

δ

ΔU = Xd Ig

UgEp ~If

Xd

Ep

UG =

con

st.

Load

IG

ΔU = IG • Xd

Fig. bFig. a

Ep ϕIG

Load angle

Generator on load

If a load is applied to the machine which has been excited in no-load operation, the output voltage Ug drops, because the load current, via the synchronous reactance, results in a voltage drop ΔU. This voltage drop is considerable at machine nominal current. In order to ensure that the generator voltage is also kept stable under load, the voltage drop must be compensated by increasing the excitation current. This compensation takes place automatically if voltage regulators are used. The generator voltage is thereby kept stable through adjustment of the excitation current. This is one of the fundamental functions of the excitation systemIn order to find the excitation current required for a specific load point, a vector diagram (Fig. b) can be drawn for the simple equivalent circuit diagram. Here, the generator voltage UG is left constant and the voltage drop ΔU is drawn in. For a purely ohmic load, this voltage drop ΔU is perpendicular to the load current IG and is applied to the generator voltage. The resulting voltage of the two vectors UG and ΔU in turn represent the induced voltage Ep, which is proportional to the excitation current.This means that a relationship has been found between the excitation current and the generator load current. If one imagines the machine current IG to be reduced to 0, then Ep and UG match. The length of the Ep vector is known to be a measure for the excitation current, which for IG = 0 corresponds to the no load field current, which is determine from the no-load characteristic. In this way, the necessary excitation can be determine for any load point. The broken lines show the vector diagram for inductive load. According to this, the excitation must be increased in order to compensate the voltage drop.


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2.3 Operation of the Synchronous Machine

The power chart of the synchronous machine

ActivePower

Reactivepower

Motor

GeneratorOperation

P

+ Q- Q 1xd

-1 +1

1 pu

overexcited

underexcited

STurbine Power

ϕ

P(Ep)~If

δ

1

The Power Chart of the synchronous machineThe vector diagram for the synchronous machine was shown with voltage and current vectors. In practice, power vectors tend to be used in order to assess the operating behaviour of the synchronous machine. For this purpose, we can draw the power diagram with the two power axes: the active power axis and the reactive power axis. The nominal apparent power (1 pu) of the synchronous machine thereby appears as a circle. The active or reactive power can thereby assume both positive and negative values. Negative active power means, for example, motor operation. The power vector diagram is obtained from the voltage vector diagram as follows:- All values are expressed in so-called Per Unit (pu) values. For example, thegenerator nominal voltage is 1 pu, the generator nominal power is 1pu etc.

- To obtain the power values from the voltage values, one multiplies the voltagevectors by the value UG/Xd according to Ohm‘s law. This gives us the power vectors.For example, the vector ΔU = IG • Xd becomes the power vector S = IG • Ug . Thepower vector S thus corresponds to the apparent power of the synchronous machine.One can proceed analogously with the other voltage vectors.

The power vectors can be entered in the so-called power diagram with the active power axis and the reactive power axis as shown in the figure above. If the synchronous reactance is expressed in per units, the 1/Xd point is the starting point for the air gap power P(Ep), which for UG=1pu is still proportional to the field current If.The operating point (1) represented in the above diagram only lies in the active power axis, i.e. only active power is output. If the synchronous machine is coupled to the electrical network and the excitation current is increased, reactive power is output into the network in addition to the active power. In this case, the machine operates within the overexcited range. Another important variable is the so-called load angle. This angle also actually occurs as a mechanical angle between the magnetic rotary field generated by the stator windings and the magnetic field generated by the rotor winding. As soon as the machine takes up active power, this angle increases. If active power is present, this angle is also influenced by the excitation current. If, for example, the machine is de-excited, the load angle becomes greater. The question arises here as to how great this angle may become for the machine to still rotate synchronously with the rotary field of the stator.


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The synchronizing torque

δ=45

ωel

ωmech

ωel

ωmech

δ=90

ωel

ωmech

δ=0

ω

"rubber band"

T95_0154.DRW

o o o

FDrive

Fsyn

r

Fig. a) Fig. b) Fig. c)

Fig. d)

P = T• ωT = F• r

Some equations:

P Active powerT TorqueF Forcer Rotor radius ω Speed

The synchronizing torque

The load angle is the mechanical angle between the magnetic rotary field generated by the stator windings and the magnetic field generated by the rotor winding. When the machine is synchronised to the network and not under load, the load angle δ=0° (see Fig a). As soon as the machine takes up active power, this angle increases. If active power is present, this angle is also influenced by the excitation current. If, for example, the machine is de-excited, the load angle becomes greater. The question arises here as to how great this angle may become under active load for the machine to still rotate synchronously with the rotary field of the stator. If one considers the torques acting within the machine, this is easy to understand. As shown in Fig. b) in generator operation, a drive torque or force Fdrive is generated through the drive power of the turbine which drives the rotor. In order to prevent the rotor from accelerating, a countertorque, the so-called synchronising torque or counterforce Fsyn is necessary. This force is generated by the magnetic fluxes, which in turn are influenced by the excitation. One can imagine this magnetic force as acting like a rubber band, always causing the rotor to rotate stable in synchronism with the rotary field. (Fig. d).If the load angle becomes greater through an increase in the power, the driving force also becomes greater and the rubber band is stretched further. The maximum synchronising torque which can be generated by the forces in the “rubber band“, is at a load angle of 90°. (Fig. c).


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Stability limitMd

δ

Μ d2

Md1

~

~

I

I

f2

f1Drive torque

δ δ 12

The torque characteristic of the generator

δδ sinsin ⋅⋅

=⋅⋅=d

GpGpd X

UEIEM

The torque equation

The torque characteristic of the synchronous machine

The diagram shows the curve of the synchronising torque as a function of the load angle.The maximum torque is achieved at a load angle of 90°, whereby the excitation current determines the value of the maximum. The greater the excitation current, the greater the magnetic flux and thus the synchronisingforce Fsyn in the machine. At a particular active power and excitation current, a particular load angle δ2results. If the excitation current is reduced with the active power of the machine remaining the same, the load angle increases to the value δ1.


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ActivePower

Reactivepower

Motor

GeneratorOperation

P

+ Q- Q 1xd

-1 +1

1 pu

overexcited

underexcited

safe operatingarea

The safe operating area of the synchronous machine

Drive Limit

Sn~Ifn

ϕ

Rated Power

δmax= 90°

Stability Limit

Field CurrentLimiter

The safe operating area of the synchronous machine

If the machine is operated at the nominal operating point, an excitation current is present which we call the nominal excitation current Ifn. The rotor windings and the power units of the excitation system are designed for this current, because it must be possible to operate permanently at this point. In order to prevent the rotor or the excitation from being overloaded, an excitation current limiter is used which is implemented in the excitation system. The result of this is that the operating range is limited within the overexcited range.In the active power axis, the operating range is limited by the maximum turbine ouput, which usually lies between 80% and 90% of the output of the synchronous machine.Within the underexcited range, the operating range is limited by the machine current or by the stability limit of the synchronous machine. The theoretical stability limit is reached at a load angle of δ=90°. This means that the safe operating range of the synchronous machine is determined by the turbine and the two limiters in the overexcited and underexcited range. Why the stability limit is reached at a load angle of 90° will be explained in greater detail in the following.


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P [MW]

-Q 1/Xd1/Xq

The power chart of the synchronous machine with limiters

FieldField CurrentCurrent LimiterLimiter

Minimum Minimum FieldField CurrentCurrent LimiterLimiter

StatorStator CurrentCurrent LimiterLimiter

UnderUnder excitationexcitation, P/Q , P/Q LimiterLimiter

Save operating area

The Power Chart of the synchronous machine with Limiters

The diagram shows the limitations of the operating ranges implemented in the excitation system using the example of a salient pole machine with Xd > Xq.It should be mentioned that the circle lying between 1/Xd and 1/Xq also exists with a solid pole machine, but is very small since in such a machine Xd ≈ Xq.In salient pole machines, Xd is usually significantly smaller than in solid pole machines and a wider operating range can therefore be used within the underexcited range.


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The V-curves of the synchronous machine

The V-Curves of the Synchronous Machine

The diagram shows the so-called V-Curves of a 280 MVA turbo generator. This is an other representation of the synchronous machine under load conditions. It shows the required field current versus machine current with the power factor.


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2.4 The Network

The Network

12

3

T95_0157.DRW

Tie

Regional grid

Substation

Power station

The Network

UNet

Infinite bus voltageExternal reactance

Xe

The electrical network

The electrical network is often of a very complex nature, and the question arises as to how the network can be evaluated by an observer at the power station. In order to evaluate the network characteristics for stationary operation, it is again appropriate to find an equivalent circuit diagram. Since the network is usually supplied from different sources or other power stations, the equivalent circuit diagram consists of a voltage source and a network impedance connected in series, analogously to the synchronous machine.The voltage source Unet represents the total of all generators participating in the network, which possesses an enormously high short-circuit power. This in turn means that the network voltage can be assumed to be completely fixed. The voltage source is also assigned the network reactance Xe, also called the external reactance. This represents the reactances from transmission lines and consumers present within the network. The ohmic resistances are not relevant in terms of the excitation system.


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2.4 The Network

Substitution diagram of the network with the generator

RLUNet

GXT Xe

Consumer

Infinite bus voltage

Transformer reactance External reactance

Networkvoltage

Substitution diagram of the network with the generator

If one represents the generator and the network in the form of a simplified equivalent circuit diagram, then the generator is followed by the short-circuit reactance XT of the high-voltage transformer. The consumers which consume the active power both from the generator and from the network are located on the high-voltage side. The external reactance Xe represents the reactanceswhich are present in the transmission lines. In contrast to the transformer reactance, this can change over time. These reactances increase during the night and on Sundays, when energy consumption is low.


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2.4 The Network

Calculation of reactive power

RL UNet

GXTr Xe

UG = 1.05 pu

= 0.1 pu = 0.2 pu

Consumers

UN = 1.0 pu

IQReactive Power

IQ = ?.............Q = ?............

Example of reactive power flow

This example is intended to show the variables on which the reactive power flow is dependent. For this purpose, we assume that the network voltage is 1.pu, the external reactances, as a typical value, 0.2 pu and the transformer reactance 0.1 pu, with the generator voltage being 1.05 pu. All pu values relate to the nominal power of the generator G.The resulting reactive power which is output to or drawn from the network is calculated using these values.

IQ = (UG – UN) / (XT + Xe) = (1.05 – 1) / (0.1 + 0.2) = 0.05 / 0.3 = 0.17 pu

Q = UG • IQ = 1.05 • 0.16 = 0.17 pu

For a 100 MVA machine, this means a reactive power output of 17 Mvar. If the generator voltage UG < UN then reactive power is taken up.


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2.5 Reactive Power DistributionGenerator operates to the common busbar

AVR

Generator 1

AVR

Generator 2

Common BusbarGrid

Uref1 Uref2

IQ

Q, IQ

U

Busbar voltageUref1

Uref2

IQ IQ

Generator 2 Generator 1

Reactive power distribution with two generators on the same bus bar

There are arrangements in which two generators are coupled directly to a bus bar.If the voltages of the generators are regulated by two different voltage regulators, then the voltage regulators must be given a reactive current influence. This influence is also referred to as the “droop“ characteristic or static of the voltage regulator. If this were not the case, then in the event of marginal deviations between the two voltage regulator setpoints, an uncontrolled reactive power flow would flow from the generator with the higher voltage to the generator with the lower voltage. To prevent this, the reactive current influence on the voltage regulator with the higher voltage must act in such a way that the machine voltage is reduced or that of the generator with the lower voltage increased. The example above shows the effect of the so-called reactive current influences on the two voltage regulators. The voltage of generator 1 is too high, which leads to a rise in the reactive current. As a result of the reactive current influence, the voltage decreases with increasing reactive current. In contrast to generator 1, the voltage in generator 2 increases with increasing negative reactive current. The two generator voltages match and a particular reactive current is maintained. This means the regulating circuit remains stable and the reactive current can be kept under control.


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2.5 Reactive Power Distribution

AVR

Q, IQ

U

Generator 1

Grid (HV) Generator connected to the step up transformer

ΔU

Uref

Uref

UG

UG

ΔU

Reactive power distribution if the generator is connected to the grid system

If the generator is connected to the electrical network via a transformer, then the reactive power flow is inherently stable due to the transformer reactance. The voltage regulator need not display a “droop“ characteristic or negative static, since the transformer reactance absorbs any voltage difference between the network voltage and the generator terminal voltage and limits the reactive current.The significant voltage drop ΔU=XT•IQ through the transformer reactance has a disturbing effect if the generator voltage is supposed to support the network voltage in the event of network disturbances. In such arrangements, the reactive power influence on the voltage regulators is therefore used in such a way that the voltage is partially compensated and thus displays a positive droop.With positive droop, also referred to as compensation, the generator voltage is increased with increasing reactive current into the network. This behaviourprovides the generator with a network-voltage-supporting function in the event of network disturbances.


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2.5 Reactive Power Distribution

UG

pos. static

neg. static

+Q-Q

Static behavior of AVR (Reactive power influence to AVR)


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2.6 Transient behaviour of the synchronous machine

Transient behavior of the synchronous machine

Behavior of Generator voltage in case of reactive power surgewith constant field current

S

UgX E

If = konst.

t

Ug

Ugo

t = 0

ΔU = Ig *Xd

Td’’ Td’ Tdo’

Td’’ Sub transient time constant 10…50msTd’ Transient time constant 0.5…1.5 sTdo’ Time constant

ΔU”=Xd”•IQ

Transient behavior of the synchronous machine

So far, we have only examined the stationary behaviour of the synchronous machine. In the following, we wish to examine the transient behaviour of the synchronous machine in connection with simple switching procedures and network disturbances.

The diagram shows the behaviour of the generator terminal voltage when the generator, in no-load operation, is connected to an inductive load. At the time t=0, the voltage drops, with the time constant Td“, to a value determined by the subtransient reactance Xd“ and reactive current IQ. After the elapse of the time constant Td“, the time constant Td‘ with the transient reactance Xd‘ becomes effective, then changing into the stationary condition caused by the synchronous reactance.


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Faults and surges for the generator

UNet~

G

XT XE1

AVR

XE2S

Load

infinite bus

1) Reactive power surge

2) Active power surge

3) Load rejection

4) Long distance short circuit

5) Short circuit at generator terminal

High voltage lineGenerator

Xd’

4)

3)

1, 2)5)

Faults and surges for the generator

In terms of control engineering all changes coming from outside are called disturbances. There is a wide scale of growing influence up to the severe disturbances of normal operation due to faults and surges such as:

1) Reactive power surge

2) Active power surge

3) Load rejection

4) Long distance short circuit

5) Short circuit at generator terminals

The following slides show the behaviour of the machine due to the faults above.


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Behavior of generator voltage in case of reactive power surge

with rotating exciter

t

Ug

Ugo

t = 0

ΔU = Ig *Xd

static excitation systems

Manual mode

Behavior of the generator voltage in case of reactive power surge

The behaviour of the generator voltage depends greatly on the type of excitation.

The diagram shows the voltage curve for three cases:• The voltage for a static excitation system in which the excitation current is

fed directly into the rotor winding via slip rings. In this system, the voltagerecovers again after a few 100ms.

• The voltage for an indirect excitation system with exciter machines. Thefield of the exciter machine delays reactions to changes from the voltageregulator and more time is required in order to bring the voltage back to itsoriginal value.

• The voltage for unregulated operation, i.e. with constant excitation current,as is the case in Manual mode.


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δ

UE

I · XDP = ω· M

P = U · I = U ·

Torque Equation

M - M = Θ

A

E

A

X D

E · sin δ

A Edωdt

θ Inertiaω speed

ω

PA U

Con

sum

er

PE

Active power surgewith power oscillations

Active power surge

If the load on the synchronous machine is changed through connection of an additional load, then the electrical active power changes suddenly. However, the mechanical drive power initially remains unchanged, due to its system inertia, until the drive system has adjusted itself to the changed power demand.Neglecting the power losses of the machine, the difference between the mechanical and electrical power results in an acceleration of the rotor, as can be seen from the torque equation. As a result of the acceleration, the load angle changes until the new stable operating point is attained. During this procedure, oscillations in the active power occur, as can be seen in the diagram of a simulation.


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Overvoltage relay

IQ x Xd "

t = 0 1 Sec.

t

Ug

Uo

with AVR (static excitation system)with constant field current

Generator voltage in case of reactive load rejection

Load rejection

By opening of the main circuit breaker of the machine the load will be dropped off immediately. The early invention of the automatic voltage regulator is certainly caused by the consequences of this event. It is also an important quality mark for a voltage regulator how the generator voltage varies with the time after the breaker has opened. The drop of the reactive load current to zero inevitably causes an immediate voltage rise ΔU=Ireactive • Xd”. If for instance the subtransient reactance Xd’’=0.2 p.u. the rejection of 0.5 p.u. reactive current gives an instantaneous rise of 10%. If the load on the synchronous machine is changed through connection of an additional load, then the electrical active power changes suddenly, which can not be reduced by any control action.Without AVR the voltage then rises further till the maximum value is reached defined by the synchronous reactance. The time delay corresponds to the no –load time constant Tdo’. With an AVR this further rise is more or less completely eliminated and the voltage is brought back to the initial value. How quickly this is achieved depends on whether or not the additional time constant of an exciter machine has to be overcome. Without a voltage regulator the over voltage relay of the generator protection would be activated and deexcite the generator.


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1 sec

UO

with constant field current

with voltage regulatorUG

t = 0t

Generator voltage in case of long distance short circuit

Long distance short circuit

In case of a short circuit in the grid system away from the power plant the voltage will drop immediately. The voltage regulator tries to keep the machine voltage on its setpoint. After a certain time the fault in the grid will be cleared by the line protection and the system voltage will recover. This leads to an overshoot of the machine voltage. The voltage regulator will reduce the voltage to normal again.


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2.7 Definition of Excitation Systems

Duties of the Excitation SystemMaintain the generator terminal voltage

Operate the synchronous machine within its operating limits

Prevent the synchronous machine from being in asynchronous mode

Fast response in case of network disturbances

Share reactive power with other synchronous machines connected in parallel

Stabilize power oscillations

High reliability

Duties of the Excitation System

The diagram shows all the important main duties of the excitation system, as they have been explained in the preceding diagrams.


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2.7 Definition of Excitation SystemsGlossary and Definitions (IEEE STD. 421.2)

Ifo No load field or excitation currentRequired field current to achieve 100% generator terminal voltage at rated speed

Ifn Nominal field or excitation currentRequired field current to operate the synchronous machine at rated power

Icl Ceiling field currentMaximum field current that excitation system is able to supply from its terminals for a specific time

Ufo No load field voltageRequired field voltage to obtain the no load field current considering the field resistance

Ufn Nominal field voltageRequired field voltage to obtain the rated field current considering the field resistance

Ufcl Ceiling field voltageRequired field voltage to obtain the ceiling field current

KPl Excitation Ceiling factorCeiling field voltage divided by no load field voltage Ufcl/Ufo

δ Load anglePhysical angle between rotor field and stator field

Glossary and Definitions

The diagrams show the most important abbreviations and definitions of physical values in connection with excitation, as defined in the IEEE STD. 421.2 standards.


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2.7 Definition of Excitation SystemsGlossary and Definitions cont…

ϕ Phase angleElectrical angle between machine voltage and machine current

cosϕ Power factorRatio of machine’s active power to apparent power

Xd Machine synchronous reactance in direct axeXq Machine synchronous reactance in quadrature axeRs System nominal response

The rate of increase of the excitation system output voltage divided by the nominal field voltage

Tv Excitation system voltage response timeThe time in second for the excitation voltage to attain 95% of the difference between ceiling field voltage and nominal field voltage


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3.1 Excitation System: Supply Modes

Excitation Systems „State of the Art“

~=

SM E

1 to 200 A

Rotating ExciterBrushless Excitation System

100 to 10000 A

Static Excitation System

~SM =

"State of the art" excitation systems

Generally speaking, two basic configurations of excitation systems are used nowadays.

Indirect excitation system (brushless excitation system)This excitation system basically consists of a voltage regulator with power unit, the alternating current machine and the rotary diodes for converting the alternating current generated by the exciter machine into the direct current required by the main machine.The voltage regulator output therefore first controls the field current of the exciter machine. In this machine, the field winding is in the stator. The 3-phase alternating current windings in which an AC voltage is induced through the rotation of the rotor lie on the rotor. This AC voltage is converted by means of the diodes which are rotating on the shaft. The direct current is fed, without slip rings, directly into the exciter winding of the main machine. No brushes are therefore necessary, for which reason this type of excitation system is called “brushless excitation”.

Direct excitation system (static excitation system)The static excitation system essentially consists of the voltage regulator, the power unit, a switch and the brushes with slip rings. The power supply to the excitation system is usually taken directly from the generator terminals and transformed in the power unit by means of thyristors into a direct current which is fed via a switch and slip rings to the rotor winding of the main machine. These systems are distinguished by very fast regulating performance.


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• Just positive ceiling voltagecapability

• Exciter response limited by theexciter machine time constant(>200ms)

• Field discharge with natural timeconstant

• Supply from PMG possible providing supporting of shortcircuit currents

• Relative large size of exciter machine for low speed generators

• No sliprings (less maintenanceand dust)

• Positive and negative ceiling voltagecapabilities

• Fast response (<20 ms) in both directions

• Fast field discharge by discharge resistoror inverter operation

• Size of excitation of transformer dependson field requirements only

• Shorter shaft (torsional oscillations)

• Maintenance on power rectifier themachine must not be at standstill

• Direct measurements of field quantitiesUf, If possible

Comparison: Indirect - Static Excitation System

Brushless excitation Static Excitation System

Comparison

The diagram shows a comparison of the most important advantages and disadvantages of both systems. It cannot be said straight away which is the “better” system. The most suitable system has to be determined from case to case.


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~=

SM ~

~=

SM ~

~=

SM =

DC Exciter AC Exciter withstationary diodes

AC Exciter with rotatingdiodes“Brushless”

Main types of rotating exciters

Other types of rotating exciters

The diagram shows other types of excitation systems which were used in the past but which are no longer in use nowadays. These systems have mostly been replaced with brushless or static excitation systems.


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3.1 Excitation System: Supply ModesMain supply modes

MS

Supply taken from machine terminals ( shunt supply )

SM

MS

Series Compounding System

SM- +

+ -

MSSM

Vectors Compounding System

Main supply modes

We make a distinction between several supply modes:

Shunt supply from the machine terminalsThe supply for the excitation system is taken directly from the terminals of the synchronous machine. This is used above all in static excitation systems and machines which are not operated in island network operation.

Shunt supply with compoundingIn the event of network disturbance, the network voltage can drop so that, where shunt supply is used, the supply to the excitation system suffers and thus can no longer cover the excitation requirement. This is especially undesirable in the case of island network operation. So-called compounding is therefore used. With compounding, the machine current is transformed into a voltage, thus supporting the excitation and covering the necessary excitation requirement.Two types of compounding are thereby used:

- series compounding- vector compounding

In series compounding, the transformed voltage is rectified by means of an additional rectifier and fed to the excitation system in series with the exciter voltage.In vector compounding, the transformed AC voltage is fed as AC voltage to the excitation transformer voltage and vectorially added, thus covering the excitation requirement.


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MS

Supplied from a permanent magnet generator (PMG)or from a pilot exciter

SM

~

Main supply modes (cont.)

MS

Supplied from a safe auxiliary supply

SM

Auxiliary supply

Main supply modes (cont)

Supply from Permanent Magnet Generator (PMG)Permanent Magnet Generators or externally-excited small generators on the same shaft are also used for indirect exciters. This supply is independent of the electrical grid and is distinguished by its high level of availability.

Supply from auxiliary network This supply is not often used, as it is difficult to guarantee a reliable supply. In many cases, these auxiliary networks are also supported by no-break power supply systems.


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High voltage line

~=

SM

Unit step up transformer

Excitation transformer

Sensing PT

Power Converter

Synchronous machine

AVR

Design Example of Static Excitation System

Sn = 210 MVAUn = 15.75 kVCos ϕn = 0.85fn = 50 HzIfn = 1600 AUfn = 230 VIfo = 400 AXd = 2.1

Aux. Supply

The static excitation system

This type of excitation system is often used for hydrogenerators and large turbogenerators larger than about 50 MVA with exceptions to clients requirements. The power for the excitation system is taken from the generator terminals. The automatic voltage regulator works through a semiconductor output stage, which is mostly a thyristor converter or an integrated gate bipolar transistor (IGBT) stage.

The voltage regulator with the power converter and excitation transformer as well as the field circuit breaker complete the number of the main components of a static excitation system.


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3.2 Excitation System: Basic ConfigurationsBasic Configurations

UG

T96_0005.DRW

IG

If

Uc

Uc

Uc

=Uc

Uc

Uc

Single channel (AUTO and MAN mode)

Power supply

FCR

AVR

Power supply I

AVR

Dual AUTO channel(Each channel with AUTO and MAN mode)

AVR

FCR

FCR

AVR = Autom. Voltage Reg.FCR = Field Current Reg.

Power supply II

Channel configurations in excitation systems

If there is only one chain of actions, e.g. the single channel: voltage and field current control - gate control set - pulse amplifier - fully controlled thyristorconverter, any failure in any one of the chain members will lead to a disturbance of the total function. We call this a single channel system, which has no redundancy.

This cheapest solution may be quite sufficient for small machines and if there are for instance 10 generators in one power station. The number of generating sets itself forms the redundancy in this case. An additional field current regulator permits manual control, if the control amplifier of the AVR or the potential transformer has failed.

But a complete second control chain is necessary, if a stand-by for any kind of failure in the working channel is needed. Such dual channel equipment is used rather frequently. There are various design variants and options which are dealt with in detail in the next pages.


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3.2 Excitation System: Basic Configurations

~=

SM =

Voltage set point

A

M

==

Supply

~

UNITROL AVR Single Channel System with integrated manual facilityfor indirect Excitation Systems

Field currentsetpoint

Manualmode

follow up

Autom.mode

Main functions of a single-channel system

Nowadays, single-channel or dual-channel systems are used. The diagram shows the most important functions assigned to a channel. A single channel features

- Automatic mode (AUTO)- Manual mode (MAN)

In Automatic mode, the voltage regulator with actual value reading and setpoint formation is active. The output signal controls the power unit, which can take the form of a converter with thyristors or power transistors (IGBTs). In this mode, the limiter functions which protect the machine against excessive loads are also active. In addition to the actual voltage regulator function, reactive power- or power factor-regulators are also available which can be switched on and off.If the actual value of the voltage regulator falls, this is detected and switchover to manual mode takes place automatically.

In manual mode, the actual value is formed from the measurement of the excitation current and passed with the setpoint to the excitation current regulator. The output from this regulator is passed to a switch by means of which the corresponding mode can be selected. This mode is only used for test purposes and as an emergency regulator in the event of failure of the voltage regulator. The limiter functions are not active in this mode.


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3.2 Excitation System: Basic ConfigurationsTrue-Dual Channel UNITROL AVR with 2 x Automatic & Manual modesfor Indirect Excitation Systems

SM =

Supply

~

Voltagesetpoint

A

M

==


Follow-up

~=

VOltagesetpoint

A

M

==


follow-up

~=

Autom.Mode

ManualMode

ManualMode

Autom.Mode

Channel I

Channel II

True dual-channel system

The so-called dual-channel system increases the availability of the excitation system significantly. The dual-channel system is equipped with two identical channels. Each channel includes the regulator functions present in a single-channel system, as described above.If a channel fails, the system switches over automatically to the other channel. Only one channel is in operation at one time (active channel), the other channel is in standby position (passive channel) and is continually matched to the active channel so that a smooth switchover is possible at any time. The operating personnel can select which channel is the active channel. There is no preference as to whether channel I or channel ll is the active channel.


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~=

VoltageSetpoint

A

M

==

Supply

SM


ManualMode

Follow-up

Autom.Mode

Single Channel Excitation System for Static Excitation System

System variants for static excitation systems with higher field currents

For higher power outputs it is neither economic nor technically sound to double the thyristor power stage. A solution with variable ac transformer stays out of consideration. Instead the converter is built redundant. Details will be explained later.

For this kind of equipment the electronic control channels can be designed as a single channel or double channel.

Single channel for static excitation systemsThe control signal within the automatic operating mode is supplied by the voltage control amplifier. Within the manual mode the signal comes from a closed loop field current control. Both out put signals are forwarded to the change-over switch to select from Auto to Manual operating mode. The control signal is fed to the pulse generation which generates the firing pulses for the thyristor stage. An automatic follow up functions is balancing the output of the non active regulator to the active one.


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3.2 Excitation System: Basic ConfigurationsVoltagesetpoint

A

M

==


follow-up

Voltage Setpoint

A

M

==


Follow-up

~=

Autom.Mode

ManualMode

ManualMode

Autom.Mode

Channel I

Channel II

Pulse busto converter

SM

~=

True- Dual Channel configurationfor Static Excitation

Pulse busto converter

Double channel configuration for static excitation systemsFor this kind of equipment the electronic control channels are doubled. Mainly the Dual-Channel Standard system according to figure above is employed.

Within this configuration each channel provides a automatic voltage regulator and a field current regulator where by the two channels are identical.The control signal within the channel I is supplied by the voltage control amplifier. Within the manual channel the signal comes from a closed loop field current control. Pulse generation and intermediate pulse amplification are identical for both channels. The change-over takes place purely electronically by releasing the pulses of the working channel and blocking those of the stand-by channel.

Naturally each channel is equipped with its own power pack to generate the required auxiliary voltages. A final pulse stage corresponds to each thyristorpower stage.

Channel balancing and automatic follow-up ensure smooth change over either from Auto to Manual mode or from one channel to the other.

For dual channel systems with automatic change-over to the stand-by channel it is very important to have an almost complete detection of internal failures in the static excitation.


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M

Gate controlunit

Pulse amplifiers

1 converter

Supply

To the field circuit ofthe machine

Supply

M

M

Gate controlunit Channel1

Pulse amplifiers


Gate controlunit Channel2

Pulse amplifiers

Power Converter ConfigurationsEconomy Configuration(Single Channel)

Twin Configuration(Double Channel)

Power converter configurationsThe converter lay-out is defined by the excitation needed for the synchronous machine and the corresponding redundancy requirements.The following standardized designs are available:

Simple converter configurationFor systems with a low excitation current demand (i.e. a single thyristorconverter is sufficient) or if there are no redundancy requirements, the combination of a single channel AVR and integrated pulse amplification with a single thyristor output stage is fully sufficient as shown in the figure above.

Redundant converter configurationFor excitation systems with large output currents or higher availability requirements the following two designs are available:

Redundancy concept (1+1): There are two identical converters connected in parallel of which only one is in operation at a time. By alternatively blocking and releasing the firing pulses to the corresponding converter switch-over is effected in case of failure.


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-50

-

M

Supply


Gate controlunit

Pulse amplifiers

TWIN Configuration(Dual channel system without converter redundancy)

Gate controlunit

Pulse amplifiers

Channel2

Channel1


Converter without redundancy

The size of the converter is at least the rated field current of the synchronous machine.

This means that if there is any failure in the converter, then the excitation system must generate a trip.

Example Failures may be for example:Fan failure.Thyristor Failure.Electronic PCB Failure (e.g. CIN board).Converter Over Temperature.Converter Current Measurement Failure.Snubber Fuse.


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3.2 Excitation System: Basic ConfigurationsParallel bridges with n-1 redundant configuration

Supply

To the field circuit

M

M

Final pulsestages

M

Gate controlunit ofchannel I

M

1

2

3

nPulsebus

Gate controlunit ofchannel II

Channel1

Channel2

Redundant converter configuration

With even higher output currents where the parallel connection of several thyristors is necessary the reliability of the converter is secured by theredundancy concept (n-1). This means that one more parallel converter than necessary is provided.

The two channels work through gate control set and intermediate pulse stage on a common pulse busbar. The different redundancy concept (1+1) and (n-1) is chosen because of selectivity reasons of the thyristor fuses. If two converters would operate in parallel, and if a thyristor looses its blocking capability, then a short circuit current starts to flow when the next thyristorsare fired. In this case the two new fired thyristors drive a short circuit current into the defective one. And as a consequence we have a series connection of two thyristor fuses in parallel with one thyristor fuse (of the defective thyristor). This arrangement does not assure that the single fuse will blow first. Therefore the converters must be changed over from one to the other. With more than 2 converters in parallel this selectivity is assured.


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3.2 Excitation System: Basic ConfigurationsPrinciple of Operation of the Thyristor Bridge

Thyristor symbol in circuit theory. Complete functional

thyristor circuit.

Construction of thyristor module.

Pulse coupler

Snub

ber c

ircui

t

Redundant converter configuration

With even higher output currents where the parallel connection of several thyristors is necessary the reliability of the converter is secured by theredundancy concept (n-1). This means that one more parallel converter than necessary is provided.

The two channels work through gate control set and intermediate pulse stage on a common pulse busbar. The different redundancy concept (1+1) and (n-1) is chosen because of selectivity reasons of the thyristor fuses. If two converters would operate in parallel, and if a thyristor looses its blocking capability, then a short circuit current starts to flow when the next thyristorsare fired. In this case the two new fired thyristors drive a short circuit current into the defective one. And as a consequence we have a series connection of two thyristor fuses in parallel with one thyristor fuse (of the defective thyristor). This arrangement does not assure that the single fuse will blow first. Therefore the converters must be changed over from one to the other. With more than 2 converters in parallel this selectivity is assured.


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5.1 Basics of Rectifiers

L3

1

L1

L2

2

5

6

3

4

Id

Ud

IV

3 phase rectifier

1

4

3

2

1

66

5

time


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5.1 Basics of RectifiersBasics of Rectifiers

Alpha=0°

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

Theta [°]

Output

TH1

TH2

TH3

TH4

TH5

TH6


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Alpha=30°

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

Theta [°]

Output

TH1

TH2

TH3

TH4

TH5

TH6


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Alpha=60°

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

Theta [°]

Output

TH1

TH2

TH3

TH4

TH5

TH6


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Alpha=90°

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

Theta [°]

Output

TH1

TH2

TH3

TH4

TH5

TH6


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Alpha=120°

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

Theta [°]

Output

TH1

TH2

TH3

TH4

TH5

TH6


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Alpha=150°

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

Theta [°]

Output

TH1

TH2

TH3

TH4

TH5

TH6


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Alpha=180°

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

Theta [°]

Output

TH1

TH2

TH3

TH4

TH5

TH6


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5.1 Basics of Rectifiers...for an inductive load

-1.5

-1

-0.5

0

0.5

1

1.5

0 30 60 90 120 150 180

ALPHA [ ° ]

Udalpha OUTPUT VOLTAGE

rectifier operation

inverter operation

Ud alpha = Udi0 x cos(alpha)

= 1,35 x Uv x cos(alpha)

IAC = 0.817 x IDC


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3.3 Excitation System: Field FlashingField flashing feature

AVR

Generator

Auxiliaryvoltage

~ +

DiodeBridge

Thyristorbridge

Usyn

U>40%

Field flashing breaker

Ug

t

Field flashing off

Thyristor bridgestarts to conduct

U>40%

U>10%Field flashing characteristic

Field flashing failedFCB Trip

10sField flashing OFF

5s

100%Softstart

Ug

• Order Fieldbreaker CLOSE• Order Excitation ON• Pulses to the thyristors are released • Field Flashing breaker closes if residual machine voltage is too low

• Stator voltage raises• Field flashing breaker opens• The softstart function raises the generator voltage smoothly up to its nominal value.

Sequence:

Field flashing feature

If the excitation system is supplied by shunt supply, i.e. directly from the generator terminals, then the residual voltage of the generator is sometimes not sufficient to build up the voltage. In such cases, when the excitation is switched on the excitation current is built up with the aid of field flashing. The field flashing consists of a diode bridge and a switch which connects an external auxiliary voltage to the field. It is dimensioned in such a way that the generator voltage is built up to approx. 20%. Once the generator voltage reaches approx. 30-40% of the nominal value, this switch is switched off again.The generator voltage is then built up to nominal value by the main converter. The slow build-up is thereby controlled by means of a softstart ramp implemented in the voltage regulator.


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3.4 Excitation System: Field SuppressionField Suppression Circuit (Crowbar)

5 6 3 4

7 8

LfRf

RE

If (field suppression)

If (operation)

inverter (WR)

+

- +

-+

- +

--Lf.dIf/dt

WR

Q02

Uarc

Uarc

UarcU

disc

harg

e

Components of field suppression equipmentThe main elements of a field discharge circuit are the field breaker with discharge contact or DC breaker with electronic discharge circuit, the discharge resistor and the overvoltage protection. In addition there is a certain amount of control means.

Field breaker (field discharge contactor), DC breakerGenerally the field breaker has to interrupt a direct current in a circuit with high inductance. Due to the inductive load the change of current depends on the discharge voltage which is defined by the arc voltage of the breaker.Field breakers are designed specially for this duty. They are equipped with arc chambers and electromagnetic quenching.Modern field breakers are equipped with limiting means such as auxiliary arc gaps, limiting resistors and the important distribution of the grown arc into a row of partial chambers. The result is a much quieter and more constant arc voltage. The most important criterion of a breaker is its interrupting capability. It is determined by several factors.• maximum arc voltage• maximum interrupted current• maximum arc energy.


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3.4 Excitation System: Field SuppressionStatic Field Suppression Circuit (Crowbar) with Firing units

1011

K1

K2

K3

+

T T

-R02

I>

BOD

-V334

12

567

1 3

2 4

Firing PCB

Fielddischarge I

Fielddischarge II

Freewheeling

F iel

dw

i ndi

ng

+

DCbreaker

_

V1 positive overvoltagethyristor

V2 discharge and negative overvoltagethyristor

V3 redundant discharge or free wheeling thyristor

CROWBARDC Breaker

-V2

-V1

Current Measurement

Discharge resistor

Crowbar design

Overvoltage protection (Crowbar)

The voltages occur during the deexcitation process should with a safety margin always remain below the insulation level of the winding, i.e. below the test voltage, whereby a reduction caused by aging has to be taken in account. The same voltages also appear at the converter output and stress the blocking capability of the thyristors. In addition with salient pole machines inverse induced voltages (back emf) occur during asynchronous operation, that is after falling out of step. Since an inverse current can not pass the thyristors, such voltages rise fast to high amplitudes.

Therefore practically without exception a fast acting overvoltage protection is provided, which discharges the field winding over a resistor, if a well defined voltage level is exceeded. Mostly the normal field discharge resistor is also used for this purpose. As sensing devices special pre-selected avalanche diodes (BOD) are used.

Current sensors are used to detect current in the field discharge circuit. This criteria is used to supervise the discharge circuit.


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3.4 Excitation System: Field Suppression

If, Uf[p.u.]

tt=0

With non-linear resistor

Methods for field suppressionThere are several kinds of deexcitation circuits which are partly only of historical interest. Some of them will be described in short below for the better understanding of the whole matter.Basically a field suppression circuit must accelerate the current decrease in the field winding. If we just reduce the voltage of the feeding source to zero, the current will decrease in accordance with the well-known exponential function and with the natural time constant T = L/Rf of the field circuit.By insertion of a discharge resistor in series with the field, e.g. by opening the switch Q02, the effective time constant of the circuit is reduced.We want a fast decrease of the flux. It is important to realize that we can force the flux decrease in the direct axis only. The time constants in the quadratureaxis cannot be influenced at all.If we connect a suppression resistor RE equal to the field resistance Rf in series, the effective time constants at no-load Tdo’ and with load Td’ are reduced to half the natural value. The time constant of the core TA and the one of the quadrature axis Tqo remain unchanged. The quicker the field decrease in the direct axis is achieved, for instance with the help of a non-linear suppression resistor, the more significant delayed field decrease in the quadrature axis becomes.This is a passive field suppression method where part of the magnetic energy stored in the field w = ½L• If2 is converted into heat in the discharge resistor. The supply voltage UG must be reduced quickly to zero. Otherwise the field current does not come down to zero and the resistor RE is overloaded. The arrangement is simple and uses a normal dc-breaker.


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3.4 Excitation System: Field Suppression

Invertermode

Field Suppression from no Load Condition

Field breakeropens

Field suppression resistorsA part of the field energy is converted to heat within the field suppression resistor. The size is determined by the heat storing capacity. Another important feature is rigidity of conductor, terminals and resistor itself to withstand the dynamic forces of peak currents. We distinguish linear and nonlinear (voltage dependent) resistors for field suppression.For the field of rotating exciters and for generators up to appr. 20 MVA linear resistors are employed. The field current decreases according to the well-known exponential function. For generator-fields the maximum initial value of the field current is given by the 3-phase stator terminal short circuit. The field voltage decreases proportional to the current. Its maximum value Ufc = Ifc• REmust be smaller than the insulation test voltage. On the other hand the field breaker can commutate the current onto the suppression resistor only, if its arcing voltage is higher than the sum of maximum field voltage Ufc and ceiling voltage Up of the excitation source. The energy to be stored is given by the following integral

WE = RE ∫i2(t) dt

For more than a rough approximation the calculation is rather time consuming, so that the use of a small computer program is worthwhile.To speed up to the field suppression for larger machines, voltage dependent resistors made of silicium-carbide are used. With falling voltage this material shows a marked rise of its specific resistance. The initial resistance value is made equal to the one of a linear suppression resistor, being limited by the admissible voltage. But this voltage is now lowering slowly owing to the increasing resistance, while the current is quickly reduced. The effective time constant becomes itself a function of the momentary voltage.


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3.5 Excitation System: Design

UNITROL 6000UNITROL 5000 UNITROL PUNITROL D (first Digital)

UNS2110

UNS3214

UNITROL F (Digital)UNITROL M (Analog)

SYSTEMS WITH ROTATINGEXCITERS

STATIC EXCITATION SYSTEMS

SYST

EM C

OM

PLEX

ITY

GENERATOR / EXCITATION SYSTEM RATINGS

Application Ranges of UNITROL Excitation Systems

UN1000

Application rangesThe figure above shows the type of ABB’s Unitrol excitation and AVR systems used for the wide range of synchronous machines.

UNS2110/UNS3214This AVR system is used for smaller machines up to approx. 50 MW in conjunction with indirect excitation systems. The equipment is made of analog electronic using integrated circuits.

UNITROL 1000This is an automatic voltage regulator of the latest design for synchronous generators and synchronous motors. The use of the most advanced microprocessor technology together with IGBT semiconductor technology allows it to be used in a wide area of application

UNITROL F / UNITROL MUNITROL F provides a comprehensive range of Automatic Voltage Regulators and Static Excitation Systems for high performance control of all kind of synchronous machines. UNITROL F uses microprocessor technology and replaced UNITROL M which is made of analog technology.

UNITROL 5000 / UNITROL P / UNITROL DUNITROL 5000 is used for high scale static excitation system. Its excellent performance is able to cope with all requirements in the field of excitation systems. UNITROL 5000 replaced UNITROL P which was based on the PSR Technology. In turn the UNITROL P replaced UNITROL D the first digital voltage regulator.


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3.5 Excitation System: DesignMilestones of Excitation System1908 First mechanical rolling sector regulator

1940 Oil-pressure regulator KC is added to the mechanical regulator

1950 Magnetic amplifier combined with diode bridge has been introduced

1956 The KC regulator is supplemented by load angle limiter

1957 First static excitation system with electronic AVR (Baureihe) accomplished by mercury-arc rectifier

1965 Mercury-arc rectifier replaced by thyristor converter

1967 Brushless excitation system with rotating diodes

1969 Improvement of stability by electronic power system stabilizer (PSS)

1975 UNITROL® C analog AVR-electronic replaces Baureihe-electronic

1976 UNITROL® S2210/3214 AVR with low complexity has been developed for small synchronous machines.

1977 Static excitation system for positive and negative excitation current applied to rotating synchronous compensator

1983 UNITROL® M with high integrated analog circuits applied in indirect excitation systems and direct excitation system of low complexity

1989 UNITROL® D introduces the microprocessor technologyand replaces the UNITROL® C analog technology.

1993 Second generation of numeric voltage regulator of type UNITROL® P as a free programmable regulator replaces UNITROL® D

1995 A new microprocessor based voltage regulator of type UNITROL® F replaces the old analog regulator UNITROL®M. This voltage regulator is applied for indirect excitation system and direct excitation system up to medium sized synchronous machines

1999 UNITROL® 5000 excitation system has been introduced. It combines the capacities of UNITROL® F and UNITROL® P in one high integrated system.

2001 UNITROL® 1000 microprocessor based AVR System replaces UNITROL® S2210/S3214.

2007 UNITROL® 6000 high integrated excitation system. It utilizes the reactive power control for all type and size of synchronous machines. It replaces the former UNITROL® 5000 and UNITROL® F excitation system.

02 excitation basics general a1(notes pages)

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