CONCLUSIONES

Addr. Setting Title Setting Options Default Setting Comments

2101 REF PROT. OFF

ON

Block relay for trip com-mands

OFF Restricted Earth Fault Protection

2102 REF I> BLOCK 1.0..2.5 I/InO 1.5 I/InO REF Pickup of Phase Current Blocking

2103 REF U0>RELEASE 1.0..100.0 V; 0 5.0 V REF Pickup of U0> Release

2110 I-REF> 0.05..2.00 I/InO 0.10 I/InO I-REF> Pickup

2112 T I-REF> 0.00..60.00 sec;∞ 0.00 sec T I-REF> Time Delay

2113A SLOPE 0.00..0.95 0.25 Slope of Charac. I-REF> =

f(I0-Rest)

2114A BASE POINT 0.00..2.00 I/InO 0.00 I/InO Base Point for Slope of Charac-teristic

F.No. Alarm Comments

05803 >BLOCK REF >BLOCK restricted earth fault prot.

05811 REF OFF Restricted earth fault is switched OFF

05812 REF BLOCKED Restricted earth fault is BLOCKED

05813 REF ACTIVE Restricted earth fault is ACTIVE

05840 REF I> blocked REF is blocked by phase current 05841 REF U0> releas. REF Release by U0>

05817 REF picked up REF protection picked up

05821 REF TRIP REF protection TRIP

05836 REF Adap.fact. REF: adverse Adaption factor CT

05847 I0-Diff: I0-Diff at REF-Trip

05848 I0-Res: I0-Restraint at REF-Trip

2.14 Underexcitation (Loss-of-Field) Protection (ANSI 40)

General The underexcitation or loss of field protection protects a synchronous generator/motor from asynchronous operation in the event of a malfunction in the excitation system and from local overheating of the rotor. Furthermore, it ensures that the network sta-bility is not endangered due to the underexcitation of large synchronous generators.

2.14.1 Functional Description

Underexcitation Determination

In order to detect underexcitation, the unit processes all three terminal phase currents and all three terminal voltages to form the stator circuit criterion. It also processes the excitation voltage and/or the signal from an external excitation voltage monitor to form the rotor circuit criterion.

For the stator circuit criterion, the reciprocal of the impedance (equal the admittance) is calculated from the positive sequence system of the currents and voltages. In the admittance plane, the stability limit of the machine is independent of the voltage: thus, the protection characteristic can be optimally matched to the stability characteristic of the machine. By evaluating the positive sequence system, underexcitation conditions are reliably detected even during asymmetrical faults within or outside of the machine.

Characteristics Figure 2-51 shows the loading diagram of the synchronous generator in the admit-tance plane (P/U²; Q/–U²) with the steady-state stability limit which intersects the re-active axis in close proximity to 1/X_d(reciprocal value of the synchronous direct reac-tance).

Figure 2-51 Admittance Diagram of Turbo Generators I_w

---U P

U²

--- G

= =

I_EN U_N

---I_N U_N

---1 X_d

--- –I_b

---U –Q

U²

--- –B

= =

ϑ_N

ϕ_N

U_N Rated voltage I_w Active current

I_N Nominal Current I_b Reactive current

I_EN Rated excitation current G Conductance ϑN Rated pole angle (rotor angle) B Susceptance

ϕN Rated load angle P Active power

X_d Synchronous reactance Q Reactive power U Terminal voltage

underexcited overexcited

Underexcitation (Loss-of-Field) Protection (ANSI 40)

The 7UM62 underexcitation protection provides three independent characteristics which can be freely combined. As illustrated in Figure 2-52, it is, for example, possible to model the static machine stability characteristic by means of two partial

characteristics with the same time delays (T CHAR. 1 = T CHAR. 2). The partial characteristics are distinguished by the corresponding distance from the zero point (1/

xd CHAR. 1) and (1/xd CHAR. 2) the corresponding inclination angle

α

^{1 and}

α

^2.

If the resulting characteristic (1/xd CHAR.1)/

α

1; (1/xd CHAR.2)/

α

2is exceed (in Figure 2-52 to the left side), a delayed warning (e.g. by 10 s) or a trip signal are transmitted. The delay is necessary to ensure that the voltage regulator is given enough time to increase the excitation voltage.

Figure 2-52 Stator Criterion: Pick–Up Characteristic in Admittance Diagram

A further characteristic (1/xd CHAR.3 /

α

₃can be matched to the dynamic stability characteristic of the synchronous machine. Since stable operation is impossible if this characteristic is exceeded, immediate tripping is required in this case (T CHAR 3time stage).

Note:

The generator diagram can be visualized in more than one way. Figure 2-51 shows a form that is quite common at Siemens Power Generation, with a rotation of 90° and mirroring at the active power axis.

1/xd Charact. 2

Charact. 3

Characteristic 1

α₃ δ α₂

α₁

E/U I/U

1/xd

1/xd

G [p.u.]

B [p.u.]

where

G [p.u.] P S⁄ _N U U⁄ _N

( )²

---= Conductance per unit

B [p.u.] –Q S⁄ _N U U⁄ _N

( )²

---= Susceptance per unit

CHAR.1 CHAR.2

CHAR.3

Excitation Voltage Request

In case of a faulty voltage regulator or a failure of the excitation voltage, it is possible to switch up with a short delay (time stageT SHRT Uex<, e.g. 1.5 s). To do so, the device must either be informed via a binary input of the excitation voltage failure, or the excitation voltage must be fed in via measuring transducer TD3 and a voltage divider, provided that at address3012 EXCIT. VOLT.the excitation voltage request via measuring transducer has been switchedON.

As soon as the excitation voltage drops below a settable minimum3013 Uexcit. <, short-time tripping is initiated.

Instead of the excitation voltage detection, or even in addition to it, the signal of an external excitation voltage monitoring feature can be fed in via a binary input. Here again, high-speed tripping is initiated as soon as a failure of the excitation voltage is signalled.

Low-Pass Filter As the excitation DC voltage may contain a large amount of superimposed harmonics (e.g. owing to thyristor control), an analog low pass is provided on the C-I/O-6 board in addition to the integrated digital filter. This low pass damps especially multiples of the sampling frequency, which cannot be suppressed sufficiently by the digital filter.

The jumper settings for activating this filter are described in Section 3.1.3. When the 7UM62 is delivered from the factory, the filter is on. The jumper setting must match the setting of the parameter0297 TRANSDUCER 3(see Power System Data, Section 2.3.2). If they do not match, an alarm is output, the device is reported faulty and not operative.

Undervoltage Blocking

The admittance calculation requires a minimum measurement voltage. During a severe collapse (short-circuit) or failure of the stator voltages, the protection is therefore blocked by an integrated a.c. voltage monitor whose pickup threshold3014A Umin is set on delivery to 25 V.

Figure 2-53 illustrates the logic diagram of the underexcitation protection.

Underexcitation (Loss-of-Field) Protection (ANSI 40)

Figure 2-53 Logic diagram of the Underexcitation Protection

2.14.2 Setting Hints

General The underexcitation protection is only effective and accessible if this function was set within the framework of the protective function configuration (section 2.2, address 0130,UNDEREXCIT.=Enabled). SetDisabled if the function is not required. At the address3001 UNDEREXCIT.the user can switch the functionONorOFF, or only block the trip command (Block Relay).

The correct power system data is required according to section 2.3. This is a prerequisite for the setting of the underexcitation protection.

OR

&

3003 ANGLE 1 3002 1/xd CHAR. 1

>Char. 1 BLK.

FNo. 05329

3006 ANGLE 2 3005 1/xd CHAR. 2

>Char. 2 BLK.

FNo. 05330

3009 ANGLE 3 3008 1/xd CHAR. 3

>Char. 3 BLK.

3011 T SHRT Uex<

&

The trip characteristics of the underexcitation protection in the admittance value diagram are composed of straight lines which are respectively defined by their conductance section 1/xd (=coordinate distance) and their inclination angle

α

^{. The}

straight lines (1/xd CHAR.1)/

α

1 (characteristic 1) and (1/xd CHAR.2)/

α

²

(characteristic 2) form the static underexcitation limit (see figure 2-54). (1/xd CHAR.1) corresponds to the reciprocal value of the related synchronous direct reactance.

If the voltage regulator of the synchronous machine has underexcitation limiting, the static characteristics are set in such a way that the underexcitation limiting of the voltage regulator will intervene before characteristic 1 is reached (see figure 2-56).

Figure 2-54 Underexcitation Protection Characteristics in the Admittance Plane

Characteristic-Curve Values

If the generator power diagram (Figure 2-55) in its preferred representation (abscissa

= positive reactive power; ordinate = positive active power) is transformed to the admittance plane (division by U²), the tripping characteristic can be matched

immediately to the stability characteristic of the machine. If the axis sizes are divided by the nominal apparent power, the generator diagram is indicated per unit (this diagram corresponds to a per unit representation of the admittance diagram).

1 x_d

--- 1

X_d

--- U_N

3 I⋅ _N

---⋅

=

λ₃

λ₂

α₃ α₂

α₁ 1 3 2

1/xd CH.1

1/xd CH.2

λ₂ 1/xd CH.3

G [p.u.]

B [p.u.]

Underexcitation (Loss-of-Field) Protection (ANSI 40)

.

Figure 2-55 Power Diagram of a Salient-Pole Generator, Indicated per Unit

The primary setting values can be read out directly from the diagram. The related values must be converted for the protection setting. The same conversion formula can be used if the protection setting is calculated with the predefined synchronous direct reactance.

with x_dsec Secondary per unit synchronous direct reactance, x_{d mach} Per unit synchronous direct reactance of machine, I_{N mach} Rated machine current

U_{N mach} Rated machine voltage

U_{N VT prim} Primary rated voltage of the voltage transformers I_{N CT prim} Primary rated current of the current transformers

Instead of 1/x_{d mach}the approximate valueI_K0/I_Ncan be used (withI_K0= short-circuit current at no-load excitation).

Setting example:

Machine: U_{N mach} = 6.3 kV

I_{N mach} = S_N/√3 U_N= 5270 kVA/√3⋅ 6.3 kV = 483 A x_{d mach} = 2.47 (= machine manufacturer’s indication

in Fig. 2-55) Current transformers: I_{N CT prim} = 500 A

0 Limit of the stator

winding heating

Limit of the stator winding heating

Voltage transformer: U_{N VT prim} = 6.3 kV

Multiplied by a safety factor of about 1.05, the setting value1/xd CHAR. 1at address 3002results.

For

α

1, the angle of the underexcitation limiting of the voltage regulator is selected or the inclination angle of the machine stability characteristic is used. The setting value ANGLE 1is typically situated between 60 ° and 80 °.

In most cases, the machine manufacturer prescribes a minimum excitation value for small active powers. For this purpose, characteristic 1 is cut from characteristic 2 in case of a low active-power load. Consequently,1/xd CHAR. 2is set to about 0.9 ⋅ (1/xd CHAR. 1), theANGLE 2to 90 °. The kinked tripping limit according to figure 2-54 (CHAR. 1, CHAR. 2) results in this way, if the corresponding time delaysT CHAR. 1andT CHAR. 2of both characteristics are set equally.

Characteristic 3 serves to adapt the protection to the dynamic machine stability limits.

If there are no precise indications, the user must select a value1/xd CHAR. 3 situated approximately between the synchronous direct reactance x_dand the transient reactance x_d'. However, it should be greater than 1.

A value between 80 ° and 110 ° is usually selected for the correspondingANGLE 3, which ensures that only a dynamic instability can lead to a pickup with characteristic 3. The associated time delay is set at address3010 T CHAR 3to the value suggested in Table 2-6.

Figure 2-56 Admittance Diagram of a Turbo Generator

Time Delays If the static limit curve consisting of the characteristics 1 and 2 is exceeded, the voltage regulator must first have the opportunity of enhancing the excitation. For this reason, a warning message due to this criterion is ”long-time” delayed (at least 10 s for3004 T CHAR. 1and3007 T CHAR. 2).

If the excitation voltage is missing or too low, the stator criterion picks up as well, provided that the excitation voltage request feature has been switchedONat address 3012 EXCIT. VOLT., and the voltage has dropped below the thresholdUexcit. <

set at address3013, or the absence of the excitation voltage has been signalled to the device by a binary input. In all these cases tripping is possible with a short delay.

1

Underexcitation (Loss-of-Field) Protection (ANSI 40)

This feature is set at address3011 T SHRT Uex<. The following messages and trip commands are typically assigned:

Note: If very short time delays are selected, dynamic balancing procedures may cause unwanted operations. For this reason, it is recommended to set time values of 0.05 s or higher.

Excitation Voltage Request

The excitation voltage monitoring feature is set to approx. 50 % of the no-load excita-tion voltage. If the generator is used for phase-shifting, an even lower threshold must be chosen, depending on the application in had. It should also be noted that normally a voltage divider is connected between the device and the excitation voltage.

where U_Exc No-load excitation voltage,

k_VD Transformation ratio of voltage divider Example:

U_{Exc N} = 110 V

U_{Exc 0} = 40 V

k_VD = 10: 1

2.14.2.1 Settings of the Underexcitation (Loss-of-Field) Protection Table 2-6 Setting the Underexcitation Protection

Characteristic 1 and 2 static stability

not delayed Annunciation:

Err < PU Characteristic 1 and 2 static

stability

long-time delayed

T CHAR. 1=T CHAR. 2

≈ 10 s

Trip

Exc<1 TRIP / Exc<2 TRIP

Characteristic 1 and 2 excitation voltage failure

short–time delayed T SHRT Uex< ≈ 1.5 s

Trip

Exc< Uexc < TRIP Characteristic 3

SettingxU Exc < 0.50 U_{Exc 0} k_U

Addr. Setting Title Setting Options Default Setting Comments

3001 UNDEREXCIT. OFF

ON

Block relay for trip com-mands

OFF Underexcitation Protection

3011 T SHRT Uex< 0.00..60.00 sec;∞ 0.50 sec T-Short Time Delay (Char. &

Uexc<)

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