Overexcitation Volt Hertz Protection ANSI 24

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Overexcitation (Volt/Hertz) Protection (ANSI 24) Overexcitation protection is used to detect inadmissibly high induction in generators and transformers, especially in power station unit transformers. The protection must intervene when the limit value for the protected object (e.g. unit transformer) is exceeded. The transformer is endangered, for example, if the power station block is disconnected from the system from full-load, and if the voltage regulator either does not operate or does not operate sufficiently fast to control the associated voltage rise. Similarly a decrease in frequency (speed), e.g. in island systems, can lead to an inadmissible increase in induction. An increase in induction above the rated value quickly saturates the iron core and causes large eddy current losses. · Functional Description · Setting Notes · Settings · Information List

Functional Description Measurement Method The overexcitation protection feature servers to measure the voltageU/frequency ratio f, which is proportional to the B induction and puts it in relation to the BN nominal induction. In this context, both voltage and frequency are related to nominal values of the object to be protected (generator, transformer).

The calculation is based on the maximum voltage of the three phase-to-phase voltages. The frequency range monitored extends from 10 Hz to 70 Hz.

Voltage Transformer Adaptation Any deviation between primary nominal voltage of the voltage transformers and of the object to be protected is compensated by an internal correction factor (UN For this reason pickup values and characteristic do not need to be converted to secondary values. However the system primary nominal transformer voltage and the nominal voltage of the object to be protected must be entered correctly (see Sections Power System Data 1 and Power System Data 2).

prim/UN Mach).

Characteristic Curves Overexcitation protection includes two staged characteristics and one thermal characteristic for approximate modeling of the heating of the protection object due to overexcitation. As soon as a first pickup threshold (warning stage 4302 U/f >) has been exceeded, a 4303 T U/f > time stage is started. On its expiry a warning message is transmitted. At the same time a counter switching is activated when the pickup threshold is exceeded. This weighted counter is incremented in accordance with the current U/f value resulting in the trip time for the parametrized characteristic. A trip signal is transmitted as soon as the trip counter state has been reached. The trip signal is retracted as soon as the value falls below the pickup threshold and the counter is decremented in accordance with a parametrizable cool-down time. The thermal characteristic is specified by 8 value pairs for overexcitation U/f (related to nominal values) and trip time t. In most cases, the specified characteristic for standard transformers provides sufficient protection. If this characteristic does not correspond to the actual thermal behavior of the object to be protected, any desired characteristic can be implemented by entering customer-specific trip times for the specified U/f overexcitation values. Intermediate values are determined by a linear interpolation within the device.

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Tripping Range of the Overexcitation Protection The characteristic resulting from the device default settings is shown in the Technical Data Section Overexcitation Protection. Figure Tripping Range of the Overexcitation Protection illustrates the behaviour of the protection on the assumption that within the framework of configuration the setting for the pickup threshold (parameter4302 U/f >) was chosen higher or lower than the first setting value of the thermal characteristic. The following figure shows the logic diagram for overexcitation protection. The counter can be reset to zero by means of a blocking input or a reset input.

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Logic Diagram of the Overexcitation Protection

Setting Notes · General · Independent Stages · Thermal Characteristic · Limitation · Cooling Time · Voltage Transformer Adaptation

General Overexcitation protection is only effective and available if address 143 OVEREXC. PROT. is set to Enabled during configuration. If the function is not required, it is set to Disabled. Address 4301 OVEREXC. PROT. serves to switch the function ON or OFF or to block only the trip command (Block relay). Overexcitation protection measures the voltage/frequency quotient which is proportional to the induction B. The protection must intervene when the limit value for the protected object (e.g. unit transformer) is exceeded. The transformer is for example endangered if the power station block is switched off at full-load operation and the voltage regulator does not respond fast enough or not at all to avoid related voltage increase. Similarly a decrease in frequency (speed), e.g. in island systems, can lead to an inadmissible increase in induction. In this way the U/f protection monitors the correct functioning both of the voltage regulator and of the speed regulation, in all operating states.

Independent Stages The limit-value setting at address 4302 U/f > is based on the induction limit value relation to the nominal induction (B/BN) as specified by the manufacturer of the object to be protected. A pickup message is transmitted as soon as the induction limit value U/f set at address 4302 is exceeded. A warning message is transmitted after expiry of the corresponding 4303 T U/f > time delay. The 4304 U/f >>, 4305 T U/f >> trip stage characteristic serves to rapidly switch off particularly strong overexcitations. The time set for this purpose is an additional time delay which does not include the operating time (measuring time, drop-out time).

Thermal Characteristic A thermal characteristic is superimposed on the trip stage characteristic. For this purpose, the temperature rise created by the overexcitation is approximately modeled. Not only the already mentioned pickup signal is generated on transgression of the U/f induction limit set at address 4302, but in addition a counter is activated additionally which causes the tripping after a length of time corresponding to the set characteristic.

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Thermal tripping time characteristic (with default settings) The characteristic of a Siemens standard transformer was selected as a default setting for the parameters 4306 to 4313. If the protection object manufacturer did not provide any information, the preset standard characteristic should be used. Otherwise, any trip characteristic can be specified entering parameters point-bypoint over a maximum of 7 straight lengths. To do this, the trip times t of the overexcitation values U/f = 1.05; 1.10; 1.15; 1.20; 1.25; 1.30; 1.35 and 1.40 are read out from the predefined characteristic and entered at the addresses4306 t(U/f=1.05) to 4313 t(U/f=1.40). The protection device interpolates linearly between the points.

Limitation The heating model of the object to be protected is limited to a 150 % overshoot of the trip temperature.

Cooling Time Tripping by the thermal image drops out by the time of the pickup threshold dropout. However, the counter content is counted down to zero with the cooldown time parametrized at address 4314 T COOL DOWN. In this context this parameter is defined as the time required by the thermal image to cool down from 100 % to 0 %.

Voltage Transformer Adaptation Any deviation between the primary nominal voltage of the voltage transformers and of the protected object is compensated by an internal correction factor (UN prim/UN For this it is necessary that the relevant parameters 221 Unom PRIMARY and 251 UN GEN/MOTOR were properly entered in accordance with Section Power System Data 1.

Mach).

Settings Addr.

Parameter

Setting Options

Default Setting

Comments

4301

OVEREXC. PROT.

OFF ON Block relay

OFF

Overexcitation Protection (U/f)

4302

U/f >

1.00 .. 1.20

1.10

U/f > Pickup

4303

T U/f >

0.00 .. 60.00 sec; ∞

10.00 sec

T U/f > Time Delay

4304

U/f >>

1.00 .. 1.40

1.40

U/f >> Pickup

4305

T U/f >>

0.00 .. 60.00 sec; ∞

1.00 sec

T U/f >> Time Delay

4306

t(U/f=1.05)

0 .. 20000 sec

20000 sec

U/f = 1.05 Time Delay

4307

t(U/f=1.10)

0 .. 20000 sec

6000 sec

U/f = 1.10 Time Delay

4308

t(U/f=1.15)

0 .. 20000 sec

240 sec

U/f = 1.15 Time Delay

4309

t(U/f=1.20)

0 .. 20000 sec

60 sec

U/f = 1.20 Time Delay

4310

t(U/f=1.25)

0 .. 20000 sec

30 sec

U/f = 1.25 Time Delay

4311

t(U/f=1.30)

0 .. 20000 sec

19 sec

U/f = 1.30 Time Delay

4312

t(U/f=1.35)

0 .. 20000 sec

13 sec

U/f = 1.35 Time Delay

4313

t(U/f=1.40)

0 .. 20000 sec

10 sec

U/f = 1.40 Time Delay

4314

T COOL DOWN

0 .. 20000 sec

3600 sec

Time for Cooling Down

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Information List No.

Information

Type of Information

Comments

5353

>U/f BLOCK

SP

>BLOCK overexcitation protection

5357

>RM th.rep. U/f

SP

>Reset memory of thermal replica U/f

5361

U/f> OFF

OUT

Overexcitation prot. is swiched OFF

5362

U/f> BLOCKED

OUT

Overexcitation prot. is BLOCKED

5363

U/f> ACTIVE

OUT

Overexcitation prot. is ACTIVE

5367

U/f> warn

OUT

Overexc. prot.: U/f warning stage

5369

RM th.rep. U/f

OUT

Reset memory of thermal replica U/f

5370

U/f> picked up

OUT

Overexc. prot.: U/f> picked up

5371

U/f>> TRIP

OUT

Overexc. prot.: TRIP of U/f>> stage

5372

U/f> th.TRIP

OUT

Overexc. prot.: TRIP of th. stage

5373

U/f>> pick.up

OUT

Overexc. prot.: U/f>> picked up

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