Woodward PROFESSIONAL Series User manual

XRN2 –Mains decoupling relay
Manual XRN2 (Revision D)

Woodward Manual XRN2
2 DOK-TD-XRN2, Rev. D
Woodward reserves the right to update any portion of this publication at any time.
Information provided by Woodward is believed to be correct and reliable.
However, no responsibility is assumed by Woodward unless otherwise expressly undertaken.
© Woodward 1994–2019

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 3
Contents
1. Introduction and application........................................................................................5
2. Features and characteristics .......................................................................................6
3. Design............................................................................................................................7
3.1 Connections.............................................................................................................................................7
3.1.1 Analog input circuits.............................................................................................................................7
3.1.2 Blocking input.......................................................................................................................................7
3.1.3 Reset input...........................................................................................................................................7
3.1.4 Output relays........................................................................................................................................7
3.1.5 Power Supply.......................................................................................................................................8
3.1.6 Data communication............................................................................................................................8
3.2 Front plate................................................................................................................................................9
3.2.1 Indication- and operation elements......................................................................................................9
3.2.2 Display...............................................................................................................................................10
3.2.3 LEDs..................................................................................................................................................12
3.2.4 Front plate XRN2-1............................................................................................................................12
3.2.5 Front plate XRN2-2............................................................................................................................12
3.2.6 Parameter settings.............................................................................................................................13
4. Working principle........................................................................................................14
4.1 Analog circuits .......................................................................................................................................14
4.2 4.2 Digital circuits...................................................................................................................................14
4.3 Voltage supervision ...............................................................................................................................14
4.3.1 Selection of star or delta connection .................................................................................................15
4.4 Principle of frequency supervision.........................................................................................................16
4.5 Measuring of frequency gradient (XRN2-2)...........................................................................................16
4.6 Vector surge supervision (XRN2-1).......................................................................................................17
4.6.1 Measuring principle of vector surge supervision ...............................................................................19
4.7 Voltage threshold value for frequency measuring.................................................................................23
4.8 Blocking function....................................................................................................................................23
5. Operation and setting.................................................................................................24
5.1 Push buttons..........................................................................................................................................24
5.1.1 Indication of measuring values and fault data...................................................................................25
5.2 DIP switches..........................................................................................................................................26
5.2.1 Function of the output relays .............................................................................................................26
5.3 Reset......................................................................................................................................................27
5.4 Password...............................................................................................................................................27
5.4.1 Password programming.....................................................................................................................27
5.4.2 Using the password ...........................................................................................................................27
5.5 Relay setting principle............................................................................................................................28
5.5.1 Setting of default parameters.............................................................................................................28
5.5.2 Blocking the protection functions.......................................................................................................28
5.6 Display of software version and test-TRIP ............................................................................................28
5.7 Low/High range of functions blocking and reset....................................................................................29
6. Special settings...........................................................................................................30
6.1 Adjustable parameters...........................................................................................................................30
6.2Setting procedure ..................................................................................................................................31
6.2.1 Parameter setting of over- and undervoltage supervision.................................................................31
6.2.2 Setting of nominal frequency .............................................................................................................31
6.2.3 Number of measuring repetitions (T) for frequency functions ...........................................................31
6.2.4 Threshold of frequency supervision...................................................................................................32
6.2.5 Tripping delays for the frequency elements.......................................................................................32
6.2.6 Parameter setting of vector surge supervision (XRN2-1)..................................................................32
6.2.7 Parameter setting of frequency gradient (XRN2-2)...........................................................................33
6.2.8 Voltage threshold value for frequency and vector surge measuring (df/dt at XRN2-2).....................33
6.2.9 Adjustment of the slave address........................................................................................................33
6.2.10 Setting procedure for blocking the protection functions ................................................................33
6.3 Indication of measuring values..............................................................................................................34

Woodward Manual XRN2
4 DOK-TD-XRN2, Rev. D
6.3.1 Min./Max.- values...............................................................................................................................34
6.4 Reset......................................................................................................................................................35
7. Relay testing and commissioning.............................................................................36
7.1 Power-On...............................................................................................................................................36
7.2 Testing the output relays .......................................................................................................................36
7.3 Checking the set values.........................................................................................................................37
7.4 Secondary injection test.........................................................................................................................37
7.4.1 Test equipment..................................................................................................................................37
7.4.2 Example of test circuit........................................................................................................................37
7.4.3 Checking the input circuits and measuring functions ........................................................................38
7.4.4 Checking the operating and reset-ting values of the over/undervoltage functions............................38
7.4.5 Checking the relay operating time of the over/undervoltage functions .............................................39
7.4.6 Checking the operating and reset-ting values of the over/underfrequency functions ......................39
7.4.7 Checking the relay operating time of the over/underfrequency functions ........................................39
7.4.8 Checking the vector surge function ...................................................................................................40
7.4.9 Checking the external blocking and reset functions..........................................................................41
7.5 Primary injection test .............................................................................................................................41
7.6 Maintenance ..........................................................................................................................................41
8. Technical data.............................................................................................................42
8.1 Measuring input circuits.........................................................................................................................42
8.2 Common data ........................................................................................................................................42
8.3 Setting ranges and steps.......................................................................................................................43
8.4 Output relays .........................................................................................................................................44
8.5 8.5 Power supply...................................................................................................................................44
8.6 Inputs, blockage and reset.....................................................................................................................44
8.7 System data and test specifications ......................................................................................................45
8.8 Relay case.............................................................................................................................................46
9. Order form...................................................................................................................47

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 5
1.Introduction and application
The XRN2 is a universal mains decoupling device and covers the protection requirements from VDEW and most
other utilities for the mains parallel operation of power stations.
Over/ and undervoltage protection
Over/ and underfrequency protection
Extremely fast decoupling of generator in case of mains failure (XRN2-1) or
Rate of change of frequency df/dt (XRN2-2)
Because of combination of three protectional functions in one device the XRN2 is a very compact mains de-
coupling device. Compared to the standardly used single devices it has a very good price/performance ratio.

Woodward Manual XRN2
6 DOK-TD-XRN2, Rev. D
2.Features and characteristics
Microprocessor technology with watchdog
Effective analog low pass filter for suppressing harmonics when measuring frequency and
vector surge
Digital filtering of the measured values by using discrete Fourier analysis to suppress
higher harmonics and d.c. components induced by faults or system operations
Integrated functions for voltage, frequency and vector surge in one device as well as single
voltage, frequency and vector surge devices.
Voltage supervision each with two step under-/ and overvoltage detection
Frequency supervision with three step under-/ or overfrequency (user setting)
Completely independent time settings for voltage and frequency supervision
Adjustable voltage threshold value for blocking frequency and vector surge measuring.
Display of all measuring values and setting parameters for normal operation as well as
tripping via a alphanumerical display and LEDs
Storage and indication of the tripping values
In compliance with VDE 0435, part 303 and IEC 255
For blocking the individual functions by the external blocking input, parameters can be set
according to requirement
User configurable vector surge measurement 1-of-3 or 3-of-3.
Reliable vector surge measuring by exact calculation algorithm
This manual is valid for relay software version from D01_7.23 (for XRN2-1) and D04_7.23 (for XRN2 2) on-
wards.

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 7
3.Design
3.1 Connections
Figure 3.1: Connection diagram XRN2-1 and XRN2-2
3.1.1 Analog input circuits
The analog input voltages are galvanically decoupled by the input transformers of the device, then filtered and
finally fed to the analog digital converter. The measuring circuits can be applied in star or delta connection (refer
to chapter 4.3.1).
3.1.2 Blocking input
The blocking function can be set according to requirement. By applying the auxiliary voltage to C1/C1L or
C1/C1H, the previously set relay functions are blocked (refer to 4.8 and 6.2.10).
3.1.3 Reset input
Please refer to chapter 6.4.
3.1.4 Output relays
The XRN2 has 5 output relays. One trip relay with two changeover contacts. One alarm relay with two changeo-
ver contacts and three alarm relays with one changeover contact.
Tripping 11, 12, 14 and 21, 22, 24
Indication of over-/ and undervoltage alarm 31, 32, 34 and 41, 42, 44
Indication of over-/ and underfrequency alarm 51, 52, 54
Indication of vector surge 61, 62, 64 (XRN2-1) or df/dt-alarm (XRN2-2)
Indication self supervision (internal fault of the unit) 71, 72, 74
All trip and alarm relays are normally-off relays, the re-lay for self supervision is a normally-on relay.

Woodward Manual XRN2
8 DOK-TD-XRN2, Rev. D
3.1.5 Power Supply
The XRN2 has a wide range power supply. It can connect to AC or DC supply. The connection for DC supply
don’t need to consider for polarity.
3.1.6 Data communication
For data communication with a central control system the XRN2 relay is provided with a serial interface RS485.
Simplified and fast reading and changing of parameters and measuring values can be achieved by HTL/PL-
Soft3, which will be provided on request together with the relay.
The XRN2 can be connected to other units of the PROFESSIONAL LINE via interface. If there are more than
one relay in the system, the last relay of the chain has to be provided with a line termination resistor.
Figure 3.2: Connection example with 3 users, XR ... as linked device
Figure 3.3: Connection example with 3 users, XR ... as last device

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 9
3.2 Front plate
3.2.1 Indication- and operation elements
The front plate of the XRN2-protection relay comprises the following operation and indication elements:
Alphanumerical display (4 Digits)
Push buttons for setting and other operations
LEDs for measured value indication and setting
Figure 3.4: Front plate XRN2

Woodward Manual XRN2
10 DOK-TD-XRN2, Rev. D
3.2.2 Display
Function
Display shows
Pressed pushbutton
Corre-
sponding
LED
Type of
relay
Normal operation
WW
all types
Measured operating values
Actual measured val-
ue
Min. and max. values
of voltage, frequency
and vector surge
<SELECT/RESET> one
time for each value
L1, L2, L3,
f, min, max

df
XRN2-1
XRN2-2
Setting values:
star/delta connection
Y/DELT
<SELECT/RESET><+><->
/Y
undervoltage (low set)
tripping delay of low set el-
ement
setting value in volt
setting value in
seconds
<SELECT/RESET><+><->
one time for each value
U<
tU<
undervoltage (high set)
tripping delay of high set el-
ement
setting value in volt
setting value in
seconds
<SELECT/RESET><+><->
one time for each value
U<<
tU<<
overvoltage (low set)
tripping delay of low set el-
ement
setting value in volt
setting value in
seconds
<SELECT/RESET><+><->
one time for each value
U>
tU>
overvoltage (high set)
tripping delay of high set el-
ement
setting value in volt
setting value in
seconds
<SELECT/RESET><+><->
one time for each value
U>>
tU>>
rated frequency
setting value in Hz
<SELECT/RESET><+><->
fN
frequency measuring repiti-
tion
setting value
<SELECT/RESET><+><->
T
frequency element f1
tripping delay of frequency el-
ement f1
setting value in Hz
setting value in
seconds
<SELECT/RESET><+><->
one time for each value
f1
tf1
frequency element f2
tripping delay of frequency el-
ement f2
setting value in Hz
setting value in
seconds
<SELECT/RESET><+><->
one time for each value
f2
tf2
frequency element f3
tripping delay of frequency el-
ement f3
setting value in Hz
setting value in
seconds
<SELECT/RESET><+><->
one time for each value
f3
tf3
1-of-3/3-of-3 measurement
1Ph/3Ph
<SELECT/RESET><+><->
1/3
XRN2-1
threshold for vector surge
setting value in de-
gree
<SELECT/RESET><+><->

XRN2-1
setting value df/dt
measuring repitition df/dt
setting value in Hz/s
setting value in peri-
ods
<SELECT/RESET><+><->
one time for each value
Df
dt
XRN2-2
Blocking
EXIT
<+> until max. setting val-
ue
LED of
blocked
parameter
Undervoltage blocking of
frequency and vector surge
measuring (df/dt for XRN2-
2)
setting value in Volt
<SELECT/RESET><+><->
f, , df
Slave address of serial in-
terface
1 –32
<SELECT/RESET><+><->
RS
Recorded fault data:
star--connection:
U1, U2, U3
tripping values in Volt
<SELECT/RESET><+><->
one time for each phase
L1, L2, L3,
U<, U<<,
U>, U>>
delta-connection:
U12, U23, U31
tripping values in Volt
<SELECT/RESET><+><->
one time for each phase
L1, L2, L3
U<, U<<,
U>, U>>
frequency
tripping values in Hz
<SELECT/RESET><+><->
one time for each phase
f, f1, f2, f3
rate of change of frequency
tripping value in Hz/s
<SELECT/RESET><+><->
df
XRN2-2

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 11
Function
Display shows
Pressed pushbutton
Corre-
sponding
LED
Type of
relay
vector surge
tripping value in de-
gree
<SELECT/RESET><+><->
one time for each phase
 + L1, L2
or L3
XRN2-1
Save parameter?
SAV?
<ENTER>
Save parameter!
SAV!
<ENTER> for about 3 s
Software version
First part (e.g. D02-)
Sec. part (e.g. 6.01)
<TRIP>
one time for each part
Manual trip
TRI?
<TRIP>
three times
Inquire password
PSW?
<SELECT/RESET>/
<+>/<->/<ENTER>
Relay tripped
TRIP
<TRIP> or fault tripping
Secret password input
XXXX
<SELECT/RESET>/
<+>/<->/<ENTER>
System reset
WW
<SELECT/RESET>
for about 3 s
Table 3.1: possible indication messages on the display

Woodward Manual XRN2
12 DOK-TD-XRN2, Rev. D
3.2.3 LEDs
All LEDs (except LED RS, min and max) are two-colored. The LEDs on the left side, next to the alpha-numerical
display light up green during measuring and red after tripping.
The LEDs below the push button <SELECT/RESET> are lit green during setting and inquiry procedure of the
setting values which are printed on the left side next to the LEDs. The LEDs will light up red after activation of
the setting values next to their right side.
The LED marked with letters RS lights up during setting of the slave address of the device for serial data com-
munication.
3.2.4 Front plate XRN2-1
3.2.5 Front plate XRN2-2

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 13
3.2.6 Parameter settings
Setting parameter
XRN2-1
XRN2-2
Δ/Y
X
X
U<
X
X
tU<
X
X
U<<
X
X
tU<<
X
X
U>
X
X
tU>
X
X
U>>
X
X
tU>>
X
X
fN
X
X
T
X
X
f1
X
X
tf1
X
X
f2
X
X
tf2
X
X
f3
X
X
tf3
X
X
df
X
dt
X
1/3
X

X
UB<
X
X
RS485/
Slave
X
X
Table 3.2: Sequence of parameter setting of the two relay types

Woodward Manual XRN2
14 DOK-TD-XRN2, Rev. D
4.Working principle
4.1 Analog circuits
The input voltages are galvanically insulated by the input transformers. The noise signals caused by inductive
and capacitive coupling are suppressed by an analog R-C filter circuit.
The analog voltage signals are fed to the A/D-converter of the microprocessor and transformed to digital signals
through Sample- and Hold- circuits. The analog signals are sampled with a sampling frequency of 16 x fN,
namely, a sampling rate of 1.25 ms for every measuring quantity, at 50 Hz.
4.2 4.2 Digital circuits
The essential part of the XRN2 relay is a powerful microcontroller. All of the operations, from the analog digital
conversion to the relay trip decision, are carried out by the microcontroller digitally. The relay program is located
in an EPROM (Electrically-Programmable-Read-Only-Memory). With this program the CPU of the microcontrol-
ler calculates the three phase voltage in order to detect a possible fault situation in the protected object.
For the calculation of the voltage value an efficient digital filter based on the Fourier Transformation (DFFT -
Discrete Fast Fourier Transformation) is applied to suppress high frequency harmonics and d.c. components
caused by fault-induced transients or other system disturbances. The microprocessor continuously compares
the measured values with the preset thresholds stored in the parameter memory (EEPROM). If a fault occurs an
alarm is given and after the set tripping delay has elapsed, the corresponding trip relay is activated.
The relay setting values for all parameters are stored in a parameter memory (EEPROM - Electrically Erasable
Programmable Read Only Memory), so that the actual relay settings cannot be lost, even if the power supply is
interrupted.
The microprocessor is supervised by a built-in "watch-dog" timer. In case of a failure the watchdog timer re-sets
the microprocessor and gives an alarm signal via the output relay "self supervision".
4.3 Voltage supervision
The voltage element of XRN2 has the application in protection of generators, consumers and other electrical
equipment against over/and undervoltage.
The relay is equipped with a two step independent three-phase overvoltage (U>, U>>) and undervoltage (U<,
U<<) function with completely separate time and voltage settings.
In delta connection the phase-to-phase voltages and in star connection the phase-to-neutral voltages are con-
tinuously compared with the preset thresholds.
For the overvoltage supervision the highest, for the undervoltage supervision of the lowest voltage of the three
phases are decisive for energizing.

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 15
4.3.1 Selection of star or delta connection
All connections of the input voltage transformers are led to screw terminals. The nominal voltage of the device is
equal to the nominal voltage of the input transformers. Dependent on the application the input transformers can
be connected in either delta or star. The connection for the phase-to-phase voltage is the delta connection. In
star connection the measuring voltage is reduced by 1/. During parameter setting the connection configura-
tion either Y or Δhas to be adjusted.
Figure 4.1: Input v.t.s in delta connection ( )
Figure 4.2: Input v.t.s in star connection (Y)

Woodward Manual XRN2
16 DOK-TD-XRN2, Rev. D
4.4 Principle of frequency supervision
The frequency element of XRN2 protects electrical generators, consumers or electrical operating equipment in
general against over- or underfrequency.
The relay has independent three frequency elements f1- f3with a free choice of parameters, with separate ad-
justable pickup values and delay times.
The measuring principle of the frequency supervision is based in general on the time measurement of complete
cycles, whereby a new measurement is started at each voltage zero passage. The influence of harmonics on
the measuring result is thus minimized.
Figure 4.3: Determination of cycle duration by means of zero passages.
In order to avoid false tripping during occurence of interference voltages and phase shifts the relay works with
an adjustable measuring repetition (see chapter 6.2.3)
Frequency tripping is sometimes not desired by low measured voltages which for instance occur during alterna-
tor acceleration. All frequency supervision functions can be blocked with the aid of an adjustable voltage thresh-
old UB in case the measured voltage value is below this value.
4.5 Measuring of frequency gradient (XRN2-2)
Electrical generators running in parallel with the mains, e.g. industrial internal power supply plants, should be
separated from the mains when failure in the intrasystem occurs for the following reasons:
It must be prevented that the electrical generators are damaged when mains voltage
recovering asynchrone, e.g. after a short interruption.
The industrial internal power supply must be maintained.
A reliable criterion of detecting mains failure is the measurement of the rate of change of frequency df/dt. Pre-
condition for this is a load flow via the mains coupling point. At mains failure the load flow changing then spon-
taneously leads to an increasing or decreasing frequency. At active power deficit of the internal power station a
linear drop of the frequency occurs and a linear increase occurs at power excess. Typical frequency gradients
during application of "mains de-coupling" are in the range of 0.5 Hz/s up to over 2 Hz/s. The XRN2 detects the
instantaneous frequency gradient df/dt of each mains voltage period in an interval of one half period each.
Through multiple evaluation of the frequency gradient in sequence the continuity of the directional change (sign
of the frequency gradient) is determined. Because of this special measuring procedure a high safety in tripping
and thus a high stability against transient processes, e.g. switching procedure are reached. The total switching
off time at mains failure is between 60 ms and 80 ms depending on the setting.

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 17
4.6 Vector surge supervision (XRN2-1)
The vector surge supervision protects synchronous generators in mains parallel operation due to very fast de-
coupling in case of mains failure. Very dangerous are mains auto reclosings for synchronous generators. The
mains voltage returning after 300 ms can hit the generator in asynchronous position. A very fast de-coupling is
also necessary in case of long time mains failures. Generally there are two different applications:
a) Only mains parallel operation no single operation:
In this application the vector surge supervision protects the generator by tripping the
generator circuit breaker in case of mains failure.
b) Mains parallel operation and single operation:
For this application the vector surge supervision trips the mains circuit breaker. Here it is
insured that the gen.-set is not blocked when it is required as the emergency set.
A very fast decoupling in case of mains failures for synchronous generators is known as very difficult. Volt-age
supervision units cannot be used because the synchronous alternator as well as the consumer impedance sup-
port the decreasing voltage.
For this the mains voltage drops only after some 100 ms below the pickup threshold of voltage supervision re-
lays and therefore a safe detection of mains auto reclosings is not possible with this kind of relay.
Frequency relays are partial unsuitable because only a highly loaded generator decreases its speed within 100
ms. Current relays detect a fault only when short-circuit type currents exist, but cannot avoid their development.
Power relays are able to pickup within 200 ms, but they cannot prevent power to rise to short-circuit values too.
Since power changes are also caused by sudden loaded alternators, the use of power relays can be problemat-
ic.
Whereas the XRN2-1 detects mains failures within 60 ms without the restrictions described above because they
are specially designed for applications where very fast decoupling from the mains is required.
Adding the operating time of a circuit breaker or contactor, the total disconnection time remains below 150 ms.
Basic requirement for tripping of the genera-tor/mains monitor is a change in load of more than 15 - 20% of the
rated load. Slow changes of the system frequency, for instance at regulating processes (adjustment of speed
regulator) do not cause the relay to trip.
Trippings can also be caused by short-circuits within the grid, because a voltage vector surge higher than the
preset value can occur. The magnitude of the voltage vector surge depends on the distance between the short-
circuit and the generator. This function is also of advantage to the Power Utility Company because the mains
short-circuit capacity and consequently the energy feeding the short-circuit is limited.
To prevent a possible false tripping the vector surge measuring can be blocked at a set low input voltage (refer
to 6.2.8). The undervoltage lockout acts faster then the vector surge measurement.
Vector surge tripping is blocked by a phase loss so that a VT fault (e.g. faulty VTs fuse) does not cause false
tripping.
When switching on the aux. voltage or measuring voltage , the vector surge supervision is blocked for 5 s (refer
to chapter 4.8).
Note:
In order to avoid any adverse interference voltage effects, for instance from contactors or relays, which may
cause overfunctions, XRN2-1 should be connected separately to the busbar.

Woodward Manual XRN2
18 DOK-TD-XRN2, Rev. D
Important Information on the Vector Surge Supervision:
Single Phase Vector Supervision "1-of-3" ("1Ph" on the Display)
Within the Area „Safe Area“ the trip decision depends on the set Angle „Delta phi“ only (please refer to figure
Single Phase Vector Surge Supervision).
Within the Area „Unsafe Area“ a trip decision is taken if the vector surge angle „Delta phi“ is exceeded or if the
magnitude of the voltage vector is shrinked (percental related to the rated voltage). In order to prevent faulty
tripping the effective area of the vector surge supervision can be limited by means of the parameter "UB<".
Three Phase Vector Surge Supervision "3-of-3" ("1Ph" on the Display)
Within the Area „Safe Area“ the trip decision depends on the set Angle „Delta phi“ only (please refer to figure
Three Phase Vector Surge Supervision).
Within the Area „Unsafe Area“ a trip decision is taken if the vector surge angle „Delta phi“ is exceeded or if the
magnitudes of the three voltage vectors are shrinked (percental related to the rated voltage). In order to prevent
faulty tripping the effective area of the vector surge supervision can be limited by means of the parameter "UB<".

Manual XRN2 Woodward
DOK-TD-XRN2, Rev. D 19
4.6.1 Measuring principle of vector surge supervision
When a synchronous generator is loaded, a rotor displacement angle is build between the terminal voltage
(mains voltage U1) and the synchronous internal voltage (Up). Therefore a voltage is difference ΔU is built be-
tween Up and U1 (Fig. 4.4).
Figure 4.4: Equivalent circuit at synchronous generator in parallel with the mains
Figure 4.5: Voltage vectors at mains parallel operation
The rotor displacement angle between stator and rotor is depending of the mechanical moving torque of the
generator shaft. The mechanical shaft power is balanced with the electrical feeded mains power, and therefore
the synchronous speed keeps constant (Fig. 4.5).

Woodward Manual XRN2
20 DOK-TD-XRN2, Rev. D
Figure 4.6: Equivalent circuit at mains failure
In case of mains failure or auto reclosing the generator suddenly feeds a very high consumer load. The rotor
displacement angle is decreased repeatedly and the voltage vector U1changes its direction (U1') (Fig. 4.6 and
4.7).
Figure 4.7: Voltage vectors at mains failure