Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of

Renewable Energy Generation Systems

CHAPTER 1
INTRODUCTION

With the increasing depletion of fossil fuels and environmental problems, the development
of renewable energy generation to replace traditional thermal power plants has become an
effective solution. Renewable energy is regarded as the inevitable trend of future electric
power development worldwide. With the cost reduction in renewable energy generation and
the rapid development of power electronics technology, the proportion of renewable energy
generation in the power grid has boomed. However, large-scale disconnection accidents of
renewable energy plants caused by grid faults in the power grid The associate editor
coordinating the review of this manuscript and approving it for publication was Lin Zhang .
have occurred frequently in recent years, which has been a severe challenges for the voltage
stability of power grid. These stability issue occur because power electronic converters do
not have fault ride-through (FRT) abilities due to the limitations of power electronic devices
to withstand voltage and current .

Therefore, many countries have issued renewable energy grid connection guidelines,
stipulating that renewable energy plants must have required FRT capabilities to improve grid
stability. There are two main ways to improve the FRT capabilities of renewable energy units:
improving the control strategy of the converter and the auxiliary hardware circuit method.
The improved control strategies include de-excitation control, virtual impedance control,
virtual flux control. However, the control effects of the above methods are influenced by
VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License.
For more information, see http://creativecommons.org/licenses/by/4.0/ 13251Y. Gu et al.:
Isolation and Protection of the MGP System for FRT of Renewable Energy Generation
Systems the severity of the fault and the configuration of the control parameters. The
auxiliary hardware circuit methods includes energy storage system, rotor crowbar circuits
and DC chopper circuit , STATCOM, SVC, DVR, and other FACTS devices and stator series
impedance. Although these devices can achieve FRT in renewable energy units, there are still
many cases of secondary failure due to unreasonable switching during faults. In addition, the
number of massive electrical failures caused by extreme weather around the world is
increasing each. One typical case is the 9.28 blackout in South Australia . In this event, six

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
transmission line faults occurred due to extreme weather. The first five faults did not induce
large-scale wind farm disconnection. The sixth fault caused a 505 MW wind turbine
disconnection, resulting in a blackout lasting 50 hours.

This incident occurred because renewable energy plants does— not have the capability to
realize successive FRTs over a very short time, and no effective solution has been reported
yet. The inherent defects in the converter have not been resolved by using the above methods
during grid faults. A novel method of grid-connected renewable energy called the motor-
generator pair (MGP) was proposed in. Two synchronous machines in the MGP system are
coaxially connected, and the renewable energy converters are connected to the synchronous
motor while the synchronous generator is connected to the grid. One of the advantages of the
MGP system is the isolation function of grid faults on the generator side due to the damping
effect of the synchronous machine and the isolation of the mechanical shafting in the MGP
system. During the fault, the power output of the renewable energy can be maintained steadily
while the excitation of the synchronous generator can be adjusted to support the grid voltage.
All these functions can improve the transient stability of the renewable energy power grid.
In this paper, the structure and mathematical model of MGP system are given. The voltage
feedback control method of the MGP system is introduced and the isolation, damping and
protection mechanism of the MGP system for renewable energy is analyzed. On this basis,
the isolation effects of MGP are verified and analyzed by simulation and experiment.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
CHAPTER 2

STRUCTURE AND MATHEMATICAL MODEL OF MGP SYSTEM

Different from traditional grid connection, renewable energy units can be connected to the
grid through the MGP system. As shown in Fig. 1, the MGP system consists of two
synchronous machines coaxially connected, and each machine is equipped with an
independent excitation system. The capacity of the MGP system matches that of the
connected renewable energy units. The per-unit model is often used to describe the machine.
The selections of the stator and rotor reference values for synchronous machines are as
follows: the base of stator voltage usb is the magnitude of the rated stator phase voltage; the
base of the stator current isb is the magnitude of the rated stator phase current; the base of the
angular velocity ωb is the rated

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
stator angular velocity. For the synchronous machine, xad (the d-axis armature reactance of
the synchronous machine) is selected as the rotor base value. Because the two coaxially
connected synchronous machines of MGP have the same capacity, the mathematical models
of two machines are the same without considering their operation modes of the machines.
Therefore, only one machine is used as an example for analysis when modeling the MGP
system. With the synchronous motor as an example, the per-unit voltage, current, and motion
equations in the d- and q-axis system are given as equations (1)-(3).

where:

R is diag(Rs , Rs , Rf , RDd, RDq);

Rs , Rs , Rf , RDd, RDq are the resistances of the stator winding, field winding, d-axis damper
winding, and q-axis damper winding, respectively;

Ld = Lad + Lsl and Lq = Laq + Lsl are the d- and q-axis synchronous inductances, respectively;

Lf = Lad + Lfl is the inductance of the field winding;

LDd = Lad+LDdl and LDq = Laq+LDql are the inductances of the d- and q-axis damper winding,
respectively;

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
Lad and Laq are the inductances of the d- and q-axis armature reaction inductances,
respectively;

Lsl, Lfl, LDdl and LDql are the leakage inductances of the stator winding, field winding, d-axis
damper winding, and q-axis damper winding, respectively;

ω, ωm, ωmN, and 1ωm are the angular velocity, mechanical angular velocity, rated mechanical
angular velocity, and the difference between the electric angular velocity and mechanical
angular velocity, respectively;

H is the inertia;

KD is the damping coefficient;

δM is the power angle; and

Te and Tm are the electromagnetic torque and the mechanical torque, respectively. Because
two synchronous machines are coaxially connected, the MGP system is assumed to be a
single mass block model. The speed and variation of the two machines are the same when
the MGP system runs steadily. In addition, the mechanical torques sent to the shaft are
approximately equal. Therefore, it can be assumed that the two machines have their own
electromagnetic equation and share the same motion equation, given as equation (4):

where:

HM, HG are the inertials of the motor and the generator, respectively;

TeM and TeG are the electromagnetic torques of the motor and generator, respectively; and

KDM and KDG are the damping coefficients of the motor and the generator, respectively.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
CHAPTER 3

CONTROL METHOD AND ISOLATION PROTECTION MECHANISM OF MGP

A. GRID-CONNECTION CONTROL METHOD OF MGP SYSTEM

The voltage phase difference between the two terminals of the MGP system is proportional
to the active power P transferred through the MGP system [16], so P increases with the
increase of the phase difference. The above characteristic of the MGP system is the physical
basis to realize its stable operation control. Therefore, a DC voltage feedback control method
is proposed to achieve active power transmission of renewable energy through the MGP
system. The input of the control system is the renewable energy state (wind speed, pitch
angle, illumination intensity, ambient temperature, etc.) and the DC link capacitor voltage.
The reference DC voltage Uref is calculated based on the maximum power point tracking
(MPPT) and the renewable energy state. Then, the reference frequency fref is calculated by
the PI controller from the difference-between Udc and Uref, as shown in Fig. 2.

The control process is as follows: when Udc > Uref, fref increases. The increase of fref results
in the increase in the frequency of the motor voltage UM whereas the frequency of the back
electromotive force of the two machines and the generator voltage UG remains unchanged.
Therefore, the source-grid difference increases, and P increases accordingly [16]. At the same
time, the increase of P leads to an increase in the discharging speed of the DC capacitor, and

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
Udc decreases accordingly. Udc approaches Uref gradually under the feedback control and vice
versa.

B. ISOLATION AND PROTECTION MECHANISM OF MGP SYSTEM FOR

Different from traditional FRT methods, the grid voltage fault attacks the synchronous
generator firstly. When faults occur on the grid side, the stator and rotor windings of the
synchronous generator undergo an electromagnetic transient process and induce transient
overcurrent. Compared with the grid-connected converter of renewable energy, synchronous
generator has a higher insulation level and overcurrent capacity, and can withstand several
times the rated current. In addition, the stator and rotor winding impedances of the machines
and the damping effect of the excitation system restrain the magnitude of the overcurrent,
and cause the overcurrent to decay rapidly. Because of the short transient time of voltage
fault, it can be assumed that the input power of renewable energy is approximately
unchanged. The voltage fault on the grid side leads to an imbalance in the electromagnetic
torque at both ends of the shaft. Equation (4) can be transformed into:

The variation of the rotor angular velocity depends not only on the unbalanced torque but
also on the inertia and the damping coefficient, as in (5). The MGP system consists of two
excitation systems, so the damping coefficient is the superposition of both systems.
Moreover, the inertia time constant of the mechanical system of the synchronous machine
usually reaches the second level. Therefore, the fluctuation transmitted through the shaft to
the motor is obviously decreased, which causes smaller fluctuations in the motor and on the
renewable energy side. According to the above analysis, disturbances caused by grid faults
are weakened greatly by the shaft of the MGP system which plays an important role in
protecting the renewable energy units. Moreover, the damping effect of the MGP system
restrains the fluctuations on both sides. As discussed above, grid faults are isolated and
weakened by the MGP system. For the impacts of renewable energy, since the transient
process of grid faults is very short, it can be assumed that the wind speed or strength of
illumination remains approximately constant. Therefore, the disturbance in renewable energy
can be ignored. In addition, the function of reactive power regulation can not be ignored. The
phasor diagram of synchronous generator voltage and current is shown in Fig. 3. When a
low-voltage fault occurs in the power grid, the magnitude of the generator terminal voltage

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems

UL is smaller than that of the rated voltage UN. The phasor of the stator current IL lags that

of UL, and the generator outputs inductive reactive power to the power grid; when an
overvoltage fault occurs in the power grid, UH > UN, IH is ahead of UH , and the generator
absorbs the excess inductive reactive power in the grid. The above reactive power regulations
of the generator are conducive to the rapid recovery of the grid voltage, and can reduce the
adverse impact of faults on renewable energy plants.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
CHAPTER 4

SIMULATION ANALYSIS OF ISOLATION AND PROTECTION FUNCTIONS OF

SYSTEM In order to verify the isolation and protection function of the MGP system for a
renewable energy generation system, a time-domain simulation model is built, as shown in
Fig. 4.

A simplified model of magnet synchronous generator (PMSG) is used as simulate a

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems

The output of the MGP system is approximately 3 kW before the faults. Three kinds of grid
faults are set in the power grid module according to the China National Standard [4]

and the 9.28 blackout in South Australia [14]. The phase voltage and phase current of two
machines, DC link voltage, and frequency of the MGP system are monitored, and then the
single-phase power of two machines were calculated respectively. The detailed analyses are
as follows:

A. VOLTAGE SAG CONDITION

A decrease in the grid voltage from 220 V to 44 V (0.2p.u.) is set at 15 s and the recovery to
220 V is set at 20 s. The results are shown in Fig. 5.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems

As shown in Fig. 5, the generator current increases instantaneously when voltage sag occur,
which leads to the torque imbalance in the shafting. The power of the synchronous generator
oscillates and decays under the damping effect. Due to the isolation of the shafting, the power
oscillation amplitude of the synchronous motor is much smaller than that of the synchronous
generator. The DC link voltage and frequency oscillate and attenuate due to unbalanced
power on both sides of the capacitor. However, the voltage oscillation amplitude of capacitor
is no larger than 0.37% of the reference DC voltage because of the function of the control
system. From Fig. 5 (b), it can be seen that the active power output by the generator to the
power grid fluctuate, but the voltage and current of the motor stay almost unchanged
compared to those of the generator. A major part of the increasing current is the reactive
current generated by the voltage difference, which can provide inductive reactive power
support to the grid. During voltage recovery, the active power output of the MGP system can
gradually recover to its original state after a transient process. The DC link voltage fluctuates
within a small range during the voltage sag, demonstrating that the DWTG is less influenced
by the grid voltage sag.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
B. OVERVOLTAGE CONDITION

The voltage RMS value on the grid side is set to swell from 220 V to 286 (1.3 p.u.) at 15 s
and recover to 220 V at 20 s. The results are shown in Fig. 6.

As shown in Fig. 6, when the grid voltage swells to 1.3 p.u., the generator withstands the
impact of overvoltage and overcurrent while the oscillation on the motor side can be
neglected with the help of DC voltage feedback control. Compared with the results in section
A, the voltage oscillation amplitude of the capacitor is much smaller because the voltage
change amplitude of the grid is only 3/8 of the voltage sag. The key to the HVRT of renewable
energy through the MGP system is a reliable insulation level in the generator. Due to the
sudden increase of grid voltage, the excitation current of synchronous generators can not
provide enough air gap magnetic field. The voltage difference causes the generator to absorb
reactive power, promoting grid voltage recovery. Although the change in the speed leads to
small fluctuations in the voltage and current of the motor, the DWTG can maintain stable
operation under the adjustment of the control system. After a dynamic process (similar to the
low voltage process), the active power output of the MGP system can also gradually return
to its original state.

C. MULTIPLE VOLTAGE SAGS CONDITION

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
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Referring to the blackout in South Australia, multiple voltage sags are set at 22.7 s, 48.7 s,
75 s, 96.7 s, 102.7 s, and 106.7 s respectively. The phase voltage of the power grid sags from
220 V to 176 V, 132 V, 132 V, 88 V, 88 V, and 88 V successively. Each voltage drop lasts
one second and then the voltage recovers to 220 V, as shown in Fig. 7.

As shown in Fig. 7, the magnitudes of the generator current increase with the increasing
severity of the grid voltage sag. Because of the short intervals between the last three voltage
sags, the transient process caused by the fault is superimposed, resulting in large fluctuations
in the power output by the generator, which is reflected in the increasing voltage oscillations
of the capacitor. However, the motor current and DC link voltage fluctuate slightly because
of the isolation and damping functions of the MGP system. Fig. 7 (b) shows that the active
power output of MGP system is basically stable with the large fluctuations of the first several
cycles. Because of the torque imbalance, there are also fluctuations in the motor voltage and
current, but these fluctuations are very small, and the output of DWTG can gradually return
to the previous level after the voltage recovery. Overall, the operation of the MGP system
can be stable during the six voltage sags.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
CHAPTER 5

EXPERIMENTAL VERIFICATION OF MGP ISOLATION AND PROTECTION

An experimental bench is built in the laboratory to simulate the connection of the DWTG
unit to the grid through the MGP system, as shown in Fig. 8.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
The bench includes a 10 kW DWTG emulator, a 30 kW converter, an MGP system (including
two coaxially connected STC-5 synchronous machines), and a Chroma 61845 power grid
simulator. The parameters of the synchronous machines are shown in Table 3, and other
parameters are consistent with Table 1. The DWTG emulator is used to emulate the DC
output power characteristics of the rectifier connected to the PMSG. The function of the grid
emulator is to emulate the grid and set the grid voltage fault. To verify the isolation and
protection functions of the MGP system for renewable energy units, three kinds of grid faults
are set with the power grid emulator according to the China National Standard and the 9.28
blackout in South Australia. The phase voltage and phase current of two machines, DC-bus
voltage, and frequency of the MGP system are measured by a Yokogawa scope order, and
then the output power of the MGP system is calculated. At the beginning of the experiment,
the DC voltage of the converter is 580 V and the output active power of MGP system is
approximately 1.3kW under both low-voltage and overvoltage fault. The specific
experimental process and results are as follows:

A. LOW-VOLTAGE FAULT

With the grid emulator, a decrease in the voltage from 220 V to 44 V is set at 18 s, and the
recovery to 220 V is set at 23 s. The results are shown in Fig. 9.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
Fig. 9 (a) shows that the peak value of the generator current under the fault is over 10 times
that under steady-state operation, accompanied with severe oscillations at the same time. The
experimental results are worse than the simulation

results because the experimental synchronous machine is different from the synchronous
generator of the thermal power plant in terms of the shafting structure, damping effects and
quality of windings and iron core. However, the peak value of the motor current under the
fault is no more than 2 times that under the steady-state operation. After several cycles, the
output active power of the generator is stable and the inductive reactive power output is
approximately 1 kVar, as shown in Fig. 9 (b). The fluctuations of voltage and current caused
by the torque imbalance of the rotor shafting are relatively small due to the isolation and
damping effects of the MGP system, and the output power of the DWTG emulator can be
kept stable.

B. OVERVOLTAGE FAULT

A swell of the emulated grid voltage from 220 V to 286 V is set at 19 s, and the emulated
grid voltage recovers to 220 V at 24 s. The results are shown in Fig. 10.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems
Fig. 10 (a) shows that the stator current of the generator increases instantaneously when the
voltage swells. A similar transient process during both the voltage sag and swell is found
when comparing Fig. 9 and Fig. 10. The experimental results show the fluctuation in the
output power during the low-voltage process is larger than that during the overvoltage
process due to the differences in the variation of the voltage magnitude. Overall, the
overvoltage tolerance of the generator is the key to the realization of HVRT. As in Fig. 10
(b), the power of the generator tends to be stable after rapid attenuation while the power of
the motor fluctuates within a small range.

The fluctuations of the voltage and current of the motor, DC link voltage, and frequency are
very small. The above analysis shows that the MGP system isolates the grid fault on the
generator side and maintain steady operation during grid voltage swell. In addition, the
generator absorbs inductive reactive power of approximately 2.5 kVar, which provides strong
support to maintaining the voltage stability of the power grid.

C. MULTIPLE LOW-VOLTAGE FAULTS

The DC link voltage is set as 570 V, and the output active power of the MGP system reaches
900 W. Referring to the blackout in South Australia, multiple low-voltage faults are set at
7.7 s, 33.7 s, 60.7 s, 81.7 s, 87.7 s, and 91.7 s. The six sags of the emulated grid voltage are
set from 220 V to 176 V, 132 V, 132 V, 88 V, 88 V, and 88 V successively. Each voltage
drop lasts one second and then the voltage recovers to 220 V, shown in Fig. 11.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
Renewable Energy Generation Systems

Fig. 11 shows the fluctuation magnitudes on both the motor and generator side increase with
the increasing amplitude of voltage sag during the multiple low-voltage faults. Compared
with the low-voltage faults in the South Australia grid, all six low-voltage faults are ridden
through by the MGP system while the voltage and current fluctuations of the motor are
relatively small. During the last three faults, due to the short time interval between the faults,
the transient process of two adjacent faults appears with superposition effect, which leads to
larger power fluctuations on both sides of the MGP system. Due to the isolation and
protection function of the MGP system, the output power of the DWTG emulator can still
maintain a relatively stable power grid through the MGP system. However, the oscillation
amplitudes in Fig. 11 are quite large since the mechanical connection and quality of the two
synchronous machines are poor. This problem can be solved by referring to the technology
of large capacity synchronous generator applied in the thermal power plant.

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Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of
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CONCLUSION

In this paper, the isolation and protection mechanism and reactive power support function of
the MGP system are analysed based on the rotor motion equation and the phasor diagram of
voltage and current. The following conclusions are drawn through simulation and
experiment.

When a grid fault occurs, the MGP system can isolate the fault at the generator side and
protect the renewable energy units from disconnection. Because the inertia of the MGP shaft
is on the level of second, the voltage and current fluctuations at the motor side caused by the
power imbalance are small, and the output power on both sides is relatively stable with the
adjustment of the DC voltage feedback control. When the grid voltage sags several times
over a short time, the transient process superposes to the next process due to the short fault
interval, and the synchronous generator can still withstand the impact of overcurrent caused
by each fault. With the isolation and damping effect of the MPG system, the voltage and
current of the renewable energy generation system fluctuate slightly. The excitation system
of the synchronous generator can provide not only damping but also reactive power support
during the grid faults, which is conducive to grid voltage recovery.

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