3.2. BESS Converter Control
During the LVRT process, the main target of the BESS converter control is to support the system voltage, and the frequency support and system stability enhancement should also be taken into account. The following is the generator swing equation:
where
HG is the inertial time constant;
ω is the angular frequency; and
Pm and
Pe are the mechanical power and electrical power, respectively.
When a grid fault occurs, a dramatic drop in the system voltage results in Pe sharply reducing, while Pm can be considered unchanged at the moment; hence, the rotor speed increases, i.e., the system frequency increases. From this aspect, the BESS needs to absorb active power to improve frequency regulation.
According to the requirement of LVRT in the grid code [
9], converter-interfaced equipment should provide a certain reactive current for voltage support, while there are few specific requirements for the active current. Considering the fact that the voltage drop degree can reflect the frequency deviation level during LVRT, and referring to the calculation of the reactive current in the grid code [
9], the active current absorption is designed to be proportional to the value of the AC voltage drop. Therefore, by considering the reactive current control during the LVRT process, the control rule of the BESS converter can be designed as follows:
where
Vt,BA is the converter terminal voltage;
kq and
kd are the proportional coefficients of the reactive and active current references, respectively; and
Imax is the converter current limit defined as in [
27]:
where
In is the rated converter current, and
SBA is the rated converter capacity.
It can be seen from (2) and (3) that, during LVRT, the BESS converter can provide reactive power and, in the meantime, absorb active power. Such a design can not only provide frequency support, but also help improve the system’s transient stability, which will be further demonstrated in
Section 4.
After the fault is cleared, the system frequency and voltage should return to the nominal values as soon as possible. The active and reactive power references,
Pref and
Qref, in
Figure 2 can be calculated as follows [
28]:
where
KBf and
KBV are the droop coefficients of the active and reactive power references, respectively;
fBA and
fBA0 are the measured system frequency and its rated value, respectively; and
Vt0,BA is the rated value of the converter terminal voltage.
Then, the original active and reactive current references,
and
, can be generated via outer PI loops, as shown in
Figure 2. To avoid the current exceeding the maximum value, the current references should also be limited. Traditional methods for current limitation mainly include the following: (1) reactive current injection in priority, which has been adopted during the LVRT process, and (2) active current injection in priority. However, during the recovery process, both the frequency and voltage fluctuations should be well suppressed. Hence, an adaptive current limitation method is proposed in which the active and reactive current references should be limited according to the frequency and voltage deviations. The more the frequency/voltage deviates from the nominal value, the more the active/reactive current reference should be distributed. The coordination index
M is proposed to compare the frequency and voltage deviations in the same dimension:
where
fBA,
f0,
Vt,BA, and
Vt0 are the measured and rated system frequency and converter terminal voltage values, respectively;
fmax and
fmin are the maximum and minimum allowable frequency values, respectively;
Vtmax and
Vtmin are the maximum and minimum allowable voltage values, respectively; and Δ
fdb and Δ
Vt,db are the dead-band ranges of the frequency and voltage, respectively. Specifically, if both
Mf and
MV are 0,
M equals 0. Then, the active and reactive current references can be limited, as shown below, and their maximum limitation values,
and
, changing with
fBA and
Vt,BA, are shown in
Figure 3:
where sign(
x) is the sign function and denotes the sign of the variable
x;
; and
.
Based on (7)–(11) and
Figure 3, it can be seen that
Mf and
MV reflect the system frequency and voltage deviations, while
M reflects the ratios of
Mf and
MV and is also used to adjust the limitation values of the active and reactive currents. In this way, the active and reactive powers can be provided in a proper ratio to support both the system frequency and voltage, and can thus improve the system recovery comprehensively.
Additionally, the trigger signal
SLV,BA in
Figure 2 is used to estimate whether the system is undergoing the LVRT process, and it enables the converter to switch to the corresponding control rules. The signal is obtained via a hysteresis comparator with the terminal voltage as the input signal. Once the system voltage drops to below the threshold value
Vt,LV,
SLV,BA turns to 1 and enables the converter to adopt the control rules in (2) and (3); when the system voltage rises above the threshold value
Vt,rcv,
SLV,BA turns to 0 and the converter switches to the control rules in (10) and (11). The above process corresponds to the “P-Q Coordination Inner Current Limit” and “Mode Switch” blocks in
Figure 2.
Note that the state of charge (SOC) of the BESS may have an impact on the control accuracy of the converter, which has been studied in our previous work [
26]. In this paper, the influence of the SOC on the BESS is ignored.
3.3. Receiving-End Converter Control
Similar to the BESS converter, the receiving-end converter also needs to support the system voltage, while the receiving-end converter also needs to try to maintain the DC voltage and cannot inject active power into the DC link. Hence, the control rule of the receiving-end converter during the LVRT process is as follows [
9,
12]:
where
Vt,RE is the converter terminal voltage;
Imax is the converter current limit and can also be defined as (4), with the subscripts “
BA” replaced by “
RE”; and
SRE is the rated converter capacity.
In this way, the control rules in (12) and (13) enable the receiving-end converter to provide reactive power support as a priority during LVRT. It can be seen that the severity of the system voltage drop has a strong impact on active power transmission, since a deep voltage drop may enlarge the value of and then greatly reduce and the active power output.
During the recovery process, the converter restarts to control the DC voltage. In order to suppress frequency deviation rapidly, the DC voltage can be used to transfer frequency information to the sending-end AC system and wind farm and enable them to participate in frequency regulation, and the DC voltage reference
Vdcref can be calculated as follows [
29]:
where
Vdcref0 is the rated DC voltage reference;
KV is the droop coefficient; and
fRE and
fRE0 are the measured system frequency and its rated value, respectively. In this way, the system frequency deviation can be transferred into DC voltage deviation and can be detected by the sending-end and WF converters.
As for the current reference limitation, considering that the receiving-end converter needs to control the DC voltage and transfer the active power, the active current should be injected as a priority during the recovery process, and the active and reactive current references can be obtained as follows:
where
and
are the original active and reactive current references generated via outer PI loops, as shown in
Figure 2.
Similarly, the trigger signal
SLV,RE in
Figure 2 can also be obtained via the hysteresis comparator with the same threshold parameters as in the BESS converter: when
SLV,RE = 1, the converter adopts the control rules in (12) and (13); otherwise, it adopts (15) and (16). The process also corresponds to the “P-Q Coordination Inner Current Limit” and “Mode Switch” blocks in
Figure 2.
3.4. Sending-End Systems Control
The sending-end systems include the sending-end AC system and WF. Based on the previous analysis and control rules, there are two possible reasons why the DC voltage rises: (1) the receiving-end system is under the LVRT process or (2) the receiving-end system frequency is larger than the rated value. Under either circumstance, the sending-end systems should reduce the power output. In this way, as for the sending-end converter, the power reference
PSEref changes according to the DC voltage as follows [
26]:
where
PSE0 is the initial power reference;
KP is the droop coefficient; and
Vdc,SE and
Vdc0,SE are the measured DC voltage and its rated value, respectively.
As for the WF, to avoid the protective and synchronous problems caused by LVRT in the WF [
20,
21], the WF converter will change the WF frequency reference
fWFref based on the DC voltage instead of changing the WF voltage, and then the WF can adjust its power output reference
PWFref according to the WF frequency deviation [
26,
29]. Moreover, to prevent the WF frequency from exceeding the limitation
fWFmax, a limiter is further added in the WF converter, as shown in
Figure 2. The control rules can be expressed as follows [
26,
29]:
where
fWF0 is the rated WF frequency;
Vdc,WF and
Vdc0,WF are the measured DC voltage and its rated value, respectively;
PWF0 is the initial WTG power output, generally determined by the MPPT control mode;
fWF and
fWF0 are the measured WF frequency and its rated value, respectively; and
Kf and
Kσ are the droop coefficients for the WF converter and WF, respectively.
Based on the control rules in (17)–(19), the sending-end systems can decrease the power output to reduce the power imbalance when a grid fault occurs in the receiving-end system, and they can also adaptively change the power output during the system recovery process to help suppress the frequency deviation.
3.5. DC Chopper Control
When a severe fault occurs in the receiving-end system, the power transmission will be seriously affected, and the DC voltage may rise sharply. At this time, power reduction from the sending-end systems is not enough to keep DC voltage stability. Thus, a DC chopper should be activated for energy dissipation.
The DC chopper is connected to the DC terminal of the receiving-end converter in parallel. The IGBTs and centralized braking resistor are connected in series, and PWM is used to control all the IGBTs, with the absorbed power being determined by the duty cycle [
15]. It should be noted that the adopted topology of the DC chopper may not be the best, but improving the topology is not the key point of this paper. The main purpose of this paper is to demonstrate the effect of the DC chopper and the coordination of each equipment during the LVRT and recovery processes.
It can be seen from the “Duty Cycle Loop” in
Figure 2 that the duty cycle of the PWM is determined by the DC voltage. The higher the DC voltage, the more imbalanced power should be absorbed, and the larger the duty cycle should be. Moreover, since the receiving-end converter will control the DC voltage to transmit system frequency deviation during the recovery process, a hysteresis comparator is also adopted here to avoid misoperation: when the DC voltage rises above the threshold value
Vdc,LV, the trigger signal
SDC turns to 1 and activates the DC chopper for energy dissipation; when the DC voltage drops below the threshold value
Vdc,rcv, then
SDC turns to 0 and blocks the DC chopper, which corresponds to the “Trigger Signal” block in
Figure 2.
3.6. Discussion
Based on the aforementioned design, the control rules of each equipment can coordinate with each other for frequency and voltage support during the LVRT and recovery processes, and the corresponding algorithm flow chart is shown in
Figure 4.
(1) During the LVRT process, the system’s AC voltage decreases rapidly. When detecting that the AC voltage is lower than Vt,LV, the BESS and receiving-end converters will inject a reactive current as a priority with (2), (3), (12), and (13) as per the grid code requirement. Meanwhile, the BESS will absorb certain active power with (3) to help reduce the frequency deviation and improve the system stability if the reactive power output does not reach the converter capacity.
As for the DC side, since the system voltage reduction lowers the power transmission capability of the receiving-end converter, the power imbalance may cause the DC voltage to rise. Consequently, the sending-end and WF converters can detect the DC voltage deviation and change the power outputs to reduce the imbalanced power with (17)–(19). If the DC voltage continues rising and exceeds the threshold value Vdc,LV, the DC chopper will then be activated to absorb the surplus power. In this way, the system dynamics during LVRT in both the AC and DC sides can be improved.
(2) During the recovery process, it is important to suppress the system frequency and voltage fluctuations as soon as possible. When the AC voltage rises above Vt,rcv, both the BESS and receiving-end converters turn to normal/recovery mode and can provide frequency and voltage support with (10), (11), (15), and (16). Considering that traditional current limitation methods take only active or reactive current as a priority, a new current limitation method in (7)–(9) is proposed in which the active and reactive currents can be limited according to the frequency and voltage deviation severity, enabling active and reactive powers to be provided in a proper ratio. Only the local signal measurement is needed, and the calculation is updated in real time.
Moreover, during the recovery process, the DC chopper will be blocked when detecting that the DC voltage is lower than Vdc,rcv. Additionally, the receiving-end converter can further change the DC voltage according to the system frequency with (14) so that the frequency information can be transmitted to the sending-end systems, and AC2 and WF can also participate in frequency regulation with (17)–(19), improving system recovery.
It can be seen that full use has been made of the BESS to improve both the frequency and voltage dynamics during the whole process, which illustrates its significant role in providing system support. Furthermore, it should be noted that the coordination sequence of the sending-end systems and DC chopper has been improved in the proposed control. In the conventional control, the power reduction in the sending-end systems is activated at the same time when the DC chopper is triggered, while in the proposed control, the power output from the sending-end systems will decrease as soon as the DC voltage increases, and the DC chopper will be activated later when the DC voltage exceeds the threshold value. Such a difference is significant to enable the sending-end systems to take part in the receiving-end system frequency regulation during the recovery process and reduce the surplus power that needs to be dissipated by the DC chopper.