Masu maimai na muhimmanci suna da tukar-zane na turbin da ke kula da zuba a cikin masana'antu mai gida ta shirya rarraba?
Karamin lura, wanda ya shafi da yawa da kuma yadda ake iya gina tafkin da ita ce, yana cikin abubuwa masu muhimmanci da ake taimaka a kasar China. Tana da ayyuka mai kyau (17,000 bilion ton kafofin karkashin lura) da fadada aiki mai yawa. Aiki na aiki da ake yi a kan photovoltaic, wanda ya faru ne da ake yi a wurare da kuma kan aiki da ake yi a kan hanyoyin kan gida, ta zama aiki da ake yi a wurin birnin da kuma aiki mai tsawon a kan gida.
Littattafan ya nuna bayanai game da split-winding transformers a aiki da ake yi a kan grid-connected photovoltaic power stations tun daga fahimtar ilimi zuwa misalai na injiniringa.
1 Muhimman Abubuwan Main Circuit wa Grid-Connected Photovoltaic Power Stations
Main circuit na photovoltaic power stations yana da alaka da kungiyoyin inverters: distributed inverters sun fi dacewa a wurin birnin, amma centralized inverters sun fi dacewa a desert photovoltaic power stations (don samun aiki mai tsawo a kan uniform illumination via centralized Maximum Power Point Tracking - MPPT).
Amma, ba a halayen da za a iya tabbatar da ingantaccen strings ko inverters da suka sauri, ba zan iya tabbatar da zan yi fa'idar jiki, voltage drop, da kuma cost-performance. Saboda haka, jiki daga strings zuwa combiner boxes zuwa inverters da kuma arewacin photovoltaic blocks yana cikin ratios na investment-return. Don samun aiki mai kyau, capacity na centralized inverters tana da 500 kW zuwa 630 kW.
Grid-connected photovoltaic power stations sun yanayi da uku na main circuit schemes (kamar yadda aka nuna a Figure 1). Single-string scheme (da step-up transformers) yana da tsari, amma ana bukatar transformers da suka sauri. Large-unit scheme (da step-up transformers) yana da tsari, amma ana bukatar transformers da suka sauri. Scheme na large-unit (da step-up transformers) yana cikin designs na yanzu, tana bincike da cost da kuma efficiency da ma'ana.
Littattafan ya nuna abubuwan da ke da split-winding transformers a expanded-unit wiring. Wannan transformer yana da uku na operating modes: through operation, half-through operation, da kuma split operation. Idan an yi parallel branches na split winding don iya gudanar low-voltage winding da zai yi aiki da high-voltage winding, wannan yana nufin through operation, da short-circuit impedance na transformer yana nufin through impedance X1 - 2. Idan branch na low-voltage split winding take yi aiki da high-voltage winding, wannan yana nufin half-through operation, da short-circuit impedance yana nufin half-through impedance X1 - 2'. Idan branch na split winding take yi aiki da branch na biyu, wannan yana nufin split operation, da short-circuit impedance yana nufin split impedance X2 - 2'.
2 Abubuwan da ke da Split-Winding Transformers
Don samun aiki mai kyau, an yi quantitative comparison da ordinary double-winding transformers da technical parameters na products masu matara. A maida 2500 kVA split-winding transformer: 37 ± 2×2.5% / 0.36 kV / 0.36 kV, 50 Hz, short-circuit reactance percentage 6.5%, full-through reactance percentage 6.5%, half-through reactance percentage 11.7%, split coefficient < 3.6%.Calculations give:
Full-through reactance: X1 - 2 = X1 + X2 // X2
Half-through reactance: X1 - 2' = X1 + X2
Per-unit values:
High-voltage side branch reactance:
Low-voltage side branch reactance:
2.1 Reducing Short-Circuit Current
A lokacin da short-circuit yake faru a d1 a Figure 2, short-circuit current yana da uku na components: from the system (high-voltage side, with non-decaying periodic components), non-fault branch I''p1, and fault branch I''p2. For the low-voltage circuit breaker on the fault branch, its breaking capacity considers the sum of system and non-fault branch currents. With a split-winding transformer:
System-supplied short-circuit current:
Inverter-type distributed power short-circuit current is 2–4 times the rated current (duration 1.2–5 ms, 0.06–0.25 cycles), and the non-fault branch current is ~4 kA. For an ordinary double-winding transformer (for comparability, assume uk% = 6.5, same as the full-through reactance percentage of the split-winding transformer uk1 - 2%:
The per-unit reactance is：
The system-supplied short-circuit current is：
with additional contributions from non-fault branches. Clearly, using split-winding transformers for expanded-unit wiring significantly reduces the breaking-capacity requirement for low-voltage side branch circuit breakers.
Assume that the parameters of the parallel modules are completely the same and the MPPT control parameters of the inverters are the same. Then, C1 = C2 = C, L1 = L2 = L, and the inductor current of each inverter is:
It can be seen that the inductor current of each inverter consists of two parts: The first is the load current, which is the same for both inverters; the second is the circulating current, related to the amplitude, phase, and frequency differences of the inverters’ output voltages.
Currently, the main control logic for inverters in PV power stations is Maximum Power Point Tracking (MPPT). Solar cell modules have internal and external resistances. When MPPT control makes these resistances equal at a certain moment, the PV module operates at the maximum power point. Taking Figure 3 as an example, the active power P1 and reactive power Q1 output by Inverter 1 are:
2.3 Maintaining Voltage of Non-Fault Branches
Taking Figures 2 and 3 as examples, photovoltaic power stations usually adopt a centralized inverter-transformer layout, and the cable impedance between the inverter and transformer is negligible. With an ordinary double-winding transformer, the voltage of the non-fault branch drops to zero potential. In this case, relay protection is generally used to delay the operation of the non-fault branch circuit breaker to reduce the fault removal range. However, this method may not meet the protection requirements for photovoltaic power stations. If the removal time of the fault branch exceeds the low-voltage ride-through capability of the inverter, the non-fault branch will be forced to disconnect from the grid, increasing the risk of expanding the fault range.
With a split-winding transformer, due to the existence of split impedance, the short-circuit current provided by the system is equivalent to operating in the half-through mode of the split-winding transformer. The short-circuit current supplied by the non-fault branch inverter is equivalent to the split operation mode of the split-winding transformer. At the moment of short-circuit, the output voltage U''2 of the non-fault branch inverter is I''s × X'2+ I''p2× (X''2 + X'''2). Since the high-voltage side is an infinite system, according to the previous discussion, I''s is much larger than I''p2. Therefore, the first part I''s × X'2 does not decay and is larger than the second part I''p2 × (X''2 + X'''2).
Calculations show that . The output voltage of the non-fault branch inverter can be maintained at least at about 0.5Un. According to the low-voltage ride-through requirements of the photovoltaic power
station, the removal time is greater than 1 s (50 cycles). Thus, the expanded-unit wiring with split-winding transformers can reliably meet the requirement that the non-fault branch does not disconnect from the grid within the removal time of the fault branch circuit breaker.
3 Conclusion
Split-winding transformers are widely used in engineering, especially suitable for grid-connected photovoltaic power stations. As discussed above, their advantages mainly lie in reducing short-circuit current, restricting operating circulating current, and maintaining the voltage of non-fault branches. Based on engineering design examples, this paper theoretically analyzes their application advantages in photovoltaic power stations, providing certain guiding significance for the selection of wiring forms and equipment in grid-connected photovoltaic power station projects.
