Vipi ni faida za transforma ya split-winding katika viwanja vya umeme wa solar photovoltaic?

Nishati ya jua, kama chanzo cha nishati safi na lenye ujuzi wa kurudia, ni moja ya nishati mpya muhimu zinazopendekezwa China. Ina hazina ya ufafanuli kubwa (17,000 bilioni ya nyuzi za mafuta ya kiwango) na uwezo mkubwa wa maendeleo. Uchaji wa umeme kutokana na nishati ya jua, ambao kabla hakika unafanyika kwenye eneo lisilo na umeme katika maeneo magamba, sasa unajitahidi kuelekea kwenye majengo na mipango kubwa yanayofanikiwa kwenye jangwa.

Hii kitabu kuhusu kujadili transforma za split-winding katika steshoni za umeme kutokana na nishati ya jua kwa njia ya ufafanuzi na misaalio ya uhandisi.

1 Maagizo Makuu ya Mzunguko wa Steshoni za Umeme Kutokana na Nishati ya Jua

Mzunguko mkuu wa steshoni za nishati ya jua una uhusiano mkubwa na usakinishaji wa inverters: inverters wadogo ni visivyo viunguwanavyo kwa majengo, na inverters wakubwa ni zaidi yazo kwa steshoni za nishati ya jua katika jangwa (kufanya kwa uwiano wa mwanga uniform via Maximum Power Point Tracking - MPPT).

Lakini, kuwa na strings zaidi au inverters zifuatazo si daima ni faida—faragha ya miamala, kupungua volts, na gharama-yanguku lazima zifikirie. Hivyo, faragha ya miamala kutoka kwa strings hadi kwenye combiner boxes hadi inverters na maeneo ya blocks za nishati ya jua yanatengenezwa kwa awatelezi wa mapato. Kwa ajili ya ufanisi wa kiuchumi, uwezo wa inverters wakubwa huwa unategemea kati ya 500 kW na 630 kW.

Steshoni za umeme kutokana na nishati ya jua zinatumia tatu ya mikakati ya mzunguko mkuu (kama inavyoonyeshwa kwenye Chumba 1). Mikakati ya single-string (na step-up transformers) ni rahisi lakini inahitaji idadi kubwa ya transformers. Mikakati ya unit kubwa (ambayo inachukua step-up transformers) ni mbinu msingi, ikizungumzia gharama na ufanisi vizuri.

Hii kitabu kuhusu kujadili faida za kutumia transforma za split-winding kwa wiring ya expanded-unit. Ingawa kwa transformer wa double-winding wa kawaida, sekta ya kila phase ni moja ya high-voltage winding na mbili ya low-voltage windings. Low-voltage windings huanza na volts na uwezo sawa, lakini na magnetic coupling chache tu kati yao, kama inavyoonyeshwa kwenye Chumba 2.

Transformer hii mara nyingi ana mitaala mitatu: through operation, half-through operation, na split operation. Waktu branches kadhaa za split winding zinajumlisha kwa total low-voltage winding ili kukidhi high-voltage winding, huitwa through operation, na short-circuit impedance ya transformer huitwa through impedance X_{1 - 2.} Waktu branch moja ya low-voltage split winding inakidhi high-voltage winding, huitwa half-through operation, na short-circuit impedance huitwa half-through impedance X_{1 - 2'}. Waktu branch moja ya split winding inakidhi branch nyingine, huitwa split operation, na short-circuit impedance huitwa split impedance X_{2 - 2'}.

2 Faida za Transforma za Split-Winding

Kwa mujibu wa mjadala rahisi, data za parameta teknikal za bidhaa zinazofaa zitumike kwa matumizi ya uchanganuzi wa kiasi na transformers wa double-winding wa kawaida. Chagua transformer wa split-winding wa 2500 kVA: 37 &plusmn; 2&times;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%, na split coefficient < 3.6%.Uchanganuzi:

Full-through reactance: X_{1 - 2} = X₁ + X₂ // X₂

Half-through reactance: X_{1 - 2'} = X₁ + X₂   

Per-unit values:

High-voltage side branch reactance:

Low-voltage side branch reactance:

2.1 Kuongeza Kasi ya Short-Circuit

Wakati short-circuit kwenye d₁ kwenye Chumba 2, current ya short-circuit ina components tatu: kutoka kwa system (high-voltage side, na non-decaying periodic components), non-fault branch I''_p1, na fault branch I''_p2. Kwa circuit breaker ya low-voltage kwenye fault branch, breaking capacity yake huchukua sumu ya currents za system na non-fault branch. Na transforma ya split-winding:

System-supplied short-circuit current:

Inverter-type distributed power short-circuit current ni mara 2-4 ya rated current (duration 1.2-5 ms, 0.06-0.25 cycles), na current ya non-fault branch ni ~4 kA. Kwa transformer wa double-winding wa kawaida (kwa comparability, assume u_k% = 6.5, sawa na full-through reactance percentage ya transformer wa split-winding u_{k1 - 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 U''s > I''s × X'2 = 185 V. The output voltage of the non-fault branch inverter can be maintained at least at about 0.5U_n. 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.