This issue comes up often, so here is a summary of the issues. This is not meant as psak – ask your LOR.
How early can one light candles when making “early Shabbos”?
The Tur (Orach Chaim 267, citing Rishonim) and Beis Yosef (Orach Chaim 263) rule that you can only light after Plag Hamincha. In Tur (263), he explains that one cannot light while there is still much time to the day, as in such a case it would not be clear that it is for Shabbos. This position is recorded in Shulchan Aruch (263:4) as well. The source seems to be a Gemara in Shabbos (23b) that argues one cannot light to late (namely, after sundown) or too early. This is understood to mean that it must be somewhat near Shabbos (Rambam Shabbos 5:2). Tur adds, and Rama codifies this as well, that if one lights too early, then he must blow out the candles and light again. Many explain that this is because lighting candles accomplishes both Oneg Shabbos, allowing one to enjoy on Shabbos, and Kavod Shabbos. The latter must be done in advance of Shabbos, but not too early, so that it is clear that the candles are being lit for Shabbos. [See extensively in Tzitz Eliezer 12:27. In 20:21 he argues that Shulchan Aruch would not make you relight, even if one should not light too early, unlike Rama would makes you relight. See Mishpitei Uziel 1 Orach Chaim 7 who believes this is a dispute between Rabbenu Tam and Rambam. See also Yabia Omer 2 Orach Chaim 17.]
However, the Magen Avaraham (Orach Chaim 263:11) cites Mahri Weil who believes one can light early and make the bracha later. Rabbi Akiva Eigar, while he rejects this position, believes that while it is preferable to light later, if one lights early, as long as it is for Shabbos and not for something else, it works. Biur Halacha argues that Rabbi Akiva Eigar only allows lighting after plag, and this position is taken by Olas Shabbos as well. [See also Siddur Yavetz and דה”ח.]
However, Eretz Tzvi (113) points out that the simplest understanding of Rabbi Akiva Eigar is that one is permitted to light even before plag, as long as it is done for the sake of Shabbos. He argues, fascinatingly, that the limitation of plag is only in cases where the mitzvah should be done at night. For example, Channukah candles must be lit at night, and plag is the earliest time that can be plausibly be considered night. However, by Shabbos candles, the mitzvah itself is to light before Shabbos. Therefore, if one were to accept Shabbos at plag, it would follow that he could light shortly before Plag. He further argues that 1) when you light during the day, you don’t benefit until later anyway, so this is not so different. 2) the main benefit is on Shabbos, which you still have. 3) While you should light close to when you make Shabbos, it doesn’t have to be immediate, so a few minutes before is justifiable. He cites Shalah that if you davened mincha and everything is ready for Shabbos, even when you light before plag, it is clear that it is for Shabbos, so it is permitted.
Kinyan Torah adds that nowadays, when candles are not normally lit, it is always evident that the candles are for Shabbos. Therefore, one can light before Plag.
While the generally accepted psak is that one cannot light before plag, if one must, there are those who accept a combination of the above arguments to allow lighting before plag. Shut BiMareh HaBazak (available here) rules this way, as long as one has long enough candles. Alternatively, he suggests appointing a messenger, a non-Jew (and rely on a minority position in Mahari Weil and make the bracha himself later, or set a timer on electric lights, and make the bracha when they go on later.
In conclusion, I would urge not lighting before Plag, but there are positions to rely on to light right before Plag if one is accepting early Shabbos and has no choice.