It is time to light up the last section of the Warren and get yourself a new power. First, you have to get to the sewers and that means a long trek out to the farthest reaches of the dark section of town. Stick to the rooftops and go slowly; there are a lot of turret trucks stationed around this area of the Warren.
Once again, your progress to the transformer is unopposed and requires a bit of platforming. After re-establishing the transformer circuit, you learn Polarity Wall. This shield requires no energy to maintain and protects you from most attacks. The only limitation is you can't hold it while in aiming mode and you can't climb while holding it (though you can cling to a climbing surface). To put up the Wall, hold 
.
Exit the sewer
You'll need to use the Wall a lot through the rest of the sewer. You'll come up against a number of gun turrets enclosed in fencing, which you can't shoot through. You have to navigate your way past the gun emplacements while holding the Wall to keep from being killed.
After the first gun turret, you'll reach a section of sewer with two swing arms and two turrets. Once you get past the second turret, there's a short tunnel where you can recharge. Ahead of you is a series of platforms covered by guns. These platforms fall away a few seconds after you land, so you have to hold your Wall and jump quickly from platform to platform.
The enemies you face here include a couple of machine gunners and a bomber. You have to wait for a lull in the shooting and throw some grenades or take a few shots, then get your Wall back up. Once you get past these Dust Men, you're at the substation and can re-energize the last part of the Warren and a portion of the Historic District.