并网逆变器输出偏置

最近学习调试光伏并网逆变器，但是碰到个问题：

epwm的采用UP计数波形输出基本正常，但是采用UPDOWN方式时，输出波形全部偏置到0轴以上，不知道为什么？

采用的正弦表数据，100点：1523,1617,1711,1804,1895,1985,2073,2159,2243,2323,2400,2474,2543,2609,2670,2727,
2778,2825,2866,2902,2933,2958,2977,2990,2998,2999,2995,2984,2968,2946,2918,2885,
2846,2802,2753,2699,2640,2577,2509,2437,2362,2283,2201,2117,2030,1941,1850,1757,
1664,1570,1476,1382,1288,1195,1104,1014,926,840,756,676,599,525,456,390,
329,272,221,174,133,97,66,41,22,9,1,0,4,15,31,53,
81,114,153,197,246,300,359,422,490,562,637,716,798,882,969,1058,
1149,1242,1335,1429