ddlSystemLevelDCP.SelectedIndex=2;

ddlSystemLevelDCP.SelectedValue="2";

ddlSystemLevelDCP.SelectedIndex=2;
ddlSystemLevelDCP_SelectedIndexChanged(null,null);

<asp:DropDownList ID="ddlSystemLevelDCP" runat="server"
AutoPostBack="true"
onselectedindexchanged="ddlSystemLevelDCP_SelectedIndexChanged">
</asp:DropDownList>

ddlSystemLevelDCP_SelectedIndexChanged(ddlSystemLevelDCP,EventArgs.empty);