5、接地短路
实验内容4同步发电机空载运行，常值励磁电压为400V。在电枢末端发生a相接地短路。假定短路时转子直轴与a相轴重合，并且假定转子转速保持在同步转速不变。利用ode45算法求a相电流和励磁电流的波形（根据程序绘制波形）。 % lgshort.m
function iprime =lgshort(t,i) f=60.; w=2.*pi*f;
d=0.; d=d*pi/180.; theta=w*t+d +pi/2;
% Parameters of a 500 MVA, 30 KV Synchronous Machine
LF = 2.500; LD = 0.0068; LQ = 0.0016; Ld = 0.0072; Lq = 0.0070; MF = 0.100; MD = 0.0054; MQ = 0.0026; MR = 0.1250; L0 =0.002; ra = 0.002; rF = 0.4000; rD = 0.015; rQ = 0.0150;
VF = 400; % DC field voltage V = [0; -VF; 0; 0]; % Voltage column vector K=sqrt(1.5);
Ls=(L0+Ld+Lq)/3; Lm=(Ld-Lq)/3;
R=[ra-2*w*Lm*sin(2*theta) -w*MF*sin(theta) -w*MD*sin(theta) w*MQ*cos(theta) -w*MF*sin(theta) rF 0 0 -w*MD*sin(theta) 0 rD 0 w*MQ*cos(theta) 0 0 rQ];
L=[Ls+Lm*cos(2*theta) MF*cos(theta) MD*cos(theta) MQ*sin(theta)
MF*cos(theta) LF MR 0 MD*cos(theta) MR LD 0 MQ*sin(theta) 0 0 LQ];
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Li=inv(L);
iprime= -Li*V - Li*R*i;
%************************************ fprintf('\\n Wait simulation is in process. \\n') VF = 400; rF = 0.4; iF0 = VF/rF; f = 60; w = 2.*pi*f;
deg = 0; d = deg*pi/180; t0 = 0 ; tfinal = 0.80;
i0 = [0; iF0; 0; 0;]; % Initial currents
%[t,i] = ode45('lgshort', t0, tfinal, i0); % use for Matlab 4
tspan = [t0, tfinal]; % use for Matlab 5 [t,i] = ode45('lgshort', tspan, i0); % use for Matlab 5 ia=i(:,1); iF=i(:,2); % ib = Armature current iF=Field current figure(1), plot(t,ia), xlabel('t, s'), ylabel('i_a, A'), grid
title(['Line-ground short circuit i_a, ','\\delta = ', num2str(deg),'\\circ']) figure(2), plot(t,iF), xlabel('t, s'), ylabel('i_F, A'), grid
title(['Line-ground short circuit i_F, ','\\delta = ',num2str(deg),'\\circ'])
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五、实验总结 在实验内容2中，利用SimPowerSystems/Extra library/Measurements中的“FFT模块”和“三相序分量模块”，绘制出短路电流中的直流分量和倍频分量以及正序、负序和零序分量，每种状态对应的输出如下图。
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