王志刚(1985),男,硕士,高级工程师,从事交直流配电网技术、直流变压器技术、新能源并网技术相关工作(E-mail:
王后生(1984),男,硕士,高级工程师,从事交直流配电网技术、直流变压器技术、电力电子建模分析相关工作
张青杰(1979),男,硕士,高级工程师,从事直流变压器技术相关工作
半桥三电平双有源桥(HBTL-DAB)电路工作时必须解决三电平侧上下直流母线电容均压问题,在传统对称控制模式中,可通过微调占空比的方法,实现带载情况下的电容电压均衡控制。然而在空载情况下,对称控制模式难以从控制策略上实现上下直流母线电容电压的均衡调节。针对该问题,文中提出一种针对HBTL-DAB电路的不对称控制方法,首先从理论上分析对称模式不能均压的原因,进而根据不对称方法计算空载时上下直流母线电容分别在正负半周期内传输的能量偏差。通过调节该能量偏差实现上下直流母线电容电压的均衡控制,并同时指出影响电压均衡控制效果的主要因素。最后通过仿真,对所述不对称控制方法在多种工况下的运行情况进行验证。结果表明,无论是外部持续存在不平衡因素的工况、只存在初始不平衡因素的工况,还是内部存在脉冲误差的工况,不对称控制方法都能实现空载和带载时的电压均衡控制。
To make sure the half-bridge three-level dual active bridge (HBTL-DAB) circuit works correctly, the problem of capacitor voltage equalization between the upper and lower direct current (DC) buses on the three-level side must be solved. In the traditional symmetrical control mode, the capacitor voltage equalization control under load can be achieved by fine-tuning the duty cycle. However, in the case of no-load, the symmetrical control mode is difficult to realize the balance adjustment of the upper and lower DC bus capacitor voltages from the control strategy. Therefore, in view of the above problem, an asymmetric control method for the HBTL-DAB circuit is proposed. Firstly, the reason why the symmetrical mode cannot be equalized is theoretically analyzed. And then according to the asymmetric method, energy deviation on the upper and lower DC bus capacitor between positive and negative half cycles under no-load conditions is calculated. The capacitor voltage equalization control is achieved by adjusting the energy deviation, and the main factors affecting the effect of voltage balance control are also pointed out. Finally, the asymmetric control method is verified under various working conditions by simulation. The results show that no matter in condition of existing external continuous unbalance factors, or external initial unbalance factors, or internal pulse error factors, voltage balance adjustment is achieved by the asymmetric control method under no-load and loaded conditions.
当前已投运的大多数工程中,直流变压器通常采用输入串联输出并联拓扑,即高压侧采用模块串联的形式[
目前已有较多文献研究逆变器相关中点电位平衡问题[
不借助电压平衡器,仅通过控制算法实现空载不平衡控制的方法较为少见。因此文中基于二极管中点箝位型半桥三电平双有源桥(half-bridge three-level dual active bridge, HBTL-DAB)拓扑,提出一种不对称控制策略,可以在空载、双向重载工况下具有较强的中点电压平衡能力[
常用的HBTL-DAB电路如
HBTL-DAB电路拓扑
Topology of HBTL-DAB circuit
由
针对HBTL-DAB电路,文中所述对称控制方式,是指Q1/Q2或者Q3/Q4的驱动脉冲以半周期的中点对称[
但是这种不平衡调节方式存在弊端,即调节能力随负载大小变化,在重载时调节能力最强,随着负载减小调节能力减弱,在空载时失去调节能力。对称控制模式下空载和带载时的均压仿真波形如
对称控制模式下空载和带载工况的均压效果
Balance control effect under no-load and loaded conditions in symmetrical control mode
由
而要在控制上做到主动均衡控制,须分析对称控制方法在空载时不具备调节能力的原因,进而针对性地改进控制算法。第2章将针对空载工况进行重点分析。
参考
对称控制模式下电压电流波形
Voltage and current waveforms in symmetrical control mode
稳态情况下
设定
又可计算传输电感两端的压降为:
记
列写
稳态情况下
式(9)对应到
再代入式(6)和式(7)可得:
记正半周期高电平期间的输出能量为
其中
可知,对称控制模式下,上半周期和下半周期总交换的能量为0,因此不具有电压平衡能力。
不平衡因素可分为2种工况。一种是暂态的不平衡因素,不平衡因素作用时间很短,其导致不平衡发生后立即消失,例如电路状态切换后又进入稳态。另一种是持续的不平衡,例如由于某种原因,触发脉冲不对称,称为内部不平衡因素,或者个别均压电阻失效为断路模式,称为外部不平衡因素。下面针对这2种工况分别进行分析。
针对第2章的问题,文中提出不对称脉冲均衡控制策略,即
不对称控制模式下工况1电压电流波形
Voltage and current waveforms of condition 1 in asymmetric control mode
在Q1和Q2脉冲左侧对齐之后,
工况1一般出现在上下直流母线电容电压已存在不平衡,但是造成不平衡的因素已经消失的条件下,不对称控制将逐步使得电压恢复平衡。
则:
根据式(14)和式(15)可计算出:
所以单周期正负半周能量之差为:
其中
工况2一般出现在上下直流母线电容电压已存在不平衡,并且造成不平衡的因素持续存在的条件下。不对称控制模式下工况2电压电流波形
不对称控制模式下工况2电压电流波形
Voltage and current waveforms of condition 2 in asymmetric control mode
重新定义相关变量,起始时刻
采用3.1节的方法,可计算双周期正负半周能量之差,并化简为:
由式(21)可知,不平衡控制能力主要受制于0.5-
仿真主要参数如
仿真主要参数
Main parameters of simulation
参数 | 数值 | 参数 | 数值 | |
高压侧额定电压 |
1 500 | 额定容量/kW | 80 | |
低压侧额定电压 |
750 | 开关频率 |
3 | |
高频变压器变比 | 1∶1 | 直流母线电容 |
5 | |
传输电感 |
180 | 直流母线电容 |
5 | |
隔直电容 |
200 | 直流母线电容 |
5 | |
励磁电感 |
40 | 死区时间/μs | 2 |
控制策略框图
Block diagram of control strategy
参照
对称控制模式下空载电压电流
Voltages and currents under no-load condition in symmetrical control mode
设置不对称度
不对称控制模式下空载电压电流(2 s时撤除不平衡因素)
Voltages and currents under no-load condition in symmetric control mode (withdrawal of unbalance factor at 2 s)
Δ
Upper and lower direct current bus capacitor voltages when Δ
设置外部不平衡负载同
不对称控制模式下空载和带载电压电流
Voltages and currents under no-load and with-load condition in asymmetric control mode
设置内部不平衡因素,存在脉冲误差情况下的不对称控制效果如
存在脉冲误差情况下的不对称控制效果
Asymmetric control effect in the presence of pulse error
文中针对HBTL-DAB电路在对称控制模式下不能实现空载均压的问题,进行了理论分析和公式推导。由于空载时正负半周期内,电流是对称的,所以无论如何调节占空比,半周期内向外传输的能量为零,因此不能进行均衡调节。
文中提出的不对称控制方法,打破了上述对称性,并计算出了2种空载工况下的能量偏差,指出了影响上下直流母线电容电压均衡控制效果的主要因素。文中针对不对称控制方法进行了多种工况的仿真验证,结果表明无论是外部不平衡因素持续存在,还是内部存在脉冲偏差,不对称控制方法都能获得较好的均压效果。
文中重点对空载不均压情况进行了研究,并未就不对称控制模式下的软开关情况进行分析,下一步将重点研究不对称控制对软开关效果的影响。
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