买方在何种情况下可以宣告合同无效根据联合国货物

maggiehu2016：买方在何种情况下可以宣告合同无效
根据联合国货物销售合同公约，买方在以下情况下可以宣告合同无效：
　　（a）卖方不履行其在合同或本公约中的任何义务，等于根本违反合同；或
　　（b）如果发生不交货的情况，卖方不在买方按照第四十七条第（1）款规定的额外时间内交付货物，或卖方声明他将不在所规定的时间内交付货物。
　　（2）但是，如果卖方已交付货物，买方就丧失宣告合同无效的权利，除非：
　　（a）对于迟延交货，他在知道交货后一段合理时间内这样做；
　　（b）对于迟延交货以外的任何违反合同事情：
　　（一）他在已知道或理应知道这种违反合同后一段合理时间内这样做；或
　　（二）他在买方按照第四十七条第（1）款规定的任何额外时间满期后，或在卖方声明他将不在这一额外时间履行义务后一段合理时间内这样做；或
（三）他在卖方按照第四十八条第（2）款指明的任何额外时间满期后，或在买方声明他将不接受卖方履行义务后一段合理时间内这样做。
The buyer may declare the contract avoided under such circumstances:
(a) if the failure by the seller to perform any of his obligations under the contract or this Convention amounts to a fundamental breach of contract; or
(b) in case of non-delivery, if the seller does not deliver the goods within the additional period of time fixed by the buyer in accordance with paragraph (1) of article 47 or declares that he will not deliver within the period so fixed.
(2) However, in cases where the seller has delivered the goods, the buyer loses the right to declare the contract avoided unless he does so:
(a) in respect of late delivery, within a reasonable time after he has become aware that delivery has been made;
(b) in respect of any breach other than late delivery, within a reasonable time:
(i) after he knew or ought to have known of the breach;
(ii) after the expiration of any additional period of time fixed by the buyer in accordance with paragraph (1) of article 47, or after the seller has declared that he will not perform his obligations within such an additional period; or
(iii) after the expiration of any additional period of time indicated by the seller in accordance with paragraph (2) of article 48, or after the buyer has declared that he will not accept performance.