With the advent of Blockchain and the Internet of Things (IoT), the Smart Grid is a rapidly growing technology in decentralized energy distribution and trading. However, this advancement came with some serious cyber security challenges and attacks, such as single-point failure due to a centralized architecture of smart grids, slow transaction processing, emerging cybersecurity threats, double-spending, fork, and fault tolerance. We propose a comprehensive framework for the smart grid called SmartGrid-NG to solve all these issues. Instead of using blockchain as a blackbox plugin tool, we also propose a reputation-based blockchain protocol called GridChain to increase the performance of blockchain-based smart grid systems. The security analysis illustrates that the SmartGrid-NG withstands the attacks mentioned above. The performance analysis also states that the consensus delay is reduced to 80%, throughput is increased up to 60%, and computation overhead and energy consumption are reduced up to 70%.