Intrusion Intolerance - In the world of increasing cyber threats, a compromised protective relay can put power grid resilience at risk by irreparably damaging costly power assets or by causing significant disruptions. We present the first architecture and protocols for the substation that ensure correct protective relay operation in the face of successful relay intrusions and network attacks while meeting the required latency constraint of a quarter power cycle (4.167ms). Our architecture supports other rigid requirements, including continuous availability over a long system lifetime and seamless substation integration. We evaluate our implementation in a range of fault-free and faulty operation conditions, and provide deployment tradeoffs.