Telemeasurements and telesignals coming from telemechanics devices to the operational information complex are used as the initial data for solving the problem of state assessment. The disadvantage of telemeasurement chopping is that they do not contain angles at voltages that are necessary for assessing the mode parameters in solving a number of technological problems. Implementing the Transitional Modes Monitoring System has provided opportunities to improve solving the problem of state assessing by the data of synchronous vector measurements coming from synchronous vector measurement devices. However, failures on the satellite channel of synchronous vector measurement devices, as well as high load of communication networks result in errors in the arrays of synchronous vector measurements. The question of accuracy in solving the problem of state assessing at the influence of errors in synchronous vector measurements of angles at voltages on the mode parameters is not fully studied and is at the stage of research. State assessment is done using algorithms based on mathematical methods. The existing algorithms do not make it possible to take into account synchronized vector measurements of angles at voltages and require modification. The article presents a modified algorithm developed on the basis of the Gauss–Newton method, making it possible to use telemeasurements and synchronized vector measurements together, to evaluate the impact errors in synchronized vector measurements of angles at voltages on assessing the mode parameters, to determine allowable error intervals according to conditions of ensuring correct estimation mode parameters, to make the conclusion on advisability to use synchronized vector dimensions of angles in solving the problem of state assessing. The algorithm is tested using computational experiments when evaluating such mode parameters as active and reactive load power, voltage module, power flows. On the basis of studies, the error intervals of angles have been revealed, beyond which taking into account the angles at voltages will contribute to improving the accuracy of state estimation. The parameters of the mode that are the most sensory to the increase of errors in synchronized vector measurements of angles are determined.