<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019febe6-ffc7-7532-a79e-147e8f8ea7c7,g:4f28b3da-6634-41d4-9ed8-2c285eb4190a-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:tris="http://www.tristategt.org/20251231" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2024-01-31" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.tristategt.org/20251231">
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/2024-01-31" schemaLocation="https://www.xbrl.org/dtr/type/2024-01-31/types.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/country/2026" schemaLocation="https://xbrl.sec.gov/country/2026/country-2026.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2026" schemaLocation="https://xbrl.sec.gov/dei/2026/dei-2026.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/rxp/2026" schemaLocation="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd"/>
  <xs:annotation>
    <xs:appinfo>
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        <link:roleRef roleURI="http://xbrl.sec.gov/rxp/role/ByProject" xlink:type="simple" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#ByProject"/>
        <link:presentationLink xlink:role="http://xbrl.sec.gov/rxp/role/ByProject" xlink:type="extended">
          <link:loc xlink:type="locator" xlink:label="loc_rxp_AllProjectsMember" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#rxp_AllProjectsMember"/>
          <link:loc xlink:type="locator" xlink:label="loc_tris_ColowyoMineProjectMember" xlink:href="tris-20251231.xsd#tris_ColowyoMineProjectMember"/>
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        </link:presentationLink>
        <link:roleRef roleURI="http://xbrl.sec.gov/rxp/role/ByGovernment" xlink:type="simple" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#ByGovernment"/>
        <link:presentationLink xlink:role="http://xbrl.sec.gov/rxp/role/ByGovernment" xlink:type="extended">
          <link:loc xlink:type="locator" xlink:label="loc_rxp_AllGovernmentsMember" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#rxp_AllGovernmentsMember"/>
          <link:loc xlink:type="locator" xlink:label="loc_tris_ColoradoMember" xlink:href="tris-20251231.xsd#tris_ColoradoMember"/>
          <link:presentationArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="loc_rxp_AllGovernmentsMember" xlink:to="loc_tris_ColoradoMember" xlink:type="arc" preferredLabel="http://www.xbrl.org/2003/role/label"/>
        </link:presentationLink>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
        <link:roleRef roleURI="http://xbrl.sec.gov/rxp/role/GovernmentsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#GovernmentsOnly"/>
        <link:definitionLink xlink:role="http://xbrl.sec.gov/rxp/role/GovernmentsOnly" xlink:type="extended">
          <link:loc xlink:type="locator" xlink:label="loc_rxp_AllGovernmentsMember" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#rxp_AllGovernmentsMember"/>
          <link:loc xlink:type="locator" xlink:label="loc_tris_ColoradoMember_019febe6-ffc7-727a-b76e-e84a38ee4e63" xlink:href="tris-20251231.xsd#tris_ColoradoMember"/>
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        <link:roleRef roleURI="http://xbrl.sec.gov/rxp/role/ProjectsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#ProjectsOnly"/>
        <link:definitionLink xlink:role="http://xbrl.sec.gov/rxp/role/ProjectsOnly" xlink:type="extended">
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          <link:loc xlink:type="locator" xlink:label="loc_tris_ColowyoMineProjectMember_019febe6-ffc7-72ac-87d6-a47c34f8732c" xlink:href="tris-20251231.xsd#tris_ColowyoMineProjectMember"/>
          <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rxp_AllProjectsMember" xlink:to="loc_tris_ColowyoMineProjectMember_019febe6-ffc7-72ac-87d6-a47c34f8732c" xlink:type="arc" order="1"/>
        </link:definitionLink>
        <link:roleRef roleURI="http://xbrl.sec.gov/rxp/role/ResourcesOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#ResourcesOnly"/>
        <link:definitionLink xlink:role="http://xbrl.sec.gov/rxp/role/ResourcesOnly" xlink:type="extended">
          <link:loc xlink:type="locator" xlink:label="loc_rxp_AllResourcesMember" xlink:href="https://xbrl.sec.gov/rxp/2026/rxp-2026.xsd#rxp_AllResourcesMember"/>
          <link:loc xlink:type="locator" xlink:label="loc_tris_CoalMember_019febe6-ffc7-75b0-bc12-89947407a44a" xlink:href="tris-20251231.xsd#tris_CoalMember"/>
          <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rxp_AllResourcesMember" xlink:to="loc_tris_CoalMember_019febe6-ffc7-75b0-bc12-89947407a44a" xlink:type="arc" order="1"/>
        </link:definitionLink>
        <link:roleRef roleURI="http://www.xbrl.org/2009/role/negatedPeriodStartLabel" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedPeriodStartLabel"/>
        <link:roleRef roleURI="http://www.xbrl.org/2009/role/netLabel" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/net-2009-12-16.xsd#netLabel"/>
        <link:roleRef roleURI="http://www.xbrl.org/2009/role/negatedNetLabel" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedNetLabel"/>
        <link:roleRef roleURI="http://www.xbrl.org/2009/role/negatedTerseLabel" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedTerseLabel"/>
        <link:roleRef roleURI="http://www.xbrl.org/2009/role/negatedPeriodEndLabel" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedPeriodEndLabel"/>
        <link:roleRef roleURI="http://www.xbrl.org/2009/role/negatedLabel" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedLabel"/>
        <link:roleRef roleURI="http://www.xbrl.org/2009/role/negatedTotalLabel" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedTotalLabel"/>
        <link:labelLink xlink:role="http://www.xbrl.org/2003/role/link" xlink:type="extended">
          <link:label id="lab_tris_ColowyoMineProjectMember_label_en-US" xlink:label="lab_tris_ColowyoMineProjectMember" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Colowyo Mine Project [Member]</link:label>
          <link:loc xlink:type="locator" xlink:label="loc_tris_ColowyoMineProjectMember" xlink:href="tris-20251231.xsd#tris_ColowyoMineProjectMember"/>
          <link:labelArc xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tris_ColowyoMineProjectMember" xlink:to="lab_tris_ColowyoMineProjectMember" xlink:type="arc" order="1"/>
          <link:label id="lab_tris_ColoradoMember_label_en-US" xlink:label="lab_tris_ColoradoMember" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Colorado [Member]</link:label>
          <link:loc xlink:type="locator" xlink:label="loc_tris_ColoradoMember" xlink:href="tris-20251231.xsd#tris_ColoradoMember"/>
          <link:labelArc xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tris_ColoradoMember" xlink:to="lab_tris_ColoradoMember" xlink:type="arc" order="1"/>
          <link:label id="lab_country_US_label_en-US" xlink:label="lab_country_US" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">UNITED STATES</link:label>
          <link:loc xlink:type="locator" xlink:label="loc_country_US" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
          <link:labelArc xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_country_US" xlink:to="lab_country_US" xlink:type="arc" order="1"/>
          <link:label id="lab_tris_CoalMember_label_en-US" xlink:label="lab_tris_CoalMember" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Coal [Member]</link:label>
          <link:loc xlink:type="locator" xlink:label="loc_tris_CoalMember" xlink:href="tris-20251231.xsd#tris_CoalMember"/>
          <link:labelArc xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tris_CoalMember" xlink:to="lab_tris_CoalMember" xlink:type="arc" order="1"/>
        </link:labelLink>
      </link:linkbase>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="tris_ColoradoMember" abstract="true" name="ColoradoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="tris_ColowyoMineProjectMember" abstract="true" name="ColowyoMineProjectMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="tris_CoalMember" abstract="true" name="CoalMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
</xs:schema>
